Method for controlling the operation of a sifting device, sifting device, powder preparation system and plant for producing three-dimensional workpieces

Abstract

A method for controlling the operation of a sifting device comprises the step (i) of supplying powder (56) to be sifted to a sifter through a powder inlet (22) and the step (ii) of driving the sifter for a first period of time at a first driving power, the first driving power being dimensioned in such a way that, during continuous driving of the sifter at the first driving power, the powder (56) to be sifted flows over an entire sifting surface of the sifter and / or into an oversized particle outlet (32). After the first period of time has expired, the sifter is driven in a step (iii) for a second period of time at a second driving power which is lower than the first driving power. After the second period of time has expired, steps (ii) and (iii) are repeated.

Description

[0001] Nikon SLM Solutions AG - 1 -

[0002] Method for controlling the operation of a screening device, screening device, powder processing system and plant for producing three-dimensional workpieces

[0003] The invention relates to a method for controlling the operation of a screening device and to a screening device suitable, for example, 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 invention relates to a system equipped with such a screening device for producing three-dimensional workpieces using a generative layer-by-layer manufacturing process.

[0004] 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.

[0005] 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.

[0006] 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.

[0007] 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.

[0008] 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.

[0009] 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.The invention 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 invention 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.

[0010] This object is achieved by a method for controlling the operation of a screening device having the features of claim 1, a screening device having the features of claim 13 and a system for producing three-dimensional workpieces using a generative layer construction process having the features of claim 21.

[0011] 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.

[0012] 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 screening powder throughput can be increased by a factor of approximately 4 by equipping the screening device with an ultrasonic drive device compared to a mechanically driven screening device.

[0013] 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.

[0014] 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°.

[0015] 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°.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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, moisture content of the powder, 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. In principle, the sieve surface of the sieve can be aligned coplanar to a horizontal plane. Alternatively, 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 coplanar with a horizontal plane, the powder fed through the powder inlet onto the sieve preferably forms a symmetrical cone of repose with a constant angle of repose along a circumference of the cone of repose. If, however, 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 cone of repose whose angle of repose varies along its circumference depending on the direction of inclination and the angle of inclination of the sieve surface.

[0026] 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.

[0027] 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°.

[0028] Similar to the initial drive power, the initial time interval can also assume 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 initial time interval is preferably a value empirically determined for the powder to be screened before a screening process. However, the initial time interval can also be taken from a previously created table of values ​​containing various initial time interval values ​​for different powder types and process parameters, or it can be selected based on empirical values.

[0029] When determining the first time interval, the first time interval preferably ends 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 by an oversize grain sensor. The oversize grain sensor is preferably located in the area of ​​the oversize grain outlet.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] In an independently claimable method for controlling the operation of a screening device, powder to be screened is at least temporarily continuously fed with a dosing 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 m ax) 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(am 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] Additionally or alternatively, sieve cleaning can be initiated if the powder to be sieved forms a cone of material 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 threshold value. In other words, if a cone of material 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 threshold value, this can be interpreted as an indication that the sieve mesh is so clogged that sieve cleaning is necessary.

[0040] Additionally or alternatively, sieve cleaning can also be initiated after the end of each sieving process. A sieving process can be terminated, for example, when a powder feed container for the powder to be sieved, which is connected to the powder inlet of the sieving device, is empty.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] The screening process can be stopped when m is fed =moversize + msieved, where msupplied is the mass of the powder fed in, moversize is 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 movergrain flowing into the overgrain container via the overgrain outlet can be determined by means of a second measuring device provided in an overgrain container connectable to the overgrain 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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

[0049] 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.

[0050] Accordingly, the oversize grain rate Qovercomm can also be determined continuously.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] The angle of inclination can be changed during the first and / or second time interval.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] 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 (Hi) 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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 sieve. In particular, the control unit can be configured to control the drive device and the powder inlet such that, initially, when the sieve is not driven, powder to be screened is fed through the powder inlet onto the sieve until the powder to be screened has formed a cone of material with a defined size in the region of the powder inlet on the sieve surface of the sieve. After the powder feed has ended, the sieve is driven and the powder fed onto the sieve surface of the sieve is sieved. Furthermore, the control unit can be configured to control the drive device and the powder inlet such that the sieving process is terminated when =moversize + msieved, where msupplied is the mass of the powder fed in, moversize is the mass of the oversize grain flowing into the oversize grain outlet and m ge sieved is the mass of the sieved powder.

[0076] The screening device may comprise a first measuring device for determining m ZU which is provided in a powder feed container connectable to the powder inlet of the screening device, a second measuring device for determining m oversize grain, which is provided in an oversize grain container connectable 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.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] The screening device may comprise a screen holder for receiving, in particular for

[0089] Inserting the sieve. The tilting device can be attached to the sieve holder and configured to rotate the sieve holder.

[0090] 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.

[0091] The control unit may be configured to change the tilt angle during the first and / or during the second time interval.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] 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.

[0097] 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 stepwise downwards in the vertical direction as the height of a workpiece mounted on the carrier increases. 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 pm.

[0098] 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.

[0099] Preferred embodiments of the invention will now be explained in more detail with reference to the accompanying schematic drawings, of which

[0100] Figure 1 shows a plant for producing three-dimensional workpieces by exposing layers of raw material powder to electromagnetic radiation or particle radiation, which plant is equipped with a powder processing system comprising a screening device;

[0101] Figure 2 shows a detailed view of the screening device used in the powder processing system according to Figure 1;

[0102] Figure 3 shows a loose powder bed on one level;

[0103] Figure 4 shows the screening device according to Figure 2 with continuous driving of the screen with a low (second) drive power;

[0104] Figure 5 shows the screening device according to Figure 2 with a continuous drive of the screen with a high (first) drive power;

[0105] Figure 6 shows the screening device according to Figure 2 in operation, wherein the screening device is driven periodically alternately with the first drive power and the second drive power; Figure 7 illustrates the development of the drive power (top) and the metered mass flow (bottom) as a function of time when the metered mass flow is controlled as a function of the drive power;

[0106] Figure 8 shows 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 event of a defect in a screen mesh; and

[0107] Figure 9 shows 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.

[0108] A system 100 shown in Figure 1 for producing three-dimensional workpieces by exposing layers of raw material powder to electromagnetic radiation or particle radiation comprises a process chamber 102 that is sealed from the ambient atmosphere. A powder application device 104 arranged in the process chamber 102 serves to apply layers of raw material powder to a carrier 106. Excess powder that accrues during the application of the individual powder layers to the carrier 106 is collected in a collecting container 107. The carrier 106 is displaceable in the vertical direction, so that the carrier 106 can be moved step by step in the vertical direction downwards into a build chamber 109 as the height of a workpiece 108 built on the carrier 106 increases.

[0109] The process chamber 102 is provided with a gas inlet 110 for supplying an inert gas (e.g., argon) into the process chamber 102. Furthermore, a gas outlet 112 is provided so that a continuous gas flow can be generated through the process chamber 102. The gas flow can serve to remove melt splashes and / or other unwanted contaminants, such as welding fumes, from the process chamber 102.

[0110] The system 100 further comprises an irradiation device 112, which serves to selectively direct electromagnetic radiation or particle radiation onto the powder bed applied to the carrier 106. The exemplary system 100 shown in Figure 1 comprises only one irradiation device 112. However, the system 100 can also have a plurality of irradiation devices 112. The irradiation device 112 comprises a radiation source 114, which here is particularly designed in the form of a laser source. The radiation source 114, which can comprise, for example, 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 112.However, in the system 100 shown in Figure 1, the radiation source 114 is arranged outside the irradiation device 112, wherein a laser beam 116 emitted by the radiation source 114 is guided into the irradiation device 112 via an optical fiber 118.

[0111] The irradiation unit 112 further comprises two lenses 120 and 122. In the embodiment of an irradiation unit 112 shown in Figure 1, both lenses 120 and 122 have a positive refractive power. The lens 120 collimates the laser light emitted by the optical fiber 118 to generate a collimated or substantially collimated laser beam 116. The lens 122, on the other hand, is configured to focus the collimated (or substantially collimated) laser beam 116 to a desired z-position along a z-axis.

[0112] Finally, the irradiation unit 112 comprises a scanner system with a scanner mirror 124 that can be pivoted about a pivot axis S. During operation of the system 100, the scanner system and in particular the scanner mirror 124 serve to deflect the laser beam 116 emitted by the radiation source 114 such that the beam 116 strikes the raw material powder layer applied to the carrier 106 at a desired position.

[0113] The system 100 further comprises an unpacking station 126. A build chamber 109 with a workpiece 108 arranged therein is transferred to the unpacking station 126 once the construction of the workpiece 108 is complete. The irradiation device 112 and the process chamber 102 can then be used for the construction of a new workpiece without further delay. In the unpacking station 126, the workpiece 108 can be cooled if necessary and is then unpacked, i.e., removed from the build chamber 109. This may also produce a large amount of unsolidified powder in which the workpiece 108 is embedded before unpacking.

[0114] Both the powder collected in the collection container 107 and the powder recovered in the unpacking station 126 may contain particulate contaminants as well as stuck or sintered powder agglomerates. These contaminants could lead to contamination of the powder bed and consequently to a reduced quality of the workpiece 108 if the powder is reused in an additive manufacturing process in the system 100. The system 100 therefore comprises a powder preparation system 128, which is connected to the collection container 107 and the unpacking station 126 via a powder line 130. A blower, a conveyor belt, or another suitable conveying device (not shown in Figure 1), for example, can be used to convey powder from the collection container 107 and the unpacking station 126 into the powder preparation system 128.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.

[0115] The powder processing system 128 comprises a powder feed container 132 connected to the powder line 130. The powder feed container 132 thus serves to receive the powder to be processed from the collection container 107 and the unpacking station 126. Furthermore, the powder processing system 128 comprises a sieve device 10 with a sieve formed by a sieve frame 14 and a sieve mesh 16 stretched over the sieve frame. Sieved powder is collected in a sieve container 18 after passing through the sieve mesh 16.

[0116] A lid 20 is placed on the sieve frame 14 so that the sieve container 18, like the other components of the powder preparation system 128, is sealed from the ambient atmosphere and can be flooded with an inert gas during operation of the sieve device 10. Argon, for example, can be used as an inert protective gas, which prevents undesirable oxidation of the powder 56 to be sieved in the sieve device 10. The inert gas pressure in the sieve device 10 is continuously monitored by a pressure sensor (not illustrated in the figures). If the inert gas pressure in the sieve device 10 falls below a limit value, additional protective gas is supplied to the sieve device 10 under the control of the control unit 40.

