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Title:
SIEVE CONTROL
Document Type and Number:
WIPO Patent Application WO/2021/006882
Kind Code:
A1
Abstract:
An example system comprises a container to store an amount of a first build material to be sieved; and a first conduit to output a first flow of first build material from the container to a sieve. The sieve is to sieve the first flow of first build material into a second flow of second build material, and a second conduit to output the second flow from the sieve, the second flow controlled by a first valve. An airflow generator, fluidically connected to the second conduit and the sieve, is to cause airflow through the second conduit and the sieve when the first valve is in an open position and to inhibit the airflow when the first valve is in a closed position. The system further includes a device to measure the second flow and a controller to control the first valve based on a measurement of the second flow.

Inventors:
FABREGAT BOLANT NURIA (ES)
GARCIA GOMEZ ARTURO (ES)
FERNANDEZ SANJUAN JOSEP MARIA (ES)
Application Number:
PCT/US2019/040987
Publication Date:
January 14, 2021
Filing Date:
July 09, 2019
Export Citation:
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Assignee:
HEWLETT PACKARD DEVELOPMENT CO (US)
International Classes:
B29C64/314; B33Y40/00; B65G53/04
Domestic Patent References:
WO2017197004A12017-11-16
WO2018125555A12018-07-05
Foreign References:
US20190054696A12019-02-21
US10207454B22019-02-19
Attorney, Agent or Firm:
WOODWORTH, Jeffrey C. et al. (US)
Download PDF:
Claims:
CLAIMS

WHAT IT IS CLAIMED IS:

1. A system comprising:

a container to store an amount of a first build material to be sieved; a first conduit to output a first flow of first build material from the container to a sieve;

the sieve to sieve the first flow of first build material into a second flow of second build material;

a second conduit to output the second flow from the sieve, the second flow controlled by a first valve;

an airflow generator, fluidically connected to the second conduit and the sieve, to cause an airflow through the second conduit and the sieve when the first valve is in an open position and to inhibit the airflow when the first valve is in a closed position; a device to measure the second flow; and

a controller to control the first valve based on, at least in part, a measurement of the second flow.

2. The system of claim 1, wherein the controller is to set or maintain the first valve in an open position upon determining that the second flow measurement is over a second flow threshold, wherein the second flow threshold is based on at least one of a temperature, a humidity, and a type of build material.

3. The system of claim 1, wherein the controller is to set the first valve in a closed position upon determining that the second flow measurement is below a second flow threshold, wherein the second flow threshold is based on at least one of a temperature, a humidity, and a type of build material. 4. The system of claim 1, further comprising:

a second valve in the first conduit to control the first flow of first build material; a sensor to determine whether an amount of first build material accumulated on the sieve exceeds a predetermined height threshold; and

wherein the controller is to close the second valve upon determining that the amount of first build material accumulated on the sieve exceeds the height threshold.

5. The system of claim 1, wherein the sieve is to vibrate a mesh to sieve the first flow of first build material into a second flow of second build material.

6. The system of claim 1, wherein the sieve comprises a plurality of apertures to enable particles of the first build material smaller in size than the apertures to flow therethrough.

7. The system of claim 1, further comprising:

a third conduit to input a third flow of first build material from an external build material tank to the container; and

a third valve, coupled to the controller, to control the third flow of first build material to add build material to the container.

8. The system of claim 1, further comprising a build material mixer in fluid communication with the second conduit to mix an amount of second build material and an amount of a third build material therein.

9. The system of claim 8, wherein the device to measure the second flow is a load cell to determine the change of weight of the contents of the mixer over time. 10. The system of claim 8, further comprising:

a fourth conduit in fluid communication with a fresh build material tank and the mixer to input a fourth flow of fresh build material to the mixer, wherein the third build material is a fresh build material;

a fourth valve, coupled to the controller, to control the fourth flow of fresh build material; and

wherein the controller is to control the fourth valve based on a predetermined mixing ratio.

11. The system of claim 10, further comprising a build unit enclosure to receive a build unit, and a fifth conduit in fluid communication with the mixer and the build unit enclosure to input a fifth flow of mixed build material to the build unit.

12. The system of claim 1, being part of an additive manufacturing processing station.

13. A method of controlling a throughput of a build material in a sieve comprising:

operating the sieve in a first dynamic mode based on an indication that a build material flow is over a predetermined flow threshold; and

operating the sieve in a second stating mode based on an indication that the build material flow is below the predetermined flow threshold.

14. The method of claim 13, further comprising:

wherein operating in the first dynamic mode comprises opening a valve from a hose to allow an airflow to flow through the hose; and

wherein operating in the second static mode comprises closing the valve from the hose to inhibit the airflow through the hose.

15. A non-transitory machine-readable medium storing instructions executable by a processor, the non-transitory machine-readable medium comprising:

instructions to determine a flow of build material through a sieve exhaust hose; instructions to compare the determined flow with a predetermined flow threshold;

instructions to determine whether an amount of non-sieved build material on top of the sieve exceeds a predetermined build material height threshold;

instructions to open a valve from the hose to allow an airflow to flow through the hose, if the determined flow is over the predetermined flow threshold; and

instructions to close the valve from the hose to inhibit the airflow through the hose, if the determined flow is below the predetermined flow threshold and if the amount of non-sieved build material on top of the sieve exceeds the build material height threshold.

Description:
SIEVE CONTROL

BACKGROUND

[0001] Some additive manufacturing or three-dimensional printing systems selectively solidify portions of successive layers of a powdered build material. In some examples, selective solidification may be achieved by selectively applying an energy absorbing fusing agent over each formed layer of build material and applying a fusing energy to the build material layer to cause portions thereof on which fusing agent was printed to heat up sufficiently to melt, coalesce, sinter, or otherwise fuse, and then to solidify upon cooling. Other examples directly apply energy in a point-to-point manner to portions of each layers to be solidified, for example using a laser.

