Post-processing apparatus and method

Abstract

A post-processing station (1) for recovering powder build material from a build volume produced by a 3D printer wherein the build volume comprises one or more 3D objects and build material is provided, the post-processing station comprising: a depowdering unit; a vacuum pump (60); a first cyclone (20), having an inlet, an air outlet and a solids outlet, wherein the inlet of the first cyclone is operably in fluidic communication with the depowdering unit and wherein the air outlet of the first cyclone is operably in fluidic communication with the vacuum pump; a sieving unit (30) comprising a sieve (34) in fluidic communication with the solids outlet of the first cyclone, wherein the sieve comprises a first surface to which the solids outlet of the first cyclone delivers build material, and a second surface through which sieved build material passes; a second cyclone (40), having an inlet, an air outlet and a solids outlet, wherein the inlet of the second cyclone is operably in fluidic communication with the first surface of the sieve, and the air outlet of the second cyclone is operably in fluidic communication with the vacuum pump; and a valve (102), located between the sieving unit and the inlet of the second cyclone; wherein during operation of the vacuum pump the post-processing station is configured to operate in a first, depowdering, mode, in which the valve is closed, and in a second, sieve-cleaning, mode, in which the valve is open.

Description

[0001] POST-PROCESSING APPARATUS AND METHOD

[0002] Field of the Invention

[0003] The present invention relates to a post-processing station for recovering build material from a build volume produced by a 3D printer and a method of operating the same. In particular, the present invention relates to an apparatus for and method of facilitating automatic cleaning of a sieve used to sieve recovered build material.

[0004] Background

[0005] In additive manufacturing technologies (e.g. industrial 3D printing) that are powder bed based, the unfused build material powder may be recycled. This makes efficient use of the build material and has particular advantages in print and sinter and laser sintering applications. However, the recovery of unfused build material from a build volume produced by a 3D printer in a postprocessing station can be time consuming. The present invention is concerned with increasing efficiency of this process.

[0006] Summary

[0007] The invention is set out in the appended independent claims, while particular embodiments of the invention are set out in the appended dependent claims.

[0008] According to a first aspect of the invention, there is provided a post-processing station for recovering powder build material from a build volume produced by a 3D printer wherein the build volume comprises one or more 3D objects and build material, the post-processing station comprising: a depowdering unit; a vacuum pump; a first cyclone, having an inlet, an air outlet and a solids outlet, wherein the inlet of the first cyclone is operably in fluidic communication with the depowdering unit and wherein the air outlet of the first cyclone is operably in fluidic communication with the vacuum pump; a sieving unit comprising a sieve in fluidic communication with the solids outlet of the first cyclone, wherein the sieve comprises a first surface to which the solids outlet of the first cyclone delivers build material, and a second surface through which sieved build material passes; a second cyclone, having an inlet, an air outlet and a solids outlet, wherein the inlet of the second cyclone is operably in fluidic communication with the first surface of the sieve, and the air outlet of the second cyclone is operably in fluidic communication with the vacuum pump; and a valve, located between the sieving unit and the inlet of the second cyclone; wherein during operation of the vacuum pump the post-processing station is configured to operate in a first, depowdering, mode, in which the valve is closed, and in a second, sievecleaning, mode, in which the valve is open.

[0009] By opening the valve, automatic cleaning of the sieve can advantageously be performed, as this creates a fluidic connection between the sieving unit and the second cyclone, through which any material deposited on the sieve can be passed and then collected. The build material may typically be a powder particulate, while the material deposited on the sieve may further comprise agglomerates and other debris.

[0010] As used herein, the term ‘fluidic communication’ preferably connotes the existence of any communication channel which allows the fluid flow of material, including at least solid (powder) and gas.

[0011] In some implementations, the air outlet of the first cyclone is operably in fluidic communication with the vacuum pump via the inlet to the second cyclone. This can provide a safeguard against the possibility that the sieve may become blocked, and then build material passed through to the filter, which could then become clogged. In this arrangement, in such a situation, the build material would instead pass through the second cyclone and exit via the solids outlet of the second cyclone.

[0012] Preferably, the sieve comprises a first, upper, surface, to which the solids outlet of the first cyclone delivers build material, and a second, lower, surface from which sieved build material passes into the buffer tank below the sieve, and wherein the inlet of the second cyclone is operably in fluidic communication with the first, upper, surface of the sieve. This arrangement can advantageously facilitate the cleaning of the sieve of material deposited from the depowdering unit.

[0013] In some preferable implementations, the sieving unit comprises a first sensor configured to measure an amount of accumulated build material on the first surface of the sieve, wherein the post processing station is configured to switch between the first and second mode based on the measured amount of accumulated build material on the first surface of the sieve. This can prevent the system becoming blocked with accumulated build material / powder, for example by determining when to switch to cleaning of the first surface of the sieve.

[0014] In some implementations, the post-processing station may be configured to switch between the first and second mode in dependence on a time elapsed in the first mode. The time may be predefined, for example having been determined empirically.

[0015] In some implementations, the valve may be a first valve and the post-processing station may further comprise a second valve located between the air outlet of the first cyclone and the inlet of the second cyclone, wherein in the first mode the second valve is open and in the second mode the second valve is closed. This arrangement can help to ensure that the material deposited on the sieve can be passed to and through the second cyclone by further increasing the relative resistance of the fluidic connection between the air outlet of the first cyclone and the inlet of the second cyclone.

[0016] In more general implementations, the valve may be a first valve and the post-processing station may further comprise a second valve located between the air outlet of the first cyclone and the vacuum pump, wherein in the first mode the second valve is open and in the second mode the second valve is closed. As described above, this arrangement can help to ensure that the material deposited on the sieve can be effectively cleaned from the sieve. This may refer to an arrangement in which the air outlet of the first cyclone is connected to the vacuum pump directly or indirectly (for example, via the inlet to the second cyclone).

[0017] In preferable implementations, the sieving unit may comprise an outlet configured to allow sieved build material to exit the sieving unit, and the post-processing station may further comprise a build material collection unit (e.g. for reuse in a printing process) in fluidic connection with the outlet of the sieving unit for collecting sieved (e.g. unfused) build material (i.e. build material which has passed through the sieve). This can advantageously facilitate the collection of good quality build material, which can then be recycled.

[0018] In some preferable implementations, the build material collection unit may comprise a reuse collection tank for collecting build material and a (second) sensor for determining the amount of collected sieved build material for reuse. This can enable the quantity (e.g. indicated by weight) of the (sieved) build material being collected to be determined and tracked. This can provide information regarding the progression of the depowdering process, for example, from both the weight and the rate of change of weight. In some implementations, the post-processing station may further comprise a controller configured to switch the post-processing station from the first mode to the second mode in dependence on at least one of: the weight of collected (sieved) build material; and a rate of change of the weight of collected (sieved) build material. The controller may preferably be in communication with the sensor. Readings of the collected amount, for example the weight and / or rate of change of weight, can be used to determine when a sufficient amount of good quality build material has been recovered. This can make the process more efficient by avoiding wasting time and can ensure that only the better quality power is recovered (the later powder, when the rate of change of weight is lower, is likely to be worse quality).

[0019] In some implementations, the post-processing station may further comprise a controller configured to stop the vacuum pump in dependence on determining a weight of collected (sieved) build material; preferably the controller may be further configured to output a recommendation to replace the collection tank in dependence on determining a weight of collected (sieved) build material. This can alert a user when the collection tank may need replacing. All references to a controller may refer to the same controller.

[0020] In some implementations, the post-processing station may further comprise a controller configured to switch the post-processing station from the first mode to the second mode. This may, for example, be in dependence on a time elapsed in the first mode. The time elapsed before switching may be predefined, for example predefined on the basis of empirical observations of previous post-processing.

[0021] Preferably, the depowdering unit comprises a mesh platform and the depowdering unit is fluidically connected to the inlet of the first cyclone below the mesh platform. In some preferable implementations, the depowdering unit is further fluidically connected to the vacuum pump above the mesh platform.

[0022] In preferable implementations, the sieve is vibratable, preferably the sieve is ultrasonically vibratable. This can help to significantly improve the throughput of build material.

[0023] In some implementations, the post-processing station further comprises a collection funnel in fluidic communication with an outlet of the sieving unit, preferably wherein the collection funnel is vibratable. This can help to improve the throughput of the build material through and / or out of the sieving unit. Preferably, the post-processing station further comprises a waste collection unit in fluidic communication with the solids outlet of the second cyclone. This can be used to collect the waste material removed from the first (e.g. upper) surface of the sieve. In some implementations, an outlet of the waste collection unit may be operably in fluidic connection to a waste valve configured to be connected to an external vacuum system. This can allow the waste material to be removed from the waste collection unit. In preferable implementations, the post-processing station further comprises a waste collection tank and a (third) sensor for determining a (waste) build material amount within the waste collection tank (of the waste collection unit). This can provide feedback as to when the waste collection tank might need to be emptied.

[0024] In preferable implementations, the post-processing station further comprises a filter provided in fluidic communication between the air outlet of the first cyclone and the vacuum pump and / or between the air outlet of the second cyclone and the vacuum pump. This may be a high efficiency particulate air (HEPA) filter. This can protect the vacuum pump from the build material.

[0025] In some implementations, the post-processing station may further comprise a build material conveyance device provided between the outlet of the sieving unit and the inlet to the build material collection unit, optionally between the outlet of the sieving unit and the inlet of the reuse collection tank. In some preferable implementations: a first collected build material valve may be provided between the outlet of the sieving unit and the build material conveyance device; and / or, a second collected build material valve may be provided between the build material conveyance device and the inlet to the build material collection unit. Optionally, the second collected build material valve may be provided between the build material conveyance device and the inlet to the reuse collection tank. These arrangements can help to control the passage of reuse build material from the sieving unit.

[0026] In some implementations, the post-processing station further comprises a fresh build material unit operably in fluidic communication with the build material conveyance device via a fresh build material valve; wherein the post processing station is configured to operate in a dosing mode in which the first collected build material valve is closed and the second collected build material valve and the fresh build material valve are open to allow fresh build material to flow from the fresh build material unit to the build material collection unit. This can facilitate an efficient dosing arrangement of a combination of reuse build material and fresh build material, which can then be taken for reuse in a further build. In some implementations, a controller may be configured to switch the first collected build material valve to a closed state and to switch the second collected build material valve and the fresh build material valve to an open state to allow the post processing station to operate in the dosing mode. Preferably, the controller is configured to allow the dosing mode to continue based on the determined amount of collected (sieved) build material (e.g. a measured weight in the build material collection unit) as measured by the (second) sensor. Optionally, the (second) sensor may be a load cell and the measured / determined amount may be a measured weight. In some implementations, an inlet to the build material collection unit, and / or an inlet of the reuse collection tank, may comprise a suctioning portion operably in fluidic communication to the inlet of the first cyclone or to the inlet of the second cyclone.

[0027] According to a further aspect of the invention, there is provided a method of recovering build material from a build volume produced by a 3D printer, the build volume being disposed within a depowdering unit and comprising one or more 3D objects and build material, wherein the method comprises: operating the post-processing station in a first, depowdering, mode, comprising: operating a vacuum pump in fluidic communication with the depowdering unit via a first cyclone having an inlet, an air outlet and a solids outlet, wherein the inlet is in fluidic communication with the depowdering unit and the air outlet is in fluidic communication with the vacuum pump; and fluidically connecting the solid outlet of the first cyclone with a first surface of a sieve (e.g. of a sieving unit) so as to deliver build material from the first cyclone to the first surface of the sieve and to allow sieved build material to pass from the first surface to a second surface of the sieve; and switching from the first, depowdering, mode to a second, cleaning, mode by: fluidically connecting the first surface of the sieve to the vacuum pump via a second cyclone having an inlet, an air outlet and a solids outlet, wherein the inlet is in fluidic communication with the first surface of the sieve and the air outlet is in fluidic communication with the vacuum pump.

[0028] This method may also be a method of operating a post-processing station for recovering build material. This may be the post-processing station as described above.

[0029] In preferable implementations, the switching from the first mode to the second mode further comprises closing the fluidic connection between the air outlet of the first cyclone and the vacuum pump. This can improve the suction of the fluidic connection between the sieving unit and the second cyclone. The switching from the first mode to the second mode may comprise opening a valve located between the sieve (e.g. of the sieving unit) and the inlet of the second cyclone.

[0030] In some implementations, the valve may be a first valve and the switching from the first mode to the second mode may further comprise closing a second valve located between the air outlet of the first cyclone and the vacuum pump. In some implementations, the air outlet of the first cyclone may be in fluidic communication with the vacuum pump via the inlet to the second cyclone and the second valve is located between the air outlet of the first cyclone and the inlet of the second cyclone.

[0031] In preferable implementations, the method may further comprise: measuring a weight of collected (sieved) build material that has passed into the build material collection unit or the reuse collection tank (for example, has passed through a sieve of the sieving unit); and one of: automatically switching from the first mode to the second mode in dependence on the measured weight and / or in dependence on a rate of change of the measured weight; or automatically closing the fluidic communication between the depowdering unit and the vacuum pump. In some implementations, the method may further comprise outputting an instruction to replace a container for collecting build material. This can improve the efficiency with which build material is collected.

[0032] The method may, in some implementations, further comprise: measuring time elapsed in the first mode; and automatically switching from the first mode to the second mode is in dependence on the time elapsed in the first mode.

[0033] In some implementations, the method may further comprise measuring or determining an amount of accumulated build material on the first surface of the sieve, and switching from the first mode to the second mode based on detecting that the measured amount of accumulated build material on the first surface of the sieve exceeds a predetermined threshold amount, wherein the amount is one of weight or height of accumulated build material / powder on the first surface.

[0034] Preferably, the method further comprises: providing a waste collection unit in fluidic communication with the solids outlet of the second cyclone; and sensing the amount of build material collected in the second cyclone, and preferably removing the build material collected in the second cyclone in dependence on the sensed amount. In some implementations, the method may further comprise operating the post-processing station in a dosing mode, comprising: fluidically connecting, via a fresh build material valve, a fresh built material tank of a fresh build material unit to the reuse collection tank (e.g. of a build material collection unit); and operating the reuse powder valve and fresh build material valve to allow dosed amounts of collected build material and fresh build material into the collection unit. In particular, the operating may comprise closing the build material collection valve and opening the fresh build material valve to allow a dosed amount of fresh build material into the reuse collection tank based on the measured weight of collected sieved build material in the reuse collection tank.

[0035] According to a further aspect of the invention, there is provided a controller for causing a postprocessing station to execute the method as outlined above. The controller may be configured for executing the method in the post-processing station as outlined above.

[0036] Any apparatus feature as described herein may also be provided as a method feature, and vice versa.

[0037] Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to apparatus aspects, and vice versa. Furthermore, any, some and / or all features in one aspect can be applied to any, some and / or all features in any other aspect, in any appropriate combination.

[0038] It should also be appreciated that particular combinations of the various features described and defined in any aspects of the invention can be implemented and / or supplied and / or used independently.

[0039] Brief Description of Figures

[0040] Reference is now directed to the drawings, in which:

[0041] Figure 1 is a schematic system overview of a post-processing station according to a first embodiment of the present invention;

[0042] Figure 2 is a schematic system overview of a post-processing station according to a second embodiment of the present invention;

[0043] Figure 3 is a flow chart of the method of recovering build material in the post-processing station; Figure 4 is a flow chart of a detailed method of recovering build material in the post-processing station and cleaning the post-processing station; Figure 5 is an illustration of the first embodiment of the post-processing station in the first, depowdering, mode;

[0044] Figure 6 is an illustration of the first embodiment of the post-processing station in the second, cleaning, mode;

[0045] Figure 7 is an illustration of the second embodiment of the post-processing station in the first, de-powdering, mode;

[0046] Figure 8 is an illustration of the second embodiment of the post-processing station in the second, cleaning, mode;

[0047] Figure 9 is a schematic system overview of the post-processing station with further optional features;

[0048] Figure 10a is a schematic side view of the collection tank in connection with the collection nozzle and funnel;

[0049] Figure 10b is an illustration of the collection nozzle;

[0050] Figure 11 is a variant of Figure 2 in which the three-way valve is provided in form of two separate valves; and

[0051] Figure 12 is a variant of Figure 9 illustrating controlled build material flow from the depowdering unit into the first cyclone.

[0052] In the drawings, like elements are indicated by like reference numerals throughout.

[0053] Detailed Description

[0054] A powder bed fusion process is a process in which particulate build material is deposited layer by layer, and wherein for each object layer a cross section of an object to be formed is fused or consolidated. An illustrative powder bed fusion process is a print and fuse process that comprises depositing a layer of build material (for example, a nylon material such as PA11 or PA12), followed by depositing a material which affects the absorption properties of the material in a pattern corresponding to the cross-section of the object to be printed, and then fusing this crosssection via sintering by applying a heat source. This process is repeated layer-by-layer to build up the 3D object. The resulting build volume therefore comprises the printed 3D object within a volume, or “cake”, of unfused build material. It is advantageous to recycle the unfused build material which is of sufficiently good quality material to be reused in a further build process, typically mixed in particular ratios with fresh build material. To this aim, the build volume can be placed in a post-processing system configured for the recovery of build material and the 3D printed objects. Figure 1 shows a schematic overview of a post-processing system 1 configured for recovering unfused build material from a build volume. In particular, the post-processing system 1 of Figure 1 comprises a depowdering unit 10, a first cyclone 20, a sieving unit 30, a second cyclone 40, a filter 50, a vacuum pump 60, a waste collection unit 70 and a build material collection unit 80.

[0055] The depowdering unit 10 is configured for performing a ‘break-out’ of a build volume received from a 3D printer, the build volume comprising one or more 3D printed objects 2 formed of fused build material and unfused build material 4, and the ‘break-out’ comprising the separation of these 3D objects 2 and the unfused build material 4. The depowdering unit 10 comprises a mesh floor 14 on which the build volume can be placed, the mesh floor 14 having a mesh size such that the 3D objects remain on the mesh floor 14 and the unfused build material 4 passes through. The unfused build material 4 may typically comprise small debris including clumps of partially fused build material, and so the mesh floor 14 typically has a suitably large mesh to accommodate this. The depowdering unit 10 further comprises a vibration motor 12 configured to vibrate the mesh floor 14, for example by applying pneumatic vibrations. Additionally, air nozzles 16 are typically located within the depowdering unit 10 to blow air jets onto (typically down onto) the build volume, which can further enhance and speed up the removal of build material 4 from the 3D objects 2. The air jets from the air nozzles 16 may be output in bursts in a manner that may be controllable to control the amount of unfused build material 4 removed.

[0056] The depowdering unit 10 is in fluidic connection with the vacuum pump 60 via at least the first cyclone 20 and in some implementations and modes also via the second cyclone 40. The connection to the vacuum pump 60 is via a filter 50, which can help to prevent the vacuum pump 60 from becoming contaminated or clogged with particles of build material. The filter 50 may for example be a high-efficiency particulate air (HEPA) filter, which would allow the system to run without external extraction.

[0057] The outlet 15 of the depowdering unit 10 is in fluidic connection to the inlet 22 of the first cyclone 20. This allows unfused build material 4 which has passed through the mesh floor 14 of the depowdering unit 10 to pass to the first cyclone 20. The first cyclone 20 comprises an inlet 22, an air outlet 24 at an upper portion, and a solids outlet 26 at its base. The vortex generated by the vacuum pump within the cyclone 20 separates solids (i.e. the powder build material) from the air, outputting the solids via the solids outlet 26 at the base and air via the air outlet 24 at an upper portion (for example, the top) of the cyclone 20. This is a known process and will not be explained in further detail here. The air outlet 24 is in fluidic connection with the vacuum pump 60 via the filter 50. There may also optionally be provided a valve, such as a butterfly valve, between the air outlet and the filter 50, which is open when the system is in the depowdering mode (as will be discussed in further detail later) and hence shall be referred to herein also as the depowdering mode valve 100.

[0058] As mentioned above, the build material 4 from the depowdering unit 10 comprises unfused build material, which would be suitable for recycling for use in a further build, and debris, which would not be suitable for recycling. The solids outlet 26 of the first cyclone 20 is therefore in fluidic connection with a sieving unit 30, which is configured for separating debris from the unfused build material. The sieving unit 30 comprises a sieve 34 and a vibration motor 32 for vibrating the sieve 34, to enhance the flow of material passing through the sieve. The sieve 34 comprises a first surface and second surface, which may be an upper and lower surface in gravitational direction, between which apertures form a mesh which is finer than that of the mesh floor 14 within the depowdering unit 10. This is typically the case because the mesh floor 14 of the depowdering unit is simply to hold the 3D objects and its relatively larger mesh provides the advantage of facilitating speedy break-out, while the sieve 34 is configured to separate debris from good quality fine unfused powder 6 which can be recycled. The size and material of the mesh of the sieve may be dependent on the particular build material 4 powder, for example the mesh or aperture sizes may be between 100 pm and 250 pm, for example 180 pm. The first surface and the second surface can be considered ‘opposite’ to one another as they face in opposite directions. In particular, the first surface receives build material from the solids outlet 26 of the first cyclone 20. Good quality unfused powder 6 passes through the sieve towards and through the second surface. Any powder 6 passing through the second surface of the sieve 34 can then travel towards the outlet of the sieving unit 30.

[0059] In connection with the outlet of the sieving unit 30 is provided a build material collection unit 80, which typically comprises a reuse collection tank 82 for receiving the collected build material. The transport of the sieved unfused build material to the reuse collection tank 82 is optionally facilitated and enhanced by a conveyance device, such as a powderpump 84. The transport of the build material can be controlled by an optional first build material collection valve 106a, located between the outlet of the sieving unit 30 and the conveyance device (e.g. powderpump) 84, and / or a second build material collection valve 106b, located between the conveyance device (e.g. powderpump) 84 and the reuse collection tank 82.

[0060] The system further comprises a second cyclone 40 comprising an inlet 42, an air outlet 46 and a solids outlet 48. The inlet 42 is fluidically connected to an aperture 36 in an upper portion of the sieving unit 30; in particular, in a portion located above the sieve 32. This arrangement facilitates cleaning the sieve 32 of the debris remaining on the sieve from the unfused build material. There is provided a cleaning mode valve 102 between aperture 36 of the sieving unit 30 and the inlet 42 of the second cyclone 40 to allow this connection to be open and closed. The air outlet 46 of the second cyclone is in fluidic connection with the vacuum pump 60, via the fdter 50.

[0061] The solids outlet 48 of the second cyclone 48 is in fluidic connection with a waste collection unit 70, typically comprising a waste collection tank 72, which receives the waste (debris) material removed from the first surface of the sieve 34 by the second cyclone 40. The waste collection tank 72 may comprise a sensor 74, which senses the amount of collected waste material and is configured to output an indication when a particular amount is reached, so that the waste material can be disposed of, for example by replacing the waste collection tank 72 or by another means of removing the waste material. For example, the waste collection unit 70 may be in connection with a vacuum system (for example a vacuum cleaner). This may be a non-permanent, disconnectable connection, or it may be a permanent connection to an industrial suction system. The connection may be controllable via an optional waste valve 104.

[0062] Figure 2 shows an alternative arrangement of the system 1, in which the components are the same but the connection between the first cyclone 20 and the vacuum pump 60 is made via the second cyclone 40, rather than being a direct connection. In particular, the air outlet 24 of the first cyclone is in fluidic connection with the inlet 42 of the second cyclone 40, and the air outlet 46 of the second cyclone 40 is in connection with the vacuum pump 60 (via the filter 50). This arrangement can be advantageous as it can provide protection against unclean air being output from the air outlet 24 of the first cyclone 20 (for example, in the case that the sieve 34 blocks up) and any build material or other debris then ending up in and blocking the filter 50. Instead, in the arrangement in which the air outlet 24 of the first cyclone 20 is in fluidic connection with the inlet 42 of the second cyclone 40, any build material in the air from the air outlet 24 would be separated from the air by passing through the second cyclone 40 and be deposited out of the solids outlet 48 of the second cyclone 40, while cleaned air passes through the air outlet 46 and to the vacuum pump 60 via the filter 50. In the illustrated implementation of Figure 2, the air outlet 24 of the first cyclone 20 and the upper portion of the sieving unit 30 are both connected to a three-way valve 1020 controlling the connection to the inlet 42 of the second cyclone 40. The three-way valve fulfils the functions of the cleaning mode valve 102 and the depowdering mode valve 100 of Figure 1. However, the invention could be effectively implemented by connecting the air outlet 24 to the fluidic communication between the aperture 36 of the sieving unit 30 and the inlet 42 of the second cyclone 40 at a location between a cleaning mode valve 102 and the inlet 42 instead, as will be described below with reference to Fig. 11.

[0063] Figure 3 shows an overview of the method by which the post-processing system 1 may be operated in the context of 3D printing using a powder bed fusion process. At step 210, the 3D object 2 is printed in the printer using a layer-by-layer powder bed fusion process. This results in a build volume comprising the 3D object 2 (or, typically, multiple objects) within unfused build material 4. At step 220, this build volume is removed from the printer and, at step 230, is loaded into the depowdering unit 10 of the post-processing system 1. Depowdering of the build volume is performed at step 240 and the unfused build material is recovered for recycling at step 250.

[0064] Figure 4 shows a more detailed flow of the processes within steps 240 and 250 which form the depowdering mode according to the present invention, in addition to processes 270 and 280 for removing any waste build material from the sieve unit. In particular, the depowdering step 240 comprises initiating the breakout operation, step 242, when the build volume is within the depowdering unit 10. This typically comprises placing the depowdering unit 10 in fluidic connection with and operating the vacuum pump 60. The process may further comprise vibrating the mesh floor 14 by operating the depowdering unit vibration motor 12, and / or by directing air jets onto the build volume from air nozzles 16 in order to break up the build volume and separate the 3D objects 2 from the unfused build material 4. The steps of Figure 4 will now be further illustrated and described with reference to Figures 5 to 8.

[0065] Figure 5 illustrates the system 1 of Figure 1 when in the depowdering mode and Figure 7 illustrates the system 1 of Figure 2 when in the depowdering mode; in particular, the open connections are illustrated as solid lines and the closed connections as dashed lines. The depowdering unit 10 is in fluid connection with the vacuum pump 60 via the first cyclone 20. The build material 4 which is separated from the 3D objects 2, at step 244, passes through the mesh floor 14 due to a combination of gravity, the vacuum and the air nozzles 16 (if used), and passes out of the depowdering unit 10 via the outlet 15 and to the inlet 22 of the first cyclone 20. The vortex of the first cyclone 20 carries the build material 4 down and out through the solids outlet 26 to the sieving unit 30. At optional step 246, the sieve 30 is vibrated by the sieving unit vibration motor 32 to aid in the throughput of the unfused build material 4. This may comprise the use of ultrasonic vibrations. Passing the unfused build material 4 through the relatively fine mesh of the sieve 34 ensures that only unfused build material of the correct particle size is collected. The unfused build material which passes through the sieve is good quality, fine unfused build material 6, which can be used in a further build process. At step 252, this sieved unfused (or “recovered”) build material 6 is collected, so that it can be reused. After a period of time over which the sieved unfused build material 6 is collected, it is determined at step 254, that there is no more unfused build material 6 for reuse to be sieved and collected. This can be implemented in a number of manners independently or in combination. In one implementation, the system 1 is configured to run in depowdering mode for a set time period, typically chosen on the basis of empirical experience of an average duration of time in which most of the good quality unfused build material 6 can be collected. In some implementations, a sensor, such as a load cell, may optionally be used to measure the amount, e.g. the weight or level, of collected sieved build material 6. The collection of sieved (unfused) build material 6 into the reuse collection tank 82 may be considered to be complete when the weight detected by the loadcell 83 has reached a particular value and / or when the rate of change of weight reaches a certain value. In particular, the collection may be stopped when the flow of sieved (unfused) build material 6 drops below a threshold, which is typically indicative that only the very last of the unfused build material remains. Any unfused build material 4 that remains in the depowdering unit 10 is likely to be damaged powder attached to the 3D objects 2, and thus not suitable for reuse. (In some implementations, the collection of the build material 4 may also be temporarily halted when the sieved unfused build material 6 within the reuse collection tank 82 reaches a certain level or weight, for example as determined by weight registered by a load cell 83 as illustrated in Fig. 9 and applicable to any of the implementations disclosed herein; however, this is typically only a temporary pause while the reuse collection tank 82 is replaced and does not change the operating mode.)

[0066] At step 270, the system is switched from the depowdering mode, as described above, to a cleaning mode. This process mode is configured to clean the sieve 34 of any debris and (in some instances unfused) build material 4 remaining on the first surface of the sieve 34. This is achieved, in step 272, by opening a valve, e.g. the ‘cleaning mode valve’, or first valve, 102, that fluidically connects the aperture 36 above the sieve 34 and the inlet 42 of the second cyclone 40. This opens a lower resistance flow path than that of the communication path between the air outlet 24 of the first cyclone 20 and the vacuum pump 60, and therefore facilitates the suction of any residue build material 4 which remains on the sieve 34 through the aperture 36 and into the second cyclone 40 to thus provide the cleaning mode at step 280. During the cleaning mode, the first cyclone 20 effectively acts as a simple pipe conduit. In the embodiment as illustrated in Figure 2, this step comprises changing the status of the three-way valve 1020 to open the connection between the aperture 36 and the second cyclone 40. Optionally, the connection between the air outlet 24 of the first cyclone 20 and the vacuum pump 60 (which may be via the filter 50, and in some cases also via the second cyclone 40) is closed, illustrated as optional step 274 by the dashed outline in Figure 4. This can be achieved by closing the “depowdering mode valve”, or second valve, 100 in Figure 1. Although the connection between the aperture 36 and the inlet 42 of the second cyclone 40 presents a lower resistance path to build material flow than the flow path through the first cyclone 20, closing the flow path between the air outlet 24 of the first cyclone 20 and the vacuum pump 60 (in some cases, via the second cyclone 40, as for the arrangement illustrated in Figure 2) can improve the suction of the cleaning path between the aperture 36 and the second cyclone inlet 42. As such, this can help to ensure that the build material 4 remaining on the sieve 34 is suctioned effectively.

[0067] Figure 6 illustrates the system 1 of Figure 1 in the cleaning mode, in which the depowdering mode valve 100 (“second valve”) is closed and the cleaning mode valve 102 (“first valve”) is open; in particular, the open connections are illustrated as solid lines and the closed connections as dashed lines. In some implementations, the depowdering mode valve 100 may be absent and it is sufficient to simply open the cleaning mode valve 102, because the connection between the aperture 36 and the inlet 42 of the second cyclone 40 has a sufficiently low resistance to ensure that there is enough flow through aperture 36 and that agglomerates on first surface of the sieve are suctioned into the second cyclone as outlined above. Similarly, Figure 8 illustrates the system 1 of Figure 2 in the cleaning mode, in which the three-way valve 1020 is in a configuration such that the connection between the aperture 36 in the upper portion of the sieving unit 30 and the inlet 42 of the second cyclone 40 is open, and the connection between the air outlet 24 of the first cyclone 20 and the inlet 42 of the second cyclone 40 is closed. Again, it is sufficient simply that the connection between the aperture 36 in the upper portion of the sieving unit 30 and the inlet 42 of the second cyclone 40 is open.

[0068] In step 282 of the cleaning process 280, the sieve 34 is cleaned by suctioning any waste material deposited on the first surface of sieve 34 through the aperture 36 to the inlet 42 of the second cyclone 40, and via the vortex of the second cyclone to the solids outlet 48. This method therefore provides an automatic cleaning of the sieve 34. Step 270 may be initiated during step 240 and / or step 250. This may be done at predefined time intervals and durations. Alternatively, a sensor may be provided in the sieving unit 30 that is configured to measure or determine the amount (e.g. the weight or height) of build material that accumulates on the upper surface of the sieve 34. Based on detection of a predetermined threshold in weight or height, the cleaning mode may be temporarily applied before returning to the depowdering mode. The sensor may, for example, be a weight, vibration or optical sensor.

[0069] At step 284, the waste material collected by the second cyclone is received from the solids outlet 48 into a waste collection unit 70 comprising a waste collection tank 72. The waste collection unit 70 may be connected to a vacuum suction system by which the waste material can be removed from the waste collection tank 72. This may be performed intermittently (controlled by the waste valve 104), for example it may be automatically initiated in dependence on an amount measured of the waste material within the waste collection tank 72, and upon the measured amount reaching a particular value, as determined by the sensor 74.

[0070] The method as described above may be implemented automatically by a controller. The controller is in communication with at least the cleaning mode valve 102 (or correspondingly, the three-way valve 1020) in order to switch the system from the depowdering mode to the cleaning mode. In some implementations the controller is further in communication with various sensors of the system, for example with the sensor of the sieving unit (herein also “first sensor”), the sensor of the build material collection unit 80 (herein also “second sensor”), for example the load cell 83, and / or with the sensor 74 on the waste collection unit 70 (herein also “third sensor”), the information from which can provide an indication of when to perform the switch from the depowdering mode to the cleaning mode. The controller may further be in communication with other components of the system such as with any of the valves of the system to instruct their actuation.

[0071] Alternatives and modifications

[0072] Detailed embodiments and some possible alternatives have been described above. As those skilled in the art will appreciate, a number of modifications and further alternatives can be made to the above embodiments whilst still benefiting from the inventions embodied therein. It will therefore be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto.

[0073] For example, the three-way valve 1020 of Figure 2 could be replaced with two individual valves, one on the connection between the aperture 36 of the sieving unit 30 and the inlet 42 of the second cyclone 40 and (optionally) a further one between the air outlet 24 of the first cyclone 20 and the inlet 42 of the second cyclone 40. This is illustrated in Figure 11 which is a variant of Figure 2, in which the air outlet 24 of the first cyclone 20 is connected to the inlet 42 of the second cyclone 40 via a depowdering mode valve 100. In the depowdering mode, the cleaning valve 102 is closed and the depowdering valve 100, where present, is open. In the depowdering mode, the pump 60 is operated and applies suction to the air outlet 24 of the first cyclone 20. In the cleaning mode, the cleaning valve 102 is open and the depowdering valve 100 is closed, and with the pump 60 operating, the inlet 42 of the second cyclone 40 applies suction to the first surface of the sieve 34 to remove any build material on the first surface. Such an arrangement can effectively provide similar or the same control over the connections. The depowdering mode valve 100 between the air outlet 24 of the first cyclone 20 and the inlet 42 of the second cyclone 40 is optional. For example, in at least some implementations the connection between the aperture 36 of the sieving unit 30 and the inlet 42 of the second cyclone 40 provides a build material flow path of sufficiently low resistance, or equal resistance, compared to the flow path between the air outlet 24 of the first cyclone 20 and the inlet 42 of the second cyclone. This may allow the connection between the air outlet 24 and the second cyclone inlet 42 to remain open in the cleaning mode.

[0074] Figure 9 shows the post-processing system 1, including optional modifications, which may be provided in any combination. By way of example, the depowdering unit 10 of any of the variants described herein may comprise a three-way connector 17 at its base connecting to the outlet of the depowdering unit. Instead of, as shown in previous variants of the depowdering unit 10, a simple connection between the base of depowdering unit 10 and the vacuum pump 60, the three- way connection 17 also connects to an aperture 18 in an upper part of the depowdering unit 10 via a vacuum connector 19. This arrangement can facilitate creating a slightly negative pressure within the depowdering unit 10, to counteract the effect on positive air pressure caused by the air jets from the air nozzles 16 which may otherwise lead to dust escaping from the depowdering unit through insufficient sealing.

[0075] Another arrangement of the depowdering unit 10 is shown in Figure 12, which is a variant of Figure 9, and in which any of the additional or alternative features shown are equally applicable to any of the other arrangements described herein. For example, the flow of build material 4 (i.e. the build material not forming part of the build, which may be unfused) from the depowdering unit 10 may be controlled by providing a valve 110 between the outlet 15 of the depowdering unit 10 and the inlet 22 into the first cyclone 20. In the depowdering mode, the outlet valve 110 is opened intermittently, for example at regular intervals, to permit flow of build material 4 into the first cyclone 20 and towards the sieve 34. It was found that at the start of the depowdering mode, the rate at which the build material 4 is removed from the build is so large that it tends to block the first cyclone 20 and / or resides on the first surface of the sieve 34 since it cannot fall fast enough through the sieve 34 into the reuse collection tank 82. Excessive weight of the build material 4 on the sieve 34 causes the vibrations applied by the motor 32 to be insufficient to remove the accumulated build material 4 from the first surface of the sieve 34, while applying a higher power to the motor may cause the sieve 34 to fail. The outlet valve 110 may allow a controlled flow of for example lOg / sec, or 50g / sec, or any other suitable flow rate depending on the flow resistance between the outlet 15 of the depowdering unit 10 and the sieve 34. In variants, the outlet valve 110 may be controlled to open and close based on feedback of a measurement device configured to detect the amount of build material 4 on the sieve 34. Detecting may be by any suitable means, for example by a sensor 33 such as a weight sensor configured to monitor the weight of accumulated build material on the upper surface of the sieve 34, or a vibration sensor configured to measure the amplitude of the vibrations of the sieve 34. Where the sensor 33 is a vibration sensor, the controller may be configured to receive the measured amplitudes from the vibration sensor and to compare the peak amplitude of the vibrations of the sieve 34 to the power input of the motor 32. From empirical measurements, a lookup table may be provided that maps the measured weight, or the weight determined from a frequency and / or amplitude detected, to expected weight values of the unfused build material powder 4 on the sieve 34, and upon detecting or determining that the weight or height of accumulated build material exceeds a certain predetermined threshold that indicates excessive accumulated build material 4 on the sieve 34, the valve 110 is closed. The sensor 33 may alternatively be an optical sensor mounted at a location above the first surface of the sieve 34 and configured to detect a height of accumulated build material 4 on the first surface of the sieve 34. The controller may be configured to determine, based on the height or weight of the accumulated build material 4, that a predetermined threshold height has been reached or exceeded, and to initiate the second mode of operation and / or closing of the outlet valve 110 until the height or weight of accumulated build material 4 is decreased to an amount below the predetermined threshold. In some cases this may lead to build material 4 building up in the collection funnel of the depowdering unit 10 while the outlet valve 110 is kept closed. This may be mitigated by applying a comparatively gentle depowdering mode while the valve 110 is shut compared to when it is open to reduce the amount of build material 4 falling through the mesh 14 of the depowdering unit 10. In this way, it may be prevented that too much build material 4 weighs down on the sieve 34, causing process failure and / or failure due to wear of the sieve 34 and / or motor 32. With this arrangement, if negative suction is to be provided to the upper chamber of the depowdering unit 10, the connection between the vacuum connector 19 and the vacuum pump 60 is provided via a location between the outlet valve 110 and the inlet 22 to the first cyclone 20.

[0076] The flow path 19 may beneficially be of a lower or of at least the same flow resistance to that of the flow path from the outlet 15 of the depowdering unit 10 to a connection C of the flow path 19 via the outlet valve 110. This ensures that, while the valve 110 is open, suction is still applied by the vacuum pump 60 to the interior of the depowdering unit 10 via the connection 18.

[0077] In another arrangement, instead of providing an outlet valve 110, controlled flow may be achieved by controlling the flow from the solids outlet 26 of the first cyclone 20, by a valve or any other suitable dosing mechanism, however this may require a buffer tank between the first cyclone 20 and the sieve 34.

[0078] Returning to Figure 9, also shown is an optional arrangement for the collection and reuse of sieved (unfused) build material 6 collected in the reuse collection tank 82 in combination with dosing a controlled amount of fresh build material into the reuse collection tank 82. In this implementation, the reuse collection tank 82 is provided on a load cell 83, configured to determine the weight of sieved unfused build material 6 being collected in the reuse collection tank 82. This can be used for controlled dosing of a desired ratio of sieved unfused build material 6 to virgin, or new (unused), build material. In the illustrated implementation, a fresh build material (e.g. ‘virgin’ powder) unit 90 comprising a ‘virgin’ or ‘fresh’ build material tank 92 is provided. The fresh build material tank 92 is in fluidic connection with the reuse collection tank 82 via a fresh build material valve 108 for controlling the dosing of the fresh build material (e.g. virgin powder) into the reuse collection tank 82. This arrangement facilitates the reuse collection tank 82 being filled with a measured, dosed combination of recovered collected sieved build material 6 and fresh, virgin build material. The reuse collection tank 82 may be positioned on a load cell 83 that measures the weight of build material flowing into the reuse collection tank 82. Controlled dosing of fresh build material then may be carried out based on the weight of the recovered collected sieved build material and a predetermined ratio, from which the required weight of fresh build material to be filled into the reuse collection tank 82 may be determined. The fresh build material valve 108 may be operated in response to the weight of fresh build material flowing into the collection tank 82 during a controlled dosing mode. In a variant of the dosing mode, the fresh build material may be added intermittently after a certain amount of recovered collected sieved build material has been added to the reuse collection tank 82, by operating the conveyance device, for example a powderpump 84 and / or the optional first build material collection valve 106a only intermittently based on the detected weight in the reuse collection tank 82 in the refill mode. In the dosing mode, the conveyance device (e.g. powderpump) 84 may be switched off, and / or the optional first build material collection valve 106a may be closed, and the fresh build material valve 108 opened. In a variant, the conveyance device (e.g. powderpump) 84, the first build material collection valve 106a and the fresh build material valve 108 are provided. The conveyance device (e.g. powderpump) 84 may be connected as shown in Figure 12, in which an outlet of the fresh build material tank 92 is in fluidic communication with the conveyance device (e.g. powderpump) 84 via the fresh build material valve 108. By controlling the operation of the conveyance device (e.g. powderpump) 84 and the two valves 106a, 108, the transport of fresh, virgin build material from the fresh build material tank 92 to the reuse collection tank 82 can be facilitated. The conveyance device (e.g. powderpump) 84 may be operated continuously, and the filling mode (typically provided during the depowdering mode) and dosing mode are switched by controlling the first build material collection valve 106a and the fresh build material valve 108, for example based on the measured weight by the load cell 83. The filled reuse collection tank 82 may further comprise mixing elements to stir and mix the dosed build material to a homogeneous mixture and / or the dosed build material in the reuse collection tank 82 may be used for a new build process. As an example, the reuse collection tank 82 may be removed from the system 1 for filling a build material supply container for or of a powder bed fusion apparatus. The reuse collection tank 82 may thus be a removable reuse collection tank 82. Similarly, the waste collection tank 72 may be a removable and replaceable waste collection tank 72 that may be replaced once full during or after the depowdering mode or dosing mode. The fresh build material unit 90 and / or the fresh build material tank 92 may be external or internal to the system 1, and the fresh build material tank 92 may be a removable and replaceable fresh build material tank 92.

[0079] The arrangement of the build material transfer path between the air outlet 24 of the first cyclone and the inlet 42 into the second cyclone as shown in Figure 11 may be used to modify any of the variants described herein in which the air outlet 24 is connected directly to the vacuum pump 60 via the filter 50. For example, in Figure 12, the air outlet 24 of the first cyclone may be connected to the inlet 42 of the second cyclone via the depowdering mode valve 100.

[0080] Figure 9 further shows a modified arrangement for the collection of the recovered build material 6 to the reuse collection tank 82, via a suctioning cap 85 and a funnel 86 provided between the outlet of the sieving unit 30 and the reuse collection tank 82. This arrangement is further illustrated in a schematic cross section view in Figure 10a. A perspective view of the lower surface of suctioning cap 85 is shown in Figure 10b. In particular, the filling funnel 86 is arranged on top of the aperture to the reuse collection tank 82, the upper aperture of the funnel 86 having a smaller diameter than the lower aperture. The lower aperture is adjacent the aperture of the reuse collection tank 82 and so will typically have a corresponding diameter. The funnel 86 may for example provide extra volume for the collected sieved build material 6, to allow for a lower packing density during collection before the recovered build material 6 settles to a higher density. The collected build material 6 may be settled and compacted by applying vibration, for example simply by tapping the reuse collection tank 82, before the reuse collection tank 82 is removed from the post processing system. Between the filling funnel 86 and the suctioning cap 85, a small air gap A is provided. This facilitates easy replacement of the reuse collection tank 82, for example by simply sliding the reuse collection tank 82 out of the post processing system without having to manually uncouple or disconnect the reuse collection tank 82 from the funnel. In addition, where a load cell 83 is provided to measure the build material weight in the reuse collection tank 82, the gap further ensures that only the known weight of the tank 82 and the collected sieved unfused build material 6 is measured. However, the air gap A can cause dust (of the recovered build material 6 or of fresh virgin build material) to escape during filling. This can be alleviated by the configuration of the suctioning cap 85 as illustrated in Figure 10b. In particular, the suctioning cap 85 comprises a central downward conduit 87, through which the collected build material 6 can travel into the collection tank 82, and peripheral upward suction conduits 89 connected to an external suction conduit 88 which may be connected to a vacuum system, such as to the vacuum pump 60 (e.g. via filter 50). The peripheral upward suction conduit 89 can reduce overpressure generated by the downward conduit 87 during filling and helps to draw the build material into the reuse collection tank 82 and minimise the escape of any dust.

[0081] The valves described herein may be any suitable valve type used in conveying powdery build material. Any or all of the valves may for example be butterfly valves that facilitate stopping the flow of material in either direction. In some variants, a pump may act as a valve. For example, the valve 106a between the outlet of the sieve unit 30 and the reuse collection tank 82 may not be necessary if a conveyance device, such as a powderpump 84 is present. The valve 104 at the outlet of the waste collection tank 72 may be a one-way valve that allows the flow of waste material upon connection to a suctioning device to remove the waste material from the waste collection tank 72. The reverse flow is prevented by the one-way valve.

[0082] It will be appreciated that while various valves and the conveyance device 84 are indicated in the Figures are being part of a unit, e.g. conveyance device 84 is shown as comprised within the build material collection unit 80, any or any combination of them may be intermediate devices between the various units and be connected to the respective tanks within the units as described herein.

[0083] The sensors described herein and provided to measure or sense the build material amount, such as the sensor 74 of the waste collection tank 72, or the sensor of reuse build material collection tank 82 of the build material collection unit 80, may for example be one or more of capacitive or piezoelectric sensors arranged at or near the threshold height level of build material within the reuse collection tank 82; optical sensors arranged to detect a build material level position or height; and / or a load cell, for example. A load cell 83 is for example described as type of sensor for the reuse build material collection tank 82, arranged to measure the weight of the amount of build material within the respective tank. Such sensors may be used to control the operation of the system, and a method of operating the system 1 may further comprise any or any combination of:

[0084] (i) based on the measured amount of unfused build material 4 in the funnel of the depowdering unit 10, controlling the air jets of nozzles 16 and / or the control valve 110 and / or a control valve between the first cyclone and the sieve 34 to restrict the amount of unfused build material 4 reaching or collecting on the sieve 34; and optionally controlling the amount of unfused build material 4 collecting in the funnel of the depowdering unit 10 in variants in which the outlet 15 of the depowdering unit 10 can be shut by the control valve 110;

[0085] (ii) based on the measured amount of accumulated build material on the sieve 34, in addition to any or any combination of control options under (i), controlling the power to the vibration motor 32 to enhance or reduce the vibrations of the sieve 34 so as to increase the flow of unfused build material through the sieve 34 or to reduce wear on the sieve unit respectively; and switching from the depowdering mode to the cleaning mode when the measured amount exceeds a certain (threshold) value, and switching from the cleaning mode to the depowdering mode when the measured amount falls to or below the certain value;

[0086] (iii) based on the measured amount of waste build material in the waste collection tank 72, determine one or more of: determine that the cleaning mode is to be paused and that the waste collection tank 72 requires emptying; determine during the depowdering mode from an unexpectedly high amount of build material collected in the waste collection tank 72 and / or an unexpectedly high rate of rise in the amount measured, that the sieve 34 and / or the first cyclone 20 is blocked and / or the control valve 110 is erroneously closed; (iv) based on the measured amount of recovered build material in the reuse collection tank 82 during a filling mode of the reuse collection tank 82, determine one or more of: that the depowdering mode is complete when the amount reaches a certain (threshold) value and / or the rate of change of the amount falls below a certain (threshold) value; determine that the sieve 34 and / or the first cyclone 20 is blocked and / or the control valve 110 is erroneously closed when the amount of sieved build material 6 collected in the reuse collection tank 82 during the depowdering mode is lower than expected, optionally in combination with the detected amount in the waste collection tank 72; in a dosing mode, determine, from the measured amount of recovered build material in the reuse collection tank 82, an amount of virgin build material to be dosed to the reuse collection tank 82 to achieve a predetermined ratio of sieved unfused build material 6 to virgin build material.

[0087] Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “containing”, means “including but not limited to”, and is not intended to (and does not) exclude other components, integers or steps.

Claims

CLAIMS1. A post-processing station for recovering powder build material from a build volume produced by a 3D printer wherein the build volume comprises one or more 3D objects and build material, the post-processing station comprising: a depowdering unit; a vacuum pump; a first cyclone, having an inlet, an air outlet and a solids outlet, wherein the inlet of the first cyclone is operably in fluidic communication with the depowdering unit and wherein the air outlet of the first cyclone is operably in fluidic communication with the vacuum pump; a sieving unit comprising a sieve in fluidic communication with the solids outlet of the first cyclone, wherein the sieve comprises a first surface to which the solids outlet of the first cyclone delivers build material, and a second surface through which sieved build material passes; a second cyclone, having an inlet, an air outlet and a solids outlet, wherein the inlet of the second cyclone is operably in fluidic communication with the first surface of the sieve, and the air outlet of the second cyclone is operably in fluidic communication with the vacuum pump; and a valve, located between the sieving unit and the inlet of the second cyclone; wherein during operation of the vacuum pump the post-processing station is configured to operate in a first, depowdering, mode, in which the valve is closed, and in a second, sievecleaning, mode, in which the valve is open.

2. The post-processing station of claim 1, wherein the air outlet of the first cyclone is operably in fluidic communication with the vacuum pump via the inlet to the second cyclone.

