GB2550551A

GB2550551A - Powder material mixer

Info

Publication number: GB2550551A
Application number: GB1608311.5A
Authority: GB
Inventors: Nicolau Marc; Roure Pastor Francesc; Chanclon Fernandez Ismael; Alonso Becerro Xavier; Texido Rafel
Original assignee: Hewlett Packard Development Co LP
Current assignee: Hewlett Packard Development Co LP
Priority date: 2016-05-12
Filing date: 2016-05-12
Publication date: 2017-11-29
Also published as: GB201608311D0

Abstract

A mixer for a material management station for a 3D printing system, the mixer comprising a container with a first opening to receive build material, a shaft within the container and rotatable relative to the container, and first and second mixing elements wherein the first mixing element is coupled to the shaft and extends from the shaft by a first radial distance to engage build material proximal to a surface of the container, and the second mixing element is coupled to the shaft and extends from the shaft by a second radial distance which is smaller than the first radial distance to engage build material proximal to the shaft. Ideal, the first mixing element comprises a plurality of radial blades coupled to the shaft and extend radially from the shaft, the blades being equal spaced around the circumference of the shaft with each blade having an inclination angle between 30° and 60°. The second mixer is coupled to the shaft by being coupled to at least two radial blades of the first mixing element. Ideally, the second mixing element is a spiral helix.

Description

POWDER MATERIAL MIXER

BACKGROUND

[0001] Additive manufacturing techniques, such as three-dimensional (3D) printing, relate to techniques for making 3D objects of almost any shape from a digital 3D model through additive processes, in which 3D objects are generated on a layer-by-layer basis under computer control. A large variety of additive manufacturing technologies have been developed, differing in build materials, deposition techniques and processes by which the 3D object is formed from the build material. Such techniques may range from applying ultraviolet light to photopolymer resin, to melting semi-crystalline thermoplastic materials in powder form, to electron-beam melting of metal powders.

[0002] Additive manufacturing processes usually begin with a digital representation of a 3D object to be manufactured. This digital representation is virtually sliced into layers by computer software or may be provided in pre-sliced format. Each layer represents a cross-section of the desired object, and is sent to an additive manufacturing apparatus, that in some instances is known as a 3D printer, where it is built upon a previously built layer. This process is repeated until the object is completed, thereby building the object layer-by-layer. While some available technologies directly print material, others use a recoating process to form additional layers that can then be selectively solidified in order to create the new cross-section of the object.

[0003] The build material from which the object is manufactured may vary depending on the manufacturing technique and may comprise powder material, paste material, slurry material or liquid material. The build material is usually provided in a source container from where it needs to be transferred to the building area or building compartment of the additive manufacturing apparatus where the actual manufacturing takes place. 3D printing systems utilise build materials, often containing powder materials, i.e. materials formed by powder particles, which are fused together during the printing process. These powder materials may be transported both during the printing process and after the printing process such as when excess powder material is removed from around a printed component or part. The excess powder may be recycled for use in a future printing process.

[0004] When using such recycled powder in a future printing process, it may be desirable to mix the recycled powder with fresh powder of the same build material as the recycled powder. In particular, it may be desirable to be able to achieve this mixing locally to the 3D printing system. Alternatively, it may be desirable to mix different types of build materials or to mix in additives into a build material.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] Fig. 1A shows an example of a 3D printing system; [0006] Fig. 1B schematically illustrates the material management station of the example of Figure 1A; [0007] Fig. 1C schematically illustrates a working area of the material management station of the example of Figure 1B; [0008] Fig. 2A schematically illustrates an internal circuit diagram of one example of a material management station; [0009] Fig. 2B is a table schematically illustrating valve setting information for the material management station internal circuit of Figure 2A; and [0010] Fig. 2C schematically illustrates a build material trap geometry used in tanks of the material management station of Figure 2A.Fig. 3A shows a cross section of an example mixing tank taken in a vertical plane; [0011] Fig. 3B shows a cross section of the example mixing tank of Fig. 3A taken in a different vertical plane; [0012] Fig. 3C shows a cross section of the example mixing tank of Fig. 3A taken in a horizontal plane; [0013] Fig. 3D shows an isometric view of the example mixing tank of Fig. 3A with the container removed; and [0014] Fig. 3E shows an isometric view of the shaft with first and second mixing elements of Fig. 3A.

DETAILED DESCRIPTION

[0015] As shown in Figure 1A, the three dimensional (3D) printing system 100 (or additive manufacturing system) according to one example comprises: a trolley 102, a 3D printer 104 and a material management station 106. The material management station 106 manages build material.

[0016] The trolley 102 is arranged to slot into a docking position in the printer 104 to allow the printer 104 to generate a 3D object within the trolley. The trolley is also arranged to also slot (at a different time) into a docking position 107 in the material management station 106. The trolley 102 may be docked in the material management station 106 prior to a 3D printing process to load the trolley with build material in preparation for a subsequent 3D printing process.

[0017] The build material loaded into the trolley may include recycled or recovered build material from one or more previous printing processes, fresh build material or a portion of fresh and recycled build material. Some build materials may be non-recyclable and hence in this case no recovered build material will be used to load the trolley. The build material may be or include, for example, powdered metal materials, powdered composited materials, powder ceramic materials, powdered glass materials, powdered resin material, powdered polymer materials and the like. In some examples where the build material is a powder-based build material, the term powder-based materials is intended to encompass both dry and wet powder-based materials, particulate materials and granular materials. It should be understood that the examples described herein are not limited to powder-based materials, and may be used, with suitable modification if appropriate, with other suitable build materials. In other examples, the build material may be in the form of pellets, or any other suitable form of build material, for instance.

[0018] Returning to Figure 1A, the trolley 102 may also be docked in the docking position 107 in the material management station 106 (shown without the trolley 102 docked in Figure 1A) to clean up at least some components of the trolley 102 after it has been used in a 3D printing production process. The clean-up process may involve recovery and storage in the material management station 106 of unfused build material from the previous print job for subsequent reuse. During a 3D printing process a portion of the supplied build material may be fused to form the 3D object, whilst a remaining portion of the supplied build material may remain unfused and potentially recyclable, depending upon the type of build material used. Some processing of the unfused build material may be performed by the material management station 106 prior to storage for recycling, to reduce any agglomeration for example.

[0019] It will be understood that the material management station 106 may also include an access panel (not shown) to cover the docking position 107 when the trolley 102 is fully docked with the material management station 106 and when the trolley 102 is fully removed from the material management station 106.

[0020] One material management station 106 can be used to service different 3D printers. A given 3D printer may interchangeably use various trolleys 102, for example, utilising different trolleys for different build materials. The material management station 106 can purge a trolley 102 of a given build material after a 3D printing production process, allowing it to be filled with a different build material for a subsequent 3D printing production run. Purging of the trolley 102 may also involve purging of the material management station 106 or alternatively, it may involve separation of different build materials in the material management station 106 to prevent contamination of one build material type with another.

[0021] The trolley 102 in this example has a build platform 122 on which an object being manufactured is constructed. The trolley 102 also comprises a build material store 124 situated beneath a build platform 122 in this example. The build platform 122 may be arranged to have an actuation mechanism (not shown) allowing it, when it is docked in the printer 104 and during a 3D printing production process, to gradually move down, such as in a step-wise manner, towards the base of the trolley 102 as the printing of the 3D object progresses and as the build material store 124 within the trolley 102 becomes depleted. This provides progressively more distance between the base level of the build platform 122 and the print carriages (not shown) to accommodate the 3D object being manufactured. The size of an object being printed may increase progressively as it is built up layer-by-layer in the 3D printing process in this example.

[0022] The 3D printer 104 of this example can generate a 3D object by using a build material depositor carriage (not shown) to form layers of build material onto the build platform 122. Certain regions of each deposited layer are fused by the printer 104 to progressively form the object according to object-specifying data. The object-specifying data are based on a 3D shape of the object and may also provide object property data such as strength or roughness corresponding to the whole object or part(s) of the 3D object. In examples, the desired 3D object properties may also be supplied to the 3D printer 104 via a user interface, via a software driver or via predetermined object property data stored in a memory.

[0023] After a layer of the build material has been deposited on the build platform 122 by the printer 104, a page-wide array of thermal (or piezo) printheads on a carriage (not shown) of the 3D printer 104 can traverse the build platform 122 to selectively deposit a fusing agent in a pattern based on where particles of the build material are to fuse together. Once the fusing agent has been applied, the layer of build material may be exposed to fusing energy using one or more heating elements (not shown) of the 3D printer 104. The build material deposition, fusing agent and fusing energy application process may be repeated in successive layers until a complete 3D object has been generated. The material management station 106 may be used with any additive manufacturing technique and is not limited to printers using printheads on a carriage to deposit a fusing agent as in the example described above. For example, the material management station 106 may be used with a selective laser sintering additive manufacturing technique.

[0024] Figure IB schematically illustrates the material management station 106 of the example of Figure 1A, with the trolley 102 of Figure 1A docked therein.

[0025] As shown in the example of Figure IB, the material management station 106 has two interfaces for receiving two fresh build material supply tanks (or cartridges) 114a, 114b, which may be inserted into and released from the material management station 106. In this example, each fresh build material supply tank 114a, 114b has a capacity of between about thirty and fifty litres. In one example, the build material may be a powdered semi-crystalline thermoplastic material. The provision of two fresh build material supply tanks 114a, 114b allows “hot swapping” to be performed such that if a currently active container becomes empty or close to empty of build material when the trolley 102 is being filled with build material by the material management station 106 in preparation for an additive manufacturing process, a fresh build material supply source can be dynamically changed to the other of the two tanks. The material management system 106 may have one or more weight measurement device(s) to assess how much fresh build material is present at a given time in one or more of the fresh build material supply tanks 114a, 114b. The fresh build material from the tanks 114a, 114b, may be consumed, for example, when loading the trolley 102 with build material prior to the trolley 102 being installed in the printer 104 for a 3D printing production run.

[0026] Build material is moved around within the material management station 106 in this example using a vacuum system (described below with reference to Figure 2A), which promotes cleanliness within the system and allows for recycling of at least a portion of build material between successive 3D printing jobs, where the type of build material selected for use is recyclable.

[0027] References to a vacuum system in this specification include a vacuum that is partial vacuum or a pressure that is reduced, for example, relative to atmospheric pressure. The vacuum may correspond to “negative pressure”, which can be used to denote pressures below atmospheric pressure in a circuit surrounded by atmospheric pressure. A total trolley-use time for printing of a 3D object before trolley 102 can be reused may depend upon both a printing time of the printer 104 when the trolley 102 is in the printer 104 and a cooling time of the contents of the build volume of the trolley 102. It will be understood that the trolley 102 can be removed from the printer 104 after the printing operation, allowing the printer 104 to be re-used for a further printing operation using build material within a different trolley before the total trolley-use time has elapsed. The trolley 102 can be moved to the material management station 106 at the end of the printing time. The vacuum system can be used, in some examples, to promote more rapid cooling of the contents of the build volume following a 3D print production process than would otherwise occur without the vacuum system. Alternative examples to the vacuum system can include a compressed air system.

[0028] The material management station 106 in this example has a recovered build material tank 108 (see Figure IB), located internally, where build material recovered from the trolley 102 by the vacuum system is stored for subsequent reuse, if appropriate. Some build materials may be recyclable whilst others may be non-recyclable. In an initial 3D printing production cycle, 100% fresh build material may be used. However, on second and subsequent printing cycles, depending upon build material characteristics and user choice, the build material used for the print job may comprise a proportion of fresh build material (e.g. 20%) and a portion of recycled build material (e.g. 80%). Some users may elect to use mainly or exclusively fresh build material on second and subsequent printing cycles, for example, considering safeguarding a quality of the printed object. The internal recovered build material tank 108 may become full during a post-production clean-up process, although it may become full after two or more post-production clean up processes have been performed, but not before. Accordingly, an overflow tank in the form of an external overflow tank 110 can be provided as part of the material management station 106 to provide additional capacity for recovered build material for use once the internal recovered build material tank 108 is full or close to full capacity. Alternatively, the external overflow tank 110 can be a removable tank. In this example, one or more ports are provided as part of the material management station 106 to allow for output of or reception of build material to and/or from the external overflow tank 110. A sieve 116 or alternative build material refinement device may be provided for use together with the internal recovered build material tank 108 to make unfused build material recovered from a 3D printing production process for recycling more granular, that is, to reduce agglomeration (clumping).

[0029] The material management station 106 in this example has a mixing tank (or blending tank) 112 for mixing recycled build material from the internal recovered build material tank 108 with fresh build material from one of the fresh build material supply tanks 114a, 114b for supply to the trolley 102 when it is loaded prior to a printing production process. The mixing tank (or blending tank) 112, in this example, is provided on top of the material management station 106, above the location of the build platform 122 when the trolley 102 is docked therein. The mixing tank 112 is connected to a mixer build material trap 113 (described below with reference to Figure 2A) for input of build material into the mixing tank 112.

[0030] The mixing tank 112 may also be for mixing different build materials or different grades of build material, irrespective of whether the build materials are fresh or recycled. For example, an amount of fresh build material may be mixed, in the mixing tank 112, with one or more additives. The one or more additives may have a density different to that of the fresh build material. The additives may be used for various reasons, such as to provide colour to a build material (colourant additives) or to change the material properties of a printed part. For example, an additive may be a fibre, such as carbon fibre or glass fibre, which may increase strength of a printed part. The use or choice of an additive may influence components of the mixing tank 112. For example, a mixer blade may be constructed from a material suitable for a mixture containing a fibre additive due to such an additive being more abrasive. For instance, a metal mixer blade could be used instead of a plastic mixer blade. As another example, a mixer blade could be provided with a surface finish to account for increased abrasiveness of build material.

[0031] Suitable powder-based build materials for some examples of this disclosure include at least one of polymers, crystalline plastics, semi-crystalline plastics, polyethylene (PE), polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), amorphous plastics. Polyvinyl Alcohol Plastic (PVA), Polyamide, thermo(setting) plastics, resins, transparent powders, coloured powders, metal powder, ceramics powder such as for example glass particles, and/or a combination of at least two of these or other materials wherein such combination may include different particles each of different materials or different materials in a single compound particle. Examples of blended build materials include alumide, which may include a blend of aluminium and polyamide, multi-colour powder, and plastics/ceramics blends.

[0032] The fresh build material supply tanks 114a, 114b, the external overflow tank 110 and the main body of the material management station 106 may be constructed to fit together in a modular way, permitting a number of alternative geometrical configurations for the fully assembled material management station 106. In this way, the material management station 106 is adaptable to fit into different housing spaces in a manufacturing environment.

[0033] The fresh build material supply tanks 114a, 114b may be connected to and released from the main body of the material management station 106 via respective supply tank connectors 134a, 134b. These supply tank connectors 134a, 134b may incorporate a security system to reduce the likelihood of unsuitable build material being used in the 3D printing system. In one example, suitable fresh build material supply tanks 114a, 114b are provided with a secure memory chip, which can be read by a chip reader (not shown) or other processing circuitry on the main body of the material management station 106 to verify the authenticity of any replacement supply tank (cartridge) 114a, 114b that has been installed. In this example, the chip reader may be provided on the supply tank connectors 134a, 134b and upon attachment of the fresh build material supply tanks 114a, 114b to the respective connector 134a, 134b, an electrical connection may be formed. The processing circuitry in the material management station 106 may also be used to write a measured weight of build material determined to be in the respective fresh build material supply tank(s) 114a, 114b onto the secure memory chip of the tank to store and/or update that value. Thus, the amount of authorised build material remaining in the fresh build material supply tank(s) 114a, 114b at the end of a trolley loading process can be recorded. This allows the withdrawal of particulate build material from the fresh build material supply tanks 114a, 114b beyond the quantity with which it was filled by the manufacturer to be prevented. For example, in the case of a fresh build material supply tank 114a, 114b from which the tank manufacturer’s authorised fresh build material has previously been completely withdrawn. this limits the withdrawal of further build material that may damage the printer or print quality, if the fresh build material supply tank were re-filled with alternative fresh build material.

[0034] The secure memory chip of the fresh build material supply tanks 114a, 114b can store a material type of the build material contained within the fresh build material supply tanks. In one example, the material type is the material (e.g. ceramic, glass, resin, etc.). In this way, the material management station 106 can determine the material type to be used by the material management station 106.

[0035] Figure 1C schematically illustrates a working area of the material management station 106 of the example of Figure IB, showing the build platform 122 of the trolley 102 and a build material loading hose 142, which provides a path between the mixing tank 112 of Figure IB and the build material store 124 of the trolley 102. The loading hose 142 is used for loading the trolley 102 with build material prior to the trolley 102 being used in the printer 104. Figure 1C also shows a recycling hose 144 for unpacking manufactured 3D objects, cleaning the build platform 122 of the trolley 102 and a surrounding working area within the material management station 106. In one example, the recycling hose 144 operates by suction provided via a pump 204 (see Figure 2A) and provides an enclosed path to the recovered build material tank 108 (see Figure IB) for receiving and holding build material for re-use in a subsequent 3D printing process. The recycling hose 144 may, in one example, be operated manually by a user to recover recyclable build material from and/or to clean up a working area of the material management station 106.

[0036] Figure 2A schematically illustrates an internal circuit diagram 200 of one example of a build material management system in the form of a material management station 106. The material management station 106 can be used in conjunction with the trolley 102 of Figure 1A.

[0037] As previously described, printed parts along with unfused build material can be transported from the 3D printer 104 to the material management station 106 via the trolley 102. The material management station 106 can then be used to process build material and printed parts from the trolley 102.

[0038] In another example, printed parts along with unfused build material can be transported from the 3D printer 104 to the material management station 106 via another suitable container, e.g. a box or cartridge (not shown) instead of the trolley 102. The material management station 106 may then be used to process the powder-based material and printed parts from the container.

[0039] The material management station circuit 200 includes a conduit (or guide-channel) network and a pump 204 to provide a pressure differential across the conduit network to transport unfused build material between different components, as described below with reference to Figure 2. In this example, the pump 204 is a suction pump which operates to create a pressure differential across the suction pump to produce air flow from an air inlet at substantially atmospheric pressure through the conduit network towards an upstream side of the suction pump (at a pressure below atmospheric pressure or at “negative pressure”). The pump 204 may be provided as an integral part of the material management station 106 in one example, but in another example, the material management station 106 provides a negative/reduced pressure interface, via which a suction pump may be detachably coupled or coupled in a fixed configuration. Although the description below refers to first conduit, second conduit, third conduit, etc. of the conduit network, there is no implied ordering in the number of the conduits other than to distinguish one conduit from another.

[0040] A collection hose 206 is connected to a recovered build material tank (RBMT) 208 via a working area port in a working area 203 in the form of a working area inlet port 273 and a first conduit (hose-to-RBMT conduit) 272 of the conduit network. The recovered build material tank 208 includes a recovered build material tank (RBMT) inlet area comprising a recovered build material tank (RBMT) build material trap 218b and a recovered build material tank (RBMT) material outlet. The RBMT inlet area is where a fluidised flow of build material is received for storage in the recovered build material tank 208. The first conduit 272 provides a path between the working area inlet port 273 and the RBMT inlet area. The working area inlet port 273 is to receive build material from the collection hose 206 and is provided at an end of the first conduit 272 connected to the collection hose 206. In other examples, the RBMT inlet area may communicate directly with the working area 203 or the collection hose 206 without a first conduit 272 between.

[0041] The recovered build material tank 208 in this example is provided internally to the material management station 106. A hose-to-RBMT valve 242 is positioned along the first conduit 272 for opening and closing the path through the first conduit 272. The collection hose 206 extends from the working area inlet port 273 into the working area 203. The working area 203 includes at least a portion of the trolley 102 (or other container) and can be maintained at substantially atmospheric pressure. Build material from the trolley 102 can be collected by the collection hose 206 and transported to the recovered build material tank 208 through the first conduit 272. The recovered build material tank 208 can be used for storing any unfused build material from the trolley 102 that is suitable for being used again in a further 3D printing (additive manufacturing) process. In this way, the recovered build material tank 208 can be used as a buffer storage tank to temporarily store unfused build material prior to supplying the unfused build material use in a further 3D printing (additive manufacturing) process.

[0042] A second conduit 274 (hose-to-overflow conduit) of the conduit network connects the collection hose 206 to an overflow tank 210. The overflow tank 210 includes an overflow inlet area and the second conduit 274 provides a path between the collection hose 206 and the overflow inlet area comprising, in this example, an overflow build material trap 218a (a filter). An overflow tank port in the form of an overflow tank outlet port 275 may also be provided at an end of the second conduit 274. The overflow tank 210 can be selectively sealed by an openable lid (not shown). In a sealed configuration, the overflow tank 210 is in fluid communication with one or more overflow inlet ports and overflow outlet ports of the conduit network. Furthermore, in the sealed configuration, the overflow tank 210 is not directly open to the atmosphere. Build material from the working area 203 can be transported through the second conduit 274 and overflow tank outlet port 275 into the overflow tank 210. A hose-to-overflow valve 244 is positioned along the second conduit 274 for opening and closing a path through the second conduit 274. Unfused build material from the trolley 102 (or other container) can be collected by the collection hose 206 and transported to the overflow tank 210 through the first conduit 272. The overflow tank 210 is an external tank that is removable and that can be used for storing excess recoverable (recyclable) build material when the recovered build material tank 208 is full. Alternatively, the overflow tank 210 can be used as a waste storage tank to store unfused build material from the trolley 102 that is not suitable for recycling. In a further alternative, the overflow tank 210 can be used as a purged build material storage tank to store unfused build material from the trolley 102 and from elsewhere in the material management station 106 when the material management station 106 is purged of unfused build material.

[0043] The pump 204 is connected via a third conduit (pump-to-RBMT conduit) 276 of the conduit network to the recovered build material tank 208. The third conduit 276 provides a path between the pump 204 and the RBMT inlet area. A RBMT-to-pump valve 246 is positioned along the third conduit 276 for opening and closing the path through the third conduit 276.

[0044] The pump 204 is also connected to the overflow tank 210 via a fourth conduit (pump-to-overflow conduit) 278 of the conduit network. The fourth conduit 278 provides a path between the pump 204 and the overflow inlet area. An overflow tank port in the form of an overflow tank vacuum port 279 may also be provided at an end of the fourth conduit 278. Fluid, e.g. air, can transmit through the overflow tank vacuum port 279 from the overflow inlet area towards the pump 204. An overflow-to-pump valve 248 is positioned along the fourth conduit 278 for opening and closing a path through the fourth conduit 278.

[0045] Unfused build material in the trolley 102 can be collected using the collection hose 206 and transported either to the recovered build material tank 208 or to the overflow tank 210, or both. The tank to be used at a given time can be selected by opening appropriate valves along the conduits of the circuit of Figure 2A.

[0046] The valves described herein with reference to Figure 2A may be controlled by a controller 295, which may be, for example a programmable logic controller forming a part of processing circuitry of the build material management station 106. The controller 295 may electronically open one or more valves to open one or more paths in respective conduits based on the material transport operation being performed. The controller 295 may also electronically close one or more valves to close one or more paths in respective conduits. The valves may be, for example, butterfly valves and may be actuated using compressed air. In another example, one or more valves may be opened and closed manually by a user.

[0047] The controller controls the general operation of the material management system 200. The controller may be a microprocessor-based controller that is coupled to a memory (not shown), for example via a communications bus (not shown). The memory stores machine executable instructions. The controller 295 may execute the instructions and hence control operation of the build material management system 200 in accordance with those instructions.

[0048] Figure 2B is a table schematically illustrating for each of a number of different build material source locations and build material destination locations, an appropriate valve configuration corresponding the valves as labelled in Figure 2A. A tick in an appropriate column of the table indicates that the corresponding valve is controlled to be open by the controller 295 for the particular build material transport operation. For example, when transporting build material from the recovered build material tank 208 to the mixing tank 212, the valves 256, 258 and 254 are set by the controller 295 to be open, whereas the valves 250, 244, 276, 248, 242, 262, 260, 252a and 252b are set to be closed. In alternative examples, some valves may be set to be open by simultaneity.

[0049] In an example, a recyclability indicator is determined by processing circuitry of the build material management station 106. The recyclability indicator can be indicative of whether the build material in the trolley 102 (or container) includes recyclable or recoverable material. When it is determined that the unfused build material in the trolley 102 is not recyclable or when the recovered build material tank 208 is full, the unfused build material can be transported to the overflow tank 210.

[0050] To transport the unfused build material from the trolley 102 (or container) to the overflow tank 210, the hose-to-overflow valve 244 in the second conduit 274 between the collection hose 206 and the overflow tank 210 and the overflow-to-pump valve 248 in the fourth conduit 278 between the pump 204 and the overflow tank 210 can be opened, e.g. electronically by the controller 295. When the pump is active, a differential pressure is provided from the pump to the collection hose 206. That is, a pressure at the pump 204 is lower than a pressure at the collection hose 206. The differential pressure enables build material from the trolley 102 (or container) to be transported to the overflow tank 210.Build material (and air) in proximity with an end of the collection hose 206 (at approximately atmospheric pressure) is transported from the collection hose 206, along the second conduit 274 and through the hose-to-overflow valve 244 to overflow tank 210. The overflow tank 210 is provided in the sealed configuration. At the overflow tank 210, build material separates from air flow and drops from the overflow inlet area into the overflow tank 210. Air (and any residual build material) continues along the fourth conduit 278 and through the overflow-to-pump valve 248 towards the pump 204, which is at a reduced pressure.

[0051] To help prevent unfused build material traveling through the overflow inlet area of the overflow tank 210 into the fourth conduit 278 towards the pump 204, the overflow inlet area can include an overflow build material trap 218a (e.g. a powder trap). The overflow build material trap 218a is arranged to collect build material from the second conduit 274 and divert the build material (e.g. powder) into the overflow tank 210. Thus, the overflow build material trap 218a helps prevent build material conveying past the overflow inlet area of the overflow tank 210 and entering the fourth conduit 278 via the overflow tank vacuum port 279 to travel towards the pump 204.

[0052] The overflow build material trap 218a may include a filter (e.g. a mesh), which collects build material transported from the overflow tank 210. Thus, the filter separates build material from air flow in the overflow inlet area. Holes in the filter are small enough to prevent the passage of at least 95% of build material but allow relatively free flow of air through the filter. Holes in the filter may be small enough to prevent the passage of at least 99% of build material, whilst still allowing relatively free flow of air through the filter. Build material collected by the filter may drop from the overflow inlet area into the overflow tank 210.

[0053] Recoverable unfused build material in the trolley 102 (or container) can be transported to the recovered build material tank 208 in a similar way. To transport the unfused build material from the trolley 102 to the recovered build material tank 208, the hose-to-RBMT valve 242 in the first conduit 272 between the collection hose 206 and the recovered build material tank 208 and the RBMT-to-pump valve 246 in the third conduit 276 between the pump 204 and the recovered build material tank 208 can be opened electronically by the controller 295 as described above. When the pump is active, a differential pressure is provided from the pump to the collection hose 206. That is, a pressure at the pump 204 is lower than a pressure at the collection hose 206. The differential pressure enables build material from the trolley 102 (or container) to be transported to the recovered build material tank 208. Build material (and air) in proximity with an end of the collection hose 206 (at approximately atmospheric pressure) is transported from the collection hose 206, along the first conduit 272 and through the hose-to-RBMT valve 242 to the recovered build material tank 208. At the recovered build material tank 208, build material separates from air flow and drops from the RBMT inlet area into the recovered build material tank 208. Air (and any residual build material) continues along the third conduit 276 and through the RBMT-to-pump valve 246 towards the pump 204, which is at reduced pressure relative to atmospheric pressure.

[0054] Each of the recovered build material tank 208, the overflow tank 210, and the mixing tank 212 has a build material trap 218b, 218a and 218c respectively. These build material traps 218a, 218b, 218c perform cyclonic filtration of an incoming fluidised flow of build material and air as schematically illustrated in Figure 2C. An inlet 296 of the build material trap 218 receives the fluidised flow of build material and the build material is pushed by a centrifugal force created by suction of the pump 204 to an outer wall 297 of the build material trap 218. In one example, the outer wall 297 of the build material trap 218 has a circular cross-section and the incoming build material migrates via a cyclonic action to the outer wall 297 of the build material trap 218 until the incoming air reaches an exit below, whereupon the build material particles drop down into a vacuum sealed recipient 299 in the build material trap 218. Thus the build material trap 218 separates a fluidised flow of build material into a powder component, which is deposited in the associated tank and an air component, which is sucked towards the pump 204 via an air outlet 298 in the build material trap 218 providing an interface to the pump 204. A filter (not shown) may be provided in the air outlet 298 of the build material trap 218 to reduce the likelihood of any remaining build material reaching the pump 204 in the separated air flow. The build material trap 218 provides efficient powder separation via its geometry that promotes formation of a cyclone within the build material trap in use. It offers transportation of build material in an air flow and storage of the powder in a tank, whilst diverting an air flow out of the tank towards the pump 204. The build material trap provides a filter to capture residual powder in an air flow emerging from the cyclone to prevent it from reaching the pump 204. The build material trap 218 is one example of a build material filter having a function of separating an air from a build material flow at a corresponding tank inlet area. In other examples, the air flow is separated from the fluidised build material upon arrival at a destination tank using a filter other than a cyclonic filter. For example, a diffusion filter may be used.

[0055] Returning to Figure 2A, the RBMT inlet area of the recovered build material tank 208 may also include the RBMT build material trap 218b (e.g. a powder trap) or another type of RBMT build material filter to separate build material and air from an incoming fluidised flow of build material. The RBMT build material trap 218b operates in the same or a similar way as the overflow build material trap 218a in the overflow tank 210, to help collect and divert build material into the recovered build material tank 208 to help prevent build material from traveling through the third conduit 276 towards the pump 204.

[0056] When collecting material from the trolley 102 via the collection hose 206, as described above, a user can move the end of the collection hose 206 around the working area 203 including the trolley 102 to collect as much build material from the trolley 102 as possible.

[0057] The recovered build material tank 208 is also connected via a fifth conduit (overflow-to- RBMT conduit) 280 of the conduit network. An overflow tank port in the form of an overflow tank inlet port 281 may also be provided at an end of the fifth conduit 280. Build material from the overflow tank 210 can be transported through the fifth conduit 280 and overflow tank inlet port 281 into the recovered build material tank 208.

[0058] The fifth conduit 280 between the recovered material tank 208 and the overflow tank inlet port 281 includes an overflow-to-RBMT valve 250 in the path leading to the RBMT build material trap. In the event that the recovered build material tank 208 can be refilled with recovered build material, the overflow-to-RBMT valve 250 in the fifth conduit 280 between the recovered build material tank 208 and the overflow tank 210 can be opened, along with the RBMT-to-pump valve 246 in the third conduit 276 between the recovered build material tank 208 and the pump 204. Each of the valves can be opened electronically by the controller 295, as described above. When the pump is active, a differential pressure is provided from the pump to the overflow tank 210. That is, a pressure at the pump 204 is lower than a pressure at the overflow tank 210. In this example, the overflow tank 210 is provided in an unsealed configuration and includes an air inlet (not shown) open to atmosphere to maintain approximately atmospheric pressure within the overflow tank 210. The differential pressure enables build material from the overflow tank 210 to be transported to the recovered build material tank 208. Air flows into the overflow tank 210 through the air inlet. Build material (and air) in the overflow tank is transported from the overflow tank 210, along the fifth conduit 280 and through the overflow-to-RBMT valve 250 to the recovered build material tank 208. At the recovered build material tank 208, build material separates from air flow and drops from the RBMT inlet area into the recovered build material tank 208. Air (and any residual build material) continues along the third conduit 276 and through the RBMT-to-pump valve 246 towards the pump 204, which is at a reduced pressure.

[0059] The material management station circuit 200 also includes a mixing tank 212. The mixing tank 212 can be used to mix recovered build material from the recovered build material tank 208 with fresh build material from a fresh build material supply tank 214a or 214b, ready to be used in a 3D printing process. In some examples, the mixing tank 212 may also be used to mix different materials irrespective of whether the materials are recycled or fresh materials. For example, a mixing tank 212 may be used to mix a build material with an additive. For example, a mixing tank 212 may mix together quantities of a polyamide and glass bits. As such, in the following where reference is made to mixing a fresh build material with a recyclable (or recycled) build material, the relevant description may also be considered applicable to the mixing of different materials. That is, for example, where the following refers to the mixing of fresh build material and recycled build material, it could also be thought of as referring to the mixing of a first material and a second material (and potentially, in further examples, additional materials).

[0060] Although two fresh build material supply tanks 214a, 214b are shown in this example, in other examples, one or more fresh build material supply tanks 214a, 214b may be used. More fresh build material supply tanks 214a, 214b may be used when appropriate.

[0061] Each fresh build material supply tank 214a, 214b is connected to the mixing tank 212 via a sixth conduit (a fresh build material conduit) 282 of the conduit network and a fresh build material supply tank port 283a, 283b. The fresh build material supply tank port 283a, 283b is to output build material from the respective fresh build material supply tank 214a, 214b. Each fresh build material supply tank 214a, 214b has an associated material supply tank cartridge-to-mixer valve 252a, 252b in the sixth conduit 282 between the respective fresh build material supply tank 214a, 214b and the mixing tank 212 Each fresh build material supply tank 214a, 214b also includes an air inlet valve whereby to ensure air can enter the fresh build material supply tanks 214a, 214b to maintain air pressure within the fresh build material supply tanks 214a, 214b at approximately atmospheric pressure.

[0062] The mixing tank 212 is connected via a seventh conduit (pump-to-mixer conduit) 284 of the conduit network to the pump 204. The seventh conduit 284 between the mixing tank 212 and the pump 204 includes a mixer-to-pump valve 254, which may be opened or closed to open and close the passage through the seventh conduit 284.

[0063] To transport fresh build material from the fresh build material supply tank 214a or 214b to the mixing tank 212, the material supply tank cartridge-to-mixer valve 252a or 252b and the mixer-to-pump valve 254 in the seventh conduit 284 between the mixing tank 212 and the pump 204 are opened. Each of the valves can be opened electronically by the controller 295, as described above. When the pump 204 is active, a differential pressure is provided from the pump 204 to the fresh build material supply tank 214a or 214b. That is, a pressure at the pump 204 is lower than a pressure at the fresh build material supply tank 214a or 214b. The differential pressure enables build material from the fresh build material supply tank 214a or 214b to be transported to the mixing tank 212. Build material (and air) in the fresh build material supply tank 214a or 214b is transported from the fresh build material supply tank 214a or 214b, along the sixth conduit 282 and through the cartridge-to-mixer valve 252a or 252b to the mixing tank 212. At the mixing tank 212, build material separates from air flow and drops from the mixer inlet area into the mixing tank 212. Air (and any residual build material) continues along the seventh conduit 284 and through the mixer-to-pump valve 254 towards the pump 204, which is at a reduced pressure.

[0064] The mixer inlet area of the mixing tank 212 can also include a mixer build material trap 218c (e.g. a powder trap) or any type of mixer build material filter to separate an air flow from a build material flow, which operates in the same or similar manner to as the overflow build material trap 218a and the RBMT build material trap 218b. The mixer build material trap 218c helps to collect and divert build material into the mixing tank 212, and help prevent the build material from travelling through the seventh conduit 284 towards the pump 204.

[0065] The mixing tank 212 is also connected to the recovered build material tank 208 via an eighth conduit (RBMT-to-mixer conduit) 286 of the conduit network and a ninth conduit 288 of the conduit network extending sequentially from the recovered build material tank 208 to the mixing tank 212. The ninth conduit 288 may be part of the RBMT-to-mixer conduit 286.

[0066] A sieve 216 may, in some examples, be located in the RBMT to mixer conduit 286 or between the eighth and ninth conduits 286 and 288 between the recovered build material tank 208 and the mixing tank 212. The sieve 216 may be used to separate agglomerates and larger parts of material from the recycled or recovered build material that is transported from the recovered build material tank 208. Often, agglomerates and larger parts of material are not suitable for recycling in a further 3D printing process, so the sieve may be used to remove these parts from the build material. The sieve 216 includes an air inlet (not shown) to ensure air can enter the sieve 216 to maintain air pressure within the sieve 216 at approximately atmospheric pressure. In some examples, the RBMT-to-mixer conduit 286 may not be connected to a build material outlet of the recovered build material tank 208. In other examples a conduit connecting an outlet of the recovered build material tank 208 to a build material inlet in the mixer build material trap 218c of the mixing tank 212 may form a closed circuit.

[0067] A RBMT-to-sieve valve 256 is located in the eighth conduit 286 between the recovered build material tank 208 and the sieve 216, and a sieve-to-mixer valve 258 is located in the ninth conduit 288 between the sieve 216 and the mixing tank 212. The RBMT-to-sieve valve 256 and sieve-to-mixer valve 258 may be opened or closed to open and close the passages through the eighth and ninth conduits 286, 288 between the recovered build material tank 208 and the mixing tank 212. The valves may be opened or closed electronically by the controller 295.

[0068] To transport build material from the recovered build material tank 208 to the mixing tank 212 both the RBMT-to-sieve valve 256 and the sieve-to-mixer valve 258 in the eighth and ninth conduits 286, 288 between the recovered build material tank 208 and the mixing tank 212 can be opened as well as the mixer-to-pump valve 254 in the seventh conduit 284 that connects the mixing tank 212 to the pump 204. Build material in the recovered build material tank 208 may drop down into the sieve 216 through the eighth conduit 286 by gravity, for example. When the pump 204 is active, a differential pressure is provided from the pump 204 to the sieve 216. That is, a pressure at the pump 204 is lower than a pressure at the sieve 216. The differential pressure enables build material from the recovered build material tank 208 to be transported to the sieve 216 by gravity and to the mixing tank 212 by suction. Build material in the recovered build material tank 208 is transported through the RBMT material outlet, along the eighth conduit 286 and through the RBMT-to-sieve valve 256 to the sieve 216. Build material (and air) in the sieve 216 is transported from the sieve 216, along the ninth conduit 288 and through the sieve-to-mixer valve 258 to the mixing tank 212. At the mixing tank 212, build material separates from air flow and drops from the mixer inlet area into the mixing tank 212. Air (and any residual build material) continues along the seventh conduit 284 and through the mixer-to-pump valve 254 towards the pump 204, which is at a reduced (negative) pressure.

[0069] A currently selected ratio of recycled build material from the recovered build material tank 208 and fresh build material from the fresh build material supply tank 214a or 214b can be transported to the mixing tank 212 as described above. The ratio of fresh build material to recovered build material may be any selected ratio. The ratio may depend on the type of build material and/or the type of additive manufacturing process. In a selective laser sintering process the ratio could be, for example 50% fresh to 50% recovered build material. In one example of a printhead cartridge 3D printing process, the ratio may be 80% recovered to 20% fresh build material. For some build materials 100% fresh build material may be used, but for other build materials up to 100% recovered build material may be used. The fresh build material and the recovered build material can be mixed together within the mixing tank 212 using, for example, a rotating mixing blade 213.

[0070] When mixing build materials, a certain degree of homogeneity of the mixture may be desirable. There exist various tests for determining how homogeneous a mixture of two or more build materials is. For example, the mixing of fresh build material with recycled build material may be performed to ensure that the mechanical properties of a 3D printed part, printed from the mixture, are largely consistent with the mechanical properties of a 3D printed part which is printed from fresh material alone. Therefore, a check to determine if the mixture is suitably homogeneous may be to compare the mechanical properties of a part which is printed using the mixture against those of a part which is printed using fresh build material only. In another example, two visually distinct build materials could be mixed, and then a visual test may be performed to estimate or determine the degree of mixing. In yet another example, a sample of two mixed build materials may be taken and subjected to conditions such that one of the build materials in the mixture is removed (for example, for a mixture of a plastic and glass bits, the mixture could be heated to burn down to the glass bits alone), leaving the other build material. The amount (for example, the weight) of the remaining build material could then be compared to the amount of the sample of the mixed build materials to determine the proportion of the sample (by weight, volume, size etc.) that the remaining build material made up.

[0071] As has been mentioned, the level of homogeneity of a mixture may influence the mechanical properties of a 3D printed part. For example, a mixture may be homogeneous to the degree that any sample volume or quantity of the material will have the same (substantially the same) ratio (or proportion) of the mixed build materials. That is, for a mixture which is made up by 80% of material A and 20% of material B with this degree of homogeneity, any randomly obtained amount of the mixture may contain substantially 80% of material A and 20% of material B. This is in contrast to a mixture which is homogeneous to a lesser degree where, although the mixture as a whole is made up by 80% of material A and 20% of material B, one randomly selected amount of the mixture could be 90% material A and 10% material B while another randomly selected amount of the mixture could be 70% material A and 30% material B. The degree of homogeneity may depend, for example, on the mixing time (that is, the time the build materials are mixed together in the mixing tank 212), the speed of the mixing (that is, the speed by which mixing blades rotate in the mixing tank 212), and also on the build materials being mixed, as well as other possible factors.. The degree of homogeneity desired in a mixture may depend on the properties (surface, mechanical etc.) desired for a part printed using the mixture. In general, a mixture can be made more homogenous by increasing the amount of mixing, either by increasing the mixer rotation speed or the mixing time.

[0072] For a combination of build materials, different levels of homogeneity may correspond to different mixing times, for a particular mixing tank 212 configuration (for example, size, number of mixing elements etc. - as will be evident from the detailed discussion of a mixer given below). As such, in certain examples, the build management station may be provided with pre-configured settings for achieving different degrees of mixing (different degrees of homogeneity) for different build material mixtures in a minimum amount of time (which may be determined by experimentation). In other examples, the settings can be modified, updated or added to, to provide settings for different build material mixtures, degrees of homogeneity etc.

[0073] Once the fresh build material and the recovered build material are sufficiently mixed, the mixed build material can be transported from the mixing tank 212 through a mixer-to-trolley valve 260, a tenth conduit (mixer-to-trolley conduit) 290 of the conduit network, a working area port in the form of a working area outlet port 291, to the working area 203 and into the trolley 102. Build material from the mixing tank 212 can pass through the working area outlet port 291 into the working area 203. The trolley 102 (or container) can be located substantially beneath the mixing tank 212 so that gravity can aid the transport of mixed build material from the mixing tank 212, through the mixer-to-trolley valve 260, the tenth conduit 290, the working area outlet port 291 and the working area 203 to the trolley 102.

[0074] Once the trolley 102 is filled with enough build material for a given 3D print run, the trolley 102 can be returned to the 3D printer 104. An appropriate quantity of build material to fill the trolley 102 for a print job may be controlled by the controller 295 of the material management station 106 based on the material management station 106 sensing how much build material is in the trolley when the trolley is docked in the material management station 106 at the beginning of a trolley fill workflow. The controller may then fill the trolley with a particular quantity (dose) of build material requested by a user for a particular print job intended by the user. The dosing is achieved by using a fill level sensor (not shown) such as a load cell in the mixing tank 212 to output a fill level value indicative of an amount of non-fused build material in the mixing tank. The fill level sensor can be one or more load cells, or any other type of sensor such as a laser-based sensor, a microwave sensor, a radar, a sonar, a capacitive sensor, etc.,. When the fill level sensor is a load cell, the fill level value can be an electrical signal indicative of a mass of the non-fused build material in the storage container.

[0075] A number of different workflows may be implemented in the material management station 106. These workflows are managed by the user, but some level of automation may be provided by a data processor on the material management station 106. For example, the user may select a workflow from a digital display on the material management station 106.

For users having one material management station 106 and one printer 104 an example workflow cycle may be filling the trolley 102, followed by printing a 3D object, followed by unpacking the object from a build volume in the material management station 106 followed by a subsequent print operation and a corresponding unpacking of the build volume and so on. However, the material management station 106 may serve two or more printers so that successive unpacking and trolley filling operations may be performed by the material management station 106. The user may also choose to perform the trolley filling, printing and unpacking functions in a random order.

[0076] For each of the workflow operations, a user interface of the material management station 106 may guide the user to undertake particular manual operations that may be performed as part of the workflow operation. For example, to perform an unpack operation, the user interface may instruct the user to move the collection hose 206 around the collection area 203 as described previously. In addition, the material management station 106 can automatically initiate other functions of the workflow operation. For example, to perform the unpack operation, the material management station 106 can automatically operate the pump 204 whilst the user moves the collection hose 206 around the collection area 203 to recover build material from the trolley 102. Any workflow operations the material management station 106 can perform fully automatically may be signalled to the user through the user interface without requiring user confirmation to proceed. If the workflow operation could present a potential safety risk, the otherwise fully automatic workflow operation may follow user confirmation to proceed.

[0077] For example, to load the trolley 102 with build material, the user sets this workflow operation then the material management station 106 automatically launches the different operations sequentially. The material management station 106 is controlled to send build material from the recovered build material tank 208 to the mixing tank 212. The material management station 106 is further controlled to send fresh build material from at least one of the fresh build material supply tanks 214a, 214b to the mixing tank 212. The material management station 106 is subsequently controlled to blend the mixture in the mixing tank 212. The mixed build material in the mixing tank 212 can then be discharged to the trolley 102. In an example, this workflow operation is completed as a batch process, and so the cycle may be continuously repeated to completely fill the trolley 102.

[0078] In some processes, a small portion (e g. 1%) of build material can pass through the build material traps 218a, 218b, 218c (e.g. the powder traps) and can travel towards the pump 204.

[0079] An additional RBMT build material trap 220 (e.g. a powder trap) may, in some examples, be located in an eleventh conduit (pump feed conduit) 292 of the conduit network that connects each of the third, fourth and seventh conduits 276, 278 and 284 to the pump 204. The additional RBMT build material trap 220 is connected to the RBMT inlet area. The additional RBMT build material trap 220 collects build material that may have passed through any of the overflow build material trap 218a, RBMT build material trap 218b or mixer build material trap 218c to help prevent it from reaching the pump 204. Build material collected in the additional RBMT build material trap 220 can be transported into the recovered build material tank 208 by opening a trap-to-RBMT valve 262. The trap-to-RBMT valve 262 may be opened electronically by the controller 295. The RBMT build material trap 220 may operate in the same or similar way to each of the overflow, RBMT, and mixer build material traps 218a, 218b and 218c. Build material can be transported from the RBMT build material trap 220 to the recovered build material tank 208 by gravity.

[0080] A pump filter 222 may also be located in a twelfth conduit 294 of the conduit network adjacent the pump 204. This pump filter 222 helps to collect any build material that may have passed through any of the overflow build material trap 218a, RBMT build material trap 218b or mixer build material trap 218c as well as the additional RBMT build material trap 220. This helps prevent the build material from reaching the pump 204, thereby reducing the likelihood of the function of the pump 204 being impaired, which could happen if large quantities of build material were to reach it.

[0081] At any time, when the material management station 106 is to be used to process build material of a different material type, for example of a different material, the material management station circuit 200 can be controlled to implement a purging process to purge substantially all build material of a current material type from the material management station circuit 200 to the overflow tank 210. The fresh build material supply tanks 214a, 214b can be disconnected from the build material station circuit 200 and stored to prevent wastage of fresh building material of the current material type.

[0082] In one example, the purging process is carried out when unfused build material in the trolley 102 has already been collected using the collection hose 206 and transported either to the recovered build material tank 208 or to the overflow tank 210, or both. Alternatively, the purge process can include using the collection hose 206 to transport any unfused build material in the trolley 102 to the overflow tank 210, as described previously.

[0083] The purge process includes transporting any unfused build material in the recovered build material tank 208 to the overflow tank 210. To transport unfused build material from the recovered build material tank 208 to the overflow tank 210, the RBMT-to-sieve valve 256 and the sieve-to-mixer valve 258 in the eighth and ninth conduits 286, 288 between the recovered build material tank 208 and the mixing tank 212 can be opened as well as the mixer-to-trolley valve 260 in the tenth conduit 290 and the hose-to-overflow valve 244 in the second conduit 274 between the collection hose 206 and the overflow tank 210 and the overflow-to-pump valve 248 in the fourth conduit 278 between the pump 204 and the overflow tank 210. Any build material in the recovered build material tank 208 drops down into the sieve 216 through the eighth conduit 286 by gravity. The collection hose 206 can be connected directly to the tenth conduit 290 before or after any cleaning of the unfused build material in the trolley 102 has been completed. When the pump 204 is active, a differential pressure is provided from the pump 204 to the sieve 216 via the overflow-to-pump valve 248, the overflow tank 210, the hose-to-overflow valve 244, the collection hose 206, the mixer-to-trolley valve 260, the mixing tank 212 and the sieve-to-mixer valve 258. Build material in the recovered material tank 208 is transported to the sieve 216 by gravity via the eighth conduit 286 and the RBMT-to-sieve valve 256. That is, a pressure at the pump 204 is lower than a pressure at the sieve 216. The differential pressure enables build material from the recovered build material tank 208 to be transported to the sieve 216 and on to the overflow tank 210. At the overflow tank, build material separates from air flow and drops from the overflow inlet area into the overflow tank 210. Air (and any residual build material) continues along the fourth conduit 278 and through the overflow-to-pump valve 248 towards the pump 204, which is at a reduced pressure. It can be seen that any unfused build material in the sieve 216, the mixing tank 212 or in any of the eighth conduit 286, the ninth conduit 288, the tenth conduit 290 or the second conduit 274 may also be transported to the overflow tank 210. In this way, substantially all unfused build material in the material management station circuit 200 can be transported to the overflow tank 210.

[0084] Alternatively, the unfused build material in the recovered build material tank 208 can be transported to the trolley 102 as described previously. Subsequently, the unfused build material in the trolley 102 can be transported to the overflow tank 210, also as described previously. Thus, an alternative way to transport unfused build material from the recovered build material tank 208 to the overflow tank 210 can be provided without directly connecting the collection hose 206 to the tenth conduit 290.

[0085] The purge process can also include one or more further purging process elements where a sacrificial material is transported through any part of the conduit network of the material management station circuit 200 which may still contain at least an amount of unfused build material of a current material type. The sacrificial material can act to dislodge at least some of the current build material remaining in the material management station circuit 200. The sacrificial material in one example may be the build material of the different build material type to be subsequently used in the material management station 106. The sacrificial material may alternatively be an inert material (e.g., silica) which is not a build material. In this way, any small amount of sacrificial material remaining in the material management station 106 at the end of the purging process is unlikely to interfere with the further operation of the material management station 106.

[0086] After the purge process is completed, and substantially all the unfused build material in the material management station circuit 200 is in the overflow tank 210, the overflow tank 210 can then be removed from the material management station 106, for example for storage or disposal and a further overflow tank (not shown) can be connected to the material management station 106. The further overflow tank can be empty or the further overflow tank can contain build material previously purged from the (or another) material management station 106.

[0087] The purge process can be performed in response to a user input, or automatically. Where purging is performed automatically, the material management station circuit 200 can be controlled to implement the purging process when a trolley 102 containing a different material is slotted into the docking position 107 in the material management station 106. In this example, a material type is electronically recorded on a memory chip of the trolley 102 (or other container). The memory chip is readable by the processing circuitry of the material management station 106 to determine the material type of the material in the trolley 102 (or other container). Alternatively or additionally, the material management station circuit 200 can be controlled to implement the purging process when one or more fresh build material supply tanks 214a, 214b containing a different material type are connected to the material management station circuit 200. In this example, a material type is electronically recorded on a memory chip of the fresh build material supply tanks 214a, 214b. The memory chip is readable by the processing circuitry of the material management station 106 to determine the material type of the material in the fresh build material supply tanks 214a, 214b. In other examples, the material management station circuit 200 can be controlled to implement the purging process when both fresh build material supply tanks 214a, 214b are removed from the material management station circuit 200. The material management station 106 may be controlled to provide an indication to a user that the purging process can be performed based on the criteria discussed previously.

[0088] Examples of the mixing tank 212 will now be described in detail, referring to the example shown in Figs. 3A to 3E where appropriate. Fig. 3A shows a cross section of the mixing tank 212 in a vertical plane extending along the length of the tank along a driven shaft. Figs. 3B and 3C are cross sections of mixing tank 212 in alternative planes along the indicated lines and viewed in the direction of the arrows. Fig. 3D is an isometric view of mixing tank 212 with container 315 removed. Fig. 3E is an isometric view of the shaft 340, first mixing elements 360 and second mixing elements 370 of the mixing tank 212.

[0089] As described above, the mixing tank 212 is for mixing build materials. The materials for mixing may be a fresh build material and a recycled build material, as defined above. Additionally or alternatively, the materials may be a first build material and a second build material, which are differentiated in a manner other than being a fresh build material and a recycled build material. For example, as described above, the first build material may be a fresh build material while the second build material could be an additive or different build material, intended to provide a printed part with features beyond those arising from use of the first build material alone.

[0090] In an example, a mixing tank 212 may include a container 310. The container 310 may, for example, have a substantially U-shaped cross section, an example of which may be seen in Fig. 3B (which depicts a vertical cross-section across the length of the mixing container shown in Fig. 3A). Furthermore, the container 310 could be made out of plastic, for example constructed by a plastic injection or formed using other processes.