[0002] Upon completion of a 3D print job, some systems may re-use the non-melted, uncoalesced, not-sintered, and/or not-fused additive manufacturing build material in subsequent print jobs. In some examples, the re-used build material is mixed with fresh build material prior launching the subsequent print job.

BRIEF DESCRIPTION OF THE DRAWINGS

[0003] The present application may be more fully appreciated in connection with the following detailed description of non-limiting examples taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout and in which:

[0004] Fig. 1 is a schematic diagram showing an example of a system to control the sieving mode of a sieve.

[0005] Fig. 2A is a flow diagram illustrating an example method to control the sieving mode of a sieve.

[0006] Fig. 2B is a flow diagram illustrating another example method to control the sieving mode of a sieve. [0007] Fig. 3 is a schematic diagram showing another example of a system to control the sieving mode of a sieve.

[0008] Fig. 4 is a schematic diagram showing another example of a system to control the sieving mode of a sieve.

[0009] Fig. 5 is a schematic diagram showing another example of a system to control the sieving mode of a sieve.

[0010] Fig. 6A is a schematic diagram showing an example of an additive manufacturing processing station to control the sieving mode of a sieve.

[0011] Fig. 6B is a schematic diagram showing an example of 3D printer to control the sieving mode of a sieve.

[0012] Fig. 7 is a block diagram illustrating an example of a processor-based system to control the sieving mode of a sieve.

DETAILED DESCRIPTION

[0013] The following description is directed to various examples of additive manufacturing, or three-dimensional printing, apparatus and processes to generate high quality 3D objects. Throughout the present disclosure, the terms "a" and "an" are intended to denote at least one of a particular element. In addition, as used herein, the term "includes" means includes but not limited to, the term "including" means including but not limited to. The term "based on" means based at least in part on.

[0014] For simplicity, it is to be understood that in the present disclosure, elements with the same reference numerals in different figures may be structurally the same and may perform the same functionality

[0015] Some examples of additive manufacturing use build material to generate 3D objects from a virtual 3D object model of the 3D object to be generated. In an example, the 3D object model may be a Computer Aid Design (CAD) file. The 3D object model may, in another example, be sliced into a plurality of 2D slices corresponding to virtual cross sections of the object to be generated. Each slice may correspond to a physical build material layer. The 3D object model may also be defined in a vector-type format from which 2D rasterized images may be generated from each slice of the object model. Additionally, the 3D object model may be transformed into a file comprising instructions or data indicating which locations of build material layers are to be treated, e.g. solidified, to generate the object.

[0016] Suitable powder-based build materials for use in examples herein may include, where appropriate, at least one of polymers, metal powder, ceramics powder such as for example, poliamides, Thermoplastic Polyurethane (TPU), and stainless-steel. Some additive manufacturing systems use build material in, for example, a powdered or granular form.

[0017] Different powders may have different characteristics, such as different average particle sizes, different minimum and maximum particle sizes, different coefficients of friction, different angle of repose, and the like. In some examples non-powdered build materials may be used such as gels, pastes, and slurries.

[0018] As mentioned above, some additive manufacturing or three-dimensional printing systems selectively solidify portions of successive layers of a powdered build material to generate one or a plurality of 3D objects in a build chamber. Upon completion of the printing process, a build material extraction mechanism extracts the non-solidified build material from the build chamber, thereby freeing the generated objects from the non-solidified build material. In some examples, the build chamber may be an integral part of a 3D printer. In other examples, the build chamber is comprised in a removable build unit which is attachable and detachable from the 3D printer.

[0019] In the present disclosure, the terms "solidified" and "non-solidified" should be interpreted in their respective broad meaning. A solidified object may include a completely solidified object and a partially solidified object. Solidified build material is the build material that is intended to be used as part of the 3D object to be generated. Conversely, a non- solidified build material is the build material that is not intended to be solidified and form part of a 3D object. Dependent on build material type, some non-solidified build material may become agglomerated during a 3D print process.

[0020] In some examples, the build material extraction mechanism is a built-in part of the 3D printer. In other examples, the build material extraction mechanism is part of a separate build material processing station. In an example, the build material extraction mechanism may comprise a pump or fan to create an airflow source to extract a mix of air and non-solidified build material from the build chamber. In some examples, the mix of build material and air may travel through a conduit between the build chamber and the pump. In an example, the build chamber end of the conduit may be handled by a user to manually extract the build material from the build chamber. In another example, the build chamber end of the conduit may be in fluid communication with the build chamber to automatically extract the build material from the build chamber without the interaction of the user.

[0021] During generation of a 3D object, non-solidified build material in a build chamber may be exposed to agents and/or heat. The exposure of agents and/or heat may cause some of the non-solidified build material particles to stick together, and therefore to form agglomerations (e.g., chunks) of build material which are bigger in size than the individual build material particles. This effect may be commonly referred to as caking. In some examples, the non- solidified build material may also comprise other elements, such as, broken solidified parts from a previously generated 3D object or the like.

[0022] The non-solidified build material may be re-used, at least partially, in the generation of further 3D objects. The non-solidified build material that is intended to be re-used, may be referred hereinafter as recycled build material. The recycled build material may have a level of degraded mechanical properties as opposed to fresh build material due to, for example, the aforementioned exposure to agents and heat. In some examples, recycled build material may be mixed with fresh build material at a predetermined ratio to enhance the mechanical properties of the mix of build material to be used in the generation of future 3D objects.