[0117] A detailed view of the sieving device 10 is shown in Figure 2. A powder inlet 22 of the sieving device 10 comprises a metering device 24 and a valve 26, so that a controlled supply of powder from the powder feed container 132 to the sieve is possible via the powder inlet 22. A metering sensor 23 is provided in the region of the powder inlet 22, the function of which will be explained in more detail below. The sieve container 18 has a downwardly tapered cross-section. Sieved powder received in the sieve container 18 can therefore be discharged from the sieve container by gravity via a sieved powder outlet 28 arranged in the region of a lower section of the sieve container 18.The sieved powder outlet 28 is connected to a sieved powder container 134 of the powder preparation system 128 and comprises a valve 30 so that a controlled discharge of sieved powder from the sieve container 18 into a sieved powder container 134 of the powder preparation system 128 is possible via the sieved powder outlet 28.

[0118] The screening device 10 further comprises an oversize grain outlet 32, to which a valve 34 is assigned. An oversize grain sensor 33 is provided in the region of the oversize grain outlet 32, which can detect powder particles flowing into the oversize grain outlet 32. Oversize grain that is too coarse to pass through the screen mesh 16 toward the screen container 18 can be removed from the screening device 10 via the oversize grain outlet 24 in a controlled manner and fed to an oversize grain container 136 of the powder processing system 128. The powder inlet 22 and the oversize grain outlet 32 ​​are arranged in the region of opposite side edges of the screening surface defined by the screen mesh 16. Furthermore, the sieve mesh 16 and thus the sieve surface of the sieve defined by the sieve mesh 16 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 32 ​​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 22 toward the oversize grain outlet 32, thereby promoting the spreading of powder supplied via the powder inlet 22 across the sieve surface and the removal of oversize grain into the oversize grain outlet 32. Just as the removal of sieved powder is also carried out by gravity, the removal of oversize grain from the sieve device 10 is also carried out by gravity.

[0119] The screening device 10 is further equipped with a drive device 36 for driving the screen. The drive device 36 engages the screen frame and, during operation of the screening device 10, causes the screen frame 14 and thus the screen mesh 16 to vibrate. The preferred embodiment of a screening device 10 shown here is equipped with a drive device 36 in the form of an ultrasonic drive device, which is designed to subject the screen to ultrasonic vibrations. Furthermore, a vibrator 38 is provided, which also engages the screen frame 14 and serves to vibrate the screen frame 14 and thus the screen mesh 16 for the purpose of cleaning the screen.

[0120] The operation of the screening device 10 is controlled by a control unit 40. The control unit 40 can be a control unit that is exclusively assigned to the screening device 10. Alternatively, however, it is also conceivable for the control unit 40 to be integrated into a higher-level control unit, for example, a control unit for controlling the powder processing system 128 and / or a control unit for controlling the system 100 for producing a three-dimensional workpiece.

[0121] Finally, the screening device 10 comprises a first measuring device 42, a second measuring device 40, and a third measuring device 46. The first measuring device 42, which here is designed in the form of one or more weighing cells, is arranged in the powder feed container 132 of the powder processing system 28 and serves to record the mass of the powder fed to the powder inlet 22 of the screening device 10 from the powder feed container 32. The second measuring device 44, which here is also designed in the form of one or more weighing cells, is arranged in the oversize container 136 of the powder processing system 28 and serves to record the mass of the oversize material that flowed into the oversize container 136 via the oversize outlet 32 ​​of the screening device 10.The third measuring device 46, which is again designed in the form of one or more weighing cells, is finally arranged in the sieved powder container 134 of the powder preparation system 28 and serves to determine the mass of the sieved powder that flows from the sieved powder outlet 28 of the sieving device 10 into the sieved powder container 134.

[0122] As can be seen in Figure 3, in a loose bed of a powder 56 on a plane, an angle of repose aa is formed, which angle 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.

[0123] When powder 56 to be screened is fed through the powder inlet 22 onto the screen of the screening device 10, a repose cone forms on the screen, i.e., on the screen mesh 16. 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 36 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.

[0124] When the sieve, as shown in Figure 4, is driven with a low drive power of the drive device 36, the cone of material forming on the sieve below the powder inlet 22 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 10, the sieve surface utilization is correspondingly low, since only a small section a of the sieve surface adjacent to the powder inlet 22 is actually impinged upon by powder 56. In contrast, a section b of the sieve surface that is not impinged upon by powder and consequently defines a "safety distance" between the section a of the sieve surface impinged upon by powder and the oversize grain outlet 32 ​​is comparatively large.

[0125] The inclination of the sieve surface in the direction of the oversize grain outlet 32 ​​means that the repose cone is no longer symmetrically shaped, as in Figure 3, but is adapted to the orientation of the sieve surface and has a variable angle of repose, which is smaller in a circumferential section of the repose cone facing the oversize grain outlet 32 ​​than in a circumferential section of the repose cone facing away from the oversize grain outlet 32. However, an angle of inclination aa-w of the sieve surface of the sieve relative to the horizontal plane E (see Figure 2) is smaller than the angle of repose aa of the repose cone formed by the powder 56 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 22, a stable repose cone is still formed and the powder 56 does not flow uncontrollably over the sieve surface.

