阅读说明：本技术 增材制造粉末再循环系统 (Additive manufacturing powder recirculation system ) 是由 戴维·爱德华·毕比 D·J·惠顿 于 2020-04-29 设计创作，主要内容包括：一种增材制造粉末再循环设备(1),包括：粉末再循环回路(120),该粉末再循环回路具有：用于接收来自增材制造设备的粉末的入口(114)；用于向该增材制造设备供应粉末的出口(154)；以及在该入口(114)与该出口(154)之间延伸的粉末流路径。该粉末流路径中的分流阀(200,400)被配置为选择性地将粉末流置于与下游粉末再循环回路或在粉末再循环回路(1)外部的料斗(140)处于流体连通。(An additive manufacturing powder recycling apparatus (1) comprising: a powder recirculation loop (120) having: an inlet (114) for receiving powder from an additive manufacturing apparatus; an outlet (154) for supplying powder to the additive manufacturing apparatus; and a powder flow path extending between the inlet (114) and the outlet (154). A diverter valve (200, 400) in the powder flow path is configured to selectively place the powder flow in fluid communication with a downstream powder recