Powder-conveying system for conveying raw-material powder to an installation for manufacturing a three-dimensional workpiece

Abstract

The invention relates to a powder-conveying system for conveying raw-material powder to an installation for manufacturing a three-dimensional workpiece by irradiating layers of the raw-material powder with electromagnetic radiation or particle radiation. The powder-conveying system comprises: a conveyor line which is designed to convey a gas flow, at least along sections, and a powder flow driven by the gas flow, at least along sections; and a conveyor device which is designed to convey the gas flow through the conveyor line. The powder-conveying system also comprises: a first tank, which is connected to the conveyor line, for supplying the installation with powder for an additive manufacturing process; and at least one overflow container, which is connected to the conveyor line, for receiving excess powder accumulating during the additive manufacturing process. The powder-conveying system also comprises: a buffer container which is connected to the conveyor line for supplying a sieve device with powder to be sieved; and the sieve device for sieving the powder to be sieved and for dispensing sieved powder. The powder-conveying system comprises an interface for an external tank, which interface is connected to the conveyor line, for introducing fresh or impure powder into the powder-conveying system. The powder-conveying system also comprises a controller for controlling the powder-conveying system, so that the system carries out at least one of the following conveying processes: a. conveying the sieved powder into the first tank; b. conveying powder from the at least one overflow container into the buffer container; and c. conveying powder from the external tank into the buffer container.

Description

[0001] Powder conveying system for conveying raw material powder to a system for producing a three-dimensional workpiece

[0002] The invention relates to a powder conveying system for conveying raw material powder to a plant for producing a three-dimensional workpiece, in particular for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation (for example a plant for selective laser melting or sintering).

[0003] In additive (or generative) processes for producing three-dimensional workpieces, and in particular in generative layered construction processes, it is known to apply an initially formless or shape-neutral molding compound of a raw material (e.g., a raw material powder) layer by layer to a carrier and to solidify it by site-specific irradiation (e.g., by melting or sintering) in order to ultimately obtain a workpiece of the desired shape. The irradiation can be carried out using electromagnetic radiation, for example, in the form of laser radiation, or by particle radiation, for example, in the form of electron beams. In an initial state, the molding compound can initially be in the form of granules, powder, or liquid molding compound and, as a result of the irradiation, can be solidified selectively or, in other words, site-specifically. In particular, the molding compound can be a bulk material, such as raw material powder.The molding compound can comprise, for example, ceramic, metal, or plastic materials, as well as mixtures of these. One variant of additive layering processes involves so-called laser beam melting in a powder bed (also known as selective laser melting), in which metallic and / or ceramic raw material powder materials, in particular, are solidified into three-dimensional workpieces under the irradiation of a laser beam.

[0004] For the production of individual workpiece layers using selective laser melting, it is also known to apply raw material powder in the form of a layer of raw material powder to a carrier and to irradiate it selectively and in accordance with the geometry of the workpiece layer currently being produced. The laser radiation penetrates the raw material powder and solidifies it, for example, as a result of heating, which causes melting or sintering. Once a workpiece layer has solidified, a new layer of unprocessed raw material powder is applied to the previously produced workpiece layer. Known coating arrangements or powder application devices can be used for this purpose. Subsequently, the now uppermost and still unprocessed raw material powder layer is irradiated again. Consequently, the workpiece is built up successively layer by layer, with each layer defining a cross-sectional area and / or a contour of the workpiece.In this context, it is also known to use CAD or comparable workpiece data in order to produce the workpieces essentially automatically.

[0005] In order to carry out the three-dimensional manufacturing process in a process chamber of the plant, it is necessary that a coater (i.e., a powder application device) of the plant be provided with fresh, uncontaminated raw material powder. Furthermore, it is known that raw material powder that has already been in the process chamber is collected in one or more overflow tanks, then sieved, and finally made available to the process once more as sieved raw material powder.

[0006] In known systems, the powder is transported manually at least in sections and / or the handling of the powder requires that an operator of a powder conveying system has to intervene manually in the conveying process. However, this entails health risks, e.g. through inhalation of the powder, and increases the risk of powder contamination, e.g. through contact of the powder with ambient air. It is therefore desirable to provide a powder conveying system that enables powder transport that is as automatic as possible, in particular between several source or destination tanks and / or that is low-maintenance and / or requires as little user intervention as possible and / or that enables powder transport in which the powder comes into no contact with the ambient atmosphere or only comes into contact with it as little as possible.

[0007] The object of the invention is therefore to provide an improved powder conveying system which solves at least one of the problems described above or a problem related thereto.

[0008] This object is achieved by a powder conveying system having the features of the independent patent claim. Further embodiments are specified in the subclaims.

[0009] According to a first aspect, the invention therefore relates to a powder conveying system for conveying raw material powder to a system for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation. The powder conveying system comprises a conveying line configured to convey, at least in sections, a gas stream and, at least in sections, a powder stream driven by the gas stream, and a conveying device configured to convey the gas stream through the conveying line. The powder conveying system further comprises a first tank connected to the conveying line for supplying the system with powder for an additive manufacturing process and at least one overflow container connected to the conveying line for receiving excess powder from the additive manufacturing process.The powder conveying system further comprises a buffer container connected to the conveying line for supplying a screening device with powder to be screened, and it comprises the screening device for screening the powder to be screened and for dispensing screened powder. The powder conveying system comprises an interface connected to the conveying line for an external tank for introducing fresh or contaminated powder into the powder conveying system. The powder conveying system comprises a control device for controlling the powder conveying system so that it carries out at least one of the following conveying processes: a powder conveying the screened powder into the first tank; b powder conveying from the at least one overflow container into the buffer container; and c powder conveying from the external tank into the buffer container.

[0010] The powder conveying system can be configured to convey the powder pneumatically, at least partially or entirely. Thus, one or more conveying sections can also be configured, for example, mechanically (e.g., in the form of a screw conveyor) or as gravity conveyors. The system can, in particular, be a system for selective laser melting or sintering, which, for example, has one or more of the features described above. Alternatively, the system can be a system for selective electron beam melting.

[0011] The conveyor line may, for example, comprise one or more pipes and / or one or more hoses and / or one or more connectors. The conveyor line may be powder-tight, and in particular gas- and powder-tight, so that no gas or powder can enter or exit the conveyor line (laterally) except through the openings in the conveyor line.

[0012] The fact that the conveying line is configured in sections to convey a powder stream driven by the gas flow can mean that only a section of an entire conveying circuit formed by the conveying line is configured to convey a gas-powder mixture. The remaining part of the conveying circuit essentially conveys only gas (conveying gas), for example, air, protective gas, or an air-protective gas mixture.

[0013] The control device can, for example, comprise a computer. The control device can comprise a processor and a memory, wherein a program is stored in the memory which, when executed, causes the processor to carry out a method according to the details described below. In particular, it should be noted that all method steps described below can be carried out by the control unit. Thus, for example, if it is described below that the conveyor device is stopped, this can mean that the control device is set up to control the conveyor device such that it stops. As a further example, it can be described that a measured value from a sensor is taken into account. This can mean that the control unit is set up to receive the value from the respective sensor and, if necessary, to react depending on the control value.

[0014] The terms "tank," "storage," and "container" are used synonymously herein. In particular, the terms "overflow container" and "overflow tank" have the same meaning. All of the above-mentioned terms ("tank," "storage," and "container") each refer to a container designed to hold a predetermined maximum amount of powder, whereby the use of the respective term does not make any (limiting) statement about the size of this maximum powder quantity of the respective container. However, the different containers are referred to using specific terms, for example, "first tank," "buffer container," "main storage," and "external tank." However, these individual terms are not to be understood as limiting, but merely serve to differentiate the various containers from one another. For example, the "buffer container" could also be referred to as the "second tank."Thus, no structural differences are intended, for example, between a "tank" and a "container." In particular, a corresponding tank can also be formed by a section of the conveyor line or a section of a cyclone, if this section is designed to hold a predetermined amount of powder.

[0015] For each of the conveying processes a, b, and c, associated sections of the conveying line can be provided. In other words, each conveying process a, b, and c can be assigned an associated section of the conveying line through which conveying gas flows during the conveying of the respective conveying process. Thus, in connection with the conveying processes a, b, and c, one can also speak of corresponding conveying circuits a, b, and c. The conveying circuits a, b, and c can share sections of the conveying line. In other words, there can be at least one section of the conveying line that is used by at least two of the conveying processes a, b, and c.

[0016] Thus, the following applies: Each of the conveying processes a, b, or c can be assigned a corresponding conveying circuit a, b, or c, respectively. Each conveying circuit corresponds to a closed section of the conveying line. At least two of the conveying circuits can share one or more sections of the conveying line, i.e., they can use this section(s) jointly.

[0017] Given the above understanding, the terms "support group" and "support process" are used synonymously in some places in this disclosure. When a specific support process is mentioned, it is clear that a corresponding support group is implicitly present, and vice versa.

[0018] Conveying according to conveying processes a, b and c can be initiated or enabled by opening and / or closing corresponding valves. For this purpose, corresponding valves can be provided at suitable points in the conveying line. If, for example, conveying gas is to be conveyed through a specific conveying circuit, valves located in the conveying line along this conveying circuit can be opened. At the same time, valves that separate the specific conveying circuit from other conveying circuits can be closed so that no conveying gas flows through one or more conveying circuits through which no powder is to be conveyed. Additionally or alternatively, sections of conveying processes a, b and c or even one or more of conveying processes a, b and c can be designed entirely as non-pneumatic conveying processes.Conveying by means of a screw conveyor can be started, for example, by starting the screw conveyor motor and stopped by stopping the motor. Conveying by means of gravity conveying can be started, for example, by opening a flap or valve and stopped by closing the flap or valve.

[0019] The first tank can be arranged above a process chamber of the system. The first tank can be arranged inside a housing of the system or outside of it. In addition to the first tank, another container (herein also an intermediate tank) can be arranged above the process chamber, wherein powder can be supplied to the intermediate tank from the first tank. In particular, the first tank can be located above the intermediate tank. The first tank can be located outside a housing of the system, and the intermediate tank can be located inside the housing.

[0020] Either (a) all of the conveying processes ac may contain pneumatic conveying processes, wherein a conveyed powder is conveyed by the gas flow, or (b) at least one of the conveying processes ac, in particular the conveying process b, may not contain a pneumatic conveying process.

[0021] In case (b), the conveying process which does not include a pneumatic conveying process may include conveying by a screw conveyor and / or conveying by gravity conveying.

[0022] Thus, there is at least one embodiment in which all of the conveying processes ac are configured as pneumatic conveying lines, which are designed to convey the gas flow and, at least in sections, a powder flow driven by the gas flow. However, powder can be fed (in particular, metered) into the respective conveying process ac via a conveyor screw. Furthermore, one of the conveying processes ac, for example, process b, can be carried out entirely non-pneumatically, for example, via a correspondingly provided conveyor screw and / or via gravity conveying.

[0023] The powder conveying system may further comprise a main reservoir connected to the conveying line for receiving the screened powder. The conveying process a may convey the screened powder from the main reservoir into the first tank.

[0024] If no main storage facility is available, the conveying process a can, for example, remove the sieved powder directly from the sieve and convey it to the first tank, particularly by means of pneumatic conveying. To remove the powder from the sieve, the sieve can have a corresponding container for the sieved powder.

[0025] The following explanations apply in particular to a powder conveying system in which all conveying processes ac are configured as pneumatic conveying processes. However, if technically feasible, the following explanations can equally apply to a powder conveying system in which at least one of the conveying processes ac is configured as non-pneumatic conveying (e.g., as a screw conveyor).

[0026] The powder conveying system can enable closed powder conveying. In particular, the powder conveying system can enable powder conveying in which conveyed powder does not leave the powder conveying system throughout several consecutive additive manufacturing processes of the system. The term "closed powder conveying" can thus be understood and replaced by the description of powder conveying in which conveyed powder does not leave the powder conveying system throughout several consecutive additive manufacturing processes of the system. This minimizes operator intervention.

[0027] The powder conveying system can be configured to maintain an inert gas atmosphere within the conveying line, particularly during one of the conveying processes a to c. The inert gas can be nitrogen or argon. The conveying line can thus be inert gas-tight, and one or more inert gas sources can be provided to flood the conveying line with inert gas.

[0028] The powder conveying system may further comprise a pressure equalization tank coupled to the conveying line. The pressure equalization tank can reduce excess pressure within the conveying line.

[0029] The pressure equalization tank may be coupled to the conveying line downstream of the conveying device and upstream of the connections of the first tank, the at least one overflow tank, the buffer tank, the main storage tank and the external tank.

[0030] The powder conveying system may further comprise at least one metering device, in particular comprising a conveyor screw, for metered feeding of the powder to be conveyed into the conveying line from the at least one overflow tank, from the main storage and / or from the external tank.

[0031] As an alternative to a screw conveyor, the dosing device can comprise, for example, a dosing screw, a vibrating conveyor or a tube chain conveyor.

[0032] The powder conveying system may further comprise at least one separation device, in particular comprising a cyclone, for separating the conveyed powder from the conveying line and for feeding the powder into the first tank and / or into the buffer container.

[0033] As an alternative to a cyclone, the separation device may comprise, for example, a centrifugal separator, a filter separator or an electrostatic precipitator.

[0034] At least one of the following tanks may be coupled to a pressure equalization line: the first tank, the at least one overflow tank, the buffer tank, the main storage tank and the external tank.

[0035] The processes presented below, in particular for controlling the powder conveying system according to the first aspect, can also be used independently of the system of the first aspect. In particular, one or more of the following aspects can be used in a powder conveying system that enables powder to be conveyed from a source tank to a destination tank by means of pneumatic conveying.

[0036] The control device may be configured to perform a flow check, comprising checking a flow through the conveying line.

[0037] The powder conveying system may further comprise a velocity sensor for measuring a velocity of the gas flow in the conveying line. Flow checking comprises opening valves of the powder conveying system to allow flow through at least one conveying circuit associated with one of the conveying processes a, b, and c. Flow checking further comprises setting a conveying velocity of the gas flow to a predetermined range or value and determining whether a velocity value of the gas flow measured by the velocity sensor is greater than a predetermined limit.

[0038] Adjusting the conveying speed may include: determining whether a speed value measured by the speed sensor is within the predetermined value range; if not, determining whether the speed value is below the predetermined value range; if the speed value is below the predetermined value range, increasing the power of the conveying device; and if the speed value is not below the predetermined value range, decreasing the power of the conveying device. The powder conveying system may further include a pressure sensor for measuring a pressure of the gas stream and a filter for filtering remaining powder particles from the gas stream.The flow check may include: determining a total pressure loss based on sensor data from the pressure sensor if it is determined that the total pressure loss exceeds a predetermined limit, performing a filter cleaning of the filter and again determining a total pressure loss, and if it is determined that the total pressure loss does not exceed the predetermined limit, conveying powder in at least one of the conveying processes a, b and c.

[0039] The flow check performed by the control device may further include, if it is determined that the total pressure loss exceeds the predetermined limit, outputting an error before performing the filter cleaning.

[0040] The powder conveying system may include at least one oxygen sensor for measuring an oxygen content of the gas stream. The control device may be configured to perform an oxygen content check, and the oxygen content check may include measuring an oxygen content using the at least one oxygen sensor.

[0041] The powder conveying system may comprise a first oxygen sensor for measuring the oxygen content of the gas stream and a second oxygen sensor, redundant to the first oxygen sensor, for measuring the oxygen content of the gas stream.

[0042] The control device can be configured to carry out a pipeline inerting of the conveying line if the measurement of the oxygen content of the gas stream shows that the oxygen content is above a predetermined limit value.

[0043] Alternatively or additionally, the material can also be flooded with protective gas.

[0044] The powder conveying system may further comprise a vent valve for venting gas from the conveying line to an environment or an external volume. Pipeline inerting may comprise opening the vent valve. A filter may be provided downstream of the vent valve (i.e., downstream of the vent valve).

[0045] The pipeline inerting may further comprise: evacuating at least a portion of the conveying line, leak testing of the conveying line, normalizing the conveying line, and checking an oxygen content in the conveying line.

[0046] Evacuation may include checking whether a measured pressure within the conveying line is below a predetermined evacuation pressure.

[0047] The powder conveying system may further include a velocity sensor for measuring a velocity of the gas flow in the conveying line. Evacuating may include: determining whether a velocity value measured by the velocity sensor is within a predetermined value range; if not, determining whether the velocity value is below the predetermined value range; if the velocity value is below the predetermined value range, increasing the power of the conveying device; and if the velocity value is not below the predetermined value range, decreasing the power of the conveying device.

[0048] The leak test may include: closing the drain valve, stopping the conveying device, determining whether a system pressure increase per time exceeds a predefined limit, if the system pressure increase per time exceeds the predefined limit, issuing an error message, and if the system pressure increase per time does not exceed the predefined limit, proceeding to normalize the conveying line.

[0049] The powder conveying system may include a first pressure sensor on an inlet side of the conveying device and a second pressure sensor on an outlet side of the conveying device. Determining the system pressure increase may include considering a sum of the measured values ​​from the first and second pressure sensors.

[0050] Normalizing the production line may include: flooding the production line with inert gas and measuring an oxygen content in the production line.

[0051] The powder conveying system can further include restarting the pipeline inertization if the measured oxygen content in the conveying line exceeds a predetermined limit. Optionally, inert gas can be added before restarting the pipeline inertization. If the predetermined limit is still exceeded, the pipeline inertization is restarted. If the limit is then no longer exceeded, conveying can be started, for example, or the pipeline inertization can be skipped.

[0052] Checking the oxygen content in the production line may include restarting the pipeline inerting if the measured oxygen content exceeds a predetermined limit a predetermined number of times.

[0053] The control device can be configured to carry out a conveying operation according to at least one of the conveying processes a, b and c, wherein the conveying operation comprises opening at least one valve which is arranged in a conveying circuit associated with the respective conveying process.

[0054] The control device can be configured to carry out a conveying control during conveying, which comprises: monitoring an oxygen content in the conveying line, monitoring a pressure in the conveying line, monitoring a conveying speed of the gas flow, monitoring at least one measured characteristic of the powder conveying system, and checking a termination condition for terminating the conveying.

[0055] Monitoring the oxygen content in the production line may include determining whether a measured oxygen content exceeds a predetermined limit, and if the measured oxygen content exceeds the predetermined limit, opening a valve to supply inert gas to the production line.

[0056] Monitoring the pressure in the production line may include determining whether a measured pressure in the production line is within a predetermined range; if the measured pressure in the production line is less than the predetermined range, opening a valve to supply inert gas; and if the measured pressure in the production line is higher than the predetermined range, opening a vent valve to vent gas from the production line.

[0057] Monitoring the conveying speed of the gas flow may include: determining a gas density of the gas flow based on at least one measured characteristic of the gas flow, determining a powder mass flow conveyed through the conveying line based on a control value applied to a metering device of an originating tank of the conveyance, determining a desired speed of the gas flow based on the gas density and based on the bulk material mass flow, and controlling the conveying device to convey the gas flow at the determined desired speed.

[0058] Monitoring at least one measured characteristic of the powder conveying system may comprise: controlling a dosing device of a source tank of the conveyance with a predetermined control value, determining whether the at least one measured characteristic of the powder conveying system is above a predetermined maximum value for the respective characteristic, and if the at least one measured characteristic is above the predetermined maximum value, reducing the control value of the dosing device by a predetermined value so that the dosing device delivers a smaller dose of powder per unit time into the gas stream.

[0059] The control device can further be configured to, if the at least one measured characteristic value is below the predetermined maximum value, increase the control value of the dosing device by a predetermined value so that the dosing device releases a higher dose of powder per unit of time into the gas stream.

[0060] The at least one parameter may include at least one of the following parameters: conveying speed, pump outlet pressure, pump power and dose of powder per time.

[0061] Checking a termination condition for terminating the conveying may include terminating the conveying by stopping the conveying device if at least one of the following events is detected: a fill level of a source tank from which powder is taken for conveying falls below a predetermined limit, a fill level of a target tank into which the powder is conveyed exceeds a predetermined limit, a predetermined maximum conveying time is exceeded and a total pressure loss of a conveying gas exceeds a predetermined limit.

[0062] The control device can be configured to perform pipe cleaning and filter cleaning after completion of conveying according to conveying process a, b, and / or c. The pipe cleaning comprises flowing a gas stream through the conveying line, and the filter cleaning comprises blowing off a filter provided in the conveying line with compressed air. The powder conveying system can further comprise at least one dew point sensor for measuring a relative humidity within the conveying line and / or within a tank of the powder conveying system. The control unit can be configured to initiate automatic powder drying if the measured relative humidity value exceeds a predetermined limit.

[0063] Powder drying can be carried out using a drying unit of the powder conveying system. For this purpose, the drying unit can comprise a heating unit and / or a moisture absorption agent. Furthermore, a vacuum generation unit can be used as the drying unit. Powder drying can be stopped when the dew point sensor measures a predetermined residual moisture content, i.e., when a measured moisture value falls below a predetermined threshold.

[0064] A dew point sensor may be provided on at least one of the following components of the powder conveying system: main tank, external tank, and conveyor.

[0065] At least one oxygen sensor may be provided for measuring an oxygen concentration in at least one of the following components of the powder conveying system: first tank, overflow tank, buffer tank, main reservoir, external tank, and screening device. The control device may be configured to initiate flooding of the conveying line with inert gas if it is determined that at least one of the provided oxygen sensors measures an oxygen concentration above a predetermined limit.

[0066] At least one pressure sensor for measuring a pressure can be provided on at least one of the following components of the powder conveying system: first tank, overflow tank, buffer container, main reservoir, external tank, and screening device. The control device can be configured to perform at least one of the following steps if it is determined that at least one of the provided pressure sensors measures a pressure increase per predetermined time unit above a predetermined limit: issuing a warning, flooding the conveying line or an affected section of the powder conveying system with inert gas, measuring an oxygen content in the affected section of the powder conveying system, and performing a filter cleaning. At least one temperature sensor for measuring a temperature can be provided on at least one motor of the powder conveying system, in particular a motor of a conveyor screw and / or the conveying device.The control device can be configured to initiate a shutdown of the conveying device if it is determined that at least one of the provided temperature sensors measures a temperature value above a predetermined limit value.

[0067] The control device can be configured to initiate a shutdown of the conveying device if at least one of the following events is detected: a torque of a motor of a dosing device exceeds a predetermined limit value, a valve of the powder conveying system is in an actual position that does not correspond to its target position, and a fault in a sensor is detected.

[0068] A fill level sensor for measuring a fill level of the at least one overflow tank can be provided on the at least one overflow tank, in particular in the form of one or more load cells, wherein a fill level sensor for measuring a fill level of the intermediate tank is provided on an intermediate tank which is arranged above the process chamber and which is designed to supply the process chamber with powder and which is designed to be supplied with powder from the first tank, in particular in the form of one or more load cells.The control device can be configured to continue an additive manufacturing process of the system in the event of a failure of the powder conveyance by the conveying device until at least one of the following events occurs: the fill level sensor of the overflow tank detects that the fill level of the overflow tank exceeds a predetermined limit value, and the fill level sensor of the intermediate tank detects that the fill level of the intermediate tank falls below a predetermined limit value.

[0069] The control device can be configured to carry out a powder conveyance of a predetermined powder quantity according to one of the conveying processes a, b or c and then to stop the conveyance.

[0070] The control device can be configured to determine a priority value for each of the conveying processes after each conveying operation according to one of the conveying processes a, b, and c, based on predetermined fill level limits of the source tanks and the destination tanks. The control device can be configured to subsequently perform conveying according to the conveying process with the highest priority value.

[0071] The system described herein for producing a three-dimensional workpiece by irradiating layers of raw material powder with electromagnetic radiation or particle radiation can, for example, comprise a carrier device for applying the powder in multiple layers to form a powder bed. Furthermore, one or more powder application devices can be provided for applying the powder and, if necessary, for applying powder of different materials. A separate powder application device can be provided for each material. The carrier device can be moved vertically downwards by means of a lifting device so that the uppermost powder layer always remains at the same height relative to a build chamber of the system. Furthermore, the system can comprise one or more irradiation units.The irradiation units each comprise a beam source (in particular a laser beam source) and an optics system with one or more optical components for shaping and deflecting the beam (e.g., beam expander, focusing unit, scanner device, F-theta lens). Alternatively, the beam source can be located outside the respective irradiation unit, with the beam being fed to the irradiation unit via an optical guide (e.g., fiber optic cable).

[0072] The invention is explained below with reference to the accompanying figures. They depict:

[0073] Figure 1: a schematic overview of a powder conveying system for conveying raw material powder to a system for producing a three-dimensional workpiece;

[0074] Figure 2: a flow diagram of a higher-level function of the powder conveying system;

[0075] Figure 3: a flow test of the conveyor circuit a or a conveyor in the conveyor circuit a, with active elements of the powder conveyor system according to Fig. 1 highlighted in bold;

[0076] Figure 4: a flow test of the conveying circuit b or a conveying operation in the conveying circuit b, with active elements of the powder conveying system according to Figure 1 highlighted in bold; Figure 5: a flow test of the conveying circuit c or a conveying operation in the conveying circuit c, with active elements of the powder conveying system according to Figure 1 highlighted in bold;

[0077] Figure 5a: a schematic overview of an alternative embodiment of a powder conveying system for conveying raw material powder to a system for producing a three-dimensional workpiece;

[0078] Figure 6: a flow chart of a leak test;

[0079] Figure 7: a pipe evacuation, with active elements of the powder conveying system according to Fig. 1 highlighted in bold;

[0080] Figure 8: a flow chart of an oxygen content check;

[0081] Figure 9: a flow diagram of an adjustment of a conveying gas velocity;

[0082] Figure 10: a flowchart of a limit control

[0083] Figure 11: a plant for producing a three-dimensional workpiece by irradiating layers of a raw material powder with electromagnetic radiation or particle radiation, which is equipped with a bulk material conveying system;

[0084] Figure 12: a schematic representation of a dosing device in the form of a conveyor screw, wherein parameters of the conveyor screw are indicated for calculating a mass flow conveyed by the conveyor screw; and

[0085] Figure 13: a flowchart of a limit control process;

[0086] Figure 14: a system for producing three-dimensional workpieces by exposing layers of raw material powder to electromagnetic radiation or particle radiation, which is equipped with a powder processing system including a screening device; Figure 15: a detailed view of the screening device used in the powder processing system according to Figure 14;

[0087] Figure 16: a loose powder bed on one level;

[0088] Figure 17: the screening device according to Figure 15 with a continuous drive of the screen with a low (second) drive power;

[0089] Figure 18: the screening device according to Figure 15 with a continuous drive of the screen with a high (first) drive power;

[0090] Figure 19: the screening device according to Figure 15 in operation, wherein the screening device is driven periodically alternately with the first drive power and the second drive power;

[0091] Figure 20: the development of the drive power (top) and the dosing mass flow (bottom) as a function of time when the dosing mass flow is controlled as a function of the drive power;

[0092] Figure 21: the development of a step response of a sum of a screened powder mass flow and an oversize mass flow to a dosing mass flow in the case of a defect in a screen mesh; and

[0093] Figure 22: an alternative embodiment of a screening device comprising a housing and a screen mounted so as to be tiltable relative to the housing, the angle of inclination of which screen can be adjusted relative to the horizontal plane.

[0094] Fig. 1 shows a schematic representation of a powder conveying system which is used for conveying raw material powder in a system 1100 for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation.

[0095] In the system shown in Fig. 1, multiple conveying processes can be carried out within the system, i.e., conveying processes from different source tanks to different destination tanks. The exact structure and functionality of the powder conveying system is described below. Not shown in Fig. 1 (but part of the system) is a control device for the powder conveying system. This control device controls the individual components of the system shown in Fig. 1, in particular the valves, dosing devices, and conveying device described below. Furthermore, the control device receives and processes data from the sensors described below.

[0096] The system 1100 corresponds, for example, to a generally known system for additive manufacturing using selective laser melting or selective laser sintering. Only the process chamber 1011 of the system 1100 is shown in Fig. 1. Raw material powder is fed from an intermediate tank 2102 (also called a hopper) of the system 1100 to the process chamber 1011 (more precisely, a powder application device or coater in the process chamber 1011) by means of a loader 2104.