[0023] In some examples, the recycled build material may be treated before mixing it with the fresh build material. The treatment may include the removal of the agglomerations (e.g., chunks) of build material and the other elements comprised therein (e.g., parts of previously generated 3D objects). Some examples may use a sieve to treat the recycled build material. Increasing the throughput of recycled material through sieve may help increase the throughput of a 3D printing system.

[0024] Referring now to the drawings, Fig. 1 shows a schematic diagram showing an example of a system 100 to control a sieving mode of a sieve 130.

[0025] The system 100 comprises a container 110 to store an amount of a first build material 115 to be sieved. In some examples, the first build material 115 may be recycled build material. In other examples, the first build material 115 may be any other suitable build material.

[0026] In an example, the container 110 comprises a housing to store the first build material 115 therein. The housing may have a suitable thickness based on the chosen material and the maximum first build material 115 amount to be stored therein. The container 110 defines a shape suitable to store an amount of first build material 115 therein, for example, a spherical shape, an elliptical shape, a polyhedral shape, or the like. Additionally, the lower part of the container 110 may be used for discharging build material and may have funnel-shape, conical shape, or any suitable shape that enables the housing to discharge build material.

[0027] The container 110 may have an opening (not shown) to couple with a first conduit 120. The first conduit 120 is also coupled to an upper portion of a sieve 130. The first conduit 120 is to output a first flow of first build material 115 from the container to a sieve. The first flow of first build material 115 may be driven through the first conduit 120 by the effect of gravity or by any suitable flow controlling means (e.g., valve, vacuum source, pump, fan, or a combination thereof). The first conduit 115 (and any further conduit defined herein) may define a cross- section profile that enables the transfer of build material, for example, a circular profile, an elliptical profile, a polygonal profile, or the like.

[0028] In an additional example, the container 110 and the first conduit 120 may be designed in such a way that the container 110 and the first conduit 120 provide a higher throughput of build material than the highest sieving throughput of the sieve such that the container 110 and the first conduit 120 are not the bottleneck in the sieving throughput. [0029] In the present disclosure, the term "conduit" comprises any mechanism suitable for transporting a gas fluid such as air, build material, and/or a mix of a gas fluid and build material. In some examples, a conduit may be a pipe, a hose, or a duct.

[0030] The system 100 further comprises the sieve 130 to sieve the first build material 115 into a second flow of second build material (not shown). The sieve 130 may comprise sieving means to sieve the first build material 115 into a second flow of second build material. The sieving means may comprise a mesh 135 comprising a plurality of apertures of a predetermined size.

[0031] The size of the plurality of apertures from the mesh 135 may enable that the particles of the first build material 115 (e.g., recycled build material particles) which are smaller in size than the apertures to flow therethrough, thereby generating a second flow of second build material. Additionally, in some examples, the sieve 130 may comprise a vibration mechanism that causes the mesh 135 to vibrate at a predetermined frequency to enhance the flowability of the first build material 115 through the apertures of the mesh 135. In an example, the aperture size of the apertures corresponding to a first subset of the plurality of apertures may be different to the aperture size of the apertures corresponding to a second subset of the plurality of apertures. As mentioned above, the first build material may comprise, at least in part, agglomerations of build material particles and/or parts of 3D printed objects from a previous print job larger in size than the apertures. The aperture size of the mesh 150 may inhibit agglomerations of build material particles and/or parts of 3D printed objects from passing through the apertures, and thereby not forming part of the second flow of second build material.

[0032] In an example, the mesh 135 comprises a plurality of apertures of the range of about 220 microns to about 375 microns, for example about 250 microns or about 355 microns. In another example, the mesh 135 comprises a plurality of apertures of the range of about 200 microns to about 450 microns. In yet another example, the mesh 135 comprises a plurality of apertures of the range of about 40 microns to about 370 microns, for example about 50 microns or about 80 microns. A plurality of examples of ranges of the plurality of apertures have been disclosed, however any specific aperture from the range defined from about 30 microns to about 1 millimeter may be used without departing from the scope of the present disclosure.

[0033] In some examples, the sieve 130 may comprise a vibration mechanism (not shown) that causes the mesh 135 to vibrate. The vibration from the mesh 135 fluidizes the first build material 115 to be sieved (e.g., break the agglomerated build material structures) and thereby increases the sieve 130 throughput.

[0034] Additionally, the sieve 130 may comprise waste exhaust means (not shown), to remove the non-sieved elements from the sieve 130 to an external waste compartment. In an example, the waste exhaust means may comprise an additional conduit located at the top part of the mesh 130 coupled to a vacuum source, so that the vacuum source can remove the un-sieved elements from the sieve 130 to the external waste compartment.

[0035] The system 100 further comprises a second conduit 140, or hose, to output the second flow from the sieve 130. The second conduit 140 comprises a first valve 145 to control the second flow based on the position of the first valve 145 (e.g., between a fully open position and a fully closed position).

[0036] The system 100 also includes an airflow generator 150 selectively fluidically connected (e.g., through fluid connection 155) to the second conduit 140 and, by extension, fluidically connected to the sieve 130. The airflow generator 150 may be a pump, a fan, a vacuum source, or any suitable device that is enabled to cause an airflow or reduced pressure. In an example, the airflow flows through the second conduit 140 from the sieve 130 and towards the airflow generator 150. In another example, the airflow generator 150 may be a vacuum source to generate at least a partial vacuum or reduced pressure in the second conduit 140 and, by extension, to the sieve 130. The airflow generator 150 is to cause an airflow through the second conduit 140 and the sieve 130 when the first valve 145 is in an open position. However, the first valve 145, when in a closed position, inhibits, at least in part, the airflow caused by the airflow generator 150 in the second conduit 140. The first valve 145 may be an electro mechanical valve or any other suitable valve that can be operated by a controller, such as controller 170. [0037] In the examples disclosed herein, reference to opening the valve may involve opening the valve completely or opening the valve to an intermediate position restricting the flow through the valve to at least 85% of the maximum flow through the valve. Additionally, reference to closing the valve may involve closing the valve completely or closing the valve to an intermediate position restricting the flow through the valve to below 85% of the maximum flow through the valve.