[0091] The container 310 may have a first opening 320 through which build material is received. The first opening 320 may, for example, be connected to a build material supply tank by a conduit. Referring to the above, the first opening 320 may receive material from a fresh build material supply tank 214a, 214b and a recyclable build material supply tank 208. Some examples of the mixing tank 212 can include a container 310 having one first opening 320 through which all build material is received, while other examples of the mixing tank 212 include a container 310 having a plurality of first openings 320 each connecting to a different build material supply tank and so through which a different supply of build material is received. In certain examples, the first opening 320 may be located at or near a top or upper part of the container 310, for example in an upper wall or apex of the container 310.

[0092] The container 310 may further have a second opening 330 by which mixed build material is able to exit or flow from the container 310 (or to allow for removal of build material from the container 310). In some examples, the container 310 includes one second opening 330 while in other examples the container 310 includes a plurality of second openings 330. In certain examples, the second opening may be positioned at or near a bottom surface 315 of the container 310 or a lower part of the container 310, for example in the floor of the container 310. The second opening 330 may be coupled to a mixer-to-trolley valve 260. Mixed build material may therefore be transported from the container 310, through the second opening 330, the mixer-to-trolley valve 260, a tenth conduit 290 (mixer-to-trolley conduit) of a conduit network, a working area port in the form of a working area outlet port 291, a working area 203 and into a troiiey 102. Transportation of the mixed buiid matehai from the container 310 through the second opening and other iisted components may be aided by gravity. However, this is one exampie and other options for aiding the transportation of mixed buiid materiai from the mixing tank 212 exist, such as a vacuum.

[0093] The mixing tank 212 may inciude a rotatabie shaft 340, that is, a shaft 340 rotatabie reiative to the container 310. in an exampie, the shaft 340 may generaiiy run paraiiei to a surface of the container 310. For exampie, the shaft 340 may run paraiiei to the bottom surface 315 of the container 310. That is, the shaft 340 may extend iongitudinaiiy through the container 310, and may extend paraiiei to the bottom surface 315 of the container 310 or at an acute angie to the bottom surface 315. The shaft 340 may be constructed of any suitabie materiai. For exampie, the shaft 340 may be constructed from metai. At one end, the shaft 340 may be connected to drive eiement 350, for exampie a motor and transmission system 350. The motor and transmission system 350 are intended to effect a rotation of the shaft 340, that is, to drive rotation of the shaft 340 reiative to the container 310. The motor and transmission system 350 may be iocated outside the container 310, and so it is possibie the motor and transmission system 350 is separate from the mixing tank 212 and is simpiy the source of torque for the shaft 340.

[0094] To connect with the motor and transmission system 350, the shaft 340 may therefore pass through a waii of the container 310, for exampie an end waii of the container 310, seaied appropriateiy for instance using a grommet. As an aiternative exampie, the motor and transmission system 350 may extend into the container 310 and provide an area, on an end waii of the container 310, to which the shaft 340 may connect. The other end of the shaft 340 may pass through (or be attached to) the other end waii of the container 310, where the other end of the shaft 340 may be retained in a manner which suitabiy aiiows for rotation of the shaft 340. Therefore, in an exampie the rotatabie shaft 340 may extend across the entire iength of the bottom surface 315 of the container 310. However, in other exampies the shaft 340 couid extend part of the way across the iength of the container 310. For instance, the end of the shaft 340 not coupied to the motor and transmission system 350 may be suspended or supported from the roof or the fioor (or a side waii) of the container 310, in a manner which stiii aiiows for rotation of the shaft 340 reiative the container 310.

[0095] The mixing tank 212 may inciude first mixing eiement 360 coupied to the shaft 340 and extending from the shaft 340 by a first radiai distance to engage buiid materiai proximai to a surface of the container 310. in some exampies, the surface of the container 310 may be a bottom surface 315 of the container 310. in reference to Fig. 3B and the exampie of a verticai cross section across the iength of the shaft shown therein, the bottom surface 315 may not be fiat and instead have a semi-circuiar cross section. As such, the first mixing element 360 may also engage build material which is proximal to any part of this surface including the point where the sidewalls (that is, the vertical walls in Fig. 3B) meet the semicircular bottom surface 315. As such, in certain examples, the first mixing element 360 may also engage build material which is proximal to the sides of the container 310.

[0096] The first mixing element 360 may comprise a radial blade coupled to the rotatable shaft 340. In one example, the radial blade may be a plastic injection, however, other materials may be used depending, for instance, on the materials intended to be mixed. For example, the radial blades may be made of metal (for example an aluminium injection). The radial blade may be included in a first mixing element 360 which in certain examples, as will be described below, may include a plurality of radial blades.

[0097] In one example, the radial blade extends from the shaft by the first radial distance. The radial blade serves to engage with build material along its full length, although the shape of the radial blade may be selected to provide most engagement proximal to a surface of the container 310 such as the bottom surface 315. Specifically, in this example, the radial blade extends to within a small distance from the bottom surface 315 of the container 310, thereby leaving a gap between the radial blade and the bottom surface 315. This distance may be defined according to the size of particles of a build material to be received in the container 310 for mixing. For example, the distance (or gap) may be 1mm or 2mm, if a particle size if between 20 and 200 microns. As a result, the radial blade may engage build material which is proximal to the bottom surface 315 of the container 310.

[0098] The radial blade may include a blade head. In an example, the blade head may be connected to the shaft by at least part of a blade stem part of the radial blade.

[0099] Upon engaging build material proximal to the bottom surface 315 when rotating, a first mixing element 360 (such as a radial blade) may lift the build material from the bottom surface 315. That is, the blade head of the radial blade may contact the build material that lies in its rotational path and force the contacted build material into higher regions of the container 310.

[0100] The radial blade, or the blade head of the radial blade, may be disposed at a particular angle (or inclination angle) with respect to the shaft. For example, in the following it may be considered that an angle of 0 degrees corresponds to a blade head part of a radial blade which is perpendicular to the shaft, while an angle of 90 degrees corresponds to a blade head which is parallel to the shaft. An inclination angle of 0 degrees would mean that as the blade rotated through the mixture only the cross sectional area of the blade would engage the material and minimal mixing would occur. Furthermore, the blade would not cause movement of the mixture along the axis of the shaft. An inclination angle of 90 degrees would result in the whole surface of the blade engaging the material to cause movement of the material circumferentially around the shaft. No axial movement would be caused. As an example, the radial blade may have an inclination angle between 30 degrees and 60 degrees, for instance 45 degrees. Such an intermediate angle will result in axial movement as well as circumferential movement. The inclination angle of a radial blade can be related to the forces imparted by the radial blade to any build material that the radial blade may come into contact with. For example a radial blade with a small or no angle may impart little axial movement or force to build material at the bottom of the container 310 that the radial blade may engage with while rotating. Conversely, if the angle is too large then a relatively small axial force may be imparted. Any axial movement may be in addition to the lifting of the build material that results from the general rotation and build material engaging of the first mixing element 360.

[0101] An example of angles of radial blades or blade heads of radial blades can be seen in Figs. 3D and 3E. Here, it may be estimated that the radial blades are at an inclination angle of approximately 45 degrees relative to the shaft 340, thereby being half-way between parallel to the shaft 340 and perpendicular to the shaft 340.

[0102] Additionally, the blade head part may be shaped to conform to the shape of the surface of the container 310. For example, with respect to the shape of the container 310 shown in the cross section in Fig. 3B, the blade head may be formed to follow the curve of the bottom surface 315 of the container 310 (this curve resembling the trough of a U-shape). This may be further seen in Fig. 3D, where the curved outer surface of that which may be considered as the blade head part of the radial blades is visible.

[0103] Of course, other examples may exist where the entirety of the radial blade may be considered a blade head which extends from the shaft to be proximal to the surface, where the blade head will engage with build material proximal to the container wall [0104] Upon rotation of the shaft, the radial blade also rotates. If a build material has been introduced into the container 310, a portion of the build material may collect in the bottom part of the container 310, for example around the bottom surface 315. As the radial blade rotates, it may contact with some of this portion of the build material and cause lifting of at least part of the contacted build material. This may result in the contacted build material being driven into upper regions of the container 310 away from the bottom surface 315. Additionally, the rotating radial blade may also impart an axial force to the contacted build material. In some examples, this axial force causes the contacted build material to be moved in a direction away from the bottom of the bottom surface 315 of the container 310. By doing this, the radial blade may facilitate movement of build materials which are introduced into the mixing tank 212.

[0105] For example, if two build materials are supplied to the mixing tank 212, a portion of one or both build materials may collect at the bottom surface of the container 310. Additionally, in other examples, if build the build materials are introduced into the mixing tank 212 (that is, the container 310) sequentially, then the first supplied build material may collect near the bottom surface 315, with the second supplied build material being deposited above the first supplied build material. Additionally, in yet other examples, even if the two build materials are supplied simultaneously, one of the build materials may be denser and so the more of this denser build material may end up disposed near the bottom surface 315. This consequence of one build material being denser than another build material may be present during mixing, with the denser build material more likely to gather proximal to the bottom surface 315. By rotation of the shaft, the radial blade comes into contact with some of this portion and imparts a force on the contacted build material. It may be expected that a component of this force is in a direction away from the bottom surface 315, and so part of the contacted build material is raised upwards in the container 310. This movement of the contacted portion may alter the homogeneity of the mixture of the two build materials. Furthermore, by moving build material away from the bottom surface 315 and potentially closer to the shaft 340 (by lifting the contact build material), the contacted build material may be brought into contact with a second mixing element 370 (described in detail below) for further mixing. Of course, in view of the first mixing element 360 extending from the shaft, a part of the first mixing element 360 nearer to the shaft 340 (that is, away from a part of the first mixing element 360 which is proximal to build material at the bottom surface 315) may also contact the lifted build material and so potentially result in mixing.

[0106] As mentioned above, and as an alternative to configurations where only one first radial blade is provided for a first mixing element 360, a plurality of radial blades (or a first mixing element comprising a plurality of radial blades) may be provided at a particular position along the shaft 340. In such examples, each radial blade extends from a different position around the circumference of the shaft 340. In one example, the radial blades may be positioned equidistant from one another along the circumference of the shaft 340. For example, a first mixer element 360 may comprise two radial blades which are offset from each other at 180 degrees around the shaft 340, while in another example a first mixer element 360 may comprise four radial blades which are disposed at 90 degrees intervals around the shaft 340. Other options are possible. An example of this can be seen in Figs. 3D and 3E, where individual first mixing elements 360 comprising two radial blades offset by 180 degrees are shown.

[0107] Additionally, in certain examples the arrangement of the radial blades may be inconsistent for the radial blades of the first mixing element 360. That is, where a first radial blade of a first mixing element 360 may face a first direction along the shaft 340, a second radial blade of the same first mixing element 360, positioned subsequently along the circumference of the shaft 340 from the first radial blade, may face the opposite direction along the shaft 340. The arrangement of the radial blades may, in some examples, take into account the location of the second opening 330 in the container 310. That is, the radial blades may be disposed, in general, to provide a force to engaged build material which transports the build material in the general direction of the second opening 330. As such, for example, radial blades on one side of the second opening 330 may have the opposite angular arrangement to radial blades located on the other side of the second opening 330.