[0126] If, however, the sieve is driven with a high drive power, as shown in Figure 5, the powder 56 spreads across the sieve surface, i.e., the angle of repose aar of the repose cone, which the powder 56 fed onto the sieve through the powder inlet forms on the sieve surface, decreases with increasing drive power of the drive device 36 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 56 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 32.This results in powder 56, as illustrated in Figure 5, flowing unscreened into the oversize grain outlet 32 ​​during continuous sieving at a high drive power. As a result, powder 56, which is actually fine-grained enough to pass through the sieve mesh 16, is lost unused.

[0127] In a method for controlling the operation of the screening device 10, illustrated in more detail in Figure 6, the powder 56 to be screened is therefore first fed onto the screen through the powder inlet 22 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 6 top and middle). The first drive power is dimensioned such that, if the screen were continuously driven with the first drive power, as shown in Figure 5, the powder 56 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 56 would be lost unscreened through the oversize grain outlet 32. Therefore, driving the screen with the first drive power is limited in time.

[0128] 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 which is lower than the first drive power (see Figure 6 below). The second drive power is in particular dimensioned such that, when the sieve is continuously driven with the second drive power, as shown in Figure 4, the powder 56 to be sieved forms a repose cone on the sieve surface of the sieve in the region of the powder inlet 22, 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 22. If the sieve were continuously driven with the second drive power, it would be ensured that no or almost no powder 56 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.

[0129] 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 occur in the non-driven state, with powder trickling through the sieve due to gravity.

[0130] 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 50 contained in the powder 56, are distributed over the sieve surface (see Figure 6, top), so that the sieving process can then proceed with a high sieve surface utilization, with the oversize grain 50 being transported by gravity toward the oversize grain outlet 32 ​​(see Figure 6, center). In contrast, during the second time interval, the sieve surface utilization is low. Furthermore, the oversize grain 50 "accumulates" in columnar fashion inside the cone of material.As a result, powder particles that are pressed into the sieve mesh 16 by back pressure and vibration of the sieve can form stuck grains and clog the sieve mesh.

[0131] 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 50 toward the oversize grain outlet 32 ​​can be ensured. However, there always remains a section b of the sieve surface not loaded with powder, and consequently a "safety distance" between the section A of the sieve surface loaded with powder and the oversize grain outlet 32. Furthermore, the sieve mesh 16 is loaded with a smaller powder mass overall and is therefore less stressed.

[0132] 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.

[0133] When determining the first time interval, the first time interval ends at the latest when powder 56 to be screened flows into the oversize grain outlet 32, i.e., the first time interval is selected such that, while the screen is being driven at the first drive power within the first time interval, no loss of powder to be screened occurs. The flow of powder to be screened into the oversize grain outlet 32 ​​is detected by the oversize grain sensor 33. In particular, the first time interval is dimensioned such that, by the end of the first time interval, 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. The first time interval therefore provides a "time safety reserve" so that the powder does not spread across the entire screen area during the first time interval.

[0134] When determining the second time interval, the second time interval is terminated at the latest when powder 56 to be screened forms a repose cone of a defined size on the sieve surface of the sieve in the area of ​​the powder inlet 22. The formation of a repose cone of a defined size is detected by the dosing sensor 23, which is triggered when a tip of the repose cone protrudes into a detection range of the dosing sensor 23. Limiting the second time interval in this way prevents the repose cone from becoming too large during the second time interval and clogging the powder inlet 22.

[0135] During operation of the screening device 10, the powder 56 to be screened is fed, at least temporarily, continuously through the powder inlet 22 onto the screen. A metered mass flow is set by a corresponding control of the metering device 24 and / or a corresponding control of the valve 26 by the control unit 40. 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.

[0136] 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 56 to be sieved through the powder inlet 22 when driving the sieve with the second drive power until the powder 56 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 22 and consequently the metering sensor 23 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 56 to be screened through the powder inlet 22 when driving the screen with the first drive power until the powder 56 to be screened has formed a cone of material with a defined size in the area of ​​the powder inlet 22 on the screen surface of the screen and powder 56 to be screened beyond this flows into the oversize outlet. When determining the screening throughput when driving the screen with the first drive power rh(a m ax), the dosing mass flow of the powder 56 to be screened through the powder inlet 22 is consequently increased until the dosing sensor 23 and the oversize particle sensor 33 are triggered.

[0137] 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.

[0138] If the sieve mesh 16 becomes clogged during the sieving process, for example due to stuck grains or cold welds, the sieve throughput decreases. Therefore, the continuous dosing mass flow rhdos is reduced if, during the sieving process, the powder 56 to be sieved forms a repose cone of a defined size in the area of ​​the powder inlet 22 on the sieve surface of the sieve, and the dosing sensor 23 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 56 to be sieved nevertheless forms a repose cone of a defined size in the area of ​​the powder inlet 22 on the sieve surface of the sieve, and the dosing sensor 23 is consequently triggered, the control unit 40 interprets this as an indication that the sieve mesh 16 is clogged and initiates sieve cleaning.When the control unit 40 has detected that sieve cleaning is to be initiated, the powder feed through the powder inlet 22 is first stopped. Powder 56 still present in the sieve is sieved before sieve cleaning begins. After sieve cleaning has started, the sieve is driven at maximum drive power. Additionally or alternatively, after sieve cleaning has started, the vibrator 38 can be activated, wherein an angle of attack of the vibrator 38 on the sieve, a drive amplitude of the vibrator 38, and a drive frequency of the vibrator 38 can be variably adjusted. In addition or alternatively to sieve cleaning initiated as a result of a blockage of the sieve mesh 16, sieve cleaning can also be initiated after the end of each sieving process, when the powder feed container 132 is empty.