[0038] The system 100 also comprises a device 160 to measure the second flow of second build material. The device 160, for example, may be any suitable kind of flowmeter. Examples of flowmeters may be, at least in part, an obstruction type flowmeter (i.e., differential pressure flowmeter or variable area flowmeter), an inferential flowmeter (i.e., turbine), an electromagnetic flowmeter, a positive-displacement flowmeter (i.e. which accumulates a fixed volume and then counts the number of times the volume is filled to measure the flow), a dynamic flowmeter (i.e., vortex shedding), an anemometer, an ultrasonic flowmeter (e.g., Doppler effect), a mass flowmeter (e.g., Coriolis), and the like. In other examples, the device 160 may allow the determination of a flow in an indirect manner, an example of which is described below with reference to Fig. 5.

[0039] When the container 115 comprises first build material 115 the second flow of second build material may indicate how well the sieve 130 sieves, thereby the second flow may indicate the throughput of build material through the sieve 130. In some circumstances the throughput of the sieve 130 may be higher if the sieve 130 has an airflow. Herein, sieving in the presence of an airflow is referred to as 'dynamic sieving' and may be implemented by setting the valve 145 in an open position. In other circumstances the throughput of the sieve 130 may be higher if the sieve 130 does not have any airflow therethrough. Herein, sieving in the absence of an airflow is referred to as 'static sieving' and may be implemented by setting the valve 145 in a closed position.

[0040] When operating in a dynamic sieving mode, second build material is pneumatically transferred in a continuous manner to the next stage of the process (e.g., to a mixer). In a static sieving mode, however, build material is stored in the conduit 140 and on the sieve 130 when the valve 145 is closed and is only pneumatically transferred to the next stage of the process periodically.

[0041] As described below, in some circumstances, static sieving may provide a higher throughput than dynamic sieving.

[0042] Some types of build materials may, depending on specific temperature and humidity conditions, flow better through the mesh 135 in a static mode. In some examples, the first build material 115 received on top of the mesh 135 may accumulate forming a bridge or 'powder arch', thereby reducing the contact surface between the mesh 135 and the first build material 115 accumulated on top of the mesh 135. In other examples, the first build material on top of the mesh 135 may accumulate forming a well or 'rat hole' of first build material 115, thereby reducing the contact surface between the mesh 135 and the first build material 115 accumulated on top of the mesh 135 corresponding to the surface of the formed well.

[0043] The formation of the aforementioned powder flow reduction phenomena (e.g., bridges and wells) may be subject to the temperature, humidity, and type of build material. When a sieve sieves some types of build materials in a static mode, the bridges and wells tend not to be generated as readily. Some materials may be better sieved using a mix of static and dynamic sieving. Precisely, upon the generation of bridges and/or wells, the shift from dynamic sieving to static sieving (e.g., shift from an existing airflow/vacuum to a non-existing airflow/vacuum) may break-up the first build material 115 bridges and wells structures, thereby increasing the contact surface between the mesh 135 and the first build material 115 accumulated on top of the mesh 135 and increasing the throughput of the sieve 130. As described herein, the system 100 automatically shifts from dynamic to static sieving based on how the build material flows through the sieve 130.

[0044] System 100 further comprises a controller 170. The controller 170 may, for example, be any combination of hardware and programming to control the first valve 145 based on, at least in part, a measurement of the second flow. In some examples, the controller 170 is to implement the functionalities resulting from the execution of the method 200 of Fig. 2A and/or 2B. In some examples herein, such combinations of hardware and programming may be implemented in a number of different ways. For example, the programming of modules may be processor-executable instructions stored on at least one non-transitory machine-readable storage medium and the hardware for modules may include at least one processor to execute those instructions. In some examples described herein, multiple modules may be collectively implemented by a combination of hardware and programming, as described above. In other examples, the functionalities of the controller 170 may be, at least partially, implemented in the form of electronic circuitry.

[0045] Fig. 2A and 2B are a flow diagrams illustrating respective examples of methods 200A and 200B to control a sieving mode of a sieve 130. Methods 200A and 200B are described below as being executed or performed by a controller, such as the controller 170 of FIG. 1. Methods 200A and 200B may be implemented in the form of executable instructions stored on a machine-readable storage medium and executed by a single processor or a plurality of processors, and/or in the form of any electronic circuitry, for example digital and/or analog ASIC. In some implementations of the present disclosure, methods 200A and 200B may include more or less elements than are shown in FIG. 2A and 2B. In some implementations, some of the elements of method 200A and 200B may, at certain times, be performed in parallel and/or may repeat.

[0046] Methods 200A and 200B may be performed by controller 170 to control some of the elements of system 100. System 100 may comprise first build material 115 in the container 110 and the airflow generator 150 may be activated. In some examples, the first valve 145 may be initially open. In other examples, the first valve 145 may be initially closed. In yet other examples, the first valve 145 may be initially in an intermediate position between the open position and the closed position.

[0047] Referring now to method 200A, at block 220A, the controller 170 determines a flow of build material at a second conduit 140. The flow of build material may be measured or determined by the device 160 and the data corresponding to the flow of build material may be further received by the controller 170. [0048] At block 240A, the controller 170 compares the determined flow of build material from block 220, with a predetermined flow threshold. The predetermined flow threshold indicates the threshold between acceptable build material flows that are compliant with some sieve 130 throughput requirements, and unacceptable build material flows that are not compliant with sieve 130 throughput requirements. The predetermined flow threshold may be based in at least one of, the type of build material, the temperature, the humidity, the printing mode (e.g., fast mode, high quality mode), and any other quality requirements. In some examples, the controller 170 may check a look-up table including the predetermined flow threshold based on the type of build material, the temperature, the humidity, the printing mode, and any other quality requirements.

[0049] If the determined flow (block 220A) is over the predetermined flow threshold, block 260A is to be performed. If the determined flow (block 220A) is below the predetermined flow threshold, block 280A is to be performed. At block 260A, the controller 170 may open the valve 145 located at the second conduit 140 thereby allowing an airflow to flow through the hose, and by extension to the sieve 130 to enter a dynamic sieving mode. At block 280A, however, the controller 170 may close the valve located at the hose, thereby inhibiting any airflow through the hose, and by extension to the sieve 130 thereby entering a static sieving mode.

[0050] Referring now to method 200B, at block 220B, the controller 170 determines whether an indication that a build material flow (e.g., determined flow from Fig. 2A block 220A) is over a predetermined flow threshold. If the indication that a build material flow is over a determined flow threshold (YES, block 220B), the controller operates the sieve in dynamic mode (block 240B). Conversely, if the indication that a build material flow is below a determined flow threshold (NO, block 220B), the controller operates the sieve in static mode (block 260B).

[0051] At block 240B, the controller 170 operates the sieve 130 in a first dynamic mode. In some examples, when operating in dynamic mode, the controller 170 is to open the valve 145 from the second conduit 140 to allow an airflow to flow through the hose. [0052] At block 260B, the controller 170 operates the sieve 130 in static mode. In some examples, when operating in static mode, the controller 170 is to close the valve 145 from the second conduit 140 to inhibit any airflow through the second conduit 140.

[0053] Fig. 3 is a schematic diagram showing another example of a system 300 to control the sieving mode of a sieve 130.

[0054] System 300 comprises a second valve 325 in the first conduit 120 to control the first flow of first build material 115. The second valve 325 may be the same as or similar to the first valve 145. The second valve 325 is connectable and operable through the controller 170. In some examples, the second valve 325 is an electro-mechanical valve. In an example, the controller 170 may open the second valve 325, thereby allowing a first flow of first build material 115 from the container 110 to be fed into the sieve 130. In another example, the controller 170 may close the second valve 325, thereby inhibiting the fist build material 115 from the container 110 to reach the sieve 130. In yet another example, the controller 170 may partially open or partially close the second valve 325, thereby partially allowing a first flow of first build material 115 to be fed into the sieve 130.

[0055] System 300 comprises a sensor 335 connected to the controller 170. The sensor 335 is to determine whether an amount of first build material 337 accumulated on the mesh 135 from the sieve 130 exceeds a predetermined height threshold (indicated as dotted lines 339). The functionality of the sensor 335 may be implemented in a number of different ways, for example using a capacitive sensor, an optical sensor, a diapason, a helix, and the like. In an example, an optical sensor may be installed at a height (referenced as HT) corresponding to the predetermined height threshold. In another example, an optical sensor may be installed at the top part of the sieve to measure whether the height of build material below the sensor is compliant with the predetermined height threshold.

[0056] The predetermined height threshold indicates the maximum acceptable height of the build material 337 that may accumulate on the mesh 135. In an example, the predetermined height threshold is based on at least one of a temperature, humidity, and type of build material. Data corresponding to the predetermined height threshold may be accessible from the controller 170 by, for example, checking a look up table.

[0057] The controller 170 is to close the second valve 325 upon determining that the amount of first build material 337 accumulated on the sieve exceeds the height threshold. By closing the second valve 325, the controller 170 regulates the maximum amount of build material 337 accumulated on the mesh 135, thereby preventing the sieve 130 from overflowing with build material, and from withstanding heavy loads corresponding to the build material 337 accumulated on the mesh 135. Furthermore, in some examples, regulating the maximum amount of build material 337 accumulated on the mesh 135, reduces the formation of bridge structures and well structures thereon.

[0058] In some examples, the controller 170 may control the first valve 145 based on the second flow measurement, and also based on the sensor 335 measurement. In an example, the controller 170 may open, or maintain in an open position, the first valve 145 (i.e., dynamic mode) upon determining that the he amount of first build material 337 accumulated on the sieve mesh 135 does not exceed the predetermined height threshold, regardless of the second flow measurement. In another example, the controller 170 may open, or maintain in an open position, the first valve 145 (i.e., dynamic mode) upon determining that the amount of first build material 337 accumulated on the sieve mesh 135 exceeds the predetermined height threshold) and upon determining that the second flow measurement is over the second flow threshold. In yet another example, the controller 170 may close, or maintain in a closed position, the first valve 145 (i.e., static mode) upon determining that the amount of first build material 337 accumulated on the sieve mesh 135 exceeds the predetermined height threshold and upon determining that the second flow measurement is below the second flow threshold.

[0059] Fig. 4 is a schematic diagram showing another example of a system 400 to control the sieving mode of a sieve 130.