[0108] In some examples, the mixing tank 212 may include a plurality of first mixing elements 360. In this case, each first mixing element 360 is coupled to the shaft 340 at a different location along the shaft 340. In certain examples, as described above, each first mixing element 360 may include a plurality of radial blades. In such examples, the radial blades of subsequent first mixing elements 360 may be offset, around the circumference of the shaft 340, from the radial blades of the previous first mixing element 360. This example configuration will be illustrated in further detail in Fig. 3B. An example of such an offset may be 90 degrees. For example, if a first mixing element 360 comprises two radial blades offset by 180 degrees from each other, a subsequent first mixing element 360, which is located next along the shaft 340, may also comprise two radial blades which are offset 180 degrees from each other and are also offset by 90 degrees from the radial blades of the first mixing element 360. This can be seen in Figs. 3D and 3E where a neighbouring pair of first mixing elements 360 can be seen to be offset from each other by 90 degrees. That is, the radial blades of one of a neighbouring pair of first mixing elements 360 are offset by 90 degrees from the radial blades of the other first mixing element 360 of the neighbouring pair of first mixing elements 360.

[0109] In addition, the angular arrangement of radial blades between neighbouring first mixing elements 360 may be the same or different. That is, for example, where the radial blades of one first mixing element 360 face forwards, backwards etc. in one arrangement, the radial blades of another first mixing element 360 may face forwards, backwards etc. in the same or another arrangement. Each first mixing element 360 may or may not include the same number of radial blades.

[0110] The mixing tank 212 may include a second mixing element 370 coupled to the shaft 340 and extending to a second radial distance from the shaft 340 to engage build material proximal to the shaft 340. The second radial distance may be smaller than the first radial distance. Here, build material proximal to the shaft 340 may simply be interpreted, for example, to refer to build material which is closer to the shaft than that engaged by the first mixing element 360 (for example, by a blade head of a radial blade) at the first radial distance. The second mixing element 370 may be suitable to engage with build material which is located around the shaft 340 of the container 310 within or around the second radial distance.

[0111] In an example, this configuration may allow for the first mixing element 360 to lift build material from the bottom surface 315 of the container 310 into the radial path of the second mixing element 370. As such, build material may not gather at the bottom surface 315 outside of the second distance and so out of contact with the second mixing element 370, due to the first mixing element 360 being able to contact such build material and allow for it to be brought into contact with the second mixing element 370. Build material lifted from the bottom surface by the first mixing element 360 may therefore be lifted back into the path of the second mixing element 370, thereby allowing for greater mixing of the build material to be achieved.

[0112] For example, the second mixing element 270 may include a spiral helix which is coupled, directly or indirectly, to the shaft 240. Note that while the term “spiral helix” is used herein, it is possible for the helix to spiral towards or away from the shaft, or be positioned in another configuration. This term “spiral helix” is used herein to refer to both a true spiral and a true helix. A spiral helix may, in one example, refer to an element which extends along a length of the container 310, is spaced apart from the shaft 340 by a predetermined distance (which may be fixed or may vary along the length of the shaft), and which twists (or curls) around the shaft 340. That is, if the predetermined distance is considered to be a radius of a circle around the shaft, then a first point along the length of the spiral helix corresponds to a first position on the circumference of this circle and a second point along the length of the spiral helix (being a point adjacent to the first point) corresponds to a second position on the circumference of this circle (being a position adjacent to the first position, either clockwise or anticlockwise from the first position depending on how the spiral helix twists around the shaft 340), Depending on the eccentricity of how the spiral helix is twisted around the shaft 340, the length of the spiral helix which corresponds to the circumference of the circle may differ (e.g., the length of spiral helix taken to make one full turn around the shaft).

[0113] An illustrative example of a spiral helix is shown in Figs. 3D and 3E. Here, it can be seen how the spiral helix (in this case coupled to several first mixing elements 370) turns around the shaft 340 as the spiral helix extends along the length of the container 310 from one first mixing element 360 to the next first mixing element 360. In these examples it can also be seen that the distance by which the spiral helix is spaced from the shaft 340 is substantially constant along the length of the spiral helix, but it is also possible for the spacing to vary.

[0114] In the illustrated example the spiral helix is formed by thin sheet material that is substantially broader than it is thick, though the material dimensions and proportions shown may be varied and/or vary along the shaft axis. Furthermore, the illustrated example shows that at points along the shaft a line extending radially from the shaft is in a plane tangential to a major surfaces of the spiral helix.

[0115] In alternative implementations such a plane may be inclined to the radial line and the angle of inclination may be fixed or variable along the length of the shaft. In the illustrated example the whole of the spiral helix is spaced apart from the shaft, though in other implementations this may not be true, at least not for the whole length of the spiral helix. Numerous further variations in the shape of each spiral helix will be readily apparent to the skilled person. Further readily apparent variation exists in the number, turn direction (clockwise or anticlockwise), relative spacing, number of attachment points and length of shaft covered (among other properties). The particular form of spiral helix or other component comprising a second mixing element will be dictated by the materials to be mixed: their material properties and volumes, together with the time for mixing and any other relevant factor concerning the use of a mixer.

[0116] For instance, if only one first mixing element 360 comprising only one radial blade were included in the mixing tank 212, then a second mixing element 370 could comprise a spiral helix coupled, at one end, to a part of the radial blade and also coupled, at the other end, to the shaft 340. In such a configuration, the second distance may be considered to refer to the point at which the spiral helix is coupled to the radial blade.

[0117] However, in another example and with reference to Fig. 3, a plurality of first mixing elements 360 may be included in the mixing tank 212. Here, a spiral helix, included in the second mixing element 370, may be coupled, at one end, to a radial blade of one first mixing element 360 and also coupled, at another end, to a radial blade of another first mixing element 360. The two radial blades involved in this coupling may be offset by 180 degrees from each other. In this configuration, the spiral helix is only indirectly coupled to the shaft 340, yet rotates with rotation of the shaft 340 due to corresponding rotation of the radial blades to which it is coupled.

[0118] In a further example, the spiral helix may be coupled to more than two first mixing elements 360. For example, the spiral helix may be coupled, at one end, to a radial blade of a first mixing element 360. The length of the spiral helix is then coupled to a radial blade of another first mixing element 360, at a position between the two ends of the spiral helix. The spiral helix may then be coupled, at the other end, to a radial blade of yet another first mixing element 360. In this specific example, therefore, a second mixing element 370 is coupled to three first mixing elements 360, but the configuration can include additional first mixing elements 360.

[0119] It should not be thought that the second mixing element is limited to being a spiral helix, however. In an example, the second mixing element could itself be a radial blade, distinguished from a radial blade of a first mixing element by not being intended to engage build material proximal to the surface of the container 310. Such a radial blade of the second mixing element could be coupled with the shaft directly, or could be coupled to the shaft indirectly by being coupled to the first mixing element in some appropriate manner.

[0120] The second mixing element 370 may be any element suitable for engaging build material within the first radial distance around the second radial distance. Said engagement causing movement of the engaged build material and so inducing mixing of build materials around the second mixing element 370. Certain examples of second mixing elements 370, such as the spiral helix, may promote mixing by additionally providing a force to engaged build material which results in the transport of the engaged build material in a direction along the shaft. As a result, engaged build material may be moved longitudinally through the tank (this potentially being in addition to the lifting of build material performed by the first mixing element 360).

[0121] In certain examples, the second mixing element 370 may include a plurality of spiral helixes. Alternatively, it could be considered that the mixing tank 212 includes a plurality of second mixing elements 370, each of which is a spiral helix. Each spiral helix may be connected in one of the manner described above; that is, connected between two or more first mixing elements 360 (such as the radial blade(s) included therein) or between a first mixing element 360 and the shaft 340 (or some other element which rotates with the shaft 340).

[0122] Referring to the previous discussion of the formation of the radial blades, an end of a spiral helix may be connected to a blade stem part of a radial blade, which is coupled to the shaft 340 at one end and joins the blade head at the other end. Alternatively, if the entirety of the radial blade is considered to be the blade head, then the end of the spiral helix may be coupled with some part of the blade head. For example, the end of the spiral helix may be coupled to a part of the blade head which is removed from the extremes of the blade head, and so may not engage with build material proximal to the bottom surface 315 of the container 310.

[0123] The specific example shown in Figs. 3A to 3E will now be considered in detail.

[0124] Figure 3 shows an example of a mixing tank 212 (or mixing tank 212) which may be implemented in the material management station 106 of the 3D printing system 100. Not all of the components included in Figs. 3A to 3C are necessary for the operation of the mixing tank 212. Such non-essential components represent optional modifications which can be included, together or independently, in examples of mixing tanks. In Fig. 3A, a cross sectional view of an example of a mixing tank 212 is shown, this being a vertical cross section taken along the length of the shaft. In Fig. 3B, an additional cross sectional view is shown this being a vertical cross section taken across the length of the shaft. In Fig. 3C, an additional cross sectional view is shown, this being a horizontal cross section taken along the length of the shaft. Approximate locations of the cross sections of Figs. 3B and 3C are shown on Fig. 3A, where it should be noted that Fig. 3C omits showing the motor and transmission system 350.

[0125] The mixing tank 212 shown in Figs. 3A to 3C includes a container 310. The container 310 has a substantially U-shaped cross section, where a top surface is provided to close the U-shape. The container 310 may be made from any suitable material, for example formed by plastic injection.

[0126] A first opening 320 is provided in the top of the container 310. As can be seen in Figs. 3A, 3B, and 3D, the first opening 320 is in the top surface of the container 310. However, the mixing tank 212 is not limited to this arrangement and instead the first opening 320 could foreseeably be located in one of the other walls of the container 320. The first opening 320 may allow for build material to be introduced into the container 320. As described above, the first opening 320 may therefore be connected, or in some manner be arranged to receive, build material from one or more sources.

[0127] A second opening 330 is provided in the bottom of the container 310. As can be seen in Figs. 3A, 3B and 3D, the second opening 330 may be provided in the bottom surface 315 of the container 310. The second opening may allow for build material to be removed from the container 310.

[0128] A shaft 340 is located in the container 310, where the shaft 340 is rotatable with respect to the container 310. The shaft is shown, in Figs. 3A and 3D, to extend across the entire length of the container 310, however a mixing tank 212 is not limited to this. The shaft 340 may be constructed from metal, however this is a non-limiting example.

[0129] The shaft 340 may be connected to a motor and transmission system 350. The motor and transmission system 350 acts as a drive element by which the shaft 320 may be rotated with respect to the container 310. As shown in Fig. 3A, an end of the shaft 340 may pass through an end wall of the container 310 such that the end of the shaft 340 may be coupled to the motor and transmission system 350. However, other configurations could be possible as described above.

[0130] A plurality of first mixing elements 360 are shown coupled to the shaft in Figs. 3A, Fig. 3B, Fig. 3D and Fig. 3E. Each first mixing element 360 is located at a different location along the shaft 340, where it is coupled to the shaft 340 such that a rotation of the shaft 340 results in a corresponding rotation of the first mixing element 360. In the examples shown in Figs. 3A to 3E, six first mixing elements 360 are coupled to the shaft.

[0131] Each first mixing element 360 comprises a plurality of radial blades, specifically two radial blades in this example as can be seen in Figs. 3B, 3D and 3E. As shown in the figure, each radial blade is offset from the other radial blade by 180 degrees around the circumference of the shaft 340. The two radial blades are therefore positioned equidistantly around the circumference of the shaft 340.

[0132] As shown in Figs. 3A, 3D and 3E, the two radial blades may also face different directions along the length of the shaft; that is, the two radial blades of a first mixing element 360 may be disposed at different angles. For example, it may be considered that the radial blade of the leftmost first mixing element 360 currently positioned above the shaft 340 may face towards to the end of the shaft 340 coupled to the motor and transmission system 350. Whereas, the other radial blade of the leftmost first mixing element 360, which is currently positioned below the shaft 340, may face towards the other end of the shaft 340. As such, the radial blades generally face towards the second opening 330.

[0133] As mentioned above, the radial blades may also be formed to correspond to the shape of the bottom surface 315 of the container 310, as shown in the vertical cross section in Fig. 3B. Additionally, a radial blade may be constructed out of plastic, for example by plastic injection, or metal, for example by aluminium injection, although this should not be considered as limiting. A radial blade may also be formed to have a certain angle (inclination angle) which may affect the how the radial blade interacts with build material located proximal to the bottom surface 315 of the container 310.

[0134] As shown in the vertical cross section of Figs. 3B, 3D and 3E, the radial blades of neighbouring first mixing elements 360 may be offset around the circumference of the shaft 340. That is, referring to the vertical cross section of Fig. 3B, the radial blades of first mixing element 360a are offset by 90 degrees to the radial blades of second mixing element 360b.

[0135] A plurality of second mixing elements 370 are shown coupled to (or between) the first mixing elements 360 in Figs. 3A, 3B, 3D and 3E, and therefore may be considered to be indirectly coupled to the shaft 340. In this example, each second mixing element is a spiral helix which is coupled between two radial blades of neighbouring first mixing elements 360.

Alternatively, it could be considered that each spiral helix is coupled to more than two first mixing elements 360, for example by being coupled to three first mixing elements 360 where a middle first mixing element 360 is, in effect, passed through by the spiral helix. In the example shown in Figs. 3A to 3E, four spiral helixes are included in the mixing tank 212, each being coupled to three first mixing elements (that is, a radial blade of each of three first mixing elements 360).

[0136] The spiral helixes are coupled to a part of the radial blades which it closer to the shaft than the end of the radial blades. That is, the spiral helixes extend to a second distance from the shaft 340 which is less than a first distance by which the radial blades extend from the shaft 340. Referring to Figs. 3A and 3E, for example, the part of the radial blade to which the spiral helix connects may be termed the blade stem, where the other part of the radial blade - the blade head - is that which is intended to engage with build material proximal to the bottom surface 315 of the container 310.

[0137] Upon rotation of the shaft 340 by the motor and transmission system 350, the radial blades (the first mixing elements) and the spiral helixes (the second mixing elements) rotate relative to the container 310. A blade head of a rotating radial blade may engage with build material which is located proximal to the bottom surface 315 of the container 310. This engagement may impart the build material with a force in the axial direction of the radial blade, moving the build material from being proximal to the bottom surface 315 to a location closer to the shaft 340 or indeed the top surface of the container 310. Additionally, a rotating spiral helix may engage with build material which is located proximal to the shaft 340 (that is, between the shaft 340 and a blade head of a radial blade), for example build material which has been moved upwards as a result of engaging with a rotating radial blade. The rotating spiral helix may therefore impart further force to the engaged build material. For example, contact with the rotating spiral helix may serve to transport the build material along the length of the container 310, depending on the configuration of the spiral helix and the rotation of the shaft 340.

[0138] The mixing tank 212 may connect to a valve, such as a mixer-to-trolley valve 260, via the second opening 330. As described above, this may further lead to a conduit (a tenth conduit 290) of a conduit network and then into a trolley 102. Mixed build material may be transported from the container 310 to the trolley 102 through this network of components by a number of methods, as described above. For example, said transporting may be aided by gravity, a vacuum, a partial vacuum etc.

[0139] The length of time taken to mix build materials may depend on a number of factors. For example, the length of time taken to mix a first build material and a second build material to a certain degree of homogeneity may differ depending on the relative volumes of each build material. In another example, the length of time may depend on the material properties of the build material(s) involved. The property of density has already been mentioned above, where the propensity of such to settle closer to the bottom surface 315 of the container 310 has been described. It may follow that, if one build material is denser than another build material, a mixing time of these materials may be longer than the mixing time for two build materials of similar density. Other build material properties could also affect the mixing time (for example, particle size).

[0140] In view of this, a fixed length of time may be set to mix a particular combination (or mixture) of build materials. Alternatively, in other examples, the homogeneity of a mixture could be measured during the mixing, where the mixing continues until the desired homogeneity is obtained. The mixing time may be of the order of 10s of seconds, perhaps 30s, 60s or 90s, though times outside of this range are possible. The mixing time relates to the use of the mixer in a batch process in which a volume of materials to be mixed is received in the container, processed and then expelled.

[0141] The length of time for mixing a particular combination to a desired degree of homogeneity may also depend on a speed of mixing. For example, for a particular build material, it may be desirable to rotate the shaft 340 at a speed which is below a particular threshold (this threshold may be related to the particular build material, and account for material properties of the build material). Therefore, while a high speed rotation of the shaft 340 (and so the mixing elements coupled to the shaft (340) could be seen to provide faster mixing than a low speed rotation, it may not be desirable to use the high speed rotation.

[0142] For example, the speed at which the build material are mixed (or the speed at which the first and second mixing elements engage with build material) may affect the temperature of the build material being mixed. Therefore, to manage the temperature of the build material being mixed in the container 310 (and so avoid undue heating, for example), the speed of the mixing may be managed. In another example, the speed of the mixing should be managed to prevent damage to the build materials being mixed. That is, if the mixing elements rotate too quickly, the size of the particles of a build material may be effected (for example, reduced by breaking down the particles) such that the mechanical properties of the build material are changed. As another example, the speed of mixing may, depending on a build material, be related to the generation of static within the container 310. In such circumstances it can be desirable to manage the generation of static, and so manage the rotation speed.

[0143] As an example, the speed by which the shaft 340 rotates may be between 15 and 50 revolutions per minute.

[0144] Mixing build materials in the mixing tank 212 may be performed as a batch process; where batches of build materials are adequately mixed and supplied to the trolley 102 one after another. However, in certain examples a continuous mixing process may be performed whereby a portion of adequately mixed build material is removed, via the second opening 330, during mixing (to be transported to the trolley 102) and then additional build material for mixing is introduced, via the first opening 320, for mixing.

[0145] All of the features disclosed in this specification (including any accompanying claims, abstract and drawings) may be combined in any combination, except combinations where at least some of such features are mutually exclusive.

[0146] Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example of a generic series of equivalent or similar features.

[0147] The present teachings are not restricted to the details of any foregoing examples. Any novel combination of the features disclosed in this specification (including any accompanying claims, abstract and drawings) may be envisaged. The claims should not be construed to cover merely the foregoing examples, but also any variants which fall within the scope of the claims.

Claims

CLAIMS:

1. A mixer for a material management station for a 3D printing system, the mixer comprising: a container having a first opening to receive build material; a shaft within the container and rotatable relative to the container; a first mixing element coupled to the shaft and extending from the shaft by a first radial distance to engage build material proximal to a surface of the container; and a second mixing element coupled to the shaft and extending from the shaft by a second radial distance which is smaller than the first radial distance to engage build material proximal to the shaft.

2. The mixer of claim 1, wherein the surface is a bottom surface of the container.

3. The mixer of claim 1, wherein the first mixing element comprises a plurality of radial blades coupled to the shaft.

4. The mixer of claim 3, wherein the radial blades extend radially from the shaft and are equally spaced around the circumference of the shaft.

5. The mixer of claim 4, wherein one radial blade of the plurality of radial blades is disposed in a first axial direction along the shaft and another radial blade of the plurality of radial blades is disposed in a second axial direction opposite to the first direction.

6. The mixer of claim 3, wherein each radial blade has an inclination angle between 30 degrees and 60 degrees relative to an axis of the shaft.

7. The mixer of claim 1, wherein the mixer comprises two first mixing elements; and wherein each first mixing element is coupled to the shaft at a different position along the shaft.

8. The mixer of claim 7, wherein each first mixing element comprises a radial blade; and wherein the radial blade of one first mixing element is offset, around the circumference of the shaft, from a radial blade of a neighbouring first mixing element.

9. The mixer of claim 7, wherein the second mixing element is indirectly coupled to the shaft by being coupled between one first mixing element and another first mixing element.

10. The mixer of claim 7, wherein the second mixing element is a spiral helix indirectly coupled to the shaft by being coupled, at one end, to the one first mixing element and coupled, at the other end, to the another first mixing element; and wherein the spiral helix, in extending from the one first mixing element to the another first mixing element, rotates around the shaft.

11. The mixer of claim 10, wherein the spiral helix is further coupled, at least one point along the length of the spiral helix, to at least one further first mixing element.

12. The mixer of claim 10, wherein each first mixing element comprises a plurality of radial blades; wherein the second mixing element comprises a plurality of spiral helixes coupled to the shaft; and wherein each spiral helix is coupled to a radial blade of one first mixing element and a radial blade of another first mixing element.

13. The mixer of claim 1, wherein the shaft is parallel with a bottom surface of the container or extends at an acute angle relative to the bottom surface of the container.

14. The mixer of claim 1, wherein the container comprises a second opening; and wherein the second opening is located in a bottom surface of the container.

15. A material management station of a 3D printing system comprising: a mixer comprising: a container having a first opening to receive build material; a shaft within the container and rotatable relative to the container; a first mixing element coupled to the shaft and extending from the shaft by a first radial distance to engage build material proximal to a surface of the container; and a second mixing element coupled to the shaft and extending from the shaft by a second radial distance which is smaller than the first radial distance to engage build material proximal to the shaft; at least one build material supply tank coupled to the first opening via a conduit; and an outlet port coupled to the second opening via a valve.