[0139] Alternatively or in addition to the continuous powder dosing described above, the powder 56 to be screened can also be fed discontinuously, at least temporarily, through the powder inlet 22 onto the screen. With discontinuous powder feeding, the powder 56 to be screened is first fed through the powder inlet 22 onto the screen while the screen is not driven, until the powder 56 to be screened has formed a cone of material of a defined size on the screen surface in the region of the powder inlet 22 and the dosing sensor 23 is triggered. The powder feed is then stopped, and the screen is driven, so that the powder 56 fed onto the screen surface is screened.

[0140] Alternatively or in addition to the continuous or discontinuous powder metering described above, the powder 56 to be screened can also be fed, at least temporarily, through the powder inlet 22 onto 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 22 onto 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 22 onto the screen during the second time interval.

[0141] In the diagrams of Figure 7, 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 56 is fed through the powder inlet 22 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.

[0142] The "sinusoidal" profile of the dosing mass flow shown in the lower diagram of Figure 7 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 7, are each of equal length, an average dosing capacity of rhdosi / 2 is achieved.

[0143] 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 32 ​​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 42, which detects the powder outflow from the powder feed container 132. The mass of the powder movergrain flowing through the oversize grain outlet into the oversize grain container 136 is determined by means of the second measuring device 44, which detects the inflow of oversize grain into the oversize grain container 38. Finally, the mass of the sieved powder msieved is determined by means of the third measuring device 46, which detects the inflow of sieved powder into the sieved powder container 134.

[0144] The oversize grain rate is a parameter that indicates the ratio between the mass of oversize grain flowing into the oversize grain outlet 32 ​​in a defined time unit and the mass of the total powder processed in the screening device 10 in the defined time unit. In particular, the oversize grain rate Qovercom can be determined according to

[0145] Quüberkorn — 1 —Calculate [rh sieved / frh sieved + m oversize)]* 100%, where rh sieved is the mass flow of the sieved powder that flows during the sieving process via the sieved powder outlet 28 of the sieving device 10 into the sieved powder container 134 and rh oversize is the mass flow of the oversize that flows during the sieving process via the oversize outlet 32 ​​of the sieving device 10 into the oversize container 136.

[0146] In the use of the screening device 10 described here for preparing raw material powder intended for processing in the system 100 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 100. 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 112, which can remain in the powder bed and thus impair the quality of the workpiece 108 to be produced. The parameters rhgesiebt and rhüberkom are therefore continuously monitored during the screening process by means of the second and third measuring devices 44, 46. From these values, the control unit 40 continuously determines the oversize grain rate Qüberkom.Furthermore, under the control of the control unit 40, a warning is issued if the oversize grain rate exceeds a limit value so that the process parameters of the system 100 can be checked if necessary.

[0147] 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 42, the oversize mass flow rhüberkom is continuously recorded by the second measuring device 44, and the sieved powder mass flow rhgesiebt is continuously recorded by the third measuring device 46. In the diagrams shown in Figure 8, 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.

[0148] The "step response" of the sum of the sieved powder mass flow rhgesiebt and the oversize mass flow rhüberkom to the metered mass flow rhdos is the time difference Δt between a time t1 at which a defined metered mass flow rhdos has been supplied to the sieving device 10 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 16. The step response is therefore a time parameter that specifies the duration of the sieving process for a specific powder mass flow.

[0149] If the step response shortens from the value Atl illustrated in the upper diagram in Figure 8 to the value At2 illustrated in the lower diagram in Figure 8, ie the curves in the diagram move closer together, this can be interpreted as an indicator of a defect, for example a tear in the screen mesh 16, if the step response falls below a first limit value. Therefore, a warning is output under the control unit 40 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.

[0150] If, on the other hand, the step response becomes longer, this indicates a lengthening of the sieving process and can be interpreted as an indicator that the sieve mesh 16 has become clogged. Therefore, under the control of the control unit 40, a sieve cleaning process is initiated 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.

[0151] Figure 9 shows a schematic side view of a screening device 10, which can be considered an alternative embodiment or a further development of the screening device 10 of Figure 2. The screening device 10 can be used in conjunction with all of the above-described embodiments of a screening device 10, a powder processing system 128, and / or a system 100 for producing three-dimensional workpieces. The elements and / or functions of the screening device 10 not described below correspond to those of the screening device 10 described above, in particular the screening device 10 of Figure 2. Thus, some of the elements of the screening device 10 of Figure 9 are not shown or are only indicated schematically, since they correspond to the elements of the screening device 10 of Figure 2 already explained in detail above.

[0152] In the upper section (a) of Figure 9, the screening device 10 is shown in a state in which the screen (consisting of screen frame 14 and screen mesh 16) is not yet installed in a housing 60 of the screening device 10. The lower section (b) of Figure 9 shows the screening device in the installed state of the screen 14, 16.