3. The post-processing station of claim 1 or 2, wherein the sieving unit comprises a first sensor configured to measure an amount of accumulated build material on the first surface of the sieve, wherein the post processing station is configured to switch between the first and second mode based on the measured amount of accumulated build material on the first surface of the sieve.

4. The post-processing station of claim 1 or 2 configured to switch between the first and second mode in dependence on a time elapsed in the first mode.

5. The post-processing station of any one of claims 2 to 4, wherein the valve is a first valve and the post-processing station further comprises a second valve located between the air outlet of the first cyclone and the inlet of the second cyclone, and wherein in the first mode the second valve is open and in the second mode the second valve is closed.

6. The post-processing station of any one of claims 2 to 4, wherein the valve is a first valve and the post-processing station further comprises a second valve located between the air outlet of the first cyclone and the vacuum pump, and wherein in the first mode the second valve is open and in the second mode the second valve is closed.

7. The post-processing station of any preceding claim, wherein the sieving unit comprises an outlet configured to allow sieved build material to exit the sieving unit, and wherein the postprocessing station further comprises a build material collection unit in fluidic connection with the outlet of the sieving unit for collecting sieved build material, optionally wherein the build material collection unit comprises a reuse collection tank for collecting sieved build material and a second sensor for determining the amount of collected sieved build material.

8. The post-processing station of claim 7, further comprising a controller configured to switch the post-processing station from the first mode to the second mode in dependence on at least one of: the amount of collected sieved build material; and a rate of change of the amount of collected sieved build material; optionally wherein the controller is configured to stop the vacuum pump in dependence on determining an amount of collected sieved build material.

9. The post-processing station of any preceding claim, further comprising the or a controller configured to switch the post-processing station from the first mode to the second mode.

10. The post-processing station of any preceding claim, wherein the depowdering unit comprises a mesh platform and wherein the depowdering unit is fluidically connected to: the inlet of the first cyclone below the mesh platform, and preferably wherein the depowdering unit is further fluidically connected to the vacuum pump above the mesh platform.

11. The post-processing station of any preceding claim, wherein the sieve is vibratable, preferably wherein the sieve is ultrasonically vibratable.

12. The post-processing station of any preceding claim, further comprising a waste collection unit in fluidic communication with the solids outlet of the second cyclone; optionally wherein the waste collection unit comprises a waste collection tank and a third sensor for determining a build material amount within the waste collection tank.

13. The post-processing station of claim 7, or of any one of claims 8 to 12 when dependent on claim 7, further comprising a build material conveyance device provided between the outlet of the sieving unit and the inlet to the build material collection unit, optionally between the outlet of the sieving unit and the inlet of the reuse collection tank.

14. The post-processing station of claim 13 or of any one of claims 8 to 12 when dependent on claim 7, wherein: a first collected build material valve is provided between the outlet of the sieving unit and the build material conveyance device; and / or a second collected build material valve is provided between the build material conveyance device and the inlet to the build material collection unit, optionally wherein the second collected build material valve is provided between the build material conveyance device and the inlet to the reuse collection tank.

15. The post-processing station of claim 13 or 14, further comprising a fresh build material unit operably in fluidic communication with the build material conveyance device via a fresh build material valve; wherein the post processing station is configured to operate in a dosing mode in which the first collected build material valve is closed and the second collected build material valve and the fresh build material valve are open to allow fresh build material to flow from the fresh build material unit to the build material collection unit.

16. The post-processing station of claim 15, comprising the or a controller, wherein the controller is configured to switch the first collected build material valve to a closed state and to switch the second collected build material valve and the fresh build material valve to an open state to allow the post processing station to operate in the dosing mode.

17. The post-processing station of claim 16 when dependent on claim 7, wherein the controller is configured to allow the dosing mode to continue based on the determined amount of collected sieved build material as measured by the second sensor, optionally wherein the second sensor is a load cell and wherein the measured amount is a measured weight.

18. The post-processing station of claim 7, or of any one of claims 8 to 17 when dependent on claim 7, wherein an inlet to the build material collection unit, or an inlet of the reuse collection tank, comprises a suctioning portion operably in fluidic communication to the inlet of the first cyclone or to the inlet of the second cyclone.

19. A method of recovering build material from a build volume produced by a 3D printer, the build volume being disposed within a depowdering unit and comprising one or more 3D objects and build material, wherein the method comprises:operating the post-processing station in a first, depowdering, mode, comprising: operating a vacuum pump in fluidic communication with the depowdering unit via a first cyclone having an inlet, an air outlet and a solids outlet, wherein the inlet is in fluidic communication with the depowdering unit and the air outlet is in fluidic communication with the vacuum pump; and fluidically connecting the solids outlet of the first cyclone with a first surface of a sieve so as to deliver build material from the first cyclone to the first surface of the sieve and to allow sieved build material to pass from the first surface to a second surface of the sieve; and switching from the first, depowdering, mode to a second, cleaning, mode by: fluidically connecting the first surface of the sieve to the vacuum pump via a second cyclone having an inlet, an air outlet and a solids outlet, wherein the inlet is in fluidic communication with the first surface of the sieve and the air outlet is in fluidic communication with the vacuum pump.

20. The method of claim 19, wherein the switching from the first mode to the second mode further comprises closing the fluidic connection between the air outlet of the first cyclone and the vacuum pump.

21. The method of claim 19 or 20, wherein the switching from the first mode to the second mode comprises opening a valve located between the sieve and the inlet of the second cyclone.

22. The method of claim 21, wherein the valve is a first valve and the switching from the first mode to the second mode further comprises closing a second valve located between the air outlet of the first cyclone and the vacuum pump.

23. The method of claim 22, wherein the air outlet of the first cyclone is in fluidic communication with the vacuum pump via the inlet to the second cyclone and the second valve is located between the air outlet of the first cyclone and the inlet of the second cyclone.

24. The method of any of claims 19 to 23, further comprising: measuring a weight of collected sieved build material that passed into the build material collection unit or reuse collection tank; and one of: automatically switching from the first mode to the second mode in dependence on the measured weight and / or in dependence on a rate of change of the measured weight; orautomatically closing the fluidic communication between the depowdering unit and the vacuum pump.

25. The method of any of claims 19 to 23, further comprising: measuring time elapsed in the first mode; and automatically switching from the first mode to the second mode is in dependence on the time elapsed in the first mode.

26. The method of any of claims 19 to 23, further comprising: measuring or determining an amount of accumulated build material on the first surface of the sieve, and switching from the first mode to the second mode based on detecting that the measured amount of accumulated build material on the first surface of the sieve exceeds a predetermined threshold amount, wherein the amount is one of weight or height.

27. The method of any of claims 19 to 26, further comprising: providing a waste collection unit in fluidic communication with the solids outlet of the second cyclone; and sensing an amount of build material collected in the second cyclone, and removing the build material collected in the second cyclone in dependence on the sensed amounts.

28. The method of claim 24, further comprising operating the post-processing station in a dosing mode, comprising: fluidically connecting, via a fresh build material valve, a fresh build material tank of a fresh build material unit to the reuse collection tank; and closing the build material collection valve and opening the fresh build material valve to allow a dosed amount of fresh build material into the reuse collection tank based on the measured weight of collected sieved build material in the reuse collection tank.

29. A controller for causing a post-processing station to execute the method of any of claims 19 to 28.

30. The controller of claim 29, wherein the controller is configured for executing the method of any of claims 19 to 28 in the post-processing station of any of claims 1 to 18.

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