[0060] The system 400 further comprises a third conduit 485 to input a third flow of first build material from an external build material tank 480 to the container 110. The external tank 480 may be a recycled build material repository in which the un-solidified build material from previous 3D print jobs is stored therein. In some examples, the external tank 480 is a bulk container. In other examples, the external tank 480 is an octabin comprising recycled build material. In yet another example, the external tank 480 is a build material container having a capacity of about 10 liters to about 50 liters, for example 30 liters. In yet another example, the external tank 480 is a build material container from about 50 liters to about 500 liters, for example 150 liters.

[0061] System 400 comprises a third valve 487 in the third conduit 485 to control the third flow of build material from the external tank 480 to the container 110. The third valve 487 may be the same as or similar to the first valve 145 and/or the second valve 325. The third valve 487 is connectable and operable through the controller 170. In some examples, the third valve 487 is an electro-mechanical valve. In an example, the controller 170 may open the third valve 487, thereby allowing a third flow of build material from the external tank 480 to be fed into the container 110. In another example, the controller 170 may close the third valve 487, thereby inhibiting the build material from the external tank 480 to reach the container 110.

[0062] Additionally, the system 400 may further comprise at least one build material blocking mechanism 457 (illustrated in dotted lines for clarity) to inhibit particulate build material from reaching the airflow generator 150. In some examples, the build material blocking mechanism 457 is a powder trap, a cyclone or any other device to inhibit particulate build material from crossing the build material blocking mechanism 457. The build material blocking mechanism 457 may comprise a plurality of blocking mechanisms connected in parallel, for example, a first cyclone connected to a second cyclone, the second cyclone being connected to the airflow generator 150. In an example, the build material blocking mechanism 457 is installed in the fluid communication channel between the airflow generator 150 and the second conduit 140 to inhibit particles of second build material in the second conduit 140 from reaching the airflow generator 150. In another example, or in addition to the previous example, the build material blocking mechanism 457 is installed in the fluid communication channel between the airflow generator 150 and the third conduit 485 to direct the first build material from the external build material tank 480 to the container and to inhibit the first build material from the external build material tank 480 from reaching the airflow generator 150. [0063] Fig. 5 is a schematic diagram showing another example of a system 500 to control the sieving mode of a sieve 130.

[0064] System 500 comprises a build material mixer 590 in fluid communication with the second conduit 140, so that when the first valve 145 is in an open position, the second flow of second build material can travel from the sieve 130 to the mixer 590. The mixer 590 is to mix its contents, for example, an amount of the second build material with an amount of a third build material (e.g., fresh build material). In some examples, the mixer 590 is a blender. In other examples, the mixer 590 comprises a rotating shaft connected to a blade that mixes the particulate contents from the mixer as the shaft rotates. Notwithstanding the previous, the mixer 590 may be any suitable device for mixing particulate material previously placed therein.

[0065] In system 500, the device to measure the second flow (i.e., device 160 from Fig. 1) is a load cell 560. The load cell 560 is to determine the second flow of second build material by determining the change of weight of the mixer 590 over time. The change of weight of the mixer 590 over time determined by the load cell 560, may correspond with the amount of second build material pneumatically fed to the mixer 590 during the same period of time. The load cell 560 is connectable and operable through the controller 170. The controller 170 may operate the load cell 560 as previously disclosed with reference to the device 160.

[0066] The system 500 further comprises a fourth conduit 585 in fluid communication with a fresh build material tank 580 and the mixer 590. The fourth conduit 585 is to input a fourth flow of fresh build material from the fresh build material tank 580 to the mixer 590. As mentioned above, the fresh build material is a build material that has not yet been involved in the generation of a 3D object. In some examples, the fresh build material tank 580 is a bulk container. In other examples, the fresh build material tank 580 is an octabin comprising fresh build material. In yet another example, the fresh build material tank 580 is a build material container from about 10 liters to about 50 liters, for example 30 liters. In yet another example, the fresh build material tank 580 is a build material container from about 50 liters to about 500 liters, for example 150 liters [0067] The system 500 further comprises a fourth valve 587 in the fourth conduit 585 to control the fourth flow of fresh build material from the fresh build material tank 580 to the mixer 590. The fourth valve 587 may be the same as or similar to the first valve 145, the second valve 325 and/or the third valve 487. The fourth valve 587 is connectable and operable through the controller 170. In some examples, the fourth valve 587 is an electro-mechanical valve. In an example, the controller 170 may open the fourth valve 587, thereby allowing a fourth flow of build material from the fresh build material tank 580 to be fed into the mixer 590. In another example, the controller 170 may close the fourth valve 587, thereby inhibiting the fresh build material from the fresh build material tank 580 to reach the mixer 590.

[0068] In some examples, the upcoming print job has build material requirements based on, for example, the printing mode and the printing technology to be used. The build material requirements may have a build material mixing ratio associated with it (e.g., mixing a first amount of recycled build material from the second flow and another amount of fresh build material from the fourth flow). In an example, the build material mixing ratio is of about 80% of recycled build material and about 20% of fresh build material. In another example, the build material mixing ratio is of about 70% of recycled build material and about 30% of fresh build material. In yet another example, the build material mixing ratio is of about 50% of recycled build material and about 50% of fresh build material. The controller 170 is to control the fourth valve 587, so that a previously selected amount of fresh build material is transferred from the build material tank 580 to the mixer 590. The previously selected amount of fresh build material may be based on the predetermined mixing ratio and the amount of recycled build material from the second flow being contained in the mixer 590.

[0069] Additionally, system 500 may further comprise a one-way valve 589 (illustrated in dotted lines) to inhibit fresh build material from the fresh build material tank 580 to any destination other than the mixer 590. The one-way valve 589 may be installed in any fluid communication channel connected to the fourth conduit 585.

