Bulk material conveying system and method for conveying bulk material

Abstract

The invention relates to a bulk-material conveying system, in particular for conveying raw-material powder in an installation 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 pipe designed to convey a gas flow and, at least in sections, a bulk-material flow driven by the gas flow, and a metering device designed to supply the gas flow with a predetermined amount of bulk material per unit of time. The amount is determined by a control value fed to the metering device. The bulk-material conveying system also comprises a conveyor device designed to convey the gas flow through the conveying pipe, and at least one measuring device for measuring at least one parameter of the gas flow. The bulk-material conveying system also comprises a control device designed to determine a gas density of the gas flow on the basis of the measured at least one parameter of the gas flow, to determine a bulk-material mass flow of the bulk-material flow on the basis of the control value fed to the metering device, to determine a target speed of the gas flow on the basis of the gas density and on the basis of the bulk-material mass flow, and to control the conveyor device to convey the gas flow at the determined target speed.

Description

[0001] Bulk material conveying system and method for conveying bulk material

[0002] The invention relates to a bulk material conveying system and a method for conveying bulk material. The invention particularly 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 (e.g., a system for selective laser melting).

[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] 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.

[0006] Particularly 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 target tank.

[0007] 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).

[0008] 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.

[0009] In particular, it is a problem to determine a suitable velocity of a gas flow for pneumatic bulk material conveying under changing conditions (e.g. operating parameters of the system and / or nature of the bulk material).

[0010] The object of the invention 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 react to a change in one or more operating parameters.

[0011] This object is achieved by a bulk material conveying system and a method for conveying bulk material having the features of the independent patent claims. Further embodiments are specified in the subclaims.

[0012] According to a first aspect, the invention 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.

[0013] 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.

[0014] 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.

[0015] 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.

[0016] 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.

[0017] 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 carry out the method according to the second aspect. 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.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] Determining the target speed v S0 The determination 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 desired velocity Vdes may be performed using the following formula: desired ^saltation "F ^safety where Vsaitation is the calculated saltation velocity of the conveyed bulk material and vsafety is a predetermined safety velocity.

[0028] 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.

[0029] 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 the particle diameter d of the bulk material. The predetermined acceleration due to gravity can be 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] Multiple parameters can be determined (in particular measured), and if at least one of the determined parameters is above a predetermined maximum value for this parameter, the control value is reduced by the predetermined value. 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 the corresponding adjustment of the control value can be carried out 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 carried out 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 carried out alternately.Reducing the control value means that the dosing device dispenses a smaller dose of bulk material per unit time.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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).

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] The bulk material conveying system can 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 can 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 can be coupled to an outlet of a sieve for sieving the raw material powder.

[0043] 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.

[0044] 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."

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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).

[0049] 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 to feed the raw material powder into the target tank.

[0050] The dosing device allows the bulk material flow to be introduced into the gas flow and the cyclone allows the bulk material to be removed from the gas flow again.

[0051] According to a second aspect, the invention 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.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] Determining the target speed v S0 n of the gas flow may include calculating a saltation velocity (Saltation Velocity) based on the determined gas density and the determined bulk material mass flow, and determining the target velocity Vsoii may be done using the following formula: target ^saltation "F ^safety where Vsaitation is the calculated saltation velocity of the conveyed bulk material and vsafety is a predetermined safety velocity. Calculating the saltation velocity may 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.

[0057] 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.

[0058] 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.

[0059] 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.

[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 bulk material per time.

[0061] The method may further comprise terminating conveying 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. 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 storage may be coupled to an outlet of a sieve for sieving the raw material powder.

[0062] The method may comprise conveying raw material powder through at least one second conveying circuit 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.

[0063] 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.

[0064] 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.

[0065] 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 with one or more optical components for shaping and deflecting the beam (e.g.

[0066] 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).

[0067] The invention is explained below with reference to the accompanying figures. They depict:

[0068] Figure 1: a system for producing a three-dimensional workpiece by

[0069] Irradiation of layers of a raw material powder with electromagnetic radiation or particle radiation, which is equipped with a bulk material conveying system;

[0070] Figure 2: a schematic representation of a dosing device in the form of a

[0071] Conveyor screw, where parameters of the conveyor screw are specified for calculating a mass flow conveyed by the conveyor screw;

[0072] Figure 3: a flowchart of a limit control process;

[0073] Figure 4: a schematic representation of a bulk material conveying system for a selective laser melting plant, which enables at least three conveying processes a, b and c;

[0074] Figure 5: a representation in which the conveying process a of the system of Fig. 4 is highlighted;

[0075] Figure 6: a representation in which the conveying process b of the system of Fig. 4 is highlighted; and

[0076] Figure 7: A diagram highlighting the conveying process c of the system of Figure 4. Figure 1 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, system 1100 is a system for producing a three-dimensional workpiece using an additive manufacturing process in which a raw material powder is used, for example, selective laser melting or selective laser sintering.