[0097] Excess powder from the additive manufacturing process can be collected in a front overflow tank 2 or a rear overflow tank 3. This particularly concerns powder that is not needed when coating a new powder layer and is thus pushed by a powder application device into one of the overflow tanks 2 or 3. An inlet valve 19 or 17 is located at an inlet of the respective powder tanks 2 or 3.

[0098] The overflow tanks 2 and 3 each comprise an outlet valve 34 and 25, respectively. Furthermore, a powder sensor 35 and 26, respectively, is provided in the area of ​​the respective outlet of the overflow tanks 2 and 3. This monitors the inlet of an associated dosing device in the form of a dosing screw (or conveyor screw) 36 and 27, respectively. A powder sensor 37 and 28, respectively, is also provided at the outlet of the respective dosing screw 36 and 27, respectively.

[0099] The overflow tanks 2 and 3 each comprise an upper powder sensor 31 or 22 and a lower powder sensor 32 or 23 for monitoring the fill level of the respective overflow tank 2 or 3. In addition, weighing cells 33 or 24 are provided on the respective overflow tanks 2 or 3. To carry out pressure equalization, a valve 30 or 21 is located on a respective upper side of the overflow tanks 2 or 3. The respective overflow tank 2 or 3 is connected to the pressure regulator via the valve 30 or 21.

[0100] 3 is connected to a delivery line. For this purpose, an additional valve 38 or 29 is located at the respective inlet to the delivery line. The process chamber is connected to a line via corresponding valves 20 or 18 for pressure equalization, which connects valve 30 or 21 to valve 38 or 29.

[0101] The powder is conveyed from the overflow tanks 2 and 3 into a buffer tank 4 via the conveyor line. A cyclone 42 is located above the buffer tank 4 to separate the powder.

[0102] After the powder has been separated in the cyclone 42, the conveying gas flows through the conveying line into a filter 41, where any remaining powder particles are filtered out. For filter cleaning, a compressed gas supply 39 is in contact with the filter 41, which can be activated by the control device. Furthermore, a valve 40 is located downstream of the filter 41. The conveying circuit leads back to the conveying device 79, which is provided in the form of a pump 79. Starting from the valve 40, the conveying gas path leads, in this order, via a pump protection filter 74, a dew point sensor 75, an oxygen sensor 76, a redundant oxygen sensor 77, and a pressure sensor 78, more precisely, a pressure sensor 78 on the pump suction side.

[0103] Starting from pump 79, the conveying gas flows further via a pressure sensor 80 on the pump pressure side (not shown separately on the pump pressure side in the schematic representation of Fig. 1). A pressure equalization tank 84 is provided downstream of the pressure sensor 80. This is equipped with an inert gas supply 82 and a safety valve 83. The pressure equalization tank 84 serves to prevent excessive overpressure on the pump pressure side, i.e., downstream of pump 79.

[0104] An exhaust valve 81 is provided between pressure sensor 80 and pressure equalization tank 84, through which gas can be vented from the conveying line. Further downstream of pressure equalization tank 84 is a velocity and temperature sensor 86. Following venting access via valves 88 and 87, there is a possibility of directing the conveying gas to convey powder from a main reservoir 6 by opening a valve 72 and / or to convey powder from an external tank 8 and / or from one or both of the overflow tanks 2 and 3 by opening a valve 73.

[0105] When valve 72 is open, the conveying gas, after powder from the main reservoir 6 has been introduced into the gas stream via valve 60, flows to a cyclone 9 located above a first tank 1. There, powder can be separated for conveying into the first tank 1. The conveying gas flows via the cyclone 42 and through filter 41 back to the pump 79, along the previously described path (past several sensors, among other things).

[0106] When valve 73 is open, powder from an external tank 8 can be added to the conveying gas flow by a metering device 70 (conveyor screw 70). Furthermore, powder from the overflow tank 3 can be added via the conveyor screw 27 and / or powder from the overflow tank 2 via the conveyor screw 36. The conveying gas-powder mixture then flows—as previously described—to the cyclone 42, where it can be conveyed into the buffer tank 4.

[0107] Powder leaving the conveyor screw 58 of the main storage 6 can be fed via a valve 60 either to the conveying gas of the conveying line which is associated with the valve 72, or alternatively or additionally via a valve 61 to the conveying gas of the conveying line which is associated with the valve 73.

[0108] Powder separated by cyclone 9 enters the first tank 1 via an inlet valve 12. From there, it is conveyed via an outlet valve 16 into an intermediate tank 2102 (also known as a hopper), for example, by gravity. From the intermediate tank 2102, it can be fed via the loader of the process chamber 1011 of the system 1100 and used in the additive manufacturing process.

[0109] Powder separated by the cyclone 42 enters a buffer tank 4 via an inlet valve 44. From the buffer tank 4, the powder can be fed (by gravity) through an outlet valve 48 to a screening device 5. The screening device 5 serves to filter out larger particles and impurities from the powder, particularly if the powder fed to the screening device 5 has already been used in an additive manufacturing process and, for example, originates from one or both of the overflow tanks 2 and 3.

[0110] The screening device 5 comprises a dosing device 49 in the form of a screw conveyor 49 for metered feeding of powder to be screened to an ultrasonic screen 50 of the screening device 5. Screened powder or particles are conveyed via a line into an oversize container 7.

[0111] The sieved powder enters the main storage 6 via an inlet valve 52. From the main storage 6, the powder can be fed via an outlet valve 56 and via the conveyor screw 58 of the conveyor line for pneumatic conveying into the first tank 1, as described above.

[0112] Furthermore, an external tank 8 can be connected to the conveyor line. This can, for example, be a movable tank that can be connected to the conveyor line via a suitable interface. Alternatively, a movable tank can be connected to the external tank 8 via an inlet valve 63 of the external tank 8, so that, for example, fresh powder can be supplied to the system.

[0113] Powder from the external tank 8 is fed to the conveyor screw 70 via an outlet valve 67 of the external tank 8 and via an inlet valve 68 of a conveyor screw 70. From there, the powder enters the conveyor line and is conveyed by the gas flow when valve 73 is open.

[0114] Each of the powder containers 1, 2, 3, 4, 6, and 8 has an inlet valve 12, 19, 17, 44, 52, and 63 and an outlet valve 14, 34, 25, 48, 56, and 67. For pressure equalization, each of the powder containers 1, 2, 3, 4, 6, and 8 further has a valve 10, 11, 30, 21, 43, 51, and 62, respectively. Via these pressure equalization valves, the respective powder container is connected via a respective pressure equalization line to the conveying line in which the gas stream of the conveying gas flows, except in the case of the first tank 1, whose pressure equalization line is connected to the intermediate tank.

[0115] The pressure equalization lines of the powder containers 2, 3, 4, 6 and 8 are connected to the conveyor line via an associated valve 38, 29, 8 (the same for containers 4 and 6) and 87.

[0116] For measuring a fill level of the powder containers 1, 2, 3, 4, 6 and 8, each of these powder containers 1, 2, 3, 4, 6 and 8 comprises an upper powder sensor 13, 31, 22, 45, 53 and 64 and a lower powder sensor 14, 32, 23, 46, 54 and 65.

[0117] Furthermore, each of the powder containers 1, 2, 3, 4, 6, and 8 is equipped with load cells 15, 33, 24, 47, 55, and 66. These serve to weigh the respective container and thus also to determine the fill level of the respective container.

[0118] In particular, to prevent and / or detect blockages, a powder sensor 35, 26, 57, and 69 is provided at the inlet of the screw conveyors 36, 27, 58, and 70, and a powder sensor 37, 28, 59, and 71 is provided at the outlet of the screw conveyors 36, 27, 58, and 70. The individual elements of the powder conveyor system shown in Fig. 1 are listed again below using their reference numerals, although this list does not claim to be complete.

[0119] 1 - First tank

[0120] 2 - front overflow tank

[0121] 3 - rear overflow tank

[0122] 4 - Lock

[0123] 5 - Sieve

[0124] 6 - Main memory

[0125] 7 - Oversize barrel (oversize container)

[0126] 8 - external tank module (external tank)

[0127] 9 - Cyclone

[0128] 10 - Pressure equalization valve for the first tank

[0129] 11 - Valve pressure equalization first tank machine side

[0130] 12 - Valve inlet first tank

[0131] 13 - upper powder sensor first tank

[0132] 14 - lower powder sensor first tank

[0133] 15 - Load cells first tank

[0134] 16 - Valve outlet first tank

[0135] 17 - Rear overflow tank inlet valve

[0136] 18 - Valve pressure equalization rear overflow tank machine side

[0137] 19 - Front overflow tank inlet valve

[0138] 20 - Valve pressure equalization front overflow tank machine side

[0139] 21 - Rear overflow tank pressure equalization valve

[0140] 22 - upper powder sensor rear overflow tank

[0141] 23 - lower powder sensor rear overflow tank

[0142] 24 - Load cells rear overflow tank

[0143] 25 - Rear overflow tank outlet valve

[0144] 26 - Powder sensor inlet dosing screw rear overflow tank

[0145] 27 - Dosing screw rear overflow tank

[0146] 28 - Powder sensor outlet dosing screw rear overflow tank

[0147] 29 - Valve pressure equalization delivery line rear overflow tank

[0148] 30 - Valve pressure equalization front overflow tank

[0149] 31 - upper powder sensor front overflow tank

[0150] 32 - lower powder sensor front overflow tank

[0151] 33 - Load cells front overflow tank 34 - Valve outlet front overflow tank

[0152] 35 - Powder sensor inlet dosing screw front overflow tank

[0153] 36 - Dosing screw front overflow tank

[0154] 37 - Powder sensor outlet dosing screw front overflow tank

[0155] 38 - Valve pressure equalization delivery line front overflow tank

[0156] 39 - Compressed gas supply filter cleaning

[0157] 40 - Valve suction side filter

[0158] 41 - Filter

[0159] 42 - Cyclone

[0160] 43 - Valve pressure equalization lock

[0161] 44 - Valve inlet lock

[0162] 45 - upper powder sensor lock

[0163] 46 - lower powder sensor lock

[0164] 47 - Load cells lock

[0165] 48 - Valve outlet lock

[0166] 49 - Dosing screw sieve

[0167] 50 - Ultrasonic sieve

[0168] 51 - Main accumulator pressure equalization valve

[0169] 52 - Main reservoir inlet valve

[0170] 53 - upper powder sensor main memory

[0171] 54 - lower powder sensor main memory

[0172] 55 - Load cells main memory

[0173] 56 - Valve outlet main reservoir

[0174] 57 - Powder sensor inlet dosing screw main storage

[0175] 58 - Dosing screw main storage

[0176] 59 - Powder sensor outlet dosing screw main storage

[0177] 60 - Valve powder outlet “a”

[0178] 61 - Valve powder outlet “b”

[0179] 62 - Valve pressure equalization external tank module

[0180] 63 - Valve inlet external tank module

[0181] 64 - upper powder sensor external tank module

[0182] 65 - lower powder sensor external tank module

[0183] 66 - Load cells external tank module

[0184] 67 - Valve outlet external tank module

[0185] 68 - Valve inlet dosing screw external tank module

[0186] 69 - Powder sensor inlet dosing screw external tank module

[0187] 70 - Dosing screw external tank module

[0188] 71 - Powder sensor outlet dosing screw external tank module 72 - Gas supply valve "a"

[0189] 73 - Gas supply valve "b" & "c"

[0190] 74 - Pump protection filter

[0191] 75 - Dew point sensor

[0192] 76 - Oxygen sensor

[0193] 77 - redundant oxygen sensor

[0194] 78 - Pressure sensor pump suction side

[0195] 80 - Pressure sensor pump pressure side

[0196] 81 - Exhaust valve

[0197] 82 - Inert gas supply pressure compensation tank

[0198] 83 - Safety valve pressure equalization tank

[0199] 84 - Pressure equalization tank

[0200] 85 - Inert gas supply delivery line

[0201] 86 - Speed ​​& Temperature Sensor

[0202] 87 - Valve pressure equalization delivery line external tank module

[0203] 88 - Valve pressure equalization delivery line main storage

[0204] In connection with the arrangement of a powder conveying system described above and shown in Fig. 1, at least three conveying processes a to c can be implemented. In other words, the bulk material conveying system comprises at least three conveying circuits a to c. The conveying circuits share, at least in sections, the conveying line in which the conveying gas circulates. Furthermore, two of the conveying processes can run in parallel, in particular the conveying processes b and c described below. This leads to a mixing (blending) of powder from the overflow tanks 2 and 3 and the external tank 8, whereby the powder mixes in the buffer container 4.

[0205] The individual conveying processes and conveying circuits are explained below. They are illustrated in Figures 3 to 5. Figure 3 shows conveying process a, Figure 4 shows conveying process b, and Figure 5 shows conveying process c. Active elements are shown in bold so that the powder or gas flow can be understood.

[0206] Conveying process a (Fig. 3):

[0207] In conveying process a, powder is conveyed from the main reservoir 6 (source tank) to the first tank 1 (destination tank). Conveying in conveying process a involves the prior opening of valves 56 and 60. The powder in the main reservoir 6 is powder that has previously been sieved by the sieving device 5. From the first tank 1, the powder can then be fed to the additive manufacturing process in the process chamber 1011. The powder is conveyed into the conveying line via the conveyor screw 58 and separated in the cyclone 9. The conveying gas flows back to the pump 79 via the filter 41.

[0208] Conveying process b (Fig. 4):

[0209] In conveying process b, powder is conveyed from overflow tank 2 (source tank) and / or overflow tank 3 (source tank) into buffer tank 4 (destination tank). Conveying in conveying process b involves the prior opening of valves 25 and 34. The powder in overflow tanks 2 and 3 is contaminated powder that was not solidified during the additive manufacturing process. The contaminated powder is fed from buffer tank 4 to screening device 5. The powder is conveyed into the conveying line via screw conveyors 36 and 27, where it is separated in cyclone 42. The conveying gas flows back to pump 79 via filter 41.

[0210] Conveying process c (Fig. 5):

[0211] In conveying process c, powder is conveyed from the external tank 8 (source tank) into the buffer tank 4 (destination tank). Conveying in conveying process c involves the prior opening of valves 67 and 68. The powder in the external tank 8 may be contaminated powder, which is fed into the process from outside. However, it may also be uncontaminated, fresh powder. The powder conveyed from the buffer tank 4 into the buffer tank 4 is fed to the screening device 5. The powder is conveyed into the conveying line via the conveyor screw 69 and separated in the cyclone 42. The conveying gas flows back to the pump 79 via the filter 41.

[0212] Fig. 5a shows a schematic representation of an alternative powder conveying system used to convey raw material powder in a system 1100 for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation. Since the powder conveying system of Fig. 5a is constructed similarly to that of Fig. 1, differing aspects of the two embodiments are described in particular below. Aspects of the powder conveying system of Fig. 5a not explained may correspond to those of the system of Fig. 1, unless otherwise stated. In particular, the same or comparable elements of the powder conveying system of Fig. 5 are identified by the same reference numerals as in Fig. 1.

[0213] In the system shown in Fig. 5a, multiple conveying processes can be performed within the system, i.e., conveying processes from different source tanks to different destination tanks. The exact structure and operation of the powder conveying system are described below.

[0214] Not shown in Fig. 5a (but integral to the system) is a control device for the powder conveying system. This control device controls the individual components of the system shown in Fig. 5a, in particular the valves, metering devices, screw conveyors, and the conveying device described below. Furthermore, the control device receives and processes data from the sensors of the system shown in Fig. 5a, which correspond to those described above in connection with Fig. 1. To avoid repetition, the sensors will therefore not be described again.

[0215] The system 1100 corresponds, for example, to a generally known system for additive manufacturing using selective laser melting or selective laser sintering. Only the process chamber 1011 of the system 1100 is shown in Fig. 5a. Raw material powder is fed from a first tank 1 of the system 1100 to the process chamber 1011 (more precisely, a powder application device or coater in the process chamber 1011) by means of a loader. Alternatively, according to the above description of Fig. 1, an additional intermediate tank can be provided between the first tank 1 and the process chamber 1011.

[0216] Excess powder from the additive manufacturing process can be collected in a front overflow tank 2 or a rear overflow tank 3, with the overflow tanks 2, 3 being shown together in Fig. 5a. This particularly concerns powder that is not needed when coating a new powder layer and is thus pushed into one of the overflow tanks 2 or 3 by a powder application device.

[0217] The powder is conveyed from the overflow tanks 2 and 3 into a buffer tank 4 via a conveyor screw 991, which is part of the conveyor line. In particular, two conveyor screws 991 can be provided—one for each of the overflow tanks 2 and 3. A valve 992 is provided at the inlet of the buffer tank 4. In the area of ​​the valve 992, the powder can fall into the buffer tank 4 by gravity.

[0218] The powder contained in the buffer container 4 can be fed to the screening device 5 by means of a screw conveyor 49. At the inlet of the screening device 5 is a valve 993, which must be open for the purpose of feeding powder into the screening device. Starting from one end of the screw conveyor 49, the powder is conveyed into the screening device 5 by gravity.

[0219] The screened oversize grain is conveyed by pneumatic conveyance via a cyclone 994 into the oversize grain container 7. A valve 995 is located at the inlet of the oversize grain container. Screened powder collects on the underside of the screening device in a volume provided for this purpose and is transported away from there by pneumatic conveyance, separated in the cyclone 9 and fed to the first tank 1.

[0220] The gas flow of the pneumatic conveying circuit is driven by a conveying device 79, which is provided in the form of a pump 79. Downstream of the pump 79, an exhaust valve 81 is provided, through which gas can be discharged from the conveying line.

[0221] When valve 996 is open, powder from an external tank 8 can be added to the conveying gas flow by a dosing device 70 (conveyor screw 70). The conveying gas-powder mixture then flows to cyclone 997, where it can be conveyed into buffer tank 4. Thus, new powder (i.e., newly added to the system) can first be sieved by sieving device 5 before being processed in process chamber 1011.

[0222] Powder separated by the cyclone 9 enters the first tank 1 via an inlet valve 12. From there, it can be fed via the loader of the process chamber 1011 of the system 1100 and used in the additive manufacturing process.

[0223] Powder separated by the cyclone 997 enters a buffer container 4 via an inlet valve 998. From the buffer container 4, the powder can be fed to the screening device 5 via the conveyor screw 49. Furthermore, it is possible to empty the powder conveying system, in particular via the powder screw 49 and the cyclone 994, by appropriately controlling the respective valves.

[0224] A valve 9911 is provided at the inlet of cyclone 9. A valve 9910 is provided at the inlet of cyclone 997. A valve 999 is provided at the inlet of cyclone 994. By opening these valves 999, 9910, and 9911, separation can be activated via the respective cyclone. Thus, a corresponding conveying process can be switched on and off, particularly via these valves 999, 9910, and 9911.

[0225] In connection with the arrangement of a powder conveying system described above and shown in Fig. 5a, at least three conveying processes a to c can be implemented. In other words, the bulk material conveying system comprises at least three conveying circuits a to c. The conveying circuits share, at least in sections, the conveying line in which the conveying gas circulates. Furthermore, two of the conveying processes can run in parallel, in particular the conveying processes b and c described below. This leads to a mixing (blending) of powder from the overflow tanks 2 and 3 and the external tank 8, whereby the powder mixes in the buffer container 4. In the example shown in Fig. 5a, the conveying process b is implemented as non-pneumatic conveying.

[0226] The individual funding processes and funding groups are explained below.

[0227] Funding process a:

[0228] In conveying process a, sieved powder is conveyed from a volume at the bottom of the sieving device 5 (source tank) into the first tank 1 (destination tank). Conveying in conveying process a includes at least the prior opening of valve 9911. The sieved powder is powder that has previously been sieved by the sieving device 5. The powder from the first tank 1 can then be fed to the additive manufacturing process in the process chamber 1011. The conveying gas flows back to the pump 79 via valve 9911.

[0229] Funding process b:

[0230] In conveying process b, powder is conveyed from the overflow tank 2 (source tank) and / or the overflow tank 3 (source tank) into the buffer tank 4 (destination tank). For this purpose, the conveyor screw 991 is switched on, and conveying takes place not pneumatically, but mechanically via the conveyor screw 991. At one end of the conveyor screw 991, the powder falls by gravity through the open valve 992 into the buffer tank 4. Conveying in conveying process b can include the prior opening of the valve 992. The powder in the overflow tanks 2 and 3 is contaminated powder that was not solidified in the additive manufacturing process. The contaminated powder is fed from the buffer tank 4 to the screening device 5 by means of the conveyor screw 49.

[0231] Funding process c:

[0232] In conveying process c, powder is conveyed from the external tank 8 (source tank) into the buffer tank 4 (destination tank). Conveying in conveying process c involves the prior opening of valves 996 and 9910. The powder in the external tank 8 may be contaminated powder, which is fed into the process from outside. However, it may also be uncontaminated, fresh powder. The powder conveyed from the buffer tank 4 into the buffer tank 4 is fed to the screening device 5. The powder is conveyed into the conveying line via the conveyor screw 70 and separated in the cyclone 997. The conveying gas flows back to the pump 79 via the valve 9910.

[0233] Further details of the powder conveying system of Fig. 1 are explained below, and in particular processes or methods are explained which can be carried out in connection with the powder conveying system. The described methods are carried out by a control unit (not shown) of the powder conveying system. For this purpose, the control device comprises a processor and a memory on which corresponding commands for carrying out the individual methods are stored. Furthermore, the control device comprises a human-machine interface and one or more interfaces for receiving sensor data. Finally, the control device comprises one or more interfaces for the (electrical) control of individual components of the powder conveying system (such as the valves, the dosing devices, etc.).

[0234] Although the following explanations are specifically directed at the powder conveying system of Fig. 1, they can also be applied to the powder conveying system of Fig. 5a. Even if not explicitly shown in Fig. 5a, the containers shown therein and described above also include corresponding sensors and / or valves for carrying out the processes described below. Embodiments of the technology presented herein can, in particular, enable inert metal powder conveying, which, by entering specific powder parameters (grain size, bulk density, etc.), universally approaches the optimal conveying point (i.e., conveying gas velocity) for common powder types and reacts to changing system influences (filter clogging, leakage, O2 rise, etc.).

[0235] The powder conveying system features an HMI (Human Machine Interface) and a PLC (Programmable Logic Controller) that can store recipes for different powder types, especially metal powders. These contain material-specific parameters such as:

[0236] Solid density

[0237] Bulk density

[0238] Grain size distribution & mean grain size

[0239] Using these material parameters, a system's feed control can approach the quasi-optimal feed point. This is defined by the saltation velocity plus a safety margin. See "Feed Gas Velocity Adjustment" in Fig. 9.

[0240] Apart from adjusting the conveying gas velocity according to Fig. 9, several processes are presented here and in particular below, each of which can run in isolation from the other processes and offers independent advantages. In particular, adjusting the conveying gas velocity is merely optional, and both the arrangement of multiple conveying circuits according to Fig. 1 and the provision of different control processes offer independent advantages.

[0241] Furthermore, it is pointed out at this point that the individual control processes presented below (for example the process of adjusting the conveying gas velocity) can also be used in a pneumatic conveying system independently of the specific arrangement according to Fig. 1.

[0242] The system in Fig. 1 has three conveying processes: a - Conveying powder from the main tank 6 to the first tank 1 b - Conveying powder from the front overflow tank 2 and the rear overflow tank 3 to lock 4 (buffer tank 4) c - Conveying powder from the external tank 8 to lock 4 The conveying processes b & c can also run in parallel, so-called "blending" (mixing powder from tanks 2 or 3 and 8).

[0243] Each container (i.e. in particular containers 1, 2, 3, 4, 6 and 8) has pressure equalization lines to the conveyor system to ensure optimal powder discharge.

[0244] A higher-level function of the powder conveying system of Fig. 1 is shown in Fig. 2.

[0245] Before each conveying cycle ("conveying" in Fig. 2), an attempt is made to establish a gas flow in the respective target pipe system, and the total pressure loss is determined. If this exceeds a defined limit, automatic filter cleaning is triggered, and another attempt is made to establish a gas flow. If this fails again within the defined operating conditions, an error is output indicating a clogged system.

[0246] If the system differential pressure is below the defined limit, the oxygen content of the gas stream is determined ("Initial O2 test" in Fig. 2). If this is below a defined limit, metal powder conveying can begin ("Conveying" in Fig. 2). If the oxygen value is above a defined limit, initial pipeline inerting is triggered.

[0247] During initial pipeline inerting, the pipeline circuit is interrupted at valves 72 and 73, and the drain valve 81 is opened. The pipelines are evacuated either separately ("a" and "b") or together. If the pressure at sensor 78 falls below a defined limit, the leak test of the pipeline system is initiated (see Fig. 6).

[0248] During the leak test (Fig. 6), pressure sensors 78 & 80 are evaluated. The system pressure increase per unit of time must not exceed a defined limit. If the limit is exceeded, normalization can continue. If the limit is exceeded, an error message is displayed on the HMI.

[0249] During normalization, the drain valve 81 is closed, the shut-off valves 72 & 73 are opened, and the inert gas supply 82 is opened. The pressure in the piping system is increased with inert gas to a defined total system pressure, and then valve 82 is closed. During the process, the oxygen content of the piping system is monitored 76 & 77. If this repeatedly exceeds the defined limit, the initial pipeline inerting is repeated. After a defined number of repetitions, the pipeline inerting can be considered a failure. An error message is displayed on the HMI.

[0250] If the oxygen content is below the defined limit, metal powder conveying ("conveying" in Fig. 2) can continue.

[0251] During the metal powder feed cycle, the corresponding valves of the required feed circuit are opened. These include the powder outlet valves of the source tank, the inlet valves of the target tank, and the internal machine lock 4. Likewise, the pressure equalization lines from the source tanks to the feed line are opened. The motors of the respective dosing devices are activated, and their speed is adjusted via the feed control system.

[0252] The conveyor control is structured in several stages and runs in a loop until defined termination criteria are reached.

[0253] The first step is oxygen monitoring. If the oxygen level measured at sensors 76 & 77 exceeds a defined limit, valve 85 opens and inert gas is supplied. If the oxygen level falls below the limit, valve 85 closes. However, if a defined time limit is exceeded by then, valve 85 remains open and the next step, pressure monitoring, proceeds.

[0254] During pressure control, sensors 78 and 80 check whether the total system pressure is within the defined range. If this range is exceeded, valve 85 opens and inert gas is supplied. If this range is exceeded, valve 81 opens and conveying gas is released. Once the defined range for the total system pressure is reached, the system continues with the calculation of the target gas flow velocity (see also Fig. 9).

[0255] The calculation of the target velocity for the gas flow is structured in three stages (see Fig. 9). The first sub-calculation determines the density of the conveying gas ("Gas Density Calculation"). This depends on the mixing ratio of inert gas, residual air, and moisture content, as well as their standard parameters (e.g., density). The mixing ratio is determined using oxygen sensors 76 & 77 and dew point sensor 75. Using the mixing ratio of the gas pressure at sensor 80 and the gas temperature at sensor 86, the gas density at the pump outlet can be determined. For powder feed points further along the pipeline, additional pressure sensors should preferably be installed at the feed points; alternatively, determined correction factors can be used.The second sub-calculation ("Mass Flow Calculation") determines the metal powder mass flow using the motor speed of the respective dosing device, the metal powder parameters from the recipe, the geometric properties of the dosing device, and a determined efficiency. The third sub-calculation ("Saltation Velocity Calculation") combines all results and determines the target gas flow velocity for the respective prevailing metal powder mass flow using additional geometric parameters such as the pipe cross-section and metal powder parameters such as the average grain size.

[0256] The target speed is controlled using a PID pump control and the speed sensor 86.

[0257] The final part of the feed control system is the limit control (see Fig. 10) under the aspect of machine protection, which results in an adjustment of the metal powder mass flow. The limit values ​​checked are: maximum feed rate (wear protection of the piping system); maximum pump outlet pressure (pump protection, component-specific); maximum pump performance (service life); and maximum dosing performance (service life).

[0258] If all limit values ​​are exceeded, the motor speed of the respective dosing device (i.e., the respective screw conveyor) is increased by a defined value. If at least one limit value is exceeded, the motor speed of the respective dosing device is reduced by a defined value. Once the limit value control is complete, the termination conditions for metal powder feeding are checked. If these are not met, the feed control loop starts again.