[0070] Fig. 6A is a schematic diagram showing an example of an additive manufacturing processing station 600A to control the sieving mode of a sieve 130. [0071] The processing station 600A comprises a build unit enclosure 680A to receive a build unit (not shown) in which the subsequent build job is to be generated. The build unit is to be transferred to a 3D printer for the generation of the build job. The processing station 600A is to fill the build unit, at least partially, with a mix of build material according to the selected build material ratio. The processing station 600A further comprises a fifth conduit 685 fluidically connecting the mixer 590 and the build unit enclosure 680A, so that when a build unit is installed in the build unit enclosure 680A, a fifth flow of mixed build material is to be inputted from the mixer 590 to the build unit.

[0072] In order to control the amount of mixed build material to be inputted from the mixer 590 to the build unit, the processing station 600A further comprises a fifth valve 687 in the fifth conduit 685. In some examples, the fifth valve 687 is connectable and operable through the controller 170. However, in other examples, the fifth valve 687 is automatically connected to a controller from the build unit (not shown) and the fifth valve 687 is operable and controlled therefrom.

[0073] Fig. 6B is a schematic diagram showing an example of 3D printer 600B to control a valve based on a measurement of a flow.

[0074] The 3D printer 600B comprises a built-in build unit 680B and a printing module 685B that is to generate the 3D objects in the build unit 680B through selective solidification. In an example, the printing module 685B performs the selective solidification by selectively applying an energy absorbing fusing agent over each formed layer of build material. In another example, the printing module 685B may apply other printing fluids, such as, UV binders or thermal binding agents. In yet another example, the printing module 685B performs the selective solidification by selectively applying a point-to-point focused energy beam (e.g., laser) or an array of point-to-point focused energy beams over each formed layer of build material.

[0075] Prior to generating the 3D objects in the build unit by the printing module 685B, the build unit 380B is filled with a mix of build material from the mixer 590. The filling mechanism may the same as or similar to the filling mechanism of Fig. 6A. A fifth flow of mixed build material is to be inputted from the mixer 590 to the build unit 680B through the fifth conduit 685. The fifth flow of mixed build material is regulated by the fifth valve 687. The fifth valve is controlled by the controller 170 from the 3D printer 600B.

[0076] FIG. 7 is a block diagram illustrating a processor-based system 700 that includes a machine-readable medium 720 encoded with example instructions to control the sieving mode of a sieve. In some implementations, the system 700 is a processor-based system and may include a processor 710 coupled to a machine-readable medium 720. The processor 710 may include a single-core processor, a multi-core processor, an application-specific integrated circuit (ASIC), a field programmable gate array (FPGA), and/or any other hardware device suitable for retrieval and/or execution of instructions from the machine-readable medium 720 (e.g., instructions 721-725) to perform functions related to various examples. Additionally, or alternatively, the processor 710 may include electronic circuitry for performing the functionality described herein, including the functionality of instructions 721-725. With respect of the executable instructions represented as boxes in FIG. 7, it should be understood that part or all of the executable instructions and/or electronic circuits included within one box may, in alternative implementations, be included in a different box shown in the figures or in a different box not shown.

[0077] The machine-readable medium 720 may be any medium suitable for storing executable instructions, such as a random-access memory (RAM), electrically erasable programmable read only memory (EEPROM), flash memory, hard disk drives, optical disks, and the like. In some example implementations, the machine-readable medium 720 may be a tangible, non- transitory medium, where the term "non-transitory" does not encompass transitory propagating signals. The machine-readable medium 720 may be disposed within the processor- based system 700, as shown in FIG. 7, in which case the executable instructions may be deemed "installed" on the system 700. Alternatively, the machine-readable medium 720 may be a portable (e.g., external) storage medium, for example, that allows system 700 to remotely execute the instructions or download the instructions from the storage medium. In this case, the executable instructions may be part of an "installation package". As described further herein below, the machine-readable medium may be encoded with a set of executable instructions 721-725. [0078] Instructions 721, when executed by the processor 710, may cause the processor 710 to determine a flow of build material through a second conduit 140. In some examples, the determination of the flow of build material may be done by the device 160. In other examples, the determination of the flow of build material may be done by the load cell 560 from Fig. 5.

[0079] Instructions 722, when executed by the processor 710, may cause the processor 710 to compare the determined flow with a predetermined flow threshold. In some examples, the predetermined flow threshold is based on at least one of a temperature, a humidity, and a type of build material.

[0080] Instructions 723, when executed by the processor 710, may cause the processor 710 to determine whether an amount of non-sieved build material on top of the sieve (e.g., sieve 130) exceeds a predetermined build material height threshold. In some examples, the determination of whether an amount of non-sieved build material exceeds the predetermined build material height threshold may be determined by a sensor (e.g., sensor 335). In some examples, the predetermined build material height threshold is based on at least one of a temperature, a humidity, and a type of build material.

[0081] Instructions 724, when executed by the processor 710, may cause the processor 710 to open a valve (e.g., valve 145) from the second conduit 140to allow an airflow to flow through the hose, if the determined flow is over the predetermined flow threshold. In some examples, the airflow may be generated by an airflow generator (e.g., airflow generator 150) fluidically connected to the second conduit 140and, by extension to the sieve.

[0082] Instructions 725, when executed by the processor 710, may cause the processor 710 to close the valve from the second conduit 140to inhibit the airflow through the hose, if the determined flow is below the predetermined flow threshold and if the amount of non-sieved build material on top of the sieve exceeds the build material height threshold.