[0077] 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.

[0078] 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 that has been applied to the carrier 1002.

[0079] 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. 1. In the exemplary powder conveying system 1001, which is shown in Fig. 1, 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.

[0080] 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.

[0081] 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. 1, 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.1, 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.

[0082] 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.

[0083] 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.

[0084] 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. 1, the measuring device 1050 is arranged immediately 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.

[0085] 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.

[0086] Finally, the bulk material conveying system 1001 comprises a control device 1040.

[0087] 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.

[0088] 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.

[0089] 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:

[0090] A. Determining a gas density of the gas stream based on the measured at least one characteristic of the gas stream;

[0091] 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

[0092] D. Controlling the conveying device to convey the gas flow at the determined target speed.

[0093] The individual steps:

[0094] Step A:

[0095] 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).

[0096] 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.

[0097] Step B:

[0098] 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.

[0099] 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.

[0100] If no corresponding calibration data is available, the dosing device 1009 can be calibrated to determine a relationship between control value and volume flow.

[0101] Furthermore, the bulk material mass flow can also be calculated. A corresponding calculation is presented below using the example of a screw conveyor.

[0102] The bulk material mass flow is calculated using the following formula:

[0103] 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.

[0104] Details on the parameters given above in the context of a screw conveyor can also be found in Fig. 2, which shows a screw conveyor and associated parameters.

[0105] Step C:

[0106] 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.

[0107] 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.

[0108] 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:

[0109] 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.

[0110] 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.

[0111] Step D:

[0112] 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.

[0113] 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.

[0114] 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.

[0115] 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.

[0116] Details of the limit control are shown in Fig. 3. In this regard, the control device 1040 is configured to determine whether at least one measured parameter of the bulk material conveying system 1001 is above a predetermined maximum value for the respective parameter. If the at least one measured parameter 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 of time.

[0117] An example of limit control is shown in Fig. 3 using specific parameters. Fig. 3 shows a flowchart of a process performed by the control device 1040 after determining the target speed and controlling the conveyor device.

[0118] The process begins with step 1202, which queries whether a measured conveying speed is below a predetermined maximum value. To measure the conveying speed, a speed sensor in the gas stream or in the gas-powder mixture stream is used, for example, sensor 1016 or 1017 in Fig. 1.

[0119] 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. 1.

[0120] 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.

[0121] 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.

[0122] 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.

[0123] 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.

[0124] 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.

[0125] The process also proceeds to the reduction of the dose of the dosing device 1008 according to step 1214 if the result in at least one of the queries 1202, 1204 and 1206 is "no" and thus a corresponding maximum value has been reached or exceeded.

[0126] After the process of Fig. 3, it is checked whether the conveying process is terminated and thus whether in particular the conveying device 1019 is stopped.

[0127] 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.

[0128] If the conveying process is not terminated, the process performed by the control device 1040 returns to step A (see above).

[0129] Fig. 4 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.

[0130] The bulk material conveying system of Fig. 4 represents a more detailed representation of a bulk material conveying system compared to Fig. 1, wherein certain components are shown and provided with reference numerals that may also be present in the system of Fig. 1, but are not explicitly described.

[0131] Furthermore, however, the system of Fig. 4 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 are described below.

[0132] 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. 4. The same applies to the limit value control of Fig. 3. For this purpose, the bulk material conveying system of Fig. 4 also comprises a control device (not shown) for controlling the individual components of the system of Fig. 4. The system 1100 corresponds, for example, to the system 1100 of Fig. 1 and - generally speaking - for example to a generally known system for additive manufacturing by means of selective laser melting or selective laser sintering. Fig. 4 only shows the process chamber of the system 1100. Raw material powder is fed by means of a loader 2104 from an intermediate tank 2102 (also: hopper) of the system 1100 to the process chamber (more precisely, a powder application device or a coater in the process chamber 1011).

[0133] 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.

[0134] 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.

[0135] 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.

[0136] 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.

[0137] The process chamber is connected to a line via corresponding valves 20 and 18 for pressure equalization, which connects valve 30 and 21 to valve 38 and 29, respectively. The powder is conveyed from the overflow tanks 2 and 3 into a buffer tank 4 via the conveying line. A cyclone 42 is located above the buffer tank 4 to separate the powder.

[0138] 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.

[0139] 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. 4). 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.

[0140] 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.

[0141] 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, which is 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, in the path described above (including via several sensors). 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 can be added via the conveyor screw 36. The conveying gas-powder mixture then flows - as described above - to the cyclone 42, where it can be conveyed into the buffer tank 4.

[0142] 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.

[0143] 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 feed. From the intermediate tank 2102, it can be fed via loader 2104 to the process chamber 1011 of system 1100 and used in the additive manufacturing process.