[0153] The screening device 10 of Figure 9 comprises a housing 60, which—similar to the lid 20—is suitable for hermetically sealing the screening device 10. Thus, a screening process can be carried out in a closed inert gas atmosphere. A powder inlet 22 is located in an upper side of the housing 60. An oversize grain outlet 32 ​​and a merely indicated screening container 18 are provided on an underside of the housing. The above-mentioned elements 22, 32, and 18 are attached to the housing 60 and are thus secured independently of any change in the angle of inclination aa-w (see below).

[0154] The sieve, consisting of sieve frame 14 and sieve mesh 16, can be inserted into the housing from the side via a flap 62. The flap 62 can be closed and, when closed, forms a gas-tight seal against the housing 60. Furthermore, a sieve receptacle 66 is provided, into which the sieve 14, 16 can be inserted and secured if necessary. This allows the sieve 14, 16 to be easily removed and reinserted, or replaced if necessary.

[0155] An inclination angle aa-w of the sieve 14, 16 (more precisely, of the sieve mesh 16) relative to the horizontal plane E is adjustable. For this purpose, an inclination device 64 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 9, the inclination device 64 is attached to the sieve holder 66 and configured to incline it relative to the housing 60. More precisely, according to the example shown, the inclination device is arranged in the center of the sieve 14, 16 and configured to rotate the sieve 14, 16 along a horizontally extending axis of rotation.

[0156] In particular, the tilt device 64 can be controlled by the control unit 40 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.

[0157] 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.

[0158] 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 16. 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 16 reaching the oversize outlet 32.

[0159] 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.

[0160] 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, for example, be attached to an inner side of the housing 60, in particular to an upper wall of the housing 60. The control unit can be configured such that the angle of inclination aa-w of the screen surface of the screen 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 propagation speed of the powder to be screened 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 propagation speed of the powder to be screened and / or the detected position of the powder front falls below a predetermined threshold.

[0161] 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.

Claims

Patent claims 1. A method for controlling the operation of a screening device (10), comprising the steps of: (i) feeding powder (56) to be screened onto a screen through a powder inlet (22); (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 (56) to be sieved flows over an entire sieve surface of the sieve and / or into an oversize grain outlet (32) when the sieve is continuously driven with the first drive power; (iii) after expiry 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 expiry of the second time interval, repeating steps (ii) and (iii).

2. Method for controlling a screening device (10) according to claim 1, wherein the second drive power is dimensioned such that the powder to be screened (56) 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 (22), 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.

3. A method for controlling the operation of a screening device (10) according to claim 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 (22) towards the oversize grain outlet (32) 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 less than an angle of repose (aa) of a repose cone formed by the powder (56) to be screened on a horizontal plane (E).

4. A method for controlling the operation of a screening device (10) according to any one of claims 1 to 3, wherein: - the first time interval is an empirically determined value for the powder to be sieved (56); and / or - when determining the first time interval, the first time interval is ended at the latest when powder (56) to be screened flows into the oversize grain outlet (32), wherein the flow of powder (56) to be screened into the oversize grain outlet (32) is detected in particular by means of an oversize grain sensor (33) provided in the region of the oversize grain outlet; and / or - the first time interval is dimensioned 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 - the second time interval is an empirically determined value for the powder (56) to be sieved; and / or - when determining the second time interval, the second time interval is ended at the latest when the powder (56) to be sieved forms a cone of material with a defined size in the region of the powder inlet (22) on the sieve surface of the sieve, wherein the formation of a cone of material with a defined size is detected in particular by means of a dosing sensor (23) provided in the region of the powder inlet (22).

5. Method for controlling the operation of a screening device (10) according to one of claims 1 to 4, wherein the powder (56) to be screened is at least temporarily continuously fed with a metered mass flow rhdos = 0.5 * (rh(amin) + rh(a m ax)) is fed through the powder inlet (22) 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.

6. A method for controlling the operation of a screening device (10) according to claim 5, wherein: - rh(amine) is determined by driving the sieve with the second Drive power of the metered mass flow of the powder to be screened (56) through the powder inlet (22) is increased until the powder to be screened (56) has formed a cone of material with a defined size in the region of the powder inlet (22) on the screen surface of the screen; and / or - rh(amax) is determined by increasing the metered mass flow of the powder (56) to be screened through the powder inlet (22) when driving the screen with the first drive power until the powder (56) to be screened has formed a cone of material with a defined size in the region of the powder inlet (22) on the screen surface of the screen and powder (56) to be screened flows into the oversize grain outlet (32), the value of rh(a m ax) is preferably multiplied by a safety factor of 0.8, 0.7, 0.6 or 0.

5.

7. A method for controlling the operation of a screening device (10) according to claim 5 or 6, wherein: - the metered mass flow (rhdos) is reduced when the powder (56) to be screened forms a cone of material with a defined size in the area of ​​the powder inlet (22) on the screen surface of the screen; and / or - sieve cleaning is initiated when the powder (56) to be sieved forms a cone of material with a defined size in the area of ​​the powder inlet (22) on the sieve surface of the sieve and / or the dosing mass flow rhdos falls below a limit value.

8. A method for controlling the operation of a screening device (10) according to claim 7, wherein upon initiation of a screening cleaning - the powder supply through the powder inlet (22) is stopped; and / or - any powder still present in the sieve is sieved before starting the sieve cleaning; and / or - after the start of the sieve cleaning, the sieve is driven with maximum drive power; and / or - after the start of the sieve cleaning, a vibrator (38) is activated which drives the sieve independently of a drive device (36) of the sieve device (10), wherein preferably an angle of attack of the vibrator (38) on the sieve, a drive amplitude of the vibrator (38) and / or a drive frequency of the vibrator (38) is variably adjustable.

9. Method for controlling the operation of a screening device (10) according to one of claims 1 to 8, wherein the powder (56) to be screened is fed at least temporarily discontinuously through the powder inlet (22) onto the screen, wherein in particular - initially, in a non-driven state of the sieve, powder (56) to be sieved is fed through the powder inlet (22) onto the sieve until the powder (56) to be sieved has formed a cone of material with a defined size in the region of the powder inlet (22) on the sieve surface of the sieve; - after the powder feed has ended, the sieve is driven and the powder fed onto the sieve surface is sieved; and - the screening process is stopped when m is fed = moversize + msieved, where msupplied is the mass of the powder supplied, moversize is the mass of the powder flowing into the oversize outlet (32) and m ge sieves is the mass of the sieved powder, and where m ZU guided is determined in particular by means of a first measuring device (42) which is provided in a powder feed container (132) connectable to the powder inlet (22) of the screening device (10), moversize is determined in particular by means of a second measuring device (44) which is provided in an oversize container (136) connectable to the oversize outlet (32) of the screening device (10), and / or m gesieved is determined in particular by means of a third measuring device (46) which is provided in a sieved powder container (134) which can be connected to a sieved powder outlet (28) of the sieving device (10).

10. A method for controlling the operation of a screening device (10) according to one of claims 1 to 9, wherein the powder (56) to be screened is fed at least temporarily through the powder inlet (22) onto the screen with a metered mass flow 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 (56) to be screened is fed through the powder inlet (22) 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 (22) onto the screen during the second time interval.

11. Method for controlling the operation of a screening device (10) according to a of claims 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.

12. A method for controlling the operation of a screening device (10) according to any one of claims 1 to 11, wherein: - the screening device (10) is sealed from the ambient atmosphere and is flooded with a protective gas during operation; and / or - additional protective gas is supplied to the screening device (10) when an inert gas pressure in the screening device (10) falls below a limit value.

13. A method for controlling the operation of a screening device (10) according to any one of claims 1 to 12, wherein: - 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 - 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 - 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.

14. A method for controlling the operation of a screening device (10) according to any one of claims 1 to 13, further comprising: - Changing an angle of inclination (aa-w) of the sieve surface of the sieve relative to a horizontal plane (E).

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

16. A method for controlling the operation of a screening device (10) according to claim 14 or 15, wherein a first angle of inclination (aa-w) 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.

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

18. A method for controlling the operation of a screening device (10) according to any one of claims 14 to 17, further comprising: - detecting a propagation speed of the powder to be screened (56) on the screen and / or a position of a powder front of the powder to be screened (56) 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.

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

20. Screening device (10) comprising: - a powder inlet (22); - a drive device (36) configured to drive the screen; and - a control unit (40) configured to control the powder inlet (22) and the drive device (36) such that: (i) powder (56) to be sieved is fed onto the sieve through the powder inlet (22); (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 (56) to be sieved flows over an entire sieve surface of the sieve and / or into an oversize grain outlet (32) when the sieve is continuously driven with the first drive power; (iii) after the first time interval has elapsed, the sieve is filled for a second time interval with a second drive power which is lower than the first drive power; (iv) after expiry of the second time interval, steps (ii) and (iii) are repeated.

21. Screening device (10) according to claim 20, wherein: - the second drive power is dimensioned such that the powder to be screened (56) 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 (22), 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 - 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 (22) towards the oversize grain outlet (32) 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 (56) to be sieved on a horizontal plane (E); and / or - the first time interval is an empirically determined value for the powder to be sieved (56); and / or - the control unit (40) is configured, when determining the first time interval, to end the first time interval at the latest when powder (56) to be screened flows into the oversize grain outlet (32), wherein the screening device (10) comprises, in particular, an oversize grain sensor () provided in the region of the oversize grain outlet for monitoring the flow of powder (56) to be screened into the oversize grain outlet (32); and / or - the control unit (40) 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 - the second time interval is an empirically determined value for the powder (56) to be sieved; and / or - the control unit (40) is configured, when determining the second time interval, to end the second time interval at the latest when the powder (56) to be screened forms a cone of material with a defined size in the region of the powder inlet (22) on the screen surface of the screen, wherein the screen device (10) in particular a dosing sensor (23) provided in the region of the powder inlet (22) for detecting the formation of a pouring cone with a defined size.

22. Sieving device (10) according to claim 20 or 21, wherein the control unit (40) is configured to control the powder inlet (22) such that the powder (56) to be sieved is at least temporarily continuously fed with a metering mass flow rhdos = 0.5 * (rh(amin) + rh(a m ax)) is fed through the powder inlet (22) onto the sieve, wherein a min 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 (40) is configured in particular: - to determine rh(amine) by increasing the metered mass flow of the powder (56) to be screened through the powder inlet (22) when driving the screen with the second drive power until the powder (56) to be screened has formed a cone of material with a defined size in the region of the powder inlet (22) on the screen surface of the screen; and / or - rh(amax) is determined by increasing the metered mass flow of the powder (56) to be screened through the powder inlet (22) when driving the screen with the first drive power until the powder (56) to be screened has formed a cone of material with a defined size in the region of the powder inlet (22) on the screen surface of the screen and powder (56) to be screened flows into the oversize grain outlet (32), 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 - to reduce the metered mass flow rhdos when the powder (56) to be screened forms a cone of material with a defined size in the area of ​​the powder inlet (22) on the screen surface of the screen; and / or - to initiate sieve cleaning when the powder (56) to be sieved forms a cone of material with a defined size in the area of ​​the powder inlet (22) on the sieve surface of the sieve and / or the dosing mass flow rhdos falls below a limit value.

23. Screening device (10) according to claim 22, wherein the control unit (40) is configured, upon initiation of a screen cleaning - to control the powder inlet (22) so that the powder supply through the powder inlet (22) is stopped; and / or - to control the drive device (36) in such a way that any powder still present in the sieve is sieved before the start of the sieve cleaning; and / or - to control the drive device (36) so that after the start of the sieve cleaning, the sieve is driven with a maximum drive power; and / or - after the start of the sieve cleaning, to activate a vibrator (38) which is configured to drive the sieve independently of the drive device (38) of the sieve device (10), wherein preferably an angle of attack of the vibrator (38) on the sieve, a drive amplitude of the vibrator (38) and / or a drive frequency of the vibrator (38) is variably adjustable.

24. Sieving device (10) according to one of claims 20 to 23, wherein the control unit (40) is configured to control the powder inlet (22) such that the powder (56) to be sieved is fed at least temporarily discontinuously through the powder inlet (22) onto the sieve, wherein the control unit (40) is in particular configured to control the drive device (36) and the powder inlet (22) such that: - initially, in a non-driven state of the sieve, powder (56) to be sieved is fed through the powder inlet (22) onto the sieve until the powder (56) to be sieved has formed a cone of material with a defined size in the region of the powder inlet (22) on the sieve surface of the sieve; - after the powder feed has ended, the sieve is driven and the powder fed onto the sieve surface is sieved; and - the screening process is stopped when m is fed = moversize + msieved, where msupplied is the mass of the powder supplied, moversize is the mass of the powder flowing into the oversize outlet (32) and m ge sieves the mass of the sieved powder, and wherein the sieving device (10) in particular comprises a first measuring device (42) for determining m ZUwhich is provided in a powder feed container (132) connectable to the powder inlet (22) of the screening device (10), in particular a second measuring device (44) for determining oversize particles, which is provided in an oversize particle container (136) connectable to the oversize particle outlet (32) of the screening device (10), and / or and / or in particular a third Measuring device (46) for determining m ge sieved, which is provided in a sieved powder container (134) connectable to a sieved powder outlet (28) of the sieving device (10).

25. Sieving device (10) according to one of claims 20 to 24, wherein the control unit (40) is configured to control the powder inlet (22) such that the powder (56) to be sieved is fed at least temporarily through the powder inlet (22) 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 (56) to be sieved is fed through the powder inlet (22) onto the sieve during the first time interval is greater than a second metered mass flow with which the powder (56) to be sieved is fed through the powder inlet (22) onto the sieve during the second time interval.

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

27. Screening device (10) according to one of claims 20 to 26, wherein: - the screening device (40) is sealed from the ambient atmosphere and is flooded with a protective gas during operation; and / or - the control unit (40) is configured to supply additional protective gas into the screening device (10) when an inert gas pressure in the screening device (10) falls below a limit value.

28. Screening device (10) according to one of claims 20 to 27, wherein the control unit (40) is configured: - 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 - 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 - to initiate a screen cleaning if 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 a second limit value.

29. Screening device (10) according to one of claims 20 to 28, comprising: - a lid (20) detachable from a sieve container (18); - a seal (48) arranged in the cover (20); and - a clamping device (50) configured to exert a clamping force on the seal (20) that holds the seal (48) in position in the lid (20).

30. Screening device (10) according to one of claims 20 to 29, comprising: - an inclination device (64) for changing an inclination angle (aa-w) of the sieve surface of the sieve relative to a horizontal plane (E).

31. Screening device (10) according to claim 30, comprising: - a housing (60) which can be closed in a gas-tight manner, wherein the sieve is arranged within the housing (60) and wherein the inclination device (64) is designed to rotate the sieve relative to the housing (60) and thereby change the angle of inclination (aa-w).

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

33. Screening device according to one of claims 30 to 32, comprising: - a sieve holder (66) for receiving, in particular for inserting, the sieve, wherein the inclination device (64) is fastened to the sieve holder (66) and is designed to rotate the sieve holder (66).

34. Screening device (10) according to one of claims 30 to 33, wherein the control unit (40) 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.

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

36. Screening device (10) according to one of claims 30 to 35, further comprising: - at least one sensor for detecting a spreading speed of the powder to be screened (56) on the screen and / or a position of a powder front of the powder to be screened (56) on the screen, wherein the control unit (40) 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 spreading speed and / or as a function of the detected position.

37. Sieving device (10) according to claim 36, wherein the control unit (40) 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 (56) to be sieved and / or the detected position of the powder front exceeds a predetermined threshold value.

38. A powder processing system (128) comprising a screening device (10) according to any one of claims 30 to 37.

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

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