[0083] The above examples may be implemented by hardware, or software in combination with hardware. For example, the various methods, processes and functional modules described herein may be implemented by a physical processor (the term processor is to be implemented broadly to include CPU, SoC, processing module, ASIC, logic module, or programmable gate array, etc.). The processes, methods and functional modules may all be performed by a single processor or split between several processors; reference in this disclosure or the claims to a "processor" should thus be interpreted to mean "at least one processor". The processes, method and functional modules are implemented as machine-readable instructions executable by at least one processor, hardware logic circuitry of the at least one processors, or a combination thereof.

[0084] As used herein, the terms "about" is used to provide flexibility to a numerical range endpoint by providing that a given value may be, for example, an additional 20% more or an additional 20% less than the endpoints of the range. The degree of flexibility of this term can be dictated by the particular variable and would be within the knowledge of those skilled in the art to determine based on experience and the associated description herein.

[0085] The drawings in the examples of the present disclosure are some examples. It should be noted that some units and functions of the procedure may be combined into one unit or further divided into multiple sub-units. What has been described and illustrated herein is an example of the disclosure along with some of its variations. The terms, descriptions and figures used herein are set forth by way of illustration. Many variations are possible within the scope of the disclosure, which is intended to be defined by the following claims and their equivalents.

[0086] There have been described example implementations with the following sets of features:

[0087] Feature set 1: A system comprising: a container to store an amount of a first build material to be sieved; a first conduit to output a first flow of first build material from the container to a sieve;

the sieve to sieve the first flow of first build material into a second flow of second build material;

a second conduit to output the second flow from the sieve, the second flow controlled by a first valve; an airflow generator, fluidically connected to the second conduit and the sieve, to cause an airflow through the second conduit and the sieve when the first valve is in an open position and to inhibit the airflow when the first valve is in a closed position; a device to measure the second flow; and

a controller to control the first valve based on, at least in part, a measurement of the second flow.

[0088] Feature set 2: A system with feature set 1, wherein the controller is to set the first valve in an open position upon determining that the second flow measurement is over a second flow threshold, wherein the second flow threshold is based on at least one of a temperature, a humidity, and a type of build material.

[0089] Feature set 3: A system with any preceding feature set 1 or 2, wherein the controller is to set the first valve in a closed position upon determining that the second flow measurement is below a second flow threshold, wherein the second flow threshold is based on at least one of a temperature, a humidity, and a type of build material.

[0090] Feature set 4: A system with any preceding feature set 1 to 3, further comprising: (i) a second valve in the first conduit to control the first flow of first build material; (ii) a sensor to determine whether an amount of first build material accumulated on the sieve exceeds a predetermined height threshold; and (iii) wherein the controller is to close the second valve upon determining that the amount of first build material accumulated on the sieve exceeds the height threshold.

[0091] Feature set 5: A system with any preceding feature set 1 to 4, wherein the sieve is to vibrate a mesh to sieve the first flow of first build material into a second flow of second build material.

[0092] Feature set 6: A system with any preceding feature set 1 to 5, wherein the sieve comprises a plurality of apertures to enable particles of the first build material smaller in size than the apertures to flow therethrough, wherein the first build material is to comprise, at least in part, agglomerations of build material particles or parts of 3D printed objects from a previous print job bigger in size than the apertures.

[0093] Feature set 7: A system with any preceding feature set 1 to 6, further comprising: (i) a third conduit to input a third flow of first build material from an external build material tank to the container; and (ii) a third valve, coupled to the controller, to control the third flow of first build material to add build material to the container.

[0094] Feature set 8: A system with any preceding feature set 1 to 7, further comprising a build material mixer in fluid communication with the second conduit to mix an amount of second build material and an amount of a third build material therein.

[0095] Feature set 9: A system with any preceding feature set 1 to 8, wherein the device to measure the second flow is a load cell to determine the change of weight of the contents of the mixer over time.

[0096] Feature set 10: A system with any preceding feature set 1 to 9 further comprising: (i) a fourth conduit in fluid communication with a fresh build material tank and the mixer to input a fourth flow of fresh build material to the mixer, wherein the third build material is a fresh build material; (ii) a fourth valve, coupled to the controller, to control the fourth flow of fresh build material; and (iii) wherein the controller is to control the fourth valve based on a predetermined mixing ratio.

[0097] Feature set 11: A system with any preceding feature set 1 to 10 further comprising a build unit enclosure to receive a build unit, and a fifth conduit in fluid communication with the mixer and the build unit enclosure to input a fifth flow of mixed build material to the build unit.

[0098] Feature set 12: A system with any preceding feature set 1 to 11, being part of an additive manufacturing processing station.

[0099] Feature set 13: A method of controlling a throughput of a build material in a sieve comprising: operating the sieve in a first dynamic mode based on an indication that a build material flow is over a predetermined flow threshold; and

operating the sieve in a second static mode based on an indication that the build material flow is below the predetermined flow threshold.

[0100] Feature set 14: the method of claim IB, further comprising: (i) wherein operating in the first dynamic mode comprises opening a valve from the hose to allow an airflow to flow through the hose; and (ii) wherein operating in the second static mode comprises closing the valve from the hose to inhibit the airflow through the hose.

[0101] Feature set 15: A non-transitory machine-readable medium storing instructions executable by a processor, the non-transitory machine-readable medium comprising: instructions to determine a flow of build material through a sieve exhaust hose; instructions to compare the determined flow with a predetermined flow threshold;

instructions to determine whether an amount of non-sieved build material on top of the sieve exceeds a predetermined build material height threshold;

instructions to open a valve from the hose to allow an airflow to flow through the hose, if the determined flow is over the predetermined flow threshold; and instructions to close the valve from the hose to inhibit the airflow through the hose, if the determined flow is below the predetermined flow threshold and if the amount of non-sieved build material on top of the sieve exceeds the build material height threshold.