[0144] 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.

[0145] 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.

[0146] The sieved powder enters the main reservoir 6 via an inlet valve 52. From the main reservoir 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. Furthermore, an external tank 8 can be connected to the conveyor line, which can be, for example, 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 fed to the system.

[0147] 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.

[0148] 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.

[0149] 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.

[0150] 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.

[0151] 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.

[0152] 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 bulk material conveying system shown in Fig. 4 are listed again below using their reference symbols; however, this list does not claim to be complete.

[0153] I - First tank 2 - front overflow tank

[0154] 3 - rear overflow tank

[0155] 4 - Lock

[0156] 5 - Sieve

[0157] 6 - Main memory

[0158] 7 - Oversize barrel (oversize container)

[0159] 8 - external tank module (external tank)

[0160] 9 - Cyclone

[0161] 10 - Pressure equalization valve for the first tank

[0162] II - Valve pressure equalization first tank machine side

[0163] 12 - Valve inlet first tank

[0164] 13 - upper powder sensor first tank

[0165] 14 - lower powder sensor first tank

[0166] 15 - Load cells first tank

[0167] 16 - Valve outlet first tank

[0168] 17 - Rear overflow tank inlet valve

[0169] 18 - Valve pressure equalization rear overflow tank machine side

[0170] 19 - Front overflow tank inlet valve

[0171] 20 - Valve pressure equalization front overflow tank machine side

[0172] 21 - Rear overflow tank pressure equalization valve

[0173] 22 - upper powder sensor rear overflow tank

[0174] 23 - lower powder sensor rear overflow tank

[0175] 24 - Load cells rear overflow tank

[0176] 25 - Rear overflow tank outlet valve

[0177] 26 - Powder sensor inlet dosing screw rear overflow tank

[0178] 27 - Dosing screw rear overflow tank

[0179] 28 - Powder sensor outlet dosing screw rear overflow tank

[0180] 29 - Valve pressure equalization delivery line rear overflow tank

[0181] 30 - Valve pressure equalization front overflow tank

[0182] 31 - upper powder sensor front overflow tank

[0183] 32 - lower powder sensor front overflow tank

[0184] 33 - Load cells front overflow tank

[0185] 34 - Valve outlet front overflow tank

[0186] 35 - Powder sensor inlet dosing screw front overflow tank 36 - Dosing screw front overflow tank

[0187] 37 - Powder sensor outlet dosing screw front overflow tank

[0188] 38 - Valve pressure equalization delivery line front overflow tank

[0189] 39 - Compressed gas supply filter cleaning

[0190] 40 - Valve suction side filter

[0191] 41 - Filter

[0192] 42 - Cyclone

[0193] 43 - Valve pressure equalization lock

[0194] 44 - Valve inlet lock

[0195] 45 - upper powder sensor lock

[0196] 46 - lower powder sensor lock

[0197] 47 - Load cells lock

[0198] 48 - Valve outlet lock

[0199] 49 - Dosing screw sieve

[0200] 50 - Ultrasonic sieve

[0201] 51 - Main accumulator pressure equalization valve

[0202] 52 - Main reservoir inlet valve

[0203] 53 - upper powder sensor main memory

[0204] 54 - lower powder sensor main memory

[0205] 55 - Load cells main memory

[0206] 56 - Valve outlet main reservoir

[0207] 57 - Powder sensor inlet dosing screw main storage

[0208] 58 - Dosing screw main storage

[0209] 59 - Powder sensor outlet dosing screw main storage

[0210] 60 - Valve powder outlet “a”

[0211] 61 - Valve powder outlet “b”

[0212] 62 - Valve pressure equalization external tank module

[0213] 63 - Valve inlet external tank module

[0214] 64 - upper powder sensor external tank module

[0215] 65 - lower powder sensor external tank module

[0216] 66 - Load cells external tank module

[0217] 67 - Valve outlet external tank module

[0218] 68 - Valve inlet dosing screw external tank module

[0219] 69 - Powder sensor inlet dosing screw external tank module

[0220] 70 - Dosing screw external tank module

[0221] 71 - Powder sensor outlet dosing screw external tank module

[0222] 72 - Gas supply valve "a"

[0223] 73 - Gas supply valve "b" & "c" 74 - Pump protection filter

[0224] 75 - Dew point sensor

[0225] 76 - Oxygen sensor

[0226] 77 - redundant oxygen sensor

[0227] 78 - Pressure sensor pump suction side

[0228] 79 - Pump

[0229] 80 - Pressure sensor pump pressure side

[0230] 81 - Exhaust valve

[0231] 82 - Inert gas supply pressure compensation tank

[0232] 83 - Safety valve pressure equalization tank

[0233] 84 - Pressure equalization tank

[0234] 85 - Inert gas supply delivery line

[0235] 86 - Speed ​​& Temperature Sensor

[0236] 87 - Valve pressure equalization delivery line external tank module

[0237] 88 - Valve pressure equalization delivery line main storage

[0238] In connection with the arrangement of a bulk material conveying system described above and illustrated in Fig. 4, 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 tank 4.