[0259] A delivery cycle is finished when: the source tank is empty, or the destination tank is full, or a defined maximum delivery time has been exceeded, or the total pressure loss exceeds a defined limit.

[0260] The fact that the source tank is empty can mean that a level sensor in the source tank of the respective conveying process reports a level that is below a predetermined limit. The fact that the target tank is full can mean that a level sensor in the target tank of the respective conveying process reports a level that is above a predetermined limit.

[0261] When a conveying cycle is terminated, the respective piping system is cleaned by a gas stream so that no blockage due to falling powder can occur in vertical sections when the gas flow is terminated (see "Pipe Cleaning" in Fig. 2). Cleaning of the piping system is completed when the load cells of the target tank detect a defined limit for the change in weight over time.

[0262] Once the pipe cleaning cycle is completed, the automatic filter cleaning (39, 40) is triggered (see "Filter cleaning" in Fig. 2).

[0263] The system features a pressure equalization tank 84 designed to reduce the overpressure at the pump outlet. When using conventional vacuum pumps, the maximum overpressure is specified by the manufacturer and must not be exceeded to protect the components. This is ensured by the limit control (see above). A pressure equalization tank is used to increase the pumping capacity.

[0264] Embodiments of the powder conveying system described herein may be capable of adapting to changing system conditions (e.g., filter wear), changing materials (e.g., material characteristics), and / or changing environmental conditions without requiring empirically determined, different conveying parameter sets. This also means maximum performance for the respective machine, material, and environmental conditions.

[0265] In addition to or alternatively to the processes described above, in particular the higher-level function shown in Fig. 2 and its subroutines, the powder conveying system of the present disclosure can have the following advantageous features or execute the following advantageous processes. Aspects of automatic condition monitoring are described below, which can be used particularly in connection with the powder conveying system of Fig. 1.

[0266] A dew point sensor is provided on at least one of the following components of the powder conveying system: main tank 6, external tank 8, and conveyor device 79. The dew point sensors are used to measure humidity during vacuum conveying and within tanks 6 and 8 via an exhaust unit. If a certain limit is exceeded (according to one embodiment, > 5%), automatic powder drying is initiated.

[0267] An oxygen sensor for measuring the oxygen concentration is provided on at least one of the following components of the powder conveying system: first tank 1, overflow tank 2 or 3, buffer tank 4, main reservoir 6, external tank 8, and sieve device 5. The oxygen sensors monitor the oxygen concentration during vacuum conveying and within all tanks and sieves. If the corresponding limit values ​​are exceeded, automatic inerting is initiated. Measurement takes place continuously during conveying, while measurements in the tanks and sieves are discontinuous at specified intervals.

[0268] A pressure sensor for measuring pressure is provided on at least one of the following components of the powder conveying system: first tank 1, overflow tank 2 or 3, buffer tank 4, main storage 6, external tank 8 and sieve device 5. The pressure sensors in the tanks and sieve are used for leakage control. If a limit value for pressure loss / increase over time is measured, a warning is issued and the inerting / O2 measurement of the corresponding system section is carried out to compensate for any missing inerting. Furthermore, the pressure sensors in the conveyor can be used for filter monitoring. If the differential pressure rises above a defined limit value, automatic filter cleaning is triggered. If the increased differential pressure is not remedied by filter cleaning, the piping system may be blocked and a warning is issued.

[0269] The motors of the dosing elements (conveyor screws) and the vacuum pump (conveying device) are temperature-monitored (using appropriate temperature sensors). If a temperature limit is exceeded, a shutdown occurs to protect the components. Furthermore, a temperature sensor is located in the conveyor line. The motors of the dosing elements are torque-monitored. If a defined torque limit is exceeded, a shutdown occurs and a warning is issued.

[0270] All pipeline valves have position feedback. If a discrepancy occurs between the target position and the actual position, the machine enters a safe state and a warning is issued for the corresponding component.

[0271] All sensors are checked for plausibility and monitored. If an error occurs, the machine enters a safe state and a warning is issued for the corresponding component.

[0272] In the following, aspects of a remaining runtime-optimized prioritization are described, which can be used in particular in connection with the powder conveying system shown in Fig. 1.

[0273] At least one or more of the containers and in particular all containers (1, 2102, 2, 3, 4, 6, 7, 8) are equipped with weighing cells which ensure continuous level measurement of the respective container.

[0274] If a disruption occurs in closed-loop operation, the tanks (2102, 2, 3) form the safety reserve for the additive manufacturing process. The intermediate tank 2102 (hopper) stores the powder for the additive manufacturing process. If this tank is empty, the process aborts. The overflow tanks (2, 3) collect the excess process powder. The empty volume forms the reserve. If at least one overflow tank is full, the process aborts.

[0275] The goal is to optimize the remaining runtime in relation to the hopper fill level and overflow reserve. It should be noted that the discharge volume per unit time from the hopper is variable and depends on the exposure time, layer thickness, and overfeed factor. The same applies to the fill volume per unit time of the overflows. Accordingly, the weight changes over time, measured at the load cells, are dynamic inputs for the algorithm.

[0276] There are three different conveying circuits or conveying processes: a. Conveying from tank 6 through lock 1 into hopper 2102 b. Conveying overflows 2 or 3 through lock 4 onto screen 50 in tank 6 c. Conveying from external tank 8 through lock 4 onto screen 50 in tank 6

[0277] A conveying cycle conveys a predetermined amount of powder (e.g. 25 l) into lock 1 or lock 4 and is terminated.

[0278] After each funding cycle, the algorithm calculates the priorities of funding circuits a, b, and c. The funding process with the highest priority is executed.

[0279] There are fixed min and max limits for each tank. Tank 6 forms the node between all cycles; according to one example, it cannot be filled to its maximum by "c," as otherwise the volume would be lost to discharge the overflows "b." According to one example, delivery circuit "b" always has priority over delivery circuit "c."

[0280] The powder conveying system of the present disclosure (in particular the powder conveying system of Fig. 1) may further be a specific case of a bulk material conveying system according to the following description. Individual aspects of the powder conveying system described herein can be combined and / or supplemented in any manner with the aspects of the bulk material conveying system described below. In particular, in connection with the powder conveying system of the present disclosure, an adjustment of the conveying speed can be carried out according to the following description.

[0281] However, the aspects of a bulk material conveying system described below may also be advantageous in their own right and constitute one or more inventions. Thus, the bulk material conveying system described below may also be used independently (i.e., independently of the powder conveying system described above).

[0282] The following disclosure relates to a bulk material conveying system and a method for conveying bulk material. In particular, the following disclosure relates to the conveying of raw material powder in a system for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation (for example, a system for selective laser melting).

[0283] In additive (or generative) processes for producing three-dimensional workpieces, and in particular in generative layered construction processes, it is known to apply an initially formless or shape-neutral molding compound of a raw material (e.g., a raw material powder) layer by layer to a carrier and to solidify it by site-specific irradiation (e.g., by melting or sintering) in order to ultimately obtain a workpiece of the desired shape. The irradiation can be carried out using electromagnetic radiation, for example, in the form of laser radiation, or by particle radiation, for example, in the form of electron beams. In an initial state, the molding compound can initially be in the form of granules, powder, or liquid molding compound and, as a result of the irradiation, can be solidified selectively or, in other words, site-specifically. In particular, the molding compound can be a bulk material, such as raw material powder.The molding compound can comprise, for example, ceramic, metal, or plastic materials, as well as mixtures of these. One variant of additive layering processes involves so-called laser beam melting in a powder bed (also known as selective laser melting), in which metallic and / or ceramic raw material powder materials, in particular, are solidified into three-dimensional workpieces under the irradiation of a laser beam.

[0284] For the production of individual workpiece layers using selective laser melting, it is also known to apply raw material powder in the form of a layer of raw material powder to a carrier and to irradiate it selectively and in accordance with the geometry of the workpiece layer currently being produced. The laser radiation penetrates the raw material powder and solidifies it, for example, as a result of heating, which causes melting or sintering. Once a workpiece layer has solidified, a new layer of unprocessed raw material powder is applied to the previously produced workpiece layer. Known coating arrangements or powder application devices can be used for this purpose. Subsequently, the now uppermost and still unprocessed raw material powder layer is irradiated again. Consequently, the workpiece is built up successively layer by layer, with each layer defining a cross-sectional area and / or a contour of the workpiece.In this context, it is also known to use CAD or comparable workpiece data in order to produce the workpieces essentially automatically.

[0285] The present disclosure is generally directed to a bulk material conveying system for conveying bulk material. In particular, the bulk material may be raw material powder for use in one of the systems described above for producing a three-dimensional workpiece. In particular, for the provision of raw material powder in an additive manufacturing process of the type described above, but also for other applications, it may be necessary to convey bulk material from a source tank to a destination tank.

[0286] For this purpose, what is typically known as pneumatic conveying is used. A conveying device (e.g., a pump or blower) generates a gas flow of conveying gas (e.g., air or a protective gas such as argon, or an air-protective gas mixture) through a conveying line. The conveying line can, in particular, form a closed circuit so that the gas flow is conveyed in a circular manner (a so-called conveying circuit). A predetermined dose of bulk material is fed into the gas flow from a source tank, for example, using a dosing device. It is also possible for bulk material to be fed into the gas flow from several source tanks simultaneously. From the point where the bulk material is fed in, a gas-bulk material mixture is conveyed through the conveying line to a point in the conveying circuit where a separation device (e.g., a cyclone) is located.With the help of the separation device, the bulk material is separated from the gas-bulk material mixture as completely as possible and fed into a target tank. For example, a cyclone can be located above the target tank, with the separated bulk material falling into the target tank due to gravity (so-called gravity conveying).

[0287] One problem with pneumatic bulk material conveying is that the system's operating parameters and / or the properties of the bulk material (e.g., the material of a bulk powder used and the associated conveying properties) can change during operation or between individual conveying processes. In this case, certain operating parameters of the bulk material conveying system must be adjusted, particularly the gas flow velocity, which is determined by the conveying device. Another operating parameter of the bulk material conveying system that may require adjustment is the amount of bulk material conveyed into the gas flow per unit time.

[0288] In particular, determining a suitable gas flow velocity for pneumatic bulk material conveying under changing conditions (e.g., operating parameters of the system and / or nature of the bulk material) poses a problem. The object of the present disclosure is therefore to provide a bulk material conveying system and a corresponding method for conveying bulk material that solve at least one of the problems described above or a related problem. In particular, a reliable and simple determination of a setpoint for the gas velocity is desirable, which can respond to a change in one or more operating parameters.

[0289] This object is achieved by a bulk material conveying system and a method for conveying bulk material having the features of the independent aspects below. Further embodiments are specified in the sub-aspects.

[0290] According to a first aspect, the disclosure therefore relates to a bulk material conveying system, in particular for conveying raw material powder in a system for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation. The bulk material conveying system comprises a conveying line configured to convey a gas stream and, at least in sections, a bulk material stream driven by the gas stream, and a dosing device configured to supply the gas stream with a predetermined dose of bulk material per unit of time. The dose is determined by a control value applied to the dosing device. The bulk material conveying system further comprises a conveying device configured to convey the gas stream through the conveying line and at least one measuring device for measuring at least one characteristic variable of the gas stream.Furthermore, the bulk material conveying system comprises a control device which is configured to determine a gas density of the gas flow based on the measured at least one characteristic variable of the gas flow, determine a bulk material mass flow of the bulk material flow based on the control value applied to the metering device, determine a desired velocity of the gas flow based on the gas density and based on the bulk material mass flow, and control the conveying device to convey the gas flow at the determined desired velocity.

[0291] The bulk material conveying system can be configured to convey the bulk material pneumatically. The system can, in particular, be a system for selective laser melting or sintering, which, for example, has one or more of the features described above. Alternatively, the system can be a system for selective electron beam melting. The conveying line can, for example, comprise one or more pipes and / or one or more hoses and / or one or more connectors. The conveying line can be powder-tight and, in particular, gas- and powder-tight, so that no gas or powder can enter or exit the conveying line (from the side) - except through the openings in the conveying line.

[0292] The fact that the conveying line is configured in sections to convey a bulk material flow driven by the gas flow may mean that only a section of an entire conveying circuit formed by the conveying line is configured to convey a gas-bulk material mixture. The remaining part of the conveying circuit essentially conveys only gas (conveying gas), for example, air, protective gas, or an air-protective gas mixture.

[0293] The control value can be a voltage, a current, a pulse, or another suitable signal (particularly an electrical signal) that can be varied to vary the dose delivered by the dosing device. The conveying device can be a pump or a blower. Furthermore, one or more filter devices can be provided in the conveying circuit formed by the conveying line, in particular for filtering bulk material or bulk material residues remaining in the gas stream.

[0294] The control device may, for example, comprise a computer. The control device may comprise a processor and a memory, wherein a program is stored in the memory which, when executed, causes the processor to perform the method according to the second aspect.

[0295] The term "bulk material mass flow" is used herein to indicate a physically quantifiable mass flow of the conveyed bulk material (in kg / s). In contrast, the term "bulk material flow" describes (merely) the presence of bulk material being conveyed through the conveying line.

[0296] The determination steps can each comprise one or more calculations. Measured variables as well as stored variables (e.g., standard values) can be incorporated into the respective calculation. The conveying device can be controlled to decrease or increase the velocity of the conveyed gas flow. In particular, increasing the voltage applied to the conveying device can, for example, lead to an increase in the velocity of the gas flow. Since the gas flow conveys the bulk material, the velocity of the gas flow also determines the velocity of the conveyed bulk material.

[0297] The gas density can be determined based on at least one of the following parameters of the gas flow: oxygen content, pressure, temperature, dew point and humidity.

[0298] For each of the parameters mentioned, a corresponding sensor can be provided in the gas stream, which is set up to measure the respective parameter.

[0299] The gas density p can be determined using the following formula: where pmess is a measured pressure of the gas stream, Tmess is a measured temperature of the gas stream, p n a predetermined normal pressure, T n is a predetermined normal temperature and pmixture is given by the following formula: where pair is a known density of air which is part of the gas stream, Pshield gas is a known density of a shielding gas which is part of the gas stream, oxygen content air is a known oxygen content of the air and oxygen content meas is a measured oxygen content of the gas stream.

[0300] Pmess and Tmess can each be measured by suitable sensors arranged in the gas stream. At least one or both of the sensors required for this purpose can be arranged downstream of the conveying device, in particular between the conveying device and a source tank from which the bulk material is conveyed. In particular, at least one or both of the sensors can be arranged immediately downstream of the conveying device. The normal pressure and the normal temperature can be predetermined standard values; for example, the normal pressure can be 1013.25 mbar and, for example, the normal temperature can be 293.15 K. However, normal pressure and / or normal temperature can also be measured values ​​of the ambient pressure or the ambient temperature.

[0301] The shielding gas can be argon, for example, where the density Pshielding gas corresponds to a known density of the gas argon. The density of air, the density of the shielding gas used, and the oxygen content of the air can be found in relevant tables in the specialist literature. The oxygen content of the air can also be a measured oxygen content of the ambient air.

[0302] Oxygen content can be measured using a suitable sensor positioned in the gas stream. The sensor required for this purpose can be positioned upstream of the conveying device, between a target tank into which the bulk material is conveyed and the conveying device. In particular, the sensor can be positioned directly upstream of the conveying device.

[0303] The control value applied to the dosing device can be a motor speed of a motor of the dosing device, in particular a motor for driving a conveyor screw of the dosing device.

[0304] However, the control value can also be a current or a voltage applied to the motor. Generally speaking, the control value can represent any (e.g., electrical) signal suitable for changing the dose delivered by the dosing device (more precisely: dose per time), in particular setting it to a predetermined value. The dosing device can, as described above, comprise a conveyor screw configured to convey bulk material from the source tank. Furthermore, the dosing device can comprise at least one of the following elements: a rotary valve, a dosing slide, and a valve, in particular a valve with a variable opening diameter.

[0305] Determining the target speed v S0 n of the gas flow may include calculating a saltation velocity based on the determined gas density and the determined bulk material mass flow. Determining the target velocity v S0 n can be done using the following formula: soll ^saltation "F ^safety where Vsaitation is the calculated saltation velocity of the conveyed bulk material and vsafety is a predetermined safety velocity.

[0306] The saltation velocity can be a velocity below which particles of the conveyed bulk material (e.g., powder) begin to fall and accumulate at the bottom of the conveying line. To ensure this does not occur, a predetermined safety velocity can be added to the calculated saltation velocity. This essentially forms a safety margin from the saltation velocity, ensuring that no deposition of bulk material occurs in the conveying line. Alternatively, to adding the safety velocity, the saltation velocity can be multiplied by a safety factor.

[0307] Calculating the saltation rate can be done using the following formula: where M s is the determined bulk material mass flow in kg / s, g is a predetermined gravitational acceleration in m / s 2is, D is a diameter of the conveyor line in m, p is the determined gas density of the gas stream in kg / m 3 and a and b are parameters depending on a particle diameter d of the bulk material.

[0308] The predetermined acceleration due to gravity can be equal to the acceleration due to gravity at the Earth's surface of 9.81 m / s 2correspond. The parameters a and b can each be a predetermined and / or pre-stored constant for a respective bulk material. Furthermore, the parameter a and / or the parameter b can be determined as a function of the particle diameter d of the bulk material used. In other words, a formula for calculating the parameter a and / or a formula for calculating the parameter b can be dependent on the particle diameter d. The particle diameter d can be taken from a specification of the bulk material used. The particle diameter d can, for example, be stored in a memory of the control device. In particular, a table can be stored in a memory of the control device in which the respective values ​​for the particle diameter d of the respective powder are stored for different powder materials.In this way, a respective particle diameter d can be used to calculate the target speed.

[0309] The control of the conveying device for conveying the gas flow can be carried out at the determined target speed using a speed sensor for measuring a speed of the gas flow and a control loop, in particular comprising a PID controller.

[0310] A PID controller is a proportional-integral-derivative controller, whereby PID control is well known in control engineering for setting and maintaining a predetermined value (in this case the gas velocity) constant.

[0311] The control device can further be configured to determine whether at least one measured characteristic of the bulk material conveying system is above a predetermined maximum value for the respective characteristic, and if the at least one measured characteristic is above the predetermined maximum value, reduce the control value of the dosing device by a predetermined value.

[0312] Several parameters can be determined (in particular measured) and if at least one of the determined parameters is above a maximum value predetermined for this parameter, the control value is reduced by the predetermined value.

[0313] The bulk material conveying system can comprise one or more sensors for measuring the respective parameter. Determining whether the at least one measured parameter is above the predetermined maximum value and adjusting the control value accordingly can be performed after the steps of determining a gas density, determining a bulk material mass flow, determining a target speed, and controlling the conveying device. These steps can be performed again following the adjustment of the control value with the new control value. Thus, the determination (and, if necessary, adjustment) of the target speed and—if necessary—the adjustment of the control value can be performed alternately. Reducing the control value means that the dosing device dispenses a smaller dose of bulk material per unit time.The control device can further be configured to increase the control value of the dosing device by a predetermined value if the at least one measured characteristic value is below the predetermined maximum value.

[0314] Multiple parameters can be determined (in particular, measured), and if all of the determined parameters are below a predetermined maximum value for the respective parameter, the control value is increased by the predetermined value. Thus, the control value can only be increased if all of the determined parameters do not exceed their respective maximum values. Increasing the control value means that the dosing device dispenses a higher dose of bulk material per unit of time.

[0315] The at least one parameter may include at least one of the following parameters: conveying speed, pump outlet pressure, pump power and dose of bulk material per time.

[0316] The respective parameter can be measured, for example, using a dedicated sensor or determined in another way. For example, the parameter can be calculated based on at least one input value (e.g., an applied voltage) applied to an element of the powder conveying system (e.g., the conveying device).

[0317] The control device can be configured to terminate a conveying operation by stopping the conveying device if at least one of the following events is detected: a source tank from which bulk material is withdrawn by means of the dosing device is empty, a target tank into which the bulk material is conveyed is full, a predetermined maximum conveying time is exceeded, and a total pressure loss of the conveying gas exceeds a predetermined limit value.

[0318] The respective event can be detected, for example, using a corresponding sensor. In detail, the events can be detected as follows: A source tank from which bulk material is withdrawn by means of the dosing device is empty. This can be determined using a corresponding level sensor (e.g. a capacitive sensor, a radar sensor, an ultrasonic sensor or an optical sensor). A target tank into which the bulk material is conveyed is full. This can be determined using a corresponding level sensor (e.g. a capacitive sensor, a radar sensor, an ultrasonic sensor or an optical sensor). A predetermined maximum conveying time has been exceeded. This can be determined using a corresponding timer that is started when conveying is started. A total pressure loss of the conveying gas exceeds a predetermined limit value. This can be determined using one or more pressure sensors in the conveying line.

[0319] The bulk material conveying system may further comprise a pressure equalization tank coupled to the conveying line downstream of the conveying device and upstream of the dosing device.

[0320] The pressure equalization tank can be configured to reduce excess pressure at an outlet of the conveying device (e.g., a pump). Thus, the pressure equalization tank can be provided at the outlet of the conveying device.

[0321] The bulk material conveying system may comprise at least a first conveying circuit for conveying raw material powder from a main reservoir into a first tank of the system for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation. A lower outlet of the first tank may be coupled to an upper inlet of an intermediate tank of the system, wherein a manufacturing process in a process chamber of the system is fed with powder from the intermediate tank. An upper inlet of the main reservoir may be coupled to an outlet of a sieve for sieving the raw material powder.

[0322] The main storage unit can, for example, be a storage unit or tank permanently installed in the bulk material conveying system. The main storage unit can be located at a lower height than the first tank. The first conveying circuit can be activated by the control device by opening at least one valve, so that the conveying device conveys gas through this first conveying circuit. The bulk material conveying system can be designed so that powder can be fed to the sieve from a buffer tank located above the sieve. Powder can, for example, be conveyed into the buffer tank by means of a second conveying circuit from an overflow tank of the system and / or from an external tank.

[0323] The terms "tank," "storage," and "container" are used synonymously herein. However, specific terms are used to refer to the different containers, such as "tank," "buffer container," "main storage," and "external tank." These individual terms are not intended to be limiting, but merely serve to differentiate the various containers from one another. For example, the "buffer container" could also be referred to as a "second tank."

[0324] The bulk material conveying system may comprise at least one second conveying circuit for conveying raw material powder from an overflow tank into a buffer tank. The overflow tank may be configured to receive excess powder from the process chamber of the system for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation. A lower outlet of the buffer tank may be coupled to an inlet of a sieve for sieving the raw material powder.

[0325] The second conveying circuit can be activated by the control device by opening at least one valve, so that the conveying device conveys gas through this second conveying circuit. The overflow tank can be located, for example, in a lateral area next to a construction cylinder of the system, with the overflow tank being open at the top so that the excess powder can be pushed into the overflow tank, for example, by means of a powder application device.

[0326] The bulk material conveying system may comprise at least a third conveying circuit for conveying raw material powder from an external tank into a buffer tank. The external tank may be detachably coupled or coupled to the bulk material conveying system. A lower outlet of the buffer tank may be coupled to an inlet of a sieve for sieving the raw material powder.

[0327] The third conveying circuit can be activated by the control device by opening at least one valve, so that the conveying device conveys gas through this third conveying circuit. The third conveying circuit can be operated simultaneously with the second conveying circuit, so that powder from the external tank and powder from the overflow tank are mixed together in an adjustable mixing ratio. The mixing ratio can be adjusted, for example, by the respective dosing devices of the respective containers (external tank, overflow tank).

[0328] The dosing device can be located at the outlet of each source tank, and a cyclone can be located at the inlet of each target tank to separate the raw material powder from the gas stream and feed it into the target tank. The dosing device can introduce the bulk material stream into the gas stream, and the cyclone can remove the bulk material from the gas stream.

[0329] According to a second aspect, the disclosure relates to a method for conveying bulk material, in particular for conveying raw material powder in a system for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation. The method comprises conveying a gas stream and a bulk material stream driven by the gas stream through a conveying line and supplying a predetermined dose of bulk material per unit of time through a dosing device. The dose can be determined by a control value applied to the dosing device. The method further comprises conveying the gas stream through the conveying line using a conveying device and measuring at least one characteristic of the gas stream using a measuring device.The method further comprises determining a gas density of the gas stream based on the measured at least one characteristic of the gas stream and determining a bulk material mass flow of the bulk material stream based on the control value applied to the metering device. The method further comprises determining a target velocity of the gas stream based on the gas density and the bulk material mass flow, and controlling the conveying device to convey the gas stream at the determined target velocity.

[0330] All of the above-described details and features of the first aspect (bulk material conveying system) can apply in connection with the method for conveying bulk material according to the second aspect (individually or in any combination with one another). The bulk material conveying system according to the first aspect can be configured to carry out the method for conveying bulk material according to the second aspect.

[0331] The gas density can be determined based on at least one of the following parameters of the gas flow: oxygen content, pressure, temperature, dew point and humidity.

[0332] The gas density p can be determined using the following formula: where pmess is a measured pressure of the gas stream, Tmess is a measured temperature of the gas stream, p n a predetermined normal pressure, T n is a predetermined normal temperature and pmixture is given by the following formula: where pair is a known density of air which is part of the gas stream, Pshield gas is a known density of a shielding gas which is part of the gas stream, oxygen content air is a known oxygen content of the air and oxygen content meas is a measured oxygen content of the gas stream.

[0333] The control value applied to the dosing device can be a motor speed of a motor of the dosing device, in particular a motor for driving a conveyor screw of the dosing device.

[0334] Determining the target speed v S0 n of the gas flow may include calculating a saltation velocity based on the determined gas density and the determined bulk material mass flow and determining the target velocity v S0 n can be done using the following formula: soll ^saltation "F ^safety where Vsaitation is the calculated saltation velocity of the conveyed bulk material and vsafety is a predetermined safety velocity.

[0335] Calculating the saltation rate can be done using the following formula: where M sis the determined bulk material mass flow in kg / s, g is a predetermined gravitational acceleration in m / s 2 is, D is a diameter of the conveyor line in m, p is the determined gas density of the gas stream in kg / m 3 and a and b are parameters depending on a particle diameter d of the bulk material.

[0336] Controlling the conveying device for conveying the gas flow at the determined target speed can be carried out using a speed sensor for measuring a speed of the gas flow and a control loop, in particular comprising a PID controller.

[0337] The method may further comprise: determining whether at least one measured characteristic of the bulk material conveying system is above a predetermined maximum value for the respective characteristic, and if the at least one measured characteristic is above the predetermined maximum value, reducing the control value of the dosing device by a predetermined value.

[0338] The method may further comprise, if the at least one measured characteristic is below the predetermined maximum value, increasing the control value of the dosing device by a predetermined value.

[0339] The at least one parameter may include at least one of the following parameters: conveying speed, pump outlet pressure, pump power and dose of bulk material per time.

[0340] The method may further comprise terminating a conveying operation by stopping the conveying device if at least one of the following events is detected: a source tank from which bulk material is withdrawn by means of the dosing device is empty, a target tank into which the bulk material is conveyed is full, a predetermined maximum conveying time is exceeded and a total pressure loss of the conveying gas exceeds a predetermined limit value.

[0341] The method may comprise conveying raw material powder through at least one first conveying circuit from a main reservoir into a first tank of the system for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation. A lower outlet of the first tank may be coupled to an upper inlet of an intermediate tank of the system, wherein a manufacturing process in a process chamber of the system is fed with powder from the intermediate tank. An upper inlet of the main reservoir may be coupled to an outlet of a sieve for sieving the raw material powder. The method may comprise conveying raw material powder through at least one second conveying circuit from an overflow tank into a buffer container.The overflow tank can be configured to receive excess powder from the process chamber of the system for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic or particle radiation. A lower outlet of the buffer tank can be coupled to an inlet of a sieve for sieving the raw material powder.

[0342] The method may comprise conveying raw material powder through at least a third conveying circuit from an external tank into a buffer container. The external tank may be releasably coupled or connectable to the bulk material conveying system. A lower outlet of the buffer container may be coupled to an inlet of a sieve for sieving the raw material powder.

[0343] The dosing device can be located at the outlet of each source tank. A cyclone can be located at the inlet of each target tank to separate the raw material powder from the gas stream and feed it into the target tank.

[0344] The system described herein for producing a three-dimensional workpiece by irradiating layers of raw material powder with electromagnetic radiation or particle radiation can, for example, comprise a carrier device for applying the powder in multiple layers to form a powder bed. Furthermore, one or more powder application devices can be provided for applying the powder and, if necessary, for applying powder of different materials. A separate powder application device can be provided for each material. The carrier device can be moved vertically downwards by means of a lifting device so that the uppermost powder layer always remains at the same height relative to a build chamber of the system. Furthermore, the system can comprise one or more irradiation units.The irradiation units each comprise a beam source (in particular a laser beam source) and an optical system with one or more optical components for shaping and deflecting the beam (e.g., beam expander, focusing unit, scanner device, F-theta lens). Alternatively, the beam source can be located outside the respective irradiation unit, with the beam being fed to the irradiation unit via an optical conductor (e.g., fiber optic cable). Fig. 11 shows a system 1100 for producing a three-dimensional workpiece by irradiating layers of a raw material powder with electromagnetic radiation or particle radiation. In other words, the system 1100 is a system for producing a three-dimensional workpiece by means of an additive manufacturing process in which a raw material powder is used, for example, selective laser melting or selective laser sintering.

[0345] Although the bulk material conveying system of the present disclosure is described below in connection with a system mentioned above, the bulk material conveying system or the method for conveying bulk material is not limited to use in connection with an additive manufacturing system. The advantages resulting from the bulk material conveying system or the associated method presented herein are generally applicable to situations in which bulk material is conveyed by means of pneumatic conveying.

[0346] The system 1100 comprises a carrier 1002 and a powder application device 1003 for applying a raw material powder 1004 to the carrier 1002. The carrier 1002 and the powder application device 1003 are located within a process chamber 1011, which can be sealed from the ambient atmosphere, ie, from the environment of the process chamber 1011. The system 1100 further comprises an irradiation device 1005 for selectively irradiating electromagnetic radiation or particle radiation onto the raw material powder 1004, which has been applied to the carrier 1002.

[0347] The process chamber 1011, i.e., the powder application device 1003, is supplied with raw material powder 1004 by means of a bulk material conveying system 1001, which is described in detail below. In the present case of powder conveying, the bulk material conveying system 1001 is a powder conveying system. The bulk material conveying system 1001 comprises a powder reservoir 1006 in which the raw material powder 1004, which is fed to the process chamber 1011, is stored. The powder reservoir 1006 is connected to a conveying line 1007 via a dosing device 1008. A gas stream 1009 is conveyed through the conveying line 1007 by means of a conveying device 1019 along a direction indicated by an arrow in Fig. 11. In the exemplary powder conveying system 1001 shown in Fig. 11, the conveying device 1009 is designed in the form of a vacuum pump.The dosing device 1008 is configured to deliver a desired dose of raw material powder 1004 into the gas stream 1009 flowing through the conveying line 1007. In particular, the dosing device 1008 comprises a first powder valve 1021, which is provided with a continuously variable flow cross-section, so that the amount of powder 1004 introduced into the gas stream 1009 per unit of time via a dosing opening 1023 of the dosing device 1008 can be continuously varied. In other words, a control value can be applied to the dosing device 1008 by a control device 1040, described further below, which determines the delivered dose per unit of time. In the case of the valve 1021 with a variable cross-section described above, the control value determines the cross-section and thus the dose. Alternatively, the valve 1021 can be replaced by a conveyor screw with an associated motor for driving the conveyor screw.In this case, the control value determines the motor speed and thus the dispensed dose. By applying a predefined control value, a desired dose of dispensed powder per unit of time can be determined.

[0348] The raw material powder-gas mixture, which flows through the conveying line 1007 downstream of the dosing device 1008, is conveyed to a cyclone 1010, which serves as a separation device. The cyclone 1010 includes an inlet 1020, which allows a tangential inflow of the raw material powder-gas mixture into a conical separation chamber 1022. Powder particles 1004, which fall out of the rotating flow of the raw material powder-gas mixture, which forms within the conical separation chamber 1022, are discharged from the cyclone 1010 via a powder outlet 1024, which is located in a lower region of the cyclone 1010. These powder particles 1004 are fed to the process chamber 1011, ie the powder application device 1003, by means of a connecting line 1026 which connects the powder outlet 1024 of the cyclone 1010 with a powder inlet 1028 of the process chamber 1011.

[0349] A second powder valve 1030 is provided in the connecting line 1026. Just like the first powder valve 1021 of the dosing device 1008, the second powder valve 1030 is also equipped with a continuously variable flow cross-section, so that the amount of powder 1004 supplied to the process chamber 1011 from the powder outlet 1024 of the cyclone 1010 can be continuously varied. The second powder valve 1030 can also (like the first powder valve) be replaced by a conveyor screw with an associated motor. In the case shown in Fig. 11, powder is thus conveyed from a source tank formed by the powder reservoir 1006 into a target tank, wherein the target tank can be viewed, for example, as the volume of the connecting line 1026 below the powder valve 1030. In the schematic representation of Fig.11, this volume represents a powder container for the powder application device 1003, from which the powder application device 1003 is supplied with powder for the application of individual layers. Alternatively, this powder reservoir (and thus the target tank) can be integrated into the powder application device 1003.

[0350] The gas separated from the powder particles 1004 in the cyclone 1010 is returned to the conveying line 1007 via a gas outlet 1032 of the cyclone 1010. The gas outlet 1032 is arranged in an upper part of the cyclone 1010.

[0351] Since the gas stream 1009 leaving the gas outlet 1032 of the cyclone 1010 may contain remaining raw material powder particles 1004, a filter unit 1014 is arranged in the conveying line 1007 downstream of the cyclone 1010. The filter unit 1014 comprises a replaceable filter 1013 configured to filter out any remaining raw material powder particles 1004 present in the gas stream 1009 leaving the gas outlet 1032.

[0352] The bulk material conveying system 1001 further comprises a measuring device 1050 for measuring a characteristic of the gas flow conveyed in the conveying line 1007. In the illustrated embodiment of Fig. 11, the measuring device 1050 is arranged directly downstream of the conveying device 1019 and is a pressure sensor for measuring a pressure (gas pressure) within the conveying line 1007. In addition to the measuring device 1050, further measuring devices 1015, 1016, 1017, and 1018 are provided along the conveying line 1007. Measuring device 1018 is a temperature sensor for measuring a temperature of the gas flow. Measuring device 1015 is an oxygen sensor for measuring an oxygen content of the gas flow. The measuring devices 1016 and 1017 can each be additional sensors for measuring one of the previously mentioned properties (pressure, temperature, oxygen content).Furthermore, the measuring devices 1016 and 1017 can each independently be a sensor for measuring a flow velocity, the presence of bulk material (e.g., a capacitive sensor or ultrasonic sensor), a dew point, etc. Apart from the measuring devices 1050 and 1015 to 1018, fewer or more sensors can also be provided. Furthermore, the measuring devices can be provided at a different location in the conveying line.

[0353] For the functioning of the control device 1040 (see below) described herein, the pressure sensor 1050, the temperature sensor 1018 and the oxygen sensor 1015 are particularly relevant.

[0354] Finally, the bulk material conveying system 1001 comprises a control device 1040.

[0355] The control device 1040 controls the operation of the elements of the bulk material conveying system, in particular all controllable elements. For example, the control device 1040 controls the conveying device 1019 and can, in particular, control it such that a predetermined flow rate of the conveyed gas is set, for example by applying a voltage value set by the control device 1040. Furthermore, the control device 1040 is capable of increasing or decreasing the flow rate by a predetermined value. The control device 1040 also controls the dosing device 1008. In particular, the control device 1040 applies a predetermined control value to the dosing device 1008, which causes the dosing device to release a predetermined dose of raw material powder into the gas stream per unit of time.Furthermore, control device 1040 is capable of increasing or decreasing a currently delivered dose by a predetermined value. Control device 1040 also receives the measurement data from all measuring devices. Furthermore, control device 1040 controls valve 1030.

[0356] The control device 1040 can further be configured to control the entire operation of the system 1010, i.e., the exposure by means of the irradiation device 1005, the powder application by means of the powder application device 1003, a lowering of the carrier 1002, etc.

[0357] To determine a suitable setpoint of the gas velocity in the delivery line 1007 and to set a velocity corresponding to the setpoint, the control device 1040 performs the following steps:

[0358] A. Determining a gas density of the gas stream based on the measured at least one characteristic of the gas stream;

[0359] B. Determining a bulk material mass flow of the bulk material flow based on the control value applied to the dosing device; C. Determining a target velocity of the gas flow based on the gas density and based on the bulk material mass flow; and

[0360] D. Controlling the conveying device to convey the gas flow at the determined target speed.

[0361] The individual steps:

[0362] Step A:

[0363] The control device 1040 calculates a gas density of the gas flow based on a measured characteristic of the gas flow. There are several ways in which a gas density of the gas flow can be determined (e.g., calculated or estimated) based on one or more measured characteristics. One possible approach is presented below as an example. The control device 1040 uses a measured pressure (measured by pressure sensor 1050), a measured temperature (measured by temperature sensor 1018), and a measured oxygen content (measured by oxygen sensor 1015).

[0364] The gas density p is determined using the following formula: where pmess is the pressure of the gas stream measured by the pressure sensor 1050, Tmess is the temperature of the gas stream measured by the temperature sensor 1018, p n a predetermined normal pressure (in the example a normal pressure of p n = 1013.25 mbar), and Tn a predetermined standard temperature (in the example, a standard temperature of 293.15 K). The values ​​for standard pressure and standard temperature can also be replaced by measured values ​​of the environment (surrounding atmosphere) of the bulk material conveying system. pMixture is calculated using the following formula: where pAir is a known density of air that is part of the gas stream, PShieldGas is a known density of a shielding gas that is part of the gas stream, OxygenContentIair is a known oxygen content of the air, and OxygenContentMeasure is the oxygen content of the gas stream measured by the oxygen sensor 1015. The density of the air can be, for example, pLutt = 1.225 kg / m 3 and in case argon is used as the shielding gas, the density of the shielding gas can be pschutzgas = 1.784 kg / m 3can be used. Oxygen content of the air can be used as 20.94%. Alternatively, the oxygen content of the ambient atmosphere can be measured and used.

[0365] Step B:

[0366] The control device 1040 determines a bulk material mass flow of the bulk material flow based on the control value applied to the dosing device 1008. In the case of the described powder conveyance, the bulk material mass flow is a powder mass flow. The mass flow is specified, for example, in kg / s.

[0367] The determination of the bulk material mass flow depends on the dosing device 1009 used and in particular on its geometry. In the case of a dosing device 1009 that has already been calibrated, for example, an associated volume flow of the conveyed bulk material can be calculated or read from a table depending on an applied control value (e.g., rotational speed of a conveyor screw). In particular, a linear relationship can exist between the control value and the volume flow, whereby a factor characterizing the linear relationship can have been previously determined by means of calibration. From the determined volume flow (m 3 / s) can be calculated using a known density (kg / m 3) of the conveyed bulk material, the desired mass flow in kg / s can be calculated. A calibration table or a calculation option for the volume flow based on a given control value can be found, for example, in a manual or a specification for the dosing device 1009.

[0368] If no corresponding calibration data is available, the dosing device 1009 can be calibrated to determine a relationship between control value and volume flow.

[0369] Furthermore, the bulk material mass flow can also be calculated. A corresponding calculation is presented below using the example of a screw conveyor.

[0370] The bulk material mass flow is calculated using the following formula:

[0371] Here, M s the bulk material mass flow, D z an outer diameter of the screw conveyor, Gw a height of the screw wall, H w,i is a first screw pitch, e is a thickness of the screw wall, n m a rotation speed of the screw conveyor, i a gear ratio and p s a bulk density.

[0372] Details on the parameters given above in the context of a screw conveyor can also be found in Fig. 12, which shows a screw conveyor and associated parameters.

[0373] Step C:

[0374] The controller 1040 determines a desired velocity of the gas flow based on the gas density determined in step A and based on the bulk material mass flow determined in step B.

[0375] Determining the target speed v S0 n of the gas flow may include calculating a saltation velocity based on the determined gas density and the determined bulk material mass flow. Determining the target velocity v S0n is carried out according to one embodiment using the following formula: soll ^saltation " ^Sicherheit where Vsaitation is the calculated saltation velocity of the conveyed bulk material and vsicherheit is a predetermined safety velocity.

[0376] The saltation velocity is a velocity below which particles of the conveyed bulk material (e.g., powder) begin to fall and accumulate at the bottom of the conveying line 1007. To ensure this does not occur, a predetermined safety velocity is added to the calculated saltation velocity. This represents a safety margin from the saltation velocity, ensuring that no deposition of bulk material occurs in the conveying line 1007. Alternatively to adding the safety velocity, a predetermined safety factor can be multiplied by the saltation velocity (e.g., 1.1 or 1.2). The saltation velocity is calculated by the control device 1040 using the following formula:

[0377] Here, M sthe determined bulk material mass flow in kg / s, g a predetermined gravitational acceleration in m / s 2 , D is a diameter of the conveying line in m, p is the determined gas density of the gas stream in kg / m 3 and a and b are parameters that each depend on a particle diameter d of the bulk material. The parameters a and b can, for example, each be stored as constants for the bulk material used in a memory of the control device. Furthermore, at least one of the two parameters a and b can be calculated, with the respective calculation depending on the particle diameter d of the bulk material used.

[0378] The particle diameter d is taken from a specification of the bulk material used, for example, a data sheet for the powder used. The particle diameter d can also be stored, for example, in a memory of the control device 1040. In particular, a table can be stored in the memory of the control device 1040, in which the respective values ​​for the particle diameter d of the respective powder are stored for different powder materials. In this way, a respective particle diameter d can be used when calculating the target speed by entering or selecting the powder used via a user interface of the control device 1040.

[0379] Step D:

[0380] The control device 1040 controls the conveying device 1019 to convey the gas flow at the desired speed determined in step C. For example, a signal with a predetermined voltage value or a predetermined frequency is applied to the conveying device 1019, wherein it is known that the voltage value or the frequency leads to the desired conveying speed of the conveying device 1019.

[0381] In the event that the conveyor device 1019 is not calibrated accordingly, a control loop can be used that takes into account a measured gas velocity value from a velocity sensor. For example, sensor 1016 or 1017 can be a corresponding velocity sensor. For example, a PID controller can be used to adjust the desired value of the target velocity.

[0382] Using the technology described above, an operating point for bulk material conveying (i.e., a conveying velocity of the conveying gas and thus a conveying velocity of the conveyed bulk material) can be set quickly, easily, and simply. This involves, on the one hand, access to data stored for different powder types in a memory of the control device 1040 (e.g., the bulk density and the particle diameter of the powder). On the other hand, it is possible to react to changing operating parameters, in particular to changes in measured values, for example, the oxygen content, the pressure, and / or the temperature within the conveying line 1007. It is also possible to react to a change in the conveyed mass flow of the bulk material by adjusting the velocity of the gas flow.

[0383] According to one exemplary embodiment, the above steps A to D, which are executed by the control device 1040, can be followed by a so-called limit control. However, this is optional, and only the determination of the target speed according to steps A to D can be performed.

[0384] Details of the limit control are shown in Fig. 13. The limit control in Fig. 13 corresponds to the limit control described above in connection with Fig.

[0385] 10 for the powder conveying system of Fig. 1. In this regard, the control device 1040 is configured to determine whether at least one measured characteristic of the bulk material conveying system 1001 is above a predetermined maximum value for the respective characteristic. If the at least one measured characteristic is above the predetermined maximum value, the control device 1040 reduces the control value of the dosing device 1008 by a predetermined value, so that it releases a smaller dose of bulk material into the gas stream per unit time.

[0386] An example of limit control is shown in Fig. 13 using specific parameters. Fig. 13 shows a flowchart of a process performed by the control device 1040 after determining the target speed and controlling the conveyor device. The process starts with step 1202, in which a query is made as to whether a measured conveyor speed is below a predetermined maximum value. A speed sensor in the gas stream or in the gas-powder mixture stream is used to measure the conveyor speed, for example, sensor 1016 or 1017 in Fig. 11.

[0387] If "yes," the process proceeds to step 1204, where it is queried whether a measured pump outlet pressure is below a predetermined maximum value. To measure the pump outlet pressure, a pressure sensor in the gas stream downstream of the pump 1019 is used, for example, sensor 1050 in Fig. 11.

[0388] If "yes," the process proceeds to step 1206, where it is queried whether a pump power consumed by the pump 1019 is below a predetermined maximum value.

[0389] If "yes," the process proceeds to step 1208, where it is queried whether a dose of bulk material dispensed by the dosing device 1008 per time is within a predetermined maximum range.

[0390] If the answer is "yes," the process is complete. You can then ask whether the feed should be stopped. If this is not the case, steps A to D discussed above are performed again, and the feed speed is adjusted if necessary.

[0391] If it is determined in step 1208 that the dispensed dose of bulk material per time is not within the predetermined maximum range ("no" after step 1208), then in step 1210 it is queried whether the dose is below the predetermined maximum range. If this is the case ("yes"), then in step 1212 the dose is increased by a predetermined value. If this is not the case ("no" after step 1210), then in step 1214 the dose is decreased by a predetermined value.

[0392] Following step 1214, step 1216 queries whether the dose is less than a predetermined minimum. If this is not the case ("no"), the process is terminated. However, if this is the case ("yes" after step 1216), filter cleaning is performed in step 1218. The process also proceeds to reduce the dose of the dosing device 1008 according to step 1214 if the result in at least one of queries 1202, 1204, and 1206 is "no," meaning that a corresponding maximum value has been reached or exceeded.

[0393] After the process of Fig. 13, it is checked whether the conveying process is terminated and thus whether in particular the conveying device 1019 is stopped.

[0394] This is the case when at least one of the following events is detected: a source tank 1006, from which bulk material is removed by means of the dosing device 1008, is empty, a target tank into which the bulk material is conveyed (or a corresponding cyclone 1010) is full, a predetermined maximum conveying time is exceeded and a total pressure loss of the conveying gas exceeds a predetermined limit value.

[0395] If the conveying process is not terminated, the process performed by the control device 1040 returns to step A (see above).

[0396] Fig. 1 shows a schematic representation of a bulk material conveying system which is used for conveying raw material powder in a plant 1100 for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation.

[0397] The bulk material conveying system of Fig. 1 represents a more detailed illustration of a bulk material conveying system compared to Fig. 11, wherein certain components are shown and provided with reference numerals that may also be present in the system of Fig. 11, but are not explicitly described.

[0398] Furthermore, however, the system of Fig. 1 also represents a more complex embodiment of a bulk material conveying system of the type described herein, particularly since multiple conveying processes can be carried out within the system, i.e., conveying processes from different source tanks to different destination tanks. The exact structure and operation of the bulk material conveying system have already been described above in connection with Fig. 1 (as a powder conveying system).

[0399] In particular, the method comprising steps A to D for adjusting a conveying speed can also be carried out in conjunction with the system of Fig. 1. The same applies to the limit value control of Fig. 13. For this purpose, the bulk material conveying system of Fig. 1 also comprises a control device (not shown) for controlling the individual components of the system of Fig. 1.

[0400] The system 1100 corresponds, for example, to the system 1100 of Fig. 11 and - generally speaking - for example to a generally known system for additive manufacturing by means of selective laser melting or selective laser sintering.

[0401] During operation (ie, conveying operation) of each of the conveying processes a to c described in connection with Fig. 1, a method can be executed that determines the gas velocity (target velocity) and adjusts the pump 79 so that it conveys the conveying gas at the determined target velocity. For this purpose, the steps A to D explained above can be performed by a control device of the bulk material conveying system.

[0402] In one embodiment, for example, sensor data from pressure sensor 80, temperature sensor 86, and oxygen sensor 76 are used to determine the gas density of the gas stream. To determine the bulk material mass flow, the geometry of the respective conveyor screw located at the outlet of the respective source tank is taken into account. Using velocity sensor 86, the velocity of the gas delivered by pump 79 is regulated, for example, using a PID controller.

[0403] In addition, a limit control according to Fig. 13 can be implemented in connection with each of the conveying processes a to c (see the description above).

[0404] Closed powder circuits according to conveying processes a to c can have the advantage, among other things, that a system operator has to intervene in the process manually as little as possible and comes into contact with the powder as little as possible, which, for example, poses health risks and, with certain powder types, can pose a risk of explosion or ignition. Automatic adjustment of the conveying speed can have the advantage of allowing an optimal operating point (i.e., an optimal conveying speed) to be set quickly and easily for different powder types and / or different process conditions.

[0405] Aspects that can be used alone and / or in combination with the powder conveying system of Fig. 1 are as follows: 1.Bulk material conveying system, in particular for conveying raw material powder in a system for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation, wherein the bulk material conveying system comprises: a conveying line which is configured to convey a gas flow and, at least in sections, a bulk material flow driven by the gas flow; a dosing device which is configured to supply the gas flow with a predetermined dose of bulk material per unit of time, wherein the dose is determined by a control value applied to the dosing device; a conveying device which is configured to convey the gas flow through the conveying line; at least one measuring device for measuring at least one characteristic variable of the gas flow; and a control device which is configured to:

[0406] Determining a gas density of the gas flow based on the measured at least one characteristic of the gas flow;

[0407] Determining a bulk material mass flow of the bulk material flow based on the control value applied to the dosing device;

[0408] Determining a target velocity of the gas flow based on the gas density and based on the bulk material mass flow; and

[0409] Controlling the conveying device to convey the gas flow at the specified target speed.

[0410] 2. Bulk material conveying system according to aspect 1, wherein the gas density is determined based on at least one of the following parameters of the gas flow: oxygen content, pressure, temperature, dew point and humidity.

[0411] 3. Bulk material conveying system according to aspect 2, wherein the gas density p is determined using the following formula: where pmess is a measured pressure of the gas stream, Tmess is a measured temperature of the gas stream, p n a predetermined normal pressure, T n is a predetermined normal temperature and pmixture is given by the following formula: where pair is a known density of air which is part of the gas stream, Pshield gas is a known density of a shielding gas which is part of the gas stream, oxygen content air is a known oxygen content of the air and oxygen content meas is a measured oxygen content of the gas stream.

[0412] 4. Bulk material conveying system according to one of aspects 1 to 3, wherein the control value applied to the dosing device is a motor speed of a motor of the dosing device, in particular a motor for driving a conveyor screw of the dosing device.

[0413] 5. Bulk material conveying system according to one of aspects 1 to 4, wherein the determination of the target speed v S0 n of the gas flow comprises calculating a saltation velocity based on the determined gas density and the determined bulk material mass flow, and wherein determining the target velocity v S0 n using the following formula: soll ^saltation "F ^safety where Vsaitation is the calculated saltation velocity of the conveyed bulk material and vsafety is a predetermined safety velocity.

[0414] 6. A bulk material conveying system according to aspect 5, wherein the saltation velocity is calculated using the following formula: where M s is the determined bulk material mass flow in kg / s, g is a predetermined gravitational acceleration in m / s 2 is, D is a diameter of the conveyor line in m, p is the determined gas density of the gas stream in kg / m 3and a and b are parameters depending on a particle diameter d of the bulk material.

[0415] 7. Bulk material conveying system according to one of aspects 1 to 6, wherein the control of the conveying device for conveying the gas flow at the determined target speed is carried out using a speed sensor for measuring a speed of the gas flow and a control loop, in particular comprising a PID controller.

[0416] 8. Bulk material conveying system according to one of aspects 1 to 7, wherein the control device is further configured to:

[0417] Determining whether at least one measured characteristic of the bulk material conveying system is above a predetermined maximum value for the respective characteristic; and if the at least one measured characteristic is above the predetermined maximum value, reducing the control value of the dosing device by a predetermined value.

[0418] 9. Bulk material conveying system according to aspect 8, wherein the control device is further configured to: if the at least one measured characteristic value is below the predetermined maximum value, increase the control value of the dosing device by a predetermined value.

[0419] 10. Bulk material conveying system according to claim 8 or 9, wherein the at least one characteristic comprises at least one of the following characteristics:

[0420] Conveying speed, pump outlet pressure, pump power and dose of bulk material per time.

[0421] 11. Bulk material conveying system according to one of aspects 1 to 10, wherein the control device is configured to:

[0422] Terminating a conveyance by stopping the conveying device if at least one of the following events is detected: a source tank from which bulk material is withdrawn by means of the dosing device is empty, a target tank into which the bulk material is conveyed is full, a predetermined maximum conveying time is exceeded and a total pressure loss of the conveying gas exceeds a predetermined limit.

[0423] 12. Bulk material conveying system according to one of aspects 1 to 11, further comprising a pressure equalization tank coupled to the conveying line downstream of the conveying device and upstream of the dosing device. 13. Bulk material conveying system according to one of aspects 1 to 12, comprising at least one first conveying circuit for conveying raw material powder from a main reservoir into a first tank of the system for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation, wherein a lower outlet of the first tank is coupled to an upper inlet of an intermediate tank of the system, wherein a manufacturing process in a process chamber of the system is fed with powder from the intermediate tank, and wherein an upper inlet of the main reservoir is coupled to an outlet of a sieve for sieving the raw material powder.

[0424] 14. Bulk material conveying system according to one of aspects 1 to 13, comprising at least one second conveying circuit for conveying raw material powder from an overflow tank into a buffer container, wherein the overflow tank is configured to receive excess powder from the process chamber of the system for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation, and wherein a lower outlet of the buffer container is coupled to an inlet of a sieve for sieving the raw material powder.

[0425] 15. Bulk material conveying system according to one of aspects 1 to 14, comprising at least one third conveying circuit for conveying raw material powder from an external tank into a buffer container, wherein the external tank is detachably coupled or coupled to the bulk material conveying system, and wherein a lower outlet of the buffer container is coupled to an inlet of a sieve for sieving the raw material powder.

[0426] 16. Bulk material conveying system according to one of aspects 1 to 15, wherein the dosing device is located at the outlet of a respective source tank and wherein a cyclone for separating the raw material powder from the gas stream and for feeding the raw material powder into the target tank is located at the inlet of a respective target tank.

[0427] 17. A method for conveying bulk material, in particular for conveying raw material powder in a plant for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation, the method comprising:

[0428] Conveying a gas stream and a bulk material stream driven by the gas stream through a conveying line;

[0429] Feeding a predetermined dose of bulk material per unit of time through a dosing device, the dose being determined by a control value applied to the dosing device;

[0430] Conveying the gas flow through the conveying line using a conveying device;

[0431] Measuring at least one characteristic of the gas flow using a measuring device;

[0432] Determining a gas density of the gas flow based on the measured at least one characteristic of the gas flow;

[0433] Determining a bulk material mass flow of the bulk material flow based on the control value applied to the dosing device;

[0434] Determining a target velocity of the gas flow based on the gas density and based on the bulk material mass flow; and

[0435] Controlling the conveying device to convey the gas flow at the specified target speed.

[0436] 18. The method according to aspect 17, wherein the gas density is determined based on at least one of the following characteristics of the gas flow: oxygen content, pressure, temperature, dew point and humidity.

[0437] 19. The method according to aspect 18, wherein the gas density p is determined using the following formula: where pmess is a measured pressure of the gas stream, Tmess is a measured temperature of the gas stream, p n a predetermined normal pressure, T nis a predetermined normal temperature and pmixture is given by the following formula: where ßAir is a known density of air which is part of the gas stream, PSchutzgas is a known density of a protective gas which is part of the gas stream, Oxygengehaltiluft is a known oxygen content of the air and Oxygengehaltmess is a measured oxygen content of the gas stream.

[0438] 20. Method according to one of aspects 17 to 19, wherein the control value applied to the dosing device is a motor speed of a motor of the dosing device, in particular a motor for driving a conveyor screw of the dosing device.

[0439] 21. The method according to any one of aspects 17 to 20, wherein determining the desired velocity Vsoii of the gas flow comprises calculating a saltation velocity based on the determined gas density and the determined bulk material mass flow, and wherein determining the desired velocity v S0 n using the following formula: soll ^saltation "F ^safety where Vsaitation is the calculated saltation velocity of the conveyed bulk material and vsafety is a predetermined safety velocity.

[0440] 22. The method of aspect 21, wherein calculating the saltation rate is performed using the following formula: where M s is the determined bulk material mass flow in kg / s, g is a predetermined gravitational acceleration in m / s 2 is, D is a diameter of the conveyor line in m, p is the determined gas density of the gas stream in kg / m 3and a and b are parameters depending on a particle diameter d of the bulk material.

[0441] 23. The method according to any one of aspects 17 to 22, wherein the control of the conveying device for conveying the gas flow at the determined target speed is carried out using a speed sensor for measuring a speed of the gas flow and a control loop, in particular comprising a PID controller. 24. The method according to any one of aspects 17 to 23, further comprising:

[0442] Determining whether at least one measured characteristic of the bulk material conveying system is above a predetermined maximum value for the respective characteristic; and if the at least one measured characteristic is above the predetermined maximum value, reducing the control value of the dosing device by a predetermined value.

[0443] 25. The method according to aspect 24, further comprising: if the at least one measured characteristic is below the predetermined maximum value, increasing the control value of the dosing device by a predetermined value.

[0444] 26. The method according to aspect 24 or 25, wherein the at least one characteristic comprises at least one of the following characteristics:

[0445] Conveying speed, pump outlet pressure, pump power and dose of bulk material per time.

[0446] 27. A method according to any one of aspects 17 to 26, further comprising:

[0447] Terminating a conveyance by stopping the conveying device if at least one of the following events is detected: a source tank from which bulk material is withdrawn by means of the dosing device is empty, a target tank into which the bulk material is conveyed is full, a predetermined maximum conveying time is exceeded and a total pressure loss of the conveying gas exceeds a predetermined limit.

[0448] 28. A method according to any one of aspects 17 to 27, comprising:

[0449] Conveying raw material powder through at least one first conveyor circuit from a main reservoir into a first tank of the system for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation, wherein a lower outlet of the first tank is coupled to an upper inlet of an intermediate tank of the system, wherein a production process in a process chamber of the system is fed with powder from the intermediate tank, and wherein an upper inlet of the main reservoir is coupled to an outlet of a sieve for sieving the raw material powder. 29. Method according to one of aspects 17 to 28, comprising:

[0450] Conveying raw material powder through at least one second conveying circuit from an overflow tank into a buffer container, wherein the overflow tank is configured to receive excess powder from the process chamber of the system for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation, and wherein a lower outlet of the buffer container is coupled to an inlet of a sieve for sieving the raw material powder.

[0451] 30. A method according to any one of aspects 17 to 29, comprising:

[0452] Conveying raw material powder through at least one third conveying circuit from an external tank into a buffer container, wherein the external tank is detachably coupled or coupled to the bulk material conveying system, and wherein a lower outlet of the buffer container is coupled to an inlet of a sieve for sieving the raw material powder.

[0453] 31. Method according to one of aspects 17 to 30, wherein the dosing device is located at the outlet of a respective source tank and wherein a cyclone for separating the raw material powder from the gas stream and for feeding the raw material powder into the target tank is located at the inlet of a respective target tank.

[0454] The screening device of the present disclosure (in particular, the screening device 5 of FIG. 1) may further be configured according to the screening device of the following disclosure. A method performed by the screening device 5 of FIG. 1 may correspond to one of the methods of the following disclosure.

[0455] Individual aspects of the screening device and / or the method described below can be applied to the screening device 5 of the above description.

[0456] However, the aspects of a screening device or an associated method described below can also be advantageous in themselves and represent one or more inventions. Thus, the screening device and / or the method described below can also be used independently (i.e., independently of the powder conveying system described above). The following disclosure relates to a method for controlling the operation of a screening device and to a screening device that is suitable, for example, for use in a powder conveying system of a system for producing three-dimensional workpieces using a generative layer building process. Furthermore, the following disclosure relates to a system equipped with such a screening device for producing three-dimensional workpieces using a generative layer building process.

[0457] In generative layered construction processes for the production of three-dimensional workpieces, particularly powder bed fusion, a raw material powder or granulate is applied layer by layer to a substrate and, depending on the desired geometry of the workpiece to be created, is selectively exposed to electromagnetic radiation, such as laser radiation or particle radiation. The radiation penetrating the powder layer causes heating and, consequently, fusion or sintering of the raw material powder particles. Subsequently, further layers of raw material powder are successively applied to the already radiation-treated and solidified layer on the substrate until the workpiece achieves the desired shape and size. The raw material powder can comprise ceramic, metal, or plastic materials, but also mixtures of these materials.Additive layering processes and in particular powder bed fusion processes can be used, for example, to produce prototypes, tools, spare parts or medical prostheses, such as dental or orthopedic prostheses, as well as to repair components based on CAD data.

[0458] A system for producing three-dimensional workpieces by selectively irradiating a raw material powder, known for example from EP 2 335 848 B1, comprises a process chamber sealed from the ambient atmosphere and a carrier arranged in the process chamber for receiving the raw material powder to be irradiated. The system further comprises an irradiation device equipped with a radiation source, in particular a laser source, and an optical unit. The optical unit serves to selectively guide an irradiation beam generated by the radiation source over the raw material powder layers applied to the carrier, depending on the geometry of the workpiece to be produced. When building a three-dimensional workpiece by selectively irradiating the powder layers applied to the carrier, the radiation energy introduced into the raw material powder causes the powder particles to melt and / or sinter.During the additive manufacturing of three-dimensional components in a powder bed (particularly through selective laser melting and / or selective laser sintering), excess powder is generated during the application of the individual powder layers, which can be collected in one or more collecting containers. Furthermore, non-excess, solidified powder can be recovered when unpacking the workpieces. The excess powder can be reused in an additive manufacturing process after appropriate reprocessing. For example, the recycled powder can be reused as raw material powder for a selective laser melting or laser sintering process, or it can be mixed with the fresh raw material powder used in this process.

[0459] However, particulate impurities as well as glued or sintered powder agglomerates in the excess powder could lead to contamination of the powder bed and thus to reduced quality of the workpiece if the excess powder is reused in an additive manufacturing process.

[0460] A key step in the recycling of excess powder is therefore sieving, which removes unwanted particulate contaminants larger than the powder particle size. In addition, the excess powder can be subjected to further powder recycling steps, such as drying, cleaning, component separation, etc., which can be performed before and / or after sieving the powder.

[0461] A device for sieving powder, suitable for use in a system for producing a three-dimensional workpiece by means of selective electron beam melting, selective laser melting, laser metal deposition, laser metal deposition, or selective laser sintering, is described in DE 20 2021 102 494. This device comprises a base unit with a vibration generator and a control unit for controlling the vibration generator. The base unit has an interface for connecting a replacement module, which comprises a sieve and a housing for holding the sieve. The replacement module can be connected to the base unit via the interface.

[0462] The following disclosure is directed to the object of providing a method for controlling the operation of a screening device and a screening device that enable efficient powder screening and are therefore particularly well suited for use in a powder conveying system of a system for producing three-dimensional workpieces using a generative layer-by-layer manufacturing process. Furthermore, the following disclosure is directed to the object of specifying a system for producing three-dimensional workpieces using a generative layer-by-layer manufacturing process that enables efficient production of high-quality workpieces.

[0463] This object is achieved by a method for controlling the operation of a screening device having the features of the following aspect 1, a screening device having the features of aspect 13 and a system for producing three-dimensional workpieces using a generative layer construction process having the features of aspect 21.

[0464] In a method for controlling the operation of a screening device, in a step (i), powder to be screened is fed through a powder inlet onto a screen. The screen can comprise a screen frame onto which a screen mesh defining a screening surface is stretched. The screen frame can be connected to a screen container or placed onto the screen container. Screened powder can then be collected in the screen container after passing through the screen mesh. The screen container can have a downwardly tapered cross-section so that screened powder received in the screen container can be discharged from the screen container by gravity via a screened powder outlet arranged in the region of a lower section of the screen container. The screening device can have an oversize outlet to discharge oversize particles that are too coarse to pass through the screen mesh toward the screen container.The powder inlet and the oversize grain outlet are preferably arranged in the area of ​​opposite side edges of the sieve surface.

[0465] The screening device is preferably equipped with a drive device for driving the screen. The drive device preferably acts on the screen frame, so that during operation of the screening device, the screen frame and thus the screen mesh are set into vibration by the drive device. The drive device can be a mechanical drive device that vibrates the screen. Preferably, however, the drive device is designed in the form of an ultrasonic drive device and is configured to subject the screen to ultrasonic vibrations. The use of an ultrasonic drive device instead of a mechanical drive device enables an increase in the screening powder throughput, i.e., a higher screening performance, and thus an increase in the efficiency of the screening device.In particular, the sieving powder throughput can be increased by a factor of approximately 4 by equipping the sieving device with an ultrasonic drive device compared to a mechanically driven sieving device.

[0466] In an ultrasonic drive system, the frequency primarily determines the drive power. While the amplitude also influences the power consumption of the drive motor, the frequency is the most important parameter for drive power, as it influences the vibrations of the screen mesh and thus the separation efficiency of the screen. A higher frequency typically leads to improved separation efficiency and thus a higher sieve powder throughput, i.e., higher screening efficiency.

[0467] When a powder is loosely poured onto a flat surface, an angle of repose forms. The angle of repose is the angle between the horizontal plane and the maximum inclination that the powder can assume without further influences, i.e. the angle at which the powder begins to flow when poured onto an inclined surface. The angle of repose depends on various factors, such as the size and shape of the powder particles. Furthermore, the angle of repose is influenced by process parameters, such as the relative humidity. A powder bed of a powder to be sieved using the sieving device can, for example, have an angle of repose between approximately 20° and approximately 40°, preferably between approximately 25° and approximately 35°, and particularly preferably approximately 30°.

[0468] The wall friction angle, on the other hand, is the angle between a horizontal plane and the incline at which the powder adheres to the wall and stops flowing. The wall friction angle depends on the type and quality of the wall as well as the surface quality of the powder particles. For example, a powder bed to be screened using the screening device can have a wall friction angle between approximately 10° and approximately 30°, preferably between approximately 15° and approximately 25°, and particularly preferably approximately 20°.

[0469] When powder is fed through the powder inlet of the sieve device, a repose cone forms on the sieve, i.e. on the sieve mesh, the shape and angle of repose of which are influenced by the vibrations acting on the repose cone and consequently by the drive power of the drive device driving the sieve. In particular, the angle of repose decreases with increasing drive power of the drive device. At the same time, the vibrations triggered by the drive device reduce wall friction, which makes it easier for the powder to slide over the sieve surface. If the sieve is driven with a low drive power of the drive device, the repose cone that forms on the sieve below the powder inlet resembles a repose cone forming on a stationary plane, the base area of ​​which only occupies a small section of the sieve surface.In such an operating state of the screening device, the screening surface utilization is low, since only a small section of the screening surface adjacent to the powder inlet is actually exposed to powder.

[0470] If, on the other hand, the sieve is driven with a high drive power, the powder spreads out over the sieve surface, i.e. the angle of repose of the cone of repose, which the powder fed onto the sieve through the powder inlet forms on the sieve surface, decreases with increasing drive power of the drive device driving the sieve, as does the wall friction angle. This increases the base area of ​​the cone of repose, so that the sieve surface utilization increases and sections of the sieve surface further away from the powder inlet and closer to the oversize grain outlet are also charged with powder. However, if the base area of ​​the cone of repose becomes too large, and in particular so large that the powder flows over the entire sieve surface and thus the entire sieve surface is charged with powder, there is a risk that powder will flow unscreened into the oversize grain outlet.As a result, powder that is actually fine-grained enough to pass through the sieve mesh is lost unused and the sieving efficiency decreases.

[0471] In the method for controlling the operation of a screening device, the screen is therefore driven in a step (ii) for a first time interval with a first drive power dimensioned such that the powder to be screened flows over an entire screening surface of the screen and / or into an oversize grain outlet when the screen is continuously driven with the first drive power. During the first time interval, the screening device is therefore driven with such a high drive power that, if maintained continuously, would, on the one hand, ensure maximum screening surface utilization, but, on the other hand, would at least be associated with a high risk of powder being lost unscreened through the oversize grain outlet.

[0472] After the first time interval has elapsed, the sieve is driven in a step (Hi) for a second time interval with a second drive power that is lower than the first drive power. After the second time interval has elapsed, steps (ii) and (iii) are repeated, i.e., the sieve is driven periodically alternately with the first higher drive power and the second lower drive power.

[0473] The initial drive power can assume different values ​​for different powder types and different process parameters, such as temperature, particle size distribution of the powder, powder moisture content, etc. The initial drive power can therefore either be determined empirically before a sieving process or taken from a previously created drive power value table for different powder types and process parameters. Alternatively, the initial drive power can also be selected based on empirical values.

[0474] During the first time interval, the high screen area utilization resulting from the high drive power and consequently a high screening performance can be benefited. Furthermore, during the first time interval, oversize grain is specifically conveyed towards the oversize grain outlet. In particular, due to the "spreading" of the cone of material across the entire screen area during the periodically recurring first time interval, oversize grain that collects inside the cone of material during the second time interval and "builds up" in columns can be removed from the screen area via the oversize grain outlet. This counteracts the formation of so-called "plug grains" caused by powder particles that are pressed into the screen mesh by back pressure and the vibration of the screen and clog the mesh.Reducing the drive power during the second time interval, on the other hand, ensures that the loss of unscreened powder through the oversize grain outlet is minimized.

[0475] The method described here for controlling the operation of a screening device therefore enables highly efficient powder screening and is therefore particularly well-suited for use in a powder conveying system of a plant for the production of three-dimensional workpieces using a generative layering process, in which large quantities of comparatively expensive (metal) powder may also have to be screened. Furthermore, the periodically changing drive power impinges on the screen mesh with a lower overall powder mass and thus places less strain on it. This reduces wear and consequently extends the service life of the mesh.

[0476] The second drive power is preferably dimensioned such that, when the sieve is continuously driven with the second drive power, the powder to be sieved forms a repose cone in the region of the powder inlet on the sieve surface of the sieve, which essentially corresponds to a repose cone forming on a stationary plane. The repose cone that forms on the sieve surface when the sieve is driven with the second drive power has, in particular, an angle of repose that is a maximum of 30%, preferably a maximum of 20%, and particularly preferably a maximum of 10% greater than the angle of repose of a repose cone that forms on a stationary plane. The base area of ​​the repose cone that forms on the sieve surface when the sieve is continuously driven with the second drive power only occupies a small section of the sieve surface of the sieve, arranged in the region of the powder inlet.During the second time interval, the screening device can therefore be driven at such a low drive power that, if maintained continuously, would ensure that no or almost no powder flows unscreened into the oversize grain outlet. However, due to the low screen area utilization, this would result in a low screening throughput and consequently a low screening performance. Thus, the second drive power can be lower than the first drive power, but still greater than zero.

[0477] In one or more embodiments, however, the second drive power may also be zero. In this case, the screen is not driven during the second time interval.

[0478] The second drive power can assume different values ​​for different powder types and different process parameters, such as temperature, particle size distribution of the powder, powder moisture content, etc. The second drive power can therefore either be determined empirically before a sieving process or taken from a previously created drive power value table for different powder types and process parameters. Alternatively, the second drive power can also be selected based on empirical values.

[0479] In principle, the sieve surface of the sieve can be aligned coplanarly with a horizontal plane. Alternatively, however, it is also conceivable for the sieve surface of the sieve to be inclined relative to a horizontal plane. An angle of repose of the cone of repose, which forms on the sieve surface of the sieve when the sieve is driven with the second drive power in the region of the powder inlet, is preferably adapted to an orientation of the sieve surface. This means that when the sieve surface of the sieve is aligned coplanarly with a horizontal plane, the powder fed onto the sieve through the powder inlet preferably forms a symmetrical cone of repose with a constant angle of repose along a circumference of the cone of repose.If, on the other hand, the sieve surface of the sieve is inclined relative to a horizontal plane, the powder fed through the powder inlet onto the sieve preferably forms a repose cone whose angle of repose varies along its circumference depending on the direction of inclination and the angle of inclination of the sieve surface.

[0480] Preferably, the sieve surface of the sieve is inclined relative to a horizontal plane such that the flow of the powder fed through the powder inlet towards the oversize grain outlet is assisted by gravity. In other words, the sieve surface of the sieve is preferably designed to slope downwards from a region below the powder inlet towards the oversize grain outlet. This not only promotes the spreading of the powder across the sieve surface, but also the removal of oversize grain from the sieve surface into the oversize grain outlet. Accordingly, the angle of repose of the repose cone, which forms on the sieve surface when the sieve is driven with the second drive power, is preferably smaller in a circumferential section of the repose cone facing the oversize grain outlet than in a circumferential section of the repose cone facing away from the oversize grain outlet.

[0481] However, the angle of inclination of the sieve surface relative to the horizontal plane is preferably less than the angle of repose of a cone of repose formed by the powder to be sieved on a horizontal plane. This ensures that, at least when the sieve is driven with the second drive power, a stable cone of repose is still formed in the region of the powder inlet and that the powder does not flow uncontrollably over the sieve surface. For example, the angle of inclination of the sieve surface relative to the horizontal plane can be approximately 10° to approximately 25°, preferably approximately 15° to approximately 20°, and particularly preferably approximately 17°.

[0482] Similar to the first drive power, the first time interval can also take on different values ​​for different powder types and different process parameters, such as temperature, particle size distribution of the powder, moisture content of the powder, etc. The first time interval is therefore preferably a value empirically determined for the powder to be screened before a screening process. However, the first time interval can also be taken from a previously created table of values ​​that contains different values ​​of the first time interval for different powder types and process parameters, or it can be selected based on empirical values. When determining the first time interval, the first time interval is preferably ended at the latest when powder to be screened flows into the oversize grain outlet. The flow of powder to be screened into the oversize grain outlet can be detected using an oversize grain sensor.The oversize sensor is preferably arranged in the area of ​​the oversize outlet.

[0483] Preferably, the first time interval is dimensioned such that, by the end of the first time interval, a sieve area utilization of approximately 70% to approximately 90%, preferably approximately 75% to approximately 85%, and particularly preferably approximately 80% of the total sieve area of ​​the sieve is not exceeded. In other words, the first time interval preferably provides a "time safety reserve" so that the powder does not spread over the entire sieve area during the first time interval. This particularly reliably prevents unscreened powder from being lost through the oversize grain outlet.

[0484] The second time interval can also take on different values ​​for different powder types and different process parameters, such as temperature, particle size distribution of the powder, moisture content of the powder, etc. Therefore, the second time interval is preferably also a value empirically determined for the powder to be sieved before a sieving process. However, the second time interval can also be taken from a previously created table of values ​​containing various values ​​for the second time interval for different powder types and process parameters, or it can be selected based on empirical values.

[0485] When determining the second time interval, the second time interval is preferably ended at the latest when powder to be screened forms a repose cone of a defined size on the screen surface in the area of ​​the powder inlet. This prevents the repose cone from becoming too large and clogging the powder inlet. On the other hand, the efficiency of the screening process is increased by the transition to the first time interval, during which the screen is driven with increased drive power and consequently the screening throughput is increased. The formation of a repose cone of a defined size can be detected, for example, by means of a dosing sensor. The dosing sensor is preferably arranged in the area of ​​the powder inlet.

[0486] The following explains the control of the powder feed through the powder inlet of the sieve, which can also be used independently of the drive control described above. In an independently claimable method for controlling the operation of a sieving device, powder to be sieved is fed at least temporarily continuously with a metered mass flow rhdos = 0.5 * (rh(amin) + rh(a m ax)) is fed through the powder inlet onto the sieve. The dosing mass flow can be adjusted, for example, by appropriately controlling a dosing device associated with the powder inlet, which may include a dosing screw, and / or by controlling a valve associated with the powder inlet. The parameters a m ax and rh(a max) define a sieve area utilization and a sieve throughput when driving the sieve with a first drive power, which is dimensioned such that the powder to be sieved flows over the entire sieve area of ​​the sieve and / or into an oversize grain outlet when the sieve is continuously driven with the first drive power. The parameters a m in and rh(a m In contrast, the screen area utilization and the screen throughput when the screen is driven with a second drive power that is lower than the first drive power. The continuous metered mass flow can therefore be determined by calculating an average of the screen throughput when the screen is driven with the first drive power and the screen throughput when the screen is driven with the second drive power.

[0487] The sieve throughput when driving the sieve with a second drive power rh(amin) is preferably determined by increasing the metered mass flow of the powder to be sieved through the powder inlet when driving the sieve with the second drive power until the powder to be sieved has formed a repose cone of a defined size on the sieve surface of the sieve in the region of the powder inlet. The formation of the repose cone of the defined size can be detected, for example, by means of the metering sensor arranged in the region of the powder inlet. In other words, when determining the sieve throughput when driving the sieve with a second drive power rh(a m in) the dosing mass flow of the powder to be screened through the powder inlet is preferably increased until the dosing sensor is triggered.

[0488] The screening throughput when driving the screen with the first drive power rh(amax) is preferably determined by increasing the metered mass flow of the powder to be screened through the powder inlet when driving the screen with the first drive power until the powder to be screened has formed a repose cone of a defined size on the screen surface of the screen in the area of ​​the powder inlet and powder to be screened flows into the oversize grain outlet. The formation of the repose cone of the defined size can, for example, be detected by means of the metering sensor arranged in the area of ​​the powder inlet. The flow of powder to be screened into the oversize grain outlet, on the other hand, can be detected by means of the oversize grain sensor arranged in the area of ​​the oversize grain outlet. In other words, when determining the screening throughput when driving the screen with the first drive power rh(a max), the dosing mass flow of the powder to be screened through the powder inlet is preferably increased until the dosing sensor and the oversize sensor are triggered.

[0489] The value of rh(a m ax) is preferably multiplied by a safety factor to prevent accidental overdosing of powder. The safety factor can be, for example, 0.8, 0.7, 0.6, or 0.5.

[0490] During the sieving process, the sieve mesh can become clogged, for example, by particles that are formed when powder particles are pressed into the sieve mesh, or by cold welding. This reduces the sieve throughput. To account for this phenomenon, the dosing mass flow rhdos can be reduced if the powder to be sieved forms a repose cone of a defined size on the sieve surface in the area of ​​the powder inlet. The formation of the repose cone of the defined size can, for example, be detected by the dosing sensor arranged in the area of ​​the powder inlet. In other words, if the dosing sensor is triggered during an ongoing sieving process, this can be interpreted as an indication of a blockage in the sieve mesh and a resulting reduced sieve throughput, and the dosing mass flow can therefore be reduced.

[0491] Additionally or alternatively, sieve cleaning can be initiated if the powder to be sieved forms a repose cone of a defined size on the sieve surface in the area of ​​the powder inlet and / or the dosing mass flow rhdos falls below a limit value. In other words, if a repose cone of a defined size still forms on the sieve surface in the area of ​​the powder inlet, even with a low dosing mass flow that falls below the limit value, this can be interpreted as an indication that the sieve mesh is so clogged that sieve cleaning is necessary. Additionally or alternatively, sieve cleaning can also be initiated after the end of each sieving process. A sieving process can be ended, for example, if a powder feed container for powder to be sieved, which can be connected to the powder inlet of the sieving device, is empty.

[0492] In addition to or as an alternative to the above options, screen cleaning can also be triggered manually, i.e., initiated by a user input. Additionally or alternatively, screen cleaning can be initiated in a time-controlled manner, for example, whenever a predetermined time (i.e., absolute time) or a predetermined operating time of the screening device (i.e., a time during which the screening device was in operation) has elapsed since the last screen cleaning.

[0493] When sieve cleaning is initiated, the powder feed through the powder inlet is preferably stopped. Furthermore, any powder still present in the sieve can be sieved before sieve cleaning begins. After sieve cleaning has started, the sieve can be driven at maximum drive power. Additionally or alternatively, after sieve cleaning has started, a vibrator can be activated, which drives the sieve independently of the drive device of the sieve. The angle of attack of the vibrator on the sieve, the drive amplitude of the vibrator, and / or the drive frequency of the vibrator are preferably variably adjustable.

[0494] Alternatively or in addition to the continuous powder dosing described above, the powder to be screened can be fed discontinuously, at least temporarily, through the powder inlet onto the sieve. With discontinuous powder feeding, powder to be screened can initially be fed through the powder inlet onto the sieve while the sieve is not driven, until the powder to be screened has formed a repository cone of a defined size on the sieve surface in the area of ​​the powder inlet. The formation of the repository cone of a defined size can in turn be detected using the dosing sensor provided in the area of ​​the powder inlet. After the powder feed has ended, the sieve can be driven and the powder fed onto the sieve surface can be sieved.

[0495] The screening process can be stopped when msupplied — moversize + mscreened, where m ZUguided the mass of the powder fed in, moberkorn the mass of the oversize grain flowing into the oversize grain outlet and m ge sieved is the mass of the sieved powder. The mass of the supplied powder m ZU can be determined, for example, by means of a first measuring device provided in a powder feed container connectable to the powder inlet of the screening device. The mass of the powder flowing into the oversize grain container via the oversize grain outlet can be determined by means of a second measuring device provided in an oversize grain container connectable to the oversize grain outlet of the screening device. The mass of the screened powder m ge sieved can finally be determined by means of a third measuring device which is provided in a sieved powder container which can be connected to the sieved powder outlet of the sieving device.

[0496] In the method for controlling the operation of a screening device, the powder feed can be either exclusively continuous or exclusively discontinuous. However, it is also conceivable for the powder feed to be partially continuous and partially discontinuous.

[0497] Alternatively or in addition to the continuous or discontinuous powder dosing described above, the powder to be screened can be fed at least temporarily through the powder inlet onto the screen with a dosing mass flow that is determined as a function of the drive power used to drive the screen. Preferably, a first dosing mass flow with which the powder to be screened is fed through the powder inlet onto the screen during the first time interval is greater than a second dosing mass flow with which the powder to be screened is fed through the powder inlet onto the screen during the second time interval. In other words, if the screen is driven with a higher drive power during the first time interval, more powder is fed through the powder inlet onto the screen than during the second time interval, during which the screen is driven with a lower drive power.This can prevent powder backlog and / or a situation in which a large amount of powder rests on the screen mesh and reduces screening performance. This allows for an increase in powder throughput.

[0498] The first and / or second dosing mass flow, as well as the length of the first and second time intervals and the first and second drive power, can be adjusted depending on the properties of the powder to be screened. The first and second dosing mass flow can each have a positive value > 0. However, it is also conceivable for the second dosing mass flow to have a value = 0, i.e., no powder is fed through the powder inlet onto the screen during the second time interval, during which the screen is driven with the lower second drive power.

[0499] The oversize grain rate is a parameter that indicates the ratio between the mass of oversize grain flowing into the oversize grain outlet in a defined time unit and the mass of the total powder processed in the screening device in the defined time unit. In particular, the oversize grain rate Qovercom can be determined according to

[0500] Quüberkorn = 1 —Calculate [rhsieved / (rhsieved + rhovercome)]* 100%, where rhsieved is the mass flow of the sieved powder flowing into the sieved powder container, which can be connected to the sieved powder outlet of the sieving device, during the sieving process, and rhovercome is the mass flow of the oversize particles flowing into the oversize particles container, which can be connected to the oversize particles outlet of the sieving device, during the sieving process. The parameters rhsieved and rhovercome can be continuously monitored during the sieving process, for example, using the second measuring device provided in the oversize particles container and the third measuring device provided in the sieved powder container. Accordingly, the oversize particle rate Qovercome can also be continuously determined.

[0501] Preferably, in the method for controlling the operation of a screening device, a warning is issued if the oversize grain rate exceeds a limit value. If the screening device is used to prepare raw material powder intended for processing in a system for producing three-dimensional workpieces using a generative layer building process, an excessively high oversize grain rate can be an indicator of unfavorable process parameters of the system. For example, an excessively high oversize grain rate can indicate that large weld spatter is being created during irradiation of the powder, which can remain in the powder bed and thus impair the quality of the workpiece to be produced. Monitoring the oversize grain rate can therefore be used to monitor the process parameters in the system for producing three-dimensional workpieces using a generative layer building process.

[0502] In a preferred embodiment of the method for controlling the operation of a screening device, the screening device is sealed from the ambient atmosphere and is flooded with protective gas during operation. This prevents unwanted oxidation of the powder being screened by the screening device. Preferably, additional protective gas can be supplied to the screening device if the inert gas pressure in the screening device falls below a threshold value. This allows leaks in the screening device to be detected and compensated.

[0503] Furthermore, during the sieving process, a step response of a sum of a sieved powder mass flow and an oversize mass flow to a metered mass flow can be monitored. The "step response" of the sum of the sieved powder mass flow and the oversize mass flow to the metered mass flow is understood to be a time difference between a point in time at which a defined metered mass flow has been supplied to the sieving device and a point in time at which a corresponding sieved powder mass flow has passed through the sieve mesh. The step response is therefore a time parameter that indicates the duration of the sieving process for a specific powder mass flow. The metered mass flow can be measured, for example, using the first measuring device provided in the powder feed container connectable to the powder inlet of the sieving device.The sum of the screened powder mass flow and the oversize mass flow can be measured, for example, using the second and third measuring devices provided in the oversize container, which can be connected to the oversize outlet of the screening device, and in the screened powder container, which can be connected to the screened powder outlet of the screening device. The metered mass flow and the sum of the screened powder mass flow and the oversize mass flow can be continuously measured. Accordingly, the step response of the sum of the screened powder mass flow and the oversize mass flow to the metered mass flow can also be continuously monitored.

[0504] If the sieve mesh becomes clogged during the sieving process, the step response of the sum of the sieved powder mass flow and the oversize mass flow increases compared to the dosing mass flow. In contrast, a shortening of the step response of the sum of the sieved powder mass flow and the oversize mass flow to the dosing mass flow, and in particular a shortening below a certain limit, represents an indicator of a defect, such as a tear in the sieve mesh. Therefore, a warning is preferably issued if the step response of the sum of the sieved powder mass flow and the oversize mass flow to the dosing mass flow falls below a first limit. Additionally or alternatively, sieve cleaning can be initiated if the step response of the sum of the sieved powder mass flow and the oversize mass flow to the dosing mass flow exceeds a second limit.This enables additional, redundant monitoring of the sieve throughput in addition to the monitoring of the formation of a cone of material exceeding a certain size in the area of ​​the powder inlet described above and, if necessary, the initiation of sieve cleaning.

[0505] The method for controlling the operation of a screening device may further comprise changing an angle of inclination of the screening surface of the screen relative to a horizontal plane.

[0506] This aspect, as well as all aspects described below in connection with a change in the angle of inclination, can be applied, on the one hand, to one of the methods described above and / or, on the other hand, independently thereof, to a method for controlling the operation of a screening device.

[0507] Thus, a method for controlling the operation of a screening device, in particular for screening powder of an additive manufacturing device (for example, a device for producing a three-dimensional workpiece by means of selective electron beam melting, selective laser melting, laser metal deposition, or selective laser sintering), may comprise the steps of: feeding powder to be screened onto a screen through a powder inlet; driving the screen; and changing an inclination angle of the screen surface of the screen relative to a horizontal plane.

[0508] Changing the angle of inclination can, for example, comprise an initial change and thus adjustment of the angle of inclination, which is carried out in particular before the step of feeding the powder to be screened. Additionally or alternatively, the angle of inclination can also be changed during ongoing operation of the screening device, for example, at the beginning of the first and / or second time interval.

[0509] The sieve can be arranged within a housing, in particular one that is sealed gas-tight, and the sieve can be rotated relative to the housing, thereby changing the angle of inclination. The gas-tight housing can be sealed gas-tight by means of a flap. The sieve can be inserted into the housing through the flap.

[0510] A first tilt angle may be set during the first time interval, and a second tilt angle may be set during the second time interval. Either (a) the first tilt angle is less than the second tilt angle, or (b) the first tilt angle is greater than the second tilt angle.

[0511] The angle of inclination can be changed during the first and / or second time interval.

[0512] The method may further comprise detecting a spreading velocity of the powder to be screened on the screen and / or a position of a powder front of the powder to be screened on the screen. Changing the angle of inclination of the screen surface relative to the horizontal plane may occur depending on the detected spreading velocity and / or depending on the detected position.

[0513] The detection can be performed, for example, using a sensor, which can in particular comprise a camera, an inductive sensor, and / or a light barrier. The sensor can be attached to the housing, in particular to an upper wall of the housing. The detection can also be performed using the control unit. The propagation speed of the powder to be screened can be a propagation speed of the powder front.

[0514] The angle of inclination can be changed in such a way that the angle of inclination is reduced when the detected spreading speed of the powder to be screened and / or the detected position of the powder front exceeds a predetermined threshold. Furthermore, the angle of inclination can be changed in such a way that the angle of inclination is increased when the detected spreading speed of the powder to be screened and / or the detected position of the powder front falls below a predetermined threshold. The angle of inclination can also be adjusted permanently, for example, within the framework of a closed control loop in which a constant spreading speed of the powder to be screened is set by adjusting the angle of inclination.

[0515] A screening device comprises a powder inlet and a drive device configured to drive the screen. Furthermore, the screening device comprises a control unit configured to control the powder inlet and the drive device such that, in a step (i), powder to be screened is fed through the powder inlet onto the screen, and in a step (ii), the screen is driven for a first time interval with a first drive power. The first drive power is dimensioned such that, when the screen is continuously driven with the first drive power, the powder to be screened flows over an entire screening surface of the screen and / or into an oversize grain outlet.Furthermore, the control unit is configured to control the powder inlet and the drive device such that in a step (iii) after expiration of the first time interval, the sieve is driven for a second time interval with a second drive power which is lower than the first drive power and in a step (iv) after expiration of the second time interval, steps (ii) and (iii) are repeated.

[0516] The second drive power is preferably dimensioned such that the powder to be sieved, when the sieve is continuously driven with the second drive power, forms a repose cone on the sieve surface of the sieve in the region of the powder inlet, which substantially corresponds to a repose cone forming on a stationary plane, wherein an angle of repose of the repose cone is adapted in particular to an orientation of the sieve surface.

[0517] Additionally or alternatively, the sieve surface of the sieve may be inclined relative to a horizontal plane such that the flow of the powder fed through the powder inlet towards the oversize grain outlet is assisted by gravity, wherein an angle of inclination of the sieve surface of the sieve relative to the horizontal plane is preferably smaller than an angle of repose of a repose cone formed by the powder to be sieved on a horizontal plane.

[0518] The first time interval can be a value empirically determined for the powder to be screened. Additionally or alternatively, the control unit can be configured to end the first time interval when determining the first time interval at the latest when powder to be screened flows into the oversize grain outlet. The screening device can comprise an oversize grain sensor provided in the region of the oversize grain outlet for monitoring the flow of powder to be screened into the oversize grain outlet. The control unit can be configured to dimension the first time interval such that a screening area utilization of approximately 70% to approximately 90%, preferably approximately 75% to approximately 85%, and particularly preferably approximately 80% of the total screening area of ​​the screen is not exceeded by the end of the first time interval.

[0519] The second time interval can be a value empirically determined for the powder to be screened. Additionally or alternatively, the control unit can be configured to end the second time interval when the powder to be screened forms a repose cone of a defined size on the screen surface of the screen in the area of ​​the powder inlet. The screening device can comprise a dosing sensor provided in the area of ​​the powder inlet for detecting the formation of a repose cone of a defined size.

[0520] In the following, an embodiment of a screening device is explained which is equipped with a control unit for controlling the powder feed through the powder inlet of the screen and can also be used independently of the screening device described above with a control unit executing a drive control.

[0521] An independently loadable screening device comprises a powder inlet and a control unit configured to control the powder inlet such that the powder to be screened is continuously fed at least temporarily with a dosing mass flow rhdos = 0.5 * (rh(amin) + rh(a m ax)) is fed through the powder inlet onto the sieve. To adjust the dosing mass flow, the control unit can, for example, be configured to control a dosing device associated with the powder inlet, which may, for example, comprise a dosing screw, and / or a valve associated with the powder inlet. The parameters a m ax and rh(a max) define a sieve area utilization and a sieve throughput when driving the sieve with a first drive power, which is dimensioned such that the powder to be sieved flows over the entire sieve area of ​​the sieve and / or into an oversize grain outlet when the sieve is continuously driven with the first drive power. The parameters a m in and rh(a m in), on the other hand, define a screen area utilization and a screen throughput when driving the screen with a second drive power that is lower than the first drive power.

[0522] The control unit may further be configured to rh(a m in) by increasing the metered mass flow of the powder to be screened through the powder inlet when driving the screen with the second drive power until the powder to be screened has formed a cone of material with a defined size in the area of ​​the powder inlet on the screen surface of the screen.

[0523] Alternatively or additionally, the control unit may be configured to rh(a m ax) by increasing the metered mass flow of the powder to be screened through the powder inlet when driving the sieve with the first drive power until the powder to be screened has formed a cone of material with a defined size on the sieve surface in the area of ​​the powder inlet and powder to be screened flows into the oversize grain outlet, whereby the value of rh(a max) is preferably multiplied by a safety factor of 0.8, 0.7, 0.6, or 0.5. The control unit can further be configured to reduce the metered mass flow rhdos if the powder to be screened forms a repose cone of a defined size in the region of the powder inlet on the screen surface of the screen. Furthermore, the control unit can be configured to initiate screen cleaning if the powder to be screened forms a repose cone of a defined size in the region of the powder inlet on the screen surface of the screen and / or the metered mass flow rhdos falls below a limit value.

[0524] The control unit can be configured, upon initiation of sieve cleaning, to control the powder inlet such that the powder supply through the powder inlet is stopped, and / or to control the drive device such that any powder still present in the sieve is sieved before sieve cleaning begins, and / or to control the drive device such that, after sieve cleaning begins, the sieve is driven with maximum drive power, and / or, after sieve cleaning begins, to activate a vibrator configured to drive the sieve independently of the drive device of the sieve device. Preferably, an angle of attack of the vibrator on the sieve, a drive amplitude of the vibrator, and / or a drive frequency of the vibrator are variably adjustable.

[0525] The control unit can further be configured to control the powder inlet such that the powder to be screened is fed, at least temporarily, discontinuously through the powder inlet onto the screen. In particular, the control unit can be configured to control the drive device and the powder inlet such that, initially, while the screen is not driven, powder to be screened is fed through the powder inlet onto the screen until the powder to be screened has formed a cone of material of a defined size in the region of the powder inlet on the screen surface of the screen. After the powder feed has ended, the screen is driven, and the powder fed onto the screen surface of the screen is screened.Furthermore, the control unit can be configured to control the drive device and the powder inlet such that the sieving process is terminated when fflfed - moversize + fflsieved, where mfed is the mass of the powder fed in, moversize is the mass of the oversize flowing into the oversize outlet and m. ge sieved is the mass of the sieved powder.

[0526] The screening device may comprise a first measuring device for determining m ZU which is provided in a powder feed container that can be connected to the powder inlet of the screening device, a second measuring device for determining mober grain, which is provided in an oversize grain container that can be connected to the oversize grain outlet of the screening device, and / or a third measuring device for determining m ge sieved, which is provided in a sieved powder container connectable to a sieved powder outlet of the sieving device.

[0527] The control unit can further be configured to control the powder inlet such that the powder to be screened is fed at least temporarily through the powder inlet onto the screen with a metering mass flow that is determined as a function of the drive power used to drive the screen, wherein in particular a first metering mass flow with which the powder to be screened is fed through the powder inlet onto the screen during the first time interval is greater than a second metering mass flow with which the powder to be screened is fed through the powder inlet onto the screen during the second time interval.

[0528] The control unit can be configured to issue a warning if an oversize grain rate, in particular a continuously measured one, exceeds a limit value.

[0529] Preferably, the screening device is sealed from the ambient atmosphere and flooded with a protective gas during operation. Additionally or alternatively, the control unit can be configured to supply additional protective gas to the screening device when the inert gas pressure in the screening device falls below a threshold value. For this purpose, the control unit can, for example, actuate a valve that controls the supply of inert gas to the screening device.

[0530] The control unit is preferably further configured to monitor a step response of a sum of a screened powder mass flow and an oversize mass flow to a metered mass flow during the screening process. Furthermore, the control unit can be configured to issue a warning if the step response of the sum of the screened powder mass flow and the oversize mass flow to the metered mass flow falls below a first limit value. Finally, the control unit can be configured to initiate screen cleaning if the step response of the sum of the screened powder mass flow and the oversize mass flow to the metered mass flow exceeds a second limit value.

[0531] The screening device can further comprise a lid that can be removed from a screening container. A seal can be arranged in the lid. For example, the seal can be arranged in the region of a side of the lid facing the screening container and can serve to seal the screening container from the ambient atmosphere when the lid is closed. Furthermore, the screening device can comprise a clamping device that is configured to exert a clamping force on the seal, which holds the seal in its position in the lid. The clamping device can, for example, comprise a clamping piece that can be pressed against the seal using an adjusting screw. The clamping device advantageously prevents the seal from falling out of the lid when the lid is removed from the screening container.

[0532] The screening device may comprise an inclination device for changing an inclination angle of the screening surface of the screen relative to a horizontal plane.

[0533] This aspect, as well as all aspects described below in connection with a change in the angle of inclination, can be applied, on the one hand, to one of the screening devices described above and / or, on the other hand, to a screening device independently thereof.

[0534] Thus, a screening device, in particular for screening powder of an additive manufacturing device (for example, a device for producing a three-dimensional workpiece by means of selective electron beam melting, selective laser melting, laser metal deposition, or selective laser sintering), may comprise: a powder inlet; a drive device configured to drive the screen; and an inclination device for changing an inclination angle of the screen surface of the screen relative to a horizontal plane.

[0535] Changing the angle of inclination can, for example, comprise an initial change and thus adjustment of the angle of inclination, which is carried out in particular before the step of feeding the powder to be screened. Additionally or alternatively, the angle of inclination can also be changed during ongoing operation of the screening device, for example, at the beginning of the first and / or second time interval.

[0536] The screening device may comprise a housing which can be closed in a gas-tight manner, wherein the screening device is arranged within the housing and wherein the inclination device is configured to rotate the screening device relative to the housing and thereby change the angle of inclination.

[0537] The powder inlet and oversize outlet can be permanently attached to the housing. The powder inlet can be attached to a top of the housing, and the oversize outlet can be attached to a bottom of the housing. Furthermore, a sieve container can be permanently attached to the bottom of the housing.

[0538] The screening device may comprise a screen holder for receiving, in particular for inserting, the screen. The tilting device may be attached to the screen holder and configured to rotate the screen holder.

[0539] The control unit may be configured to set a first tilt angle during the first time interval and a second tilt angle during the second time interval. Either (a) the first tilt angle is less than the second tilt angle, or (b) the first tilt angle is greater than the second tilt angle. The first tilt angle and the second tilt angle may be kept constant during the first and second time intervals, respectively.

[0540] The control unit may be configured to change the tilt angle during the first and / or during the second time interval.

[0541] The screening device may further comprise at least one sensor for detecting a spreading speed of the powder to be screened on the screen and / or a position of a powder front of the powder to be screened on the screen. The control unit may be configured to change the angle of inclination of the screen surface of the screen relative to the horizontal plane depending on the detected spreading speed and / or depending on the detected position.

[0542] The sensor can, in particular, comprise a camera, an inductive sensor, and / or a light barrier. The sensor can be mounted on the housing, in particular on an upper wall of the housing. Detection can also be performed using the control unit. The propagation velocity of the powder to be screened can be the propagation velocity of the powder front.

[0543] The control unit can be configured to change the angle of inclination such that the angle of inclination is reduced when the detected propagation speed of the powder to be screened and / or the detected position of the powder front exceeds a predetermined threshold value. Furthermore, the control unit can be configured to change the angle of inclination such that the angle of inclination is increased when the detected propagation speed of the powder to be screened and / or the detected position of the powder front falls below a predetermined threshold value. The control unit can also adjust the angle of inclination permanently, for example, within the framework of a closed control loop in which a constant propagation speed of the powder to be screened is set by adjusting the angle of inclination.

[0544] A powder processing system comprises a screening device as described above. The powder processing system can, for example, be designed in the form of a closed system sealed from the ambient atmosphere. Furthermore, the powder processing system can be partially or entirely flooded with a protective gas during operation. The powder processing system can comprise a powder feed container connectable to the powder inlet of the screening device, a screened powder container connectable to the screened powder outlet of the screening device, and an oversize particle container connectable to the oversize particle outlet of the screening device. The powder processing system is intended in particular for use in a system for producing three-dimensional workpieces by exposing raw material powder layers to electromagnetic radiation or particle radiation.

[0545] A system for producing three-dimensional workpieces by exposing raw material powder layers to electromagnetic radiation or particle radiation comprises a screening device and / or a powder processing system as described above.

[0546] Furthermore, the system can comprise a process chamber, in particular sealed against the ambient atmosphere, and a carrier for holding the raw material powder to be irradiated. Excess powder accumulating during the application of the individual powder layers to the carrier can be collected in one or more collecting containers. The process chamber can comprise a gas inlet for supplying a gas, in particular an inert gas, into the process chamber and a gas outlet for removing gas, possibly laden with particulate contaminants, from the process chamber. The carrier can be arranged in the process chamber. However, it is also conceivable for the process chamber to be movable over the carrier. The carrier can be a rigidly fixed carrier.Preferably, however, the carrier is displaceable in the vertical direction so that the carrier can be moved step by step downwards in the vertical direction as the height of a workpiece mounted on the carrier increases.

[0547] The raw material powder applied to the carrier, for example, by means of a powder application device movable over the carrier, is preferably a metal powder, in particular a metal alloy powder. However, the raw material powder can also be a ceramic powder or a powder containing various materials. The powder can have any suitable particle size or particle size distribution. However, it is preferred to process powders with a particle size of less than 100 μm.

[0548] The system preferably further comprises an irradiation device for selectively directing electromagnetic radiation or particle radiation onto the powder bed applied to the carrier. Furthermore, the system can comprise an unpacking station into which a workpiece accommodated in a build chamber can be transferred after its completion. In the unpacking station, the workpiece can be removed from the build chamber and the unsolidified powder surrounding it, optionally after a cooling period.

[0549] A system 100s shown in Figure 14 for producing three-dimensional workpieces by exposing raw material powder layers to electromagnetic radiation or particle radiation comprises a process chamber 102s that is sealed from the ambient atmosphere. A powder application device 104s arranged in the process chamber 102s serves to apply raw material powder layers to a carrier 106s. Excess powder that accrues during the application of the individual powder layers to the carrier 106s is collected in a collecting container 107s. The carrier 106s is displaceable in the vertical direction, so that the carrier 106s can be moved step by step in the vertical direction downwards into a construction chamber 109s as the construction height of a workpiece 108s built on the carrier 106s increases. The process chamber 102s is provided with a gas inlet 110s for supplying an inert gas (e.g. argon) into the process chamber 102s.Furthermore, a gas outlet 112s is provided so that a continuous gas flow can be generated through the process chamber 102s. The gas flow can be used to remove melt splashes and / or other unwanted contaminant particles, such as welding fumes, from the process chamber 102s.

[0550] The system 100s further comprises an irradiation device 112s, which serves to selectively direct electromagnetic radiation or particle radiation onto the powder bed applied to the carrier 106s. The exemplary system 100s shown in Figure 14 comprises only one irradiation device 112s. However, the system 100s can also have a plurality of irradiation devices 112s.

[0551] The irradiation device 112s comprises a radiation source 114s, which here is particularly designed in the form of a laser source. The radiation source 114s, which may, for example, comprise a diode-pumped ytterbium fiber laser emitting laser light with a wavelength of approximately 1070 to 1080 nm, can be integrated into the irradiation device 112s. In the system 100s shown in Figure 14, however, the radiation source 114s is arranged outside the irradiation device 112s, with a laser beam 116s emitted by the radiation source 114s being guided into the irradiation device 112s via an optical fiber 118s.

[0552] The irradiation unit 112s further comprises two lenses 120s and 122s. In the embodiment of an irradiation unit 112s shown in Figure 14, both lenses 120s and 122s have a positive refractive power. The lens 120s collimates the laser light emitted by the optical fiber 118s, thereby generating a collimated or substantially collimated laser beam 116s. The lens 122s, on the other hand, is configured to focus the collimated (or substantially collimated) laser beam 116s to a desired z-position along a z-axis.

[0553] Finally, the irradiation unit 112s comprises a scanner system with a scanner mirror 124s that can be pivoted about a pivot axis S. During operation of the system 100s, the scanner system, and in particular the scanner mirror 124s, serves to deflect the laser beam 116s emitted by the radiation source 114s such that the beam 116s impinges on the raw material powder layer applied to the carrier 106s at a desired position. The system 100s further comprises an unpacking station 126s. A build chamber 109s with a workpiece 108s arranged therein is transferred to the unpacking station 126s once the construction of the workpiece 108s is complete. The irradiation device 112s and the process chamber 102s can then be used to build a new workpiece without further delay. In the unpacking station 126s, the workpiece 108s can be cooled if necessary and is then unpacked, ie removed from the build chamber 109s.This may result in a large amount of unconsolidated powder in which the workpiece 108s is embedded before unpacking.

[0554] Both the powder collected in the collection container 107s and the powder recovered in the unpacking station 126s may contain particulate contaminants as well as bonded or sintered powder agglomerates. These contaminants could lead to contamination of the powder bed and consequently to a reduced quality of the workpiece 108s if the powder is reused in an additive manufacturing process in the system 100s. The system 100s therefore comprises a powder preparation system 128s, which is connected to the collection container 107s and the unpacking station 126s via a powder line 130s. For conveying powder from the collection container 107s and the unpacking station 126s into the powder preparation system 128s, a blower, a conveyor belt, or another suitable conveying device (not shown in Figure 14), for example, can be used.The powder preparation system is designed as a closed system, sealed against the ambient atmosphere and is completely flooded with a protective gas, such as argon, during operation.

[0555] The powder preparation system 128s comprises a powder feed container 132s connected to the powder line 130s. The powder feed container 132s thus serves to receive the powder to be prepared from the collection container 107s and the unpacking station 126s. Furthermore, the powder preparation system 128s comprises a sieve device 10s with a sieve formed by a sieve frame 14s and a sieve mesh 16s stretched over the sieve frame. Sieved powder is collected in a sieve container 18s after passing through the sieve mesh 16s.

[0556] A lid 20s is placed on the sieve frame 14s so that the sieve container 18s, like the other components of the powder preparation system 128s, is sealed from the ambient atmosphere and can be flooded with an inert gas during operation of the sieve device 10s. Argon, for example, can be used as an inert protective gas, which prevents unwanted oxidation of the powder 56s to be sieved in the sieve device 10s. The inert gas pressure in the sieve device 10s is continuously monitored by a pressure sensor not shown in the figures. If the inert gas pressure in the sieve device 10s falls below a limit value, additional protective gas is supplied to the sieve device 10s under the control of the control unit 40s.

[0557] A detailed view of the screening device 10s is shown in Figure 15. The screening device 10s of Figure 15 is used in the system illustrated in Figure 14. Alternatively, the screening device 10s can be used as the screening device 5 in the system of Figure 1.

[0558] A powder inlet 22s of the sieving device 10s comprises a metering device 24s and a valve 26s, allowing a controlled supply of powder from the powder feed container 132s to the sieve via the powder inlet 22s. A metering sensor 23s is provided in the area of ​​the powder inlet 22s, the function of which will be explained in more detail below. The sieve container 18s has a downwardly tapered cross-section. Consequently, sieved powder received in the sieve container 18s can be discharged from the sieve container by gravity via a sieved powder outlet 28s arranged in the area of ​​a lower section of the sieve container 18s.The sieved powder outlet 28s is connected to a sieved powder container 134s of the powder preparation system 128s and comprises a valve 30s, so that a controlled discharge of sieved powder from the sieve container 18s into a sieved powder container 134s of the powder preparation system 128s is possible via the sieved powder outlet 28s.

[0559] The screening device 10s further comprises an oversize grain outlet 32s, to which a valve 34s is assigned. An oversize grain sensor 33s is provided in the region of the oversize grain outlet 32s, which can detect powder particles flowing into the oversize grain outlet 32s. Oversize grain that is too coarse to pass through the screen mesh 16s toward the screen container 18s can be removed from the screening device 10s in a controlled manner via the oversize grain outlet 24s and fed to an oversize grain container 136s of the powder processing system 128s. The powder inlet 22s and the oversize grain outlet 32s are arranged in the region of opposite side edges of the screening surface defined by the screen mesh 16s. Furthermore, the sieve mesh 16s and thus the sieve surface of the sieve defined by the sieve mesh 16s is inclined relative to a horizontal plane E in such a way that the flow of the powder fed through the powder inlet towards the oversize grain outlet 32s is assisted by gravity.In other words, the sieve surface of the sieve is designed to slope downwards from an area below the powder inlet 22s toward the oversize grain outlet 32s, thereby promoting the spreading of powder fed through the powder inlet 22s across the sieve surface and the removal of oversize grain into the oversize grain outlet 32s. Just as the removal of sieved powder is carried out, the removal of oversize grain from the sieve device 10s is also gravity-driven.

[0560] The screening device 10s is further equipped with a drive device 36s for driving the screen. The drive device 36s engages the screen frame and, during operation of the screening device 10s, causes the screen frame 14s and thus the screen mesh 16s to vibrate. The preferred embodiment of a screening device 10s shown here is equipped with a drive device 36s in the form of an ultrasonic drive device, which is designed to subject the screen to ultrasonic vibrations. Furthermore, a vibrator 38s is also provided, which also engages the screen frame 14s and serves to vibrate the screen frame 14s and thus the screen mesh 16s for the purpose of cleaning the screen.

[0561] The operation of the screening device 10s is controlled by a control unit 40s. The control unit 40s can be a control unit that is exclusively assigned to the screening device 10s. Alternatively, however, it is also conceivable for the control unit 40s to be integrated into a higher-level control unit, for example, a control unit for controlling the powder processing system 128s and / or a control unit for controlling the system 100s for producing a three-dimensional workpiece.

[0562] Finally, the screening device 10s comprises a first measuring device 42s, a second measuring device 40s, and a third measuring device 46s. The first measuring device 42s, which here is designed in the form of one or more weighing cells, is arranged in the powder feed container 132s of the powder processing system 28s and serves to measure the mass of the powder fed to the powder inlet 22s of the screening device 10s from the powder feed container 32s. The second measuring device 44s, which here is also designed in the form of one or more weighing cells, is arranged in the oversize container 136s of the powder processing system 28s and serves to measure the mass of the oversize material that flowed into the oversize container 136s via the oversize outlet 32s of the screening device 10s.The third measuring device 46s, which is again designed in the form of one or more weighing cells, is finally arranged in the sieved powder container 134s of the powder preparation system 28s and serves to determine the mass of the sieved powder that flows from the sieved powder outlet 28s of the sieving device 10s into the sieved powder container 134s.

[0563] As can be seen in Figure 16, when a powder 56s is loosely packed on a plane, an angle of repose a is formed which is influenced by various factors such as the shape, density, size distribution and surface properties of the powder particles as well as by process parameters such as the relative humidity and temperature as well as by vibrations and movements acting on the repose cone.

[0564] When powder 56s to be screened is fed through the powder inlet 22s onto the screen of the screening device 10s, a repose cone forms on the screen, i.e., on the screen mesh 16s. The shape and angle of repose of the repose cone are influenced by the vibrations acting on the repose cone and consequently by the drive power of the drive device 36s driving the screen. Furthermore, the shape and angle of repose of the repose cone are influenced by the angle of inclination of the screening surface.

[0565] When the sieve, as shown in Figure 17, is driven with a low drive power of the drive device 36s, the cone of material forming on the sieve below the powder inlet 22s resembles a cone of material forming on a stationary plane, the base area of ​​which occupies only a small section a of the sieve surface. In such an operating state of the sieve device 10s, the sieve surface utilization is correspondingly low, since only a small section a of the sieve surface adjacent to the powder inlet 22s is actually impinged with powder 56s. In contrast, a section b of the sieve surface, which is not impinged with powder and consequently defines a "safety distance" between the section a of the sieve surface impinged with powder and the oversize grain outlet 32s, is comparatively large.

[0566] The inclination of the sieve surface in the direction of the oversize grain outlet 32s results in the repose cone no longer being symmetrically shaped, as in Figure 16, but rather being adapted to the orientation of the sieve surface and having a variable angle of repose, which is smaller in a circumferential section of the repose cone facing the oversize grain outlet 32s than in a circumferential section of the repose cone facing away from the oversize grain outlet 32s. However, an angle of inclination aa-w of the sieve surface of the sieve relative to the horizontal plane E (see Figure 15) is smaller than the angle of repose aa of the repose cone formed by the powder 56s to be sieved on a horizontal plane, thus ensuring that when the sieve is driven with low drive power in the area of ​​the powder inlet 22s, a stable repose cone is still formed and the powder 56s does not flow uncontrollably over the sieve surface.

[0567] If, however, the sieve is driven with a high drive power, as shown in Figure 18, the powder 56s spreads across the sieve surface, i.e., an angle of repose aar of the repose cone, which the powder 56s fed onto the sieve through the powder inlet forms on the sieve surface, decreases with increasing drive power of the drive device 36s driving the sieve. At the same time, the base area of ​​the repose cone increases, so that the section a of the sieve surface exposed to powder 56s and, consequently, the sieve surface utilization increases, until the powder finally flows over the entire sieve surface and thus the entire sieve surface is exposed to powder. A section b of the sieve surface that is not exposed to powder is then no longer present, so that the section a of the sieve surface exposed to powder no longer has a "safety distance" from the oversize grain outlet 32s.This results in powder 56s flowing unscreened into the oversize grain outlet 32s during continuous sieving at a high drive power, as illustrated in Figure 18. As a result, powder 56s, which is actually fine-grained enough to pass through the sieve mesh 16s, is lost unused.

[0568] In a method for controlling the operation of the screening device 10s, illustrated in more detail in Figure 19, the powder 56s to be screened is therefore first fed onto the screen through the powder inlet 22s in a step (i). The screen is then driven with a first drive power for a first time interval in a step (ii) (see Figure 19 top and center). The first drive power is dimensioned such that, if the screen were continuously driven with the first drive power, as shown in Figure 18, the powder 56s to be screened would flow over the entire screen surface of the screen and / or into the oversize grain outlet. The first drive power is therefore so high that, if it were continuously maintained, maximum screen surface utilization would be ensured, but there would at least be a high risk that powder 56s would be lost unscreened through the oversize grain outlet 32s. Therefore, driving the screen with the first drive power is limited in time.

[0569] Therefore, after the expiry of the first time interval, in a step (iii), the sieve is driven for a second time interval with a second drive power that is lower than the first drive power (see Figure 19 below). The second drive power is dimensioned in particular such that the powder 56s to be sieved, when the sieve is continuously driven with the second drive power, as shown in Figure 17, forms a repose cone on the sieve surface of the sieve in the region of the powder inlet 22s, which essentially corresponds to a repose cone forming on a stationary plane and whose base area only occupies a small section a of the sieve surface of the sieve arranged in the region of the powder inlet 22s. If the sieve were continuously driven with the second drive power, it would be ensured that no or almost no powder 56s flows unscreened into the oversize grain outlet.However, due to the low screen area utilization, the screen throughput and consequently the screening performance would be low. Therefore, the time required to drive the screen with the second drive power is also limited.

[0570] In the case described above, the second drive power is greater than zero and less than the first drive power. In a special case, the second drive power can also be zero, so that no active sieving takes place during the second time interval. However, passive sieving can also take place in the non-driven state, with powder trickling through the sieve due to gravity.

[0571] After the second time interval has elapsed, steps (ii) and (iii) are repeated, i.e., the sieve is driven periodically alternately with the first, higher drive power and the second, lower drive power. During the first time interval, in which the sieve is driven with the first (high) drive power, the powder particles of the powder 56 to be sieved, including the oversize grain 50s contained in the powder 56s, are distributed across the sieve surface (see Figure 19, top), so that the sieving process can then proceed with high sieve surface utilization, with the oversize grain 50s being transported by gravity toward the oversize grain outlet 32s (see Figure 19, center). In contrast, during the second time interval, the sieve surface utilization is low. Furthermore, the oversize grain 50s "accumulates" in columnar fashion inside the cone of material.As a result, powder particles that are pressed into the sieve mesh 16s by back pressure and vibration of the sieve can form stuck grains and clog the sieve mesh.

[0572] Since the first and second time intervals alternate periodically, and consequently the sieve is driven periodically alternately with the first, higher drive power and the second, lower drive power, the advantages of both drive powers can be combined. By limiting the time of the first time interval, a good distribution of the powder particles across the sieve surface and efficient removal of the oversize grain 50s toward the oversize grain outlet 32s can be ensured. However, there always remains a section b of the sieve surface that is not loaded with powder, and consequently a "safety distance" between the section A of the sieve surface that is loaded with powder and the oversize grain outlet 32s. Furthermore, the sieve mesh 16s is loaded with a lower powder mass overall and is therefore less stressed.

[0573] The first and second drive powers can take on different values ​​for different powder types and different process parameters, such as temperature, particle size distribution of the powder, moisture content of the powder, etc. The first and second drive powers are therefore either determined empirically before a sieving process, taken from a drive power value table for different powder types and process parameters, or selected based on empirical values. Likewise, the first and second time intervals can take on different values ​​for different powder types and different process parameters, such as temperature, particle size distribution of the powder, moisture content of the powder, etc. The first and second time intervals are therefore values ​​determined empirically for the powder to be sieved before a sieving process, taken from a value table, or selected based on empirical values.

[0574] When determining the first time interval, the first time interval ends at the latest when powder to be screened 56s flows into the oversize grain outlet 32s, i.e., the first time interval is selected such that no loss of powder to be screened occurs while the screen is being driven at the first drive power within the first time interval. The flow of powder to be screened into the oversize grain outlet 32s is detected by the oversize grain sensor 33s. In particular, the first time interval is dimensioned such that a screen area utilization of approximately 70% to approximately 90%, preferably approximately 75% to approximately 85%, and particularly preferably approximately 80% of the total screen area of ​​the screen is not exceeded by the end of the first time interval. The first time interval therefore provides a "time safety reserve" so that the powder does not spread over the entire screen area during the first time interval.

[0575] When determining the second time interval, the second time interval is terminated at the latest when powder 56s to be sieved forms a cone of material with a defined size in the area of ​​the powder inlet 22s on the sieve surface of the sieve. The formation of a cone of material with a defined size is detected by the dosing sensor 23s, which is triggered when a tip of the cone of material protrudes into a detection range of the dosing sensor 23s. Limiting the second time interval in this way prevents the cone of material from becoming too large during the second time interval and clogging the powder inlet 22s.

[0576] During operation of the screening device 10s, the powder 56s to be screened is fed, at least temporarily, continuously through the powder inlet 22s onto the screen. A metered mass flow is set by a corresponding control of the metering device 24s and / or a corresponding control of the valve 26s by the control unit 40s. In particular, a continuous metered mass flow rhdos is set, which corresponds to the equation rhdos = 0.5 * (rh(amin) + rh(a m ax)), where the parameters a m ax and rh(a m ax) define a screen area utilization and a screen throughput when driving the screen with the first drive power and the parameters a m in and rh(a m in) define a screen area utilization and a screen throughput when driving the screen with the second drive power.

[0577] The sieve throughput when driving the sieve with a second drive power rh(amin) is determined by increasing the metered mass flow of the powder 56s to be sieved through the powder inlet 22s when driving the sieve with the second drive power until the powder 56s to be sieved has formed a cone of material with a defined size on the sieve surface of the sieve in the area of ​​the powder inlet 22s and consequently the metering sensor 23s is triggered. The sieve throughput when driving the sieve with the first drive power rh(a max), on the other hand, is determined by increasing the metered mass flow of the powder 56s to be screened through the powder inlet 22s when the sieve is driven with the first drive power until the powder 56s to be screened has formed a cone of a defined size in the area of ​​the powder inlet 22s on the sieve surface of the sieve and powder 56s to be screened beyond this flows into the oversize outlet. When determining the sieve throughput when the sieve is driven with the first drive power rh(amax), the metered mass flow of the powder 56s to be screened through the powder inlet 22s is consequently increased until the metering sensor 23s and the oversize sensor 33s are triggered. The value of rh(a m ax) is multiplied by a safety factor of, for example, 0.8, 0.7, 0.6 or 0.5 to prevent accidental overdosing of powder.

[0578] If the sieve mesh 16s becomes clogged during the sieving process, for example due to stuck grains or cold welding, the sieve throughput decreases. Therefore, the continuous dosing mass flow rhdos is reduced if, during the sieving process, the powder 56s to be sieved forms a repose cone of a defined size on the sieve surface in the area of ​​the powder inlet 22s, and the dosing sensor 23s is triggered accordingly. However, if the continuous dosing mass flow rhdos has already been reduced to such an extent that it falls below a limit value, and the powder 56s to be sieved nevertheless forms a repose cone of a defined size on the sieve surface in the area of ​​the powder inlet 22s, and the dosing sensor 23s is consequently triggered, the control unit 40s interprets this as an indication that the sieve mesh 16s is clogged and initiates sieve cleaning.

[0579] If the control unit 40s has detected that sieve cleaning needs to be initiated, the powder feed through the powder inlet 22s is first stopped. Any powder 56s still present in the sieve is sieved before sieve cleaning begins. After sieve cleaning starts, the sieve is driven at maximum drive power. Additionally or alternatively, after sieve cleaning starts, the vibrator 38s can be activated, whereby an angle of attack of the vibrator 38s on the sieve, a drive amplitude of the vibrator 38s, and a drive frequency of the vibrator 38s can be variably adjusted. In addition or alternatively to sieve cleaning initiated as a result of a blockage of the sieve mesh 16s, sieve cleaning can also be initiated after the end of each sieving process, when the powder feed container 132s is empty.

[0580] Alternatively or in addition to the continuous powder dosing described above, the powder 56s to be screened can also be fed discontinuously, at least temporarily, through the powder inlet 22s onto the screen. With discontinuous powder feeding, the powder 56s to be screened is first fed through the powder inlet 22s onto the screen while the screen is not driven, until the powder 56s to be screened has formed a cone of material of a defined size on the screen surface in the area of ​​the powder inlet 22s and the dosing sensor 23s is triggered. The powder feed is then stopped, and the screen is driven, so that the powder 56s fed onto the screen surface is screened.

[0581] Alternatively or in addition to the continuous or discontinuous powder metering described above, the powder 56s to be screened can also be fed, at least temporarily, through the powder inlet 22s to the screen at a metered mass flow rate determined as a function of the drive power used to drive the screen. In particular, a first metered mass flow rate, with which the powder to be screened is fed through the powder inlet 22s to the screen during the first time interval, can be greater than a second metered mass flow rate, with which the powder to be screened is fed through the powder inlet 22s to the screen during the second time interval.

[0582] In the diagrams of Figure 20, the drive power L is plotted as a function of time t at the top and the metered mass flow rhdos as a function of time t at the bottom. During the periodically recurring first time intervals tl, the sieve is driven with the higher first drive power LI, whereas during the periodically recurring second time intervals t2, the sieve is driven with the lower second drive power L2. Accordingly, powder 56s is fed through the powder inlet 22s onto the sieve during the first time intervals tl with a higher first metered mass flow rhdosi and during the second time intervals t2 with a lower second metered mass flow rhdos2. In particular, the second metered mass flow rhdos2 here has the value 0, i.e., no powder is fed through the powder inlet onto the sieve during the second time intervals t2.

[0583] The "sinusoidal" profile of the dosing mass flow shown in the lower diagram of Figure 20 results from the response behavior of the dosing device when starting or stopping the movement of the dosing device, such as a dosing screw. A "rectangular" profile of the dosing mass flow is also conceivable. If the first and second time intervals, as shown in Figure 20, are each of equal length, an average dosing capacity of rhdosi / 2 is achieved.

[0584] The screening process is terminated when m is fed = moversize + msieved, where msupplied is the mass of the powder supplied, moversize is the mass of the oversize grain flowing into the oversize grain outlet 32s and m ge sieved is the mass of the sieved powder. The mass of the supplied powder m ZUis determined by means of a first measuring device 42s, which records the powder outflow from the powder feed container 132s. The mass of the powder movergrain flowing through the oversize grain outlet into the oversize grain container 136s is determined by means of the second measuring device 44s, which records the inflow of oversize grain into the oversize grain container 38s. The mass of the sieved powder msieved is finally determined by means of the third measuring device 46s, which records the inflow of sieved powder into the sieved powder container 134s. The oversize grain rate is a parameter that indicates the ratio between the mass of the oversize grain flowing into the oversize grain outlet 32s in a defined time unit and the mass of the total powder processed in the sieving device 10s in the defined time unit. In particular, the oversize grain rate Qovergrain can be determined according to

[0585] Quüberkorn = 1 —Calculate [rhsieved / frhsieved + rhovercome)]* 100%, where rhsieved is the mass flow of the sieved powder that flows during the sieving process via the sieved powder outlet 28s of the sieving device 10s into the sieved powder container 134s and rhovercome is the mass flow of the oversize particles that flows during the sieving process via the oversize particles outlet 32s of the sieving device 10s into the oversize particles container 136s.

[0586] In the use of the screening device 10s described here for preparing raw material powder intended for processing in the system 100s for producing three-dimensional workpieces using a generative layer construction process, an excessively high oversize grain rate can be an indicator of unfavorable process parameters of the system 100s. For example, an excessively high oversize grain rate can indicate that large welding spatter is generated during irradiation of the powder by the irradiation device 112s, which can remain in the powder bed and thus impair the quality of the workpiece 108s to be produced. The parameters rhscreened and rhoversize grain are therefore continuously monitored during the screening process by means of the second and third measuring devices 44s, 46s. The control unit 40s continuously determines the oversize grain rate Qoversize grain from these values.Furthermore, under the control of the control unit 40s, a warning is issued if the oversize grain rate exceeds a limit value so that the process parameters of the system 100s can be checked if necessary.

[0587] Furthermore, during the sieving process, a step response of a sum of a sieved powder mass flow rhgesiebt and an oversize mass flow rhüberkom to a metered mass flow rhdos is monitored, wherein the metered mass flow rhdos is continuously recorded by the first measuring device 42s, the oversize mass flow rhüberkom is continuously recorded by the second measuring device 44s, and the sieved powder mass flow rhgesiebt is continuously recorded by the third measuring device 46s. In the diagrams shown in Figure 21, the development of the sum of the sieved powder mass flow rhgesiebt and the oversize mass flow rhüberkom as a function of time t is represented by the dotted curves, while the development of the metered mass flow rhdos as a function of time t is represented by the dashed curves.

[0588] The "step response" of the sum of the sieved powder mass flow rhgesiebt and the oversize mass flow rhüberkom to the dosing mass flow rhdos is the time difference Δt between a time t1, at which a defined dosing mass flow rhdos has been supplied to the sieving device for 10 s, and a time t2, at which a corresponding sum of the sieved powder mass flow rhgesiebt and the oversize mass flow rhüberkom has passed the sieve mesh for 16 s. The step response is therefore a time parameter that specifies the duration of the sieving process for a specific powder mass flow.

[0589] If the step response shortens from the value Atl illustrated in the upper diagram in Figure 21 to the value At2 illustrated in the lower diagram in Figure 21, i.e., the curves in the diagram move closer together, this can be interpreted as an indicator of a defect, such as a tear in the screen mesh 16s, if the step response falls below a first limit value. Therefore, a warning is output under the control unit 40s if the step response of the sum of the screened powder mass flow rhgesiebt and the oversize mass flow rhüberkom to the dosing mass flow rhdos falls below the first limit value.

[0590] If, however, the step response becomes longer, this indicates an extension of the sieving process and can be interpreted as an indicator that the sieve mesh 16s has become clogged. Therefore, a sieve cleaning cycle is initiated under the control of the control unit 40s when the step response of the sum of the sieved powder mass flow rhgesiebt and the oversize mass flow rhüberkom to the dosing mass flow rhdos exceeds the second limit value.

[0591] Figure 22 shows a schematic side view of a screening device 10s, which can be considered an alternative embodiment or a further development of the screening device 10s of Figure 14. The screening device 10s can be used in conjunction with all of the above-described embodiments of a screening device 10s, a powder processing system 128s, and / or a system 100s for producing three-dimensional workpieces. The elements and / or functions of the screening device 10s not described below correspond to those of the above-described screening device 10s, in particular the screening device 10s of - III -

[0592] Figure 15. Thus, some of the elements of the screening device 10s of Figure 22 are not shown or are only indicated schematically, since they correspond to the elements of the screening device 10s of Figure 15 already explained in detail above.

[0593] In the upper section (a) of Figure 22, the screening device 10s is shown in a state in which the screen (consisting of screen frame 14s and screen mesh 16s) is not yet installed in a housing 60s of the screening device 10s. The lower section (b) of Figure 22 shows the screening device in the installed state of the screen 14s, 16s.

[0594] The screening device 10s of Figure 22 comprises a housing 60s, which—similar to the lid 20s—is suitable for hermetically sealing the screening device 10s. Thus, a screening process can be carried out in a closed inert gas atmosphere. A powder inlet 22s is located in an upper side of the housing 60s. An oversize grain outlet 32s and a merely indicated screening container 18s are provided on an underside of the housing. The above-mentioned elements 22s, 32s, and 18s are attached to the housing 60s and are thus secured independently of any change in the angle of inclination aa-w (see below).

[0595] The sieve, consisting of sieve frame 14s and sieve mesh 16s, can be inserted into the housing from the side via a flap 62s. The flap 62s can be closed and, when closed, forms a gas-tight seal for the housing 60s. Furthermore, a sieve receptacle 66s is provided, into which the sieve 14s, 16s can be inserted and secured if necessary. This allows the sieve 14s, 16s to be easily removed and reinserted, or replaced if necessary.

[0596] An inclination angle aa-w of the sieve 14s, 16s (more precisely, of the sieve mesh 16s) relative to the horizontal plane E is adjustable. For this purpose, an inclination device 64s is provided, which is configured to change the inclination angle aa-w. The inclination device can comprise a motor, in particular a servomotor. In the example shown in Figure 22, the inclination device 64s is attached to the sieve holder 66s and configured to incline it relative to the housing 60s. More precisely, according to the example shown, the inclination device is arranged in the center of the sieve 14s, 16s and configured to rotate the sieve 14s, 16s along a horizontally extending axis of rotation.

[0597] In particular, the tilt device 64s can be controlled by the control unit 40s such that any desired tilt angle aa-w within a predetermined angle range (e.g., 0° to 45°) can be set. According to some embodiments, a change in the tilt angle aa-w can occur so quickly that a first tilt angle is set in the first time interval and a second tilt angle is set in the second time interval. In other words, the change in the angle can occur in a change interval that is shorter than the shorter of the first and second time intervals, in particular a maximum of half as long, a maximum of 1 / 4 as long, a maximum of 1 / 8 as long, a maximum of 1 / 10 as long, a maximum of 1 / 50 as long, or a maximum of 1 / 100 as long.

[0598] However, it is also possible that a change in the inclination angle aa-w occurs continuously during the first time interval and / or during the second time interval.

[0599] Thus, the control of the screening performance (i.e., different screening performance in the first and second time intervals) can be supported by different inclination angles aa-w in the respective time intervals. In particular, a larger inclination angle can be set for the first time interval than for the second time interval. However, conversely, a smaller inclination angle can also be set for the first time interval than for the second time interval. Both options can be advantageous depending on the situation and objective. A smaller inclination angle aa-w results in less oversize being removed and the powder possibly accumulating on the screen mesh 16s. A higher inclination angle aa-w results in better removal of the oversize, but also in any "good" powder that could pass through the screen mesh 16s reaching the oversize outlet 32s.

[0600] According to some embodiments, the angle of inclination can be adjusted depending on the powder used. This can, for example, be done initially before the start of a sieving process, so that the angle of inclination remains constant throughout the sieving process. For example, a higher angle of inclination can be set for heavier materials than for lighter materials. A higher angle of inclination can also be set for powder material with non-round and / or spiky powder particles, which therefore has lower flowability, than for powder material with round powder particles and therefore higher flowability. In this way, the angle of inclination can be optimized with regard to the flow properties of the material used.

[0601] Furthermore, a sensor (not shown) can be provided, which is configured to detect a spreading speed of the powder to be screened on the screen and / or a position of a powder front of the powder to be screened on the screen. For this purpose, the sensor can comprise, for example, a camera, an inductive sensor, and / or a light barrier. The sensor can be attached, for example, to an inner side of the housing 60s, in particular to an upper wall of the housing 60s.

[0602] The control unit can be configured such that the inclination angle aa-w of the sieve surface of the sieve relative to the horizontal plane is changed depending on the detected spreading speed and / or depending on the detected position. In particular, the inclination angle aa-w can be changed such that the inclination angle aa-w is reduced when the detected spreading speed of the powder to be sieved and / or the detected position of the powder front exceeds a predetermined threshold. Similarly, the inclination angle aa-w can be changed such that the inclination angle aa-w is increased when the detected spreading speed of the powder to be sieved and / or the detected position of the powder front falls below a predetermined threshold.

[0603] This allows the angle of inclination to be controlled automatically and time-consuming tests to determine an optimal and powder-dependent angle of inclination can be avoided.

[0604] Aspects that can be used alone and / or in combination with the powder conveying system described above (see, for example, Fig. 1) are as follows:

[0605] 1. A method for controlling the operation of a screening device (10s), comprising the steps of:

[0606] (i) feeding powder to be sieved (56s) onto a sieve through a powder inlet (22s);

[0607] (ii) driving the sieve for a first time interval with a first drive power, wherein the first drive power is dimensioned such that the powder to be sieved (56s) flows over an entire sieve surface of the sieve and / or into an oversize grain outlet (32s) when the sieve is continuously driven with the first drive power;

[0608] (iii) after expiration of the first time interval, driving the sieve for a second time interval with a second drive power which is less than the first drive power; (iv) after expiration of the second time interval, repeating steps (ii) and (iii).

[0609] 2. Method for controlling a screening device (10s) according to aspect 1, wherein the second drive power is dimensioned such that the powder to be screened (56s) forms a repose cone on the screening surface of the screen when the screen is continuously driven with the second drive power in the region of the powder inlet (22s), which substantially corresponds to a repose cone forming on a stationary plane, wherein an angle of repose (aa) of the repose cone is adapted in particular to an orientation of the screening surface.

[0610] 3. A method for controlling the operation of a screening device (10s) according to aspect 1 or 2, wherein the screening surface of the screen is inclined relative to a horizontal plane (E) such that the flow of the powder fed through the powder inlet (22s) towards the oversize grain outlet (32s) is assisted by gravity, wherein an angle of inclination (aa-w) of the screening surface of the screen relative to the horizontal plane (E) is preferably smaller than an angle of repose (aa) of a repose cone formed by the powder (56s) to be screened on a horizontal plane (E).

[0611] 4. A method for controlling the operation of a screening device (lOss) according to any one of aspects 1 to 3, wherein:

[0612] - the first time interval is an empirically determined value for the powder to be sieved (56); and / or

[0613] - when determining the first time interval, the first time interval is ended at the latest when powder (56s) to be screened flows into the oversize grain outlet (32s), wherein the flow of powder (56s) to be screened into the oversize grain outlet (32s) is detected in particular by means of an oversize grain sensor (33s) provided in the region of the oversize grain outlet; and / or

[0614] - the first time interval is dimensioned so that by the end of the first time interval a screen area utilisation of approx. 70% to approx. 90%, preferably of approx.

[0615] 75% to approximately 85% and particularly preferably approximately 80% of the total sieve area of ​​the sieve is not exceeded; and / or

[0616] - the second time interval is an empirically determined value for the powder to be sieved (56s); and / or

[0617] - when determining the second time interval, the second time interval is ended at the latest when the powder to be sieved (56s) forms a repose cone with a defined size in the region of the powder inlet (22s) on the sieve surface of the sieve, wherein the formation of a repose cone with a defined size is detected in particular by means of a dosing sensor (23s) provided in the region of the powder inlet (22s).

[0618] 5. Method for controlling the operation of a screening device (10s) according to one of aspects 1 to 4, wherein the powder to be screened (56s) is at least temporarily continuously metered with a metering mass flow rhdos = 0.5 * (rh(amin) + rh(a m ax)) is fed through the powder inlet (22s) onto the sieve, wherein a m in a screen area utilization and rh(amin) is a screen throughput when driving the screen with the second drive power, and where a max is a screen area utilization and rh(amax) is a screen throughput when driving the screen with the first drive power.

[0619] 6. A method for controlling the operation of a screening device (10s) according to aspect 5, wherein:

[0620] - rh(amine) is determined by increasing the metered mass flow of the powder (56s) to be screened through the powder inlet (22s) when driving the screen with the second drive power until the powder (56s) to be screened has formed a cone of material with a defined size in the region of the powder inlet (22s) on the screen surface of the screen; and / or

[0621] - rh(amax) is determined by increasing the metered mass flow of the powder (56s) to be screened through the powder inlet (22s) when driving the screen with the first drive power until the powder (56s) to be screened has formed a cone of material with a defined size in the area of ​​the powder inlet (22s) on the screen surface of the screen and powder (56s) to be screened flows into the oversize grain outlet (32s), the value of rh(a m ax) is preferably multiplied by a safety factor of 0.8, 0.7, 0.6 or 0.5.

[0622] 7. A method for controlling the operation of a screening device (10s) according to aspect 5 or 6, wherein:

[0623] - the metered mass flow (rhdos) is reduced when the powder to be screened (56s) forms a cone of material with a defined size on the screen surface of the screen in the area of ​​the powder inlet (22s); and / or

[0624] - sieve cleaning is initiated when the powder to be sieved (56s) forms a cone of material with a defined size on the sieve surface of the sieve in the area of ​​the powder inlet (22s) and / or the dosing mass flow rhdos falls below a limit value.

[0625] 8. A method for controlling the operation of a screening device (10s) according to aspect 7, wherein upon initiation of a screening cleaning

[0626] - the powder supply through the powder inlet (22s) is stopped; and / or

[0627] - any powder remaining in the sieve is sieved before starting the sieve cleaning process; and / or

[0628] - after the start of the sieve cleaning, the sieve is driven with maximum drive power; and / or

[0629] - after the start of the sieve cleaning, a vibrator (38s) is activated which drives the sieve independently of a drive device (36s) of the sieve device (10s), wherein preferably an angle of attack of the vibrator (38s) on the sieve, a drive amplitude of the vibrator (38s) and / or a drive frequency of the vibrator (38s) is variably adjustable.

[0630] 9. Method for controlling the operation of a screening device (10s) according to one of aspects 1 to 8, wherein the powder (56ss) to be screened is fed at least temporarily discontinuously through the powder inlet (22s) onto the screen, wherein in particular

[0631] - initially, in a non-driven state of the sieve, powder (56s) to be sieved is fed through the powder inlet (22s) onto the sieve until the powder (56s) to be sieved has formed a cone of material with a defined size on the sieve surface of the sieve in the region of the powder inlet (22s);

[0632] - after the powder feed has ended, the sieve is driven and the powder fed onto the sieve surface is sieved; and

[0633] - the screening process is stopped when m is fed = moversize + msieved, where msupplied is the mass of the powder fed in, moversize is the mass of the powder flowing into the oversize outlet (32s) and m ge sieves is the mass of the sieved powder, and where m ZU guided, in particular by means of a first measuring device (42s) which is provided in a powder feed container (132s) connectable to the powder inlet (22s) of the screening device (10s), moberkorn is determined, in particular, by means of a second measuring device (44s) which is provided in an oversize container (136s) connectable to the oversize outlet (32s) of the screening device (10s), and / or m gesieved is determined in particular by means of a third measuring device (46s) which is provided in a sieved powder container (134s) which can be connected to a sieved powder outlet (28s) of the sieving device (10s).

[0634] 10. Method for controlling the operation of a screening device (10s) according to one of aspects 1 to 9, wherein the powder (56s) to be screened is fed at least temporarily with a metered mass flow through the powder inlet (22s) onto the screen, which is determined as a function of the drive power used to drive the screen, wherein in particular a first metered mass flow with which the powder (56s) to be screened is fed through the powder inlet (22s) onto the screen during the first time interval is greater than a second metered mass flow with which the powder (56) to be screened is fed through the powder inlet (22s) onto the screen during the second time interval.

[0635] 11. A method for controlling the operation of a screening device (10s) according to one of aspects 1 to 10, wherein a warning is issued when an oversize grain rate exceeds a limit value, wherein the oversize grain rate is measured in particular continuously.

[0636] 12. A method for controlling the operation of a screening device (10s) according to any one of aspects 1 to 11, wherein:

[0637] - the screening device (10s) is sealed against the ambient atmosphere and is flooded with a protective gas during operation; and / or

[0638] - additional protective gas is supplied to the screening device (10s) when an inert gas pressure in the screening device (10s) falls below a limit value.

[0639] 13. A method for controlling the operation of a screening device (10s) according to any one of aspects 1 to 12, wherein:

[0640] - during the screening process, a step response of a sum of a screened powder mass flow (rhscreened) and an oversize mass flow (rhoversize) to a dosing mass flow (rhdos) is monitored; and / or

[0641] - a warning is issued if the step response of the sum of the sieved powder mass flow (rhsieved) and the oversize mass flow (rhoversize) to the dosing mass flow (rhdos) falls below a first limit value; and / or

[0642] - a sieve cleaning is initiated when the step response of the sum of the sieved powder mass flow (rhsieved) and the oversize mass flow (rhovercome) to the dosing mass flow (rhdos) exceeds a second limit value.

[0643] 14. A method for controlling the operation of a screening device (10s) according to any one of aspects 1 to 13, further comprising:

[0644] - Changing an angle of inclination (aa-w) of the sieve surface of the sieve relative to a horizontal plane (E).

[0645] 15. A method for controlling the operation of a screening device (10s) according to aspect 14, wherein the screen is arranged within a housing (60s) which is in particular sealed in a gas-tight manner, and wherein the screen is rotated relative to the housing (60s) and the angle of inclination (aa-w) is thereby changed.

[0646] 16. A method for controlling the operation of a screening device (10s) according to aspect 14 or 15, wherein a first inclination angle (aa-w) is set during the first time interval and a second inclination angle (aa-w) is set during the second time interval, and wherein either (a) the first inclination angle is less than the second inclination angle or (b) the first inclination angle is greater than the second inclination angle.

[0647] 17. A method for controlling the operation of a screening device (10s) according to aspect 14 or 15, wherein the angle of inclination is changed during the first and / or during the second time interval.

[0648] 18. A method for controlling the operation of a screening device (10s) according to any one of aspects 14 to 17, further comprising:

[0649] - detecting a propagation speed of the powder to be screened (56s) on the screen and / or a position of a powder front of the powder to be screened (56s) on the screen, wherein the changing of the angle of inclination (aa-w) of the screen surface of the screen relative to the horizontal plane (E) takes place as a function of the detected propagation speed and / or as a function of the detected position.

[0650] 19. A method for controlling the operation of a screening device (10s) according to aspect 18, wherein the changing of the inclination angle (aa-w) is carried out such that the inclination angle (aa-w) is reduced when the detected propagation speed of the powder to be screened (56s) and / or the detected position of the powder front exceeds a predetermined threshold value.

[0651] 20. Screening device (10s) comprising:

[0652] - a powder inlet (22s);

[0653] - a drive device (36s) configured to drive the screen; and

[0654] - a control unit (40s) configured to control the powder inlet (22s) and the drive device (36s) such that:

[0655] (i) powder (56s) to be sieved is fed onto the sieve through the powder inlet (22s);

[0656] (ii) the sieve is driven for a first time interval with a first drive power, wherein the first drive power is dimensioned such that the powder to be sieved (56s) flows over an entire sieve surface of the sieve and / or into an oversize grain outlet (32s) when the sieve is continuously driven with the first drive power;

[0657] (iii) after the expiry of the first time interval, the sieve is driven for a second time interval with a second drive power which is less than the first drive power;

[0658] (iv) after expiry of the second time interval, steps (ii) and (iii) are repeated.

[0659] 21. Screening device (10s) according to aspect 20, wherein:

[0660] - the second drive power is dimensioned such that the powder to be screened (56s) forms a repose cone on the screen surface of the screen when the screen is continuously driven with the second drive power in the region of the powder inlet (22s), which substantially corresponds to a repose cone forming on a stationary plane, wherein an angle of repose (aa) of the repose cone is adapted in particular to an orientation of the screen surface; and / or

[0661] - the sieve surface of the sieve is inclined relative to a horizontal plane (E) such that the flow of the powder fed through the powder inlet (22s) towards the oversize grain outlet (32s) is assisted by gravity, wherein an angle of inclination (aa-w) of the sieve surface of the sieve relative to the horizontal plane (E) is preferably smaller than an angle of repose (aa) of a repose cone formed by the powder (56s) to be sieved on a horizontal plane (E); and / or

[0662] - the first time interval is an empirically determined value for the powder to be screened (56s); and / or - the control unit (40s) is configured, when determining the first time interval, to end the first time interval at the latest when powder to be screened (56s) flows into the oversize grain outlet (32s), wherein the screening device (10s) comprises, in particular, an oversize grain sensor provided in the region of the oversize grain outlet for monitoring the flow of powder to be screened (56s) into the oversize grain outlet (32s); and / or

[0663] - the control unit (40s) is configured to dimension the first time interval such that a sieve area utilization of approximately 70% to approximately 90%, preferably approximately 75% to approximately 85%, and particularly preferably approximately 80% of the total sieve area of ​​the sieve is not exceeded by the end of the first time interval; and / or

[0664] - the second time interval is an empirically determined value for the powder to be sieved (56s); and / or

[0665] - the control unit (40s) is configured, when determining the second time interval, to end the second time interval at the latest when the powder (56s) to be sieved forms a repose cone with a defined size in the region of the powder inlet (22s) on the sieve surface of the sieve, wherein the sieving device (10s) in particular comprises a dosing sensor (23s) provided in the region of the powder inlet (22s) for detecting the formation of a repose cone with a defined size.

[0666] 22. Sieving device (10s) according to aspect 20 or 21, wherein the control unit (40s) is configured to control the powder inlet (22s) such that the powder to be sieved (56s) is at least temporarily continuously fed with a metering mass flow rhdos = 0.5 * (rh(amin) + rh(a max)) is fed through the powder inlet (22s) onto the sieve, wherein a m in a screen area utilization and rh(amin) is a screen throughput when driving the screen with the second drive power, and where a m ax is a screen area utilization and rh(amax) is a screen throughput when driving the screen with the first drive power, wherein the control unit (40s) is configured in particular:

[0667] - to determine rh(amine) by increasing the metered mass flow of the powder (56s) to be screened through the powder inlet (22s) when driving the screen with the second drive power until the powder (56s) to be screened has formed a cone of material with a defined size in the region of the powder inlet (22s) on the screen surface of the screen; and / or

[0668] - to determine rh(amax) by increasing the metered mass flow of the powder to be screened (56s) through the powder inlet (22s) when driving the screen with the first drive power until the powder to be screened (56s) has formed a cone of material with a defined size in the area of ​​the powder inlet (22s) on the screen surface of the screen and powder to be screened (56s) flows into the oversize grain outlet (32s), wherein the value of rh(a m ax) is preferably multiplied by a safety factor of 0.8, 0.7, 0.6 or 0.5; and / or

[0669] - to reduce the dosing mass flow rhdos when the powder to be screened (56s) forms a cone of material with a defined size on the screen surface of the screen in the area of ​​the powder inlet (22s); and / or

[0670] - to initiate sieve cleaning when the powder to be sieved (56s) forms a cone of material with a defined size on the sieve surface of the sieve in the area of ​​the powder inlet (22s) and / or the dosing mass flow rhdos falls below a limit value.

[0671] 23. Screening device (10s) according to aspect 22, wherein the control unit (40s) is configured, upon initiation of a screen cleaning

[0672] - to control the powder inlet (22s) so that the powder supply through the powder inlet (22s) is stopped; and / or

[0673] - to control the drive device (36s) in such a way that any powder still present in the sieve is sieved before the start of the sieve cleaning; and / or

[0674] - to control the drive device (36s) so that after the start of the sieve cleaning, the sieve is driven with a maximum drive power; and / or

[0675] - after the start of the sieve cleaning, to activate a vibrator (38s) which is configured to drive the sieve independently of the drive device (38s) of the sieve device (10s), wherein preferably an angle of attack of the vibrator (38s) on the sieve, a drive amplitude of the vibrator (38s) and / or a drive frequency of the vibrator (38s) is variably adjustable.

[0676] 24. Sieving device (10s) according to one of aspects 20 to 23, wherein the control unit (40s) is configured to control the powder inlet (22s) such that the powder to be sieved (56s) is fed at least temporarily discontinuously through the powder inlet (22s) onto the sieve, wherein the control unit (40s) is in particular configured to control the drive device (36s) and the powder inlet (22s) such that:

[0677] - initially, in a non-driven state of the sieve, powder (56s) to be sieved is fed through the powder inlet (22s) onto the sieve until the powder (56s) to be sieved has formed a cone of material with a defined size on the sieve surface of the sieve in the region of the powder inlet (22s);

[0678] - after the powder feed has ended, the sieve is driven and the powder fed onto the sieve surface is sieved; and

[0679] - the screening process is terminated when ni fed in — ni oversize + Hl screened, where m fed in is the mass of the powder fed in, mo oversize is the mass of the powder flowing into the oversize outlet (32s) and m ge sieves the mass of the sieved powder, and wherein the sieving device (10s) in particular comprises a first measuring device (42s) for determining m ZUwhich is provided in a powder feed container (132s) connectable to the powder inlet (22s) of the screening device (10s), in particular a second measuring device (44s) for determining mober grain, which is provided in an oversize grain container (136s) connectable to the oversize grain outlet (32s) of the screening device (10s), and / or and / or in particular a third measuring device (46s) for determining m ge sieved, which is provided in a sieved powder container (134s) connectable to a sieved powder outlet (28s) of the sieving device (10s).

[0680] 25. Sieving device (10s) according to one of aspects 20 to 24, wherein the control unit (40s) is configured to control the powder inlet (22s) such that the powder (56s) to be sieved is fed at least temporarily through the powder inlet (22s) onto the sieve with a metered mass flow that is determined as a function of the drive power used to drive the sieve, wherein in particular a first metered mass flow with which the powder (56s) to be sieved is fed through the powder inlet (22s) onto the sieve during the first time interval is greater than a second metered mass flow with which the powder (56s) to be sieved is fed through the powder inlet (22s) onto the sieve during the second time interval.

[0681] 26. Screening device (10s) according to one of aspects 20 to 25, wherein the control unit (40s) is configured to issue a warning when an oversize grain rate, in particular continuously measured, exceeds a limit value.

[0682] 27. Screening device (10s) according to any one of aspects 20 to 26, wherein:

[0683] - the screening device (40s) is sealed against the ambient atmosphere and flooded with a protective gas during operation; and / or

[0684] - the control unit (40s) is configured to supply additional protective gas into the screening device (10s) when an inert gas pressure in the screening device (10s) falls below a limit value.

[0685] 28. Screening device (10s) according to one of aspects 20 to 27, wherein the control unit (40s) is configured:

[0686] - to monitor a step response of a sum of a sieved powder mass flow (rhsieved) and an oversize mass flow (rhoversize) to a dosing mass flow (rhdos) during the sieving process; and / or

[0687] - a warning will be issued if the step response of the sum of the sieved powder mass flow (rhsieved) and the oversize mass flow (rhoversize) to the dosing mass flow (rhdos) falls below a first limit value; and / or

[0688] - to initiate a sieve cleaning if the step response of the sum of the sieved powder mass flow (rhsieved) and the oversize mass flow (rhovercome) to the dosing mass flow (rhdos) exceeds a second limit value.

[0689] 29. Screening device (10s) according to any one of aspects 20 to 28, comprising:

[0690] - a lid (20s) detachable from a sieve container (18s);

[0691] - a seal (48s) arranged in the cover (20s); and

[0692] - a clamping device (50s) configured to exert a clamping force on the seal (20s) that holds the seal (48s) in position in the lid (20s).

[0693] 30. Screening device (10s) according to any one of aspects 20 to 29, comprising:

[0694] - an inclination device (64s) for changing an inclination angle (aa-w) of the sieve surface of the sieve relative to a horizontal plane (E).

[0695] 31. A screening device (10s) according to aspect 30, comprising:

[0696] - a housing (60s) which can be closed in a gas-tight manner, wherein the sieve is arranged within the housing (60s) and wherein the inclination device (64s) is designed to rotate the sieve relative to the housing (60s) and thereby change the angle of inclination (aa-w).

[0697] 32. A screening device according to aspect 30 or 31, wherein the powder inlet (22s) and the oversize grain outlet (32s) are fixedly attached to the housing (60s).

[0698] 33. A screening device according to any one of aspects 30 to 32, comprising:

[0699] - a sieve holder (66s) for receiving, in particular for inserting, the sieve, wherein the inclination device (64s) is fastened to the sieve holder (66s) and is designed to rotate the sieve holder (66s).

[0700] 34. The screening device (10s) according to any one of aspects 30 to 33, wherein the control unit (40s) is configured to set a first inclination angle (aa-w) during the first time interval and to set a second inclination angle (aa-w) during the second time interval, and wherein either (a) the first inclination angle is less than the second inclination angle or (b) the first inclination angle is greater than the second inclination angle.

[0701] 35. Screening device (10s) according to one of aspects 30 to 33, wherein the control unit (40s) is configured to change the angle of inclination during the first and / or during the second time interval.

[0702] 36. Screening device (10s) according to any one of aspects 30 to 35, further comprising:

[0703] - at least one sensor for detecting a propagation speed of the powder to be screened (56s) on the screen and / or a position of a powder front of the powder to be screened (56s) on the screen, wherein the control unit (40s) is configured to change the angle of inclination (aa-w) of the screen surface of the screen relative to the horizontal plane (E) as a function of the detected propagation speed and / or as a function of the detected position.

[0704] 37. Sieving device (10s) according to aspect 36, wherein the control unit (40s) is configured to change the angle of inclination (aa-w) such that the angle of inclination (aa-w) is reduced when the detected propagation speed of the powder to be sieved (56s) and / or the detected position of the powder front exceeds a predetermined threshold value.

[0705] 38. Powder processing system (128s) comprising a screening device (10s) according to any one of aspects 30 to 37.

[0706] 39. Plant (100s) for producing three-dimensional workpieces by exposing raw material powder layers to electromagnetic radiation or particle radiation, which comprises a screening device (10s) according to one of aspects 30 to 37 and / or a powder processing system (128s) according to aspect 38.

Claims

Patent claims 1. A powder conveying system for conveying raw material powder to a system for producing a three-dimensional workpiece by irradiating layers of the raw material powder with electromagnetic radiation or particle radiation, the powder conveying system comprising: a conveying line configured to convey, at least in sections, a gas stream and, at least in sections, a powder stream driven by the gas stream; a conveying device configured to convey the gas stream through the conveying line; a first tank connected to the conveying line for supplying the system with powder for an additive manufacturing process; at least one overflow container connected to the conveying line for receiving excess powder from the additive manufacturing process; a buffer container connected to the conveying line for supplying a screening device with powder to be screened;the screening device for screening the powder to be screened and for dispensing screened powder; an interface connected to the conveying line for an external tank for introducing fresh or contaminated powder into the powder conveying system; and a control device for controlling the powder conveying system so that it carries out at least one of the following conveying processes: a powder conveying the screened powder into the first tank; b powder conveying from the at least one overflow container into the buffer container; and c powder conveying from the external tank into the buffer container.

2. Powder conveying system according to claim 1, wherein either (a) all of the conveying processes ac contain pneumatic conveying processes, wherein a conveyed powder is conveyed by the gas flow, or (b) at least one of the conveying processes ac, in particular the conveying process b, does not contain a pneumatic conveying process.

3. Powder conveying system according to claim 2, wherein in case (b) the conveying process which does not include a pneumatic conveying process comprises conveying by a screw conveyor and / or conveying by gravity conveying.

4. Powder conveying system according to one of claims 1 to 3, further comprising a main reservoir connected to the conveying line for receiving the screened powder, wherein the conveying process a conveys the screened powder from the main reservoir into the first tank.

5. Powder conveying system according to one of claims 1 to 4, wherein the powder conveying system enables a closed powder conveying, in particular a powder conveying in which conveyed powder does not leave the powder conveying system over several consecutive additive manufacturing processes of the system.

6. Powder conveying system according to one of claims 1 to 5, wherein the powder conveying system is configured to maintain an inert gas atmosphere within the conveying line, in particular during one of the conveying processes a to c.

7. Powder conveying system according to one of claims 1 to 6 in combination with claim 4, further comprising a pressure equalization container coupled to the conveying line, which is coupled to the conveying line in particular downstream of the conveying device and upstream of the connections of the first tank, the at least one overflow container, the buffer container, the main storage tank and the external tank.

8. Powder conveying system according to one of claims 1 to 7 in combination with claim 4, further comprising at least one metering device, in particular comprising a conveyor screw, for metered feeding of the powder to be conveyed into the conveying line from the at least one overflow container, from the main storage and / or from the external tank.

9. Powder conveying system according to one of claims 1 to 8, further comprising at least one separation device, in particular comprising a cyclone, for separating the conveyed powder from the conveying line and for feeding the powder into the first tank and / or into the buffer container.

10. Powder conveying system according to one of claims 1 to 9 in combination with claim 4, wherein at least one of the following containers is coupled to a pressure equalization line: the first tank, the at least one overflow container, the buffer container, the main storage tank and the external tank.

11. Powder conveying system according to one of claims 1 to 10, wherein the control device is configured to perform a flow check, comprising checking a flow through the conveying line.

12. The powder conveying system of claim 11, further comprising a velocity meter for measuring a velocity of the gas flow in the conveying line, wherein the flow check comprises: Opening valves of the powder conveying system so that flow is possible through at least one conveying circuit belonging to one of the conveying processes a, b and c; Setting a conveying speed of the gas flow to a predetermined range or value; and Determine whether a gas flow velocity value measured by the velocity sensor is greater than a predetermined limit.

13. Powder conveying system according to claim 12, wherein adjusting the conveying speed comprises: Determining whether a speed value measured by the speed sensor is within the predetermined value range; if this is not the case, determining whether the speed value is below the predetermined value range; if the speed value is below the predetermined value range, increasing a power of the conveyor device; and if the speed value is not below the predetermined value range, decreasing the power of the conveyor device.

14. A powder conveying system according to any one of claims 11 to 13, further comprising: a pressure sensor for measuring a pressure of the gas stream; and a filter for filtering remaining powder particles from the gas stream, wherein the flow check comprises: Determining a total pressure loss based on sensor data from the pressure sensor; if it is determined that the total pressure loss exceeds a predetermined limit, performing a filter cleaning of the filter and determining a total pressure loss again; and if it is determined that the total pressure loss does not exceed the predetermined limit, conveying powder in at least one of the conveying processes a, b, and c.

15. The powder conveying system of claim 14, wherein the flow check performed by the controller further comprises: if it is determined that the total pressure loss exceeds the predetermined limit, prior to performing the filter cleaning, outputting an error.

16. Powder conveying system according to one of claims 1 to 15, comprising at least one oxygen sensor for measuring an oxygen content of the gas stream, wherein the control device is configured to perform an oxygen content check, and wherein the oxygen content check comprises measuring an oxygen content using the at least one oxygen sensor.

17. Powder conveying system according to claim 16, wherein the control device is configured to carry out a pipeline inerting of the conveying line if the measurement of the oxygen content of the gas stream shows that the oxygen content is above a predetermined limit value.

18. The powder conveying system of claim 17, further comprising a vent valve for venting gas from the conveying line to an environment or an external volume, wherein the pipeline inerting comprises opening the vent valve.

19. Powder conveying system according to claim 18, wherein the pipeline inerting further comprises: Evacuating at least one section of the production line; Leakage test of the delivery line; Normalizing the conveyor line; and Checking the oxygen content in the production line.

20. The powder conveying system of claim 19, wherein evacuating comprises checking whether a measured pressure within the conveying line is below a predetermined evacuation pressure.

21. The powder conveying system of claim 20, further comprising a velocity meter for measuring a velocity of the gas flow in the conveying line, wherein the evacuating comprises: Determining whether a speed value measured by the speed sensor is within a predetermined value range; if this is not the case, determining whether the speed value is below the predetermined value range; if the speed value is below the predetermined value range, increasing a power of the conveyor device; and if the speed value is not below the predetermined value range, decreasing the power of the conveyor device.

22. Powder conveying system according to one of claims 19 to 21, wherein the leakage test comprises: Close the drain valve; Stopping the conveyor; Determine whether a system pressure increase per time exceeds a predefined limit; if the system pressure increase per time exceeds the predefined limit, issue an error message; and if the system pressure increase per time does not exceed the predefined limit, proceed to normalize the delivery line.

23. The powder conveying system of claim 22, comprising a first pressure sensor on an inlet side of the conveying device and a second pressure sensor on an outlet side of the conveying device, wherein determining the system pressure increase comprises considering a sum of the measured values ​​of the first and second pressure sensors.

24. Powder conveying system according to one of claims 19 to 23, wherein normalizing the conveying line comprises: Flooding the production line with inert gas; and Measuring the oxygen content in the production line.

25. The powder conveying system of claim 24, further comprising: if the measured oxygen content in the conveying line exceeds a predetermined limit, restarting the pipeline inerting.

26. Powder conveying system according to one of claims 19 to 25, wherein checking the oxygen content in the conveying line comprises: If the measured oxygen content exceeds a predetermined limit a predetermined number of times, restart the pipeline inerting process.

27. Powder conveying system according to one of claims 1 to 26, wherein the control unit is configured to carry out conveying according to at least one of the conveying processes a, b and c, wherein the conveying comprises opening at least one valve which is arranged in a conveying circuit associated with the respective conveying process.

28. Powder conveying system according to claim 27, wherein the control unit is configured to perform conveying control during conveying, which comprises: Monitoring the oxygen content in the production line; Monitoring a pressure in the delivery line; Monitoring a conveying speed of the gas flow; Monitoring at least one measured parameter of the powder conveying system; and Checking a termination condition to end the funding.

29. A powder conveying system according to claim 28, wherein monitoring the oxygen content in the conveying line comprises: Determining whether a measured oxygen content exceeds a predetermined limit; and if the measured oxygen content exceeds the predetermined limit, opening a valve to supply inert gas to the production line.

30. A powder conveying system according to claim 28 or 29, wherein monitoring the pressure in the conveying line comprises: Determining whether a measured pressure in the delivery line is within a predetermined range; if the measured pressure in the delivery line is less than the predetermined range, opening a valve to supply inert gas; and if the measured pressure in the delivery line is higher than the predetermined range, opening a vent valve to vent gas from the delivery line.

31. Powder conveying system according to one of claims 28 to 30, wherein monitoring the conveying speed of the gas flow comprises: Determining a gas density of the gas flow based on at least one measured characteristic of the gas flow; Determining a powder mass flow conveyed through the conveying line based on a control value applied to a dosing device of an originating tank of the conveyance; Determining a target velocity of the gas flow based on the gas density and based on the bulk material mass flow; and Controlling the conveying device to convey the gas flow at the specified target speed 32. Powder conveying system according to one of claims 28 to 31, wherein monitoring at least one measured characteristic of the powder conveying system comprises: Controlling a dosing device of a source tank of the production with a predetermined control value; Determining whether the at least one measured characteristic of the powder conveying system is above a predetermined maximum value for the respective characteristic; and if the at least one measured characteristic is above the predetermined maximum value, reducing the control value of the dosing device by a predetermined value so that the dosing device delivers a smaller dose of powder per unit of time into the gas stream.

33. Powder conveying system according to claim 32, wherein the control device is further configured to: if the at least one measured characteristic is below the predetermined maximum value, increase the control value of the dosing device by a predetermined value so that the dosing device delivers a higher dose of powder per unit of time into the gas stream.

34. Powder conveying system according to claim 32 or 33, wherein the at least one characteristic comprises at least one of the following characteristics: Conveying speed, pump outlet pressure, pump power and dose of powder per time.

35. Powder conveying system according to one of claims 28 to 34, wherein checking a termination condition for terminating conveying comprises: Terminating the conveying by stopping the conveying device when at least one of the following events is detected: a fill level of a source tank from which powder is taken for conveying falls below a predetermined limit, a fill level of a target tank into which the powder is conveyed exceeds a predetermined limit, a predetermined maximum conveying time is exceeded and a total pressure loss of a conveying gas exceeds a predetermined limit.

36. Powder conveying system according to one of claims 1 to 35, wherein the control device is configured to carry out a pipe cleaning and a filter cleaning after completion of a conveying according to conveying process a, b and / or c, wherein the pipe cleaning comprises a flow of a gas stream through the conveying line, and wherein the filter cleaning comprises blowing off a filter provided in the conveying line with compressed air.

37. Powder conveying system according to one of claims 1 to 36, further comprising at least one dew point sensor for measuring a relative humidity within the conveying line and / or within a tank of the powder conveying system, wherein the control unit is configured to initiate automatic powder drying when the measured value of the relative humidity exceeds a predetermined limit value.

38. A powder conveying system according to claim 37 in combination with claim 4, wherein a dew point sensor is provided on at least one of the following components of the powder conveying system: main reservoir, external tank, and conveyor.

39. Powder conveying system according to one of claims 1 to 38 in combination with claim 4, wherein at least one oxygen sensor is provided for measuring an oxygen concentration at at least one of the following components of the powder conveying system: first tank, overflow container, buffer container, main storage, external tank and screening device, wherein the control device is configured to initiate flooding of the conveying line with inert gas if it is determined that at least one of the provided oxygen sensors measures an oxygen concentration above a predetermined limit value.

40. Powder conveying system according to one of claims 1 to 39 in combination with claim 4, wherein at least one pressure sensor is provided for measuring a pressure on at least one of the following components of the powder conveying system: first tank, overflow container, buffer container, main reservoir, external tank and sieve device, wherein the control device is configured to perform at least one of the following steps if it is determined that at least one of the provided pressure sensors measures a pressure increase per predetermined time unit above a predetermined limit value: Issue a warning; Flooding the conveyor line or an affected section of the powder conveyor system with inert gas; Measuring an oxygen content in the affected section of the powder conveying system; and Carry out a filter cleaning.

41. Powder conveying system according to one of claims 1 to 40, wherein at least one temperature sensor is provided for measuring a temperature on at least one motor of the powder conveying system, in particular a motor of a conveyor screw and / or the conveying device, wherein the control device is configured to initiate a shutdown of the conveying device if it is determined that at least one of the provided temperature sensors measures a temperature value above a predetermined limit value.

42. Powder conveying system according to one of claims 1 to 41, wherein the control device is configured to initiate a shutdown of the conveying device if at least one of the following events is detected: a torque of a motor of a metering device exceeds a predetermined limit; a valve of the powder conveying system is in an actual position that does not correspond to its desired position; a fault in a sensor is detected.

43. Powder conveying system according to one of claims 1 to 42, wherein a fill level sensor for measuring a fill level of the at least one overflow container is provided on the at least one overflow container, in particular in the form of one or more load cells, wherein a fill level sensor for measuring a fill level of the intermediate tank is provided on an intermediate tank which is arranged above the process chamber and which is configured to supply the process chamber with powder and which is configured to be supplied with powder from the first tank, in particular in the form of one or more load cells, wherein the control device is configured to continue an additive manufacturing process of the system in the event of a failure of the powder conveyance by the conveying device until at least one of the following events occurs: the fill level sensor of the overflow container detects that the fill level of the overflow container exceeds a predetermined limit value; and the fill level sensor of the intermediate tank detects that the fill level of the intermediate tank falls below a predetermined limit value.

44. Powder conveying system according to one of claims 1 to 43, wherein the control device is configured to carry out a powder conveyance of a predetermined amount of powder according to one of the conveying processes a, b or c and then to stop the conveyance.

45. Powder conveying system according to claim 44, wherein the control device is configured to determine, after each conveying operation according to one of the conveying processes a, b and c, a priority value for each of the conveying processes based on predetermined fill level limits of the source tanks and the destination tanks, and wherein the control device is configured to subsequently carry out conveying according to the conveying process with the highest priority value.

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