[0239] The individual conveying processes and conveying circuits are explained below. They are illustrated in Figures 5 to 7. Figure 5 shows conveying process a, Figure 6 shows conveying process b, and Figure 7 shows conveying process c. Active elements are shown in bold so that the powder or gas flow can be understood.

[0240] Conveying process a (Fig. 5):

[0241] 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. 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.

[0242] Conveying process b (Fig. 6):

[0243] 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 the overflow tanks 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 conveyor screws 36 and 27, where it is separated in cyclone 42. The conveying gas flows back to pump 79 via filter 41.

[0244] Conveying process c (Fig. 7):

[0245] 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 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.

[0246] During operation (ie, conveying operation) of each of the conveying processes a to c described above, 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.

[0247] 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.

[0248] In addition, a limit control according to Fig. 3 can be implemented in connection with each of the conveying processes a to c (see the description above).

[0249] 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.

Claims

Patent claims 1. 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 comprising: a conveying line configured to convey a gas flow and, at least in sections, a bulk material flow driven by the gas flow; a dosing device configured to supply the gas flow with a predetermined dose of bulk material per unit of time, the dose being determined by a control value applied to the dosing device; a conveying device configured to convey the gas flow through the conveying line; at least one measuring device for measuring at least one characteristic of the gas flow; and a control device configured to: Determining a gas density of the gas flow based on the measured at least one characteristic of the gas flow; Determining a bulk material mass flow of the bulk material flow based on the control value applied to the dosing device; 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.

2. Bulk material conveying system according to claim 1, 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.

3. Bulk material conveying system according to claim 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 na 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.

4. Bulk material conveying system according to one of claims 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.

5. Bulk material conveying system according to one of claims 1 to 4, wherein the determination of the target speed v S0n of the gas flow comprises calculating a saltation velocity (Saltation Velocity) based on the determined gas density and the determined bulk material mass flow, and wherein the determination of the desired velocity Vsoii is carried out using the following formula: desired ^saltation "F ^safety where Vsaitation is the calculated saltation velocity of the conveyed bulk material and vsafety is a predetermined safety velocity.

6. Bulk material conveying system according to claim 5, wherein the saltation rate 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 specific 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.

7. Bulk material conveying system according to one of claims 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.

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

9. Bulk material conveying system according to claim 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.

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: Conveying speed, pump outlet pressure, pump power and dose of bulk material per time.

11. Bulk material conveying system according to one of claims 1 to 10, wherein the control device is arranged to: 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.

12. Bulk material conveying system according to one of claims 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 claims 1 to 12, comprising at least a first conveying circuit for conveying raw material powder from a main reservoir into a first tank of the plant 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 plant, wherein a production process in a process chamber of the plant 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.

14. Bulk material conveying system according to one of claims 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.

15. Bulk material conveying system according to one of claims 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.

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

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: Conveying a gas stream and a bulk material stream driven by the gas stream through a conveying line; 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; Conveying the gas flow through the conveying line using a conveying device; Measuring at least one characteristic of the gas flow using a measuring device; Determining a gas density of the gas flow based on the measured at least one characteristic of the gas flow; Determining a bulk material mass flow of the bulk material flow based on the control value applied to the dosing device; 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.

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

19. The method according to claim 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 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.

20. Method according to one of claims 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.

21. Method according to one of claims 17 to 20, wherein the determination of the target speed v S0 n of the gas flow comprises calculating a saltation velocity (Saltation Velocity) based on the determined gas density and the determined bulk material mass flow, and wherein determining the target velocity v S0n 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.

22. The method of claim 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 3 and a and b are parameters depending on a particle diameter d of the bulk material.

23. Method according to one of claims 17 to 22, wherein the control of the conveying device for conveying the gas flow 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.

24. The method according to any one of claims 17 to 23, further comprising: 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.

25. The method of claim 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.

26. The method according to claim 24 or 25, wherein the at least one characteristic comprises at least one of the following characteristics: Conveying speed, pump outlet pressure, pump power and dose of bulk material per time.

27. The method according to any one of claims 17 to 26, further comprising: 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.

28. A method according to any one of claims 17 to 27, comprising: 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. A method according to any one of claims 17 to 28, comprising: 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.

30. A method according to any one of claims 17 to 29, comprising: 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.

31. A method according to any one of claims 17 to 30, wherein the dosing device is located at the outlet of a respective source tank and wherein a cyclone is located at the inlet of a respective target tank for separating the raw material powder from the gas stream and for feeding the raw material powder into the target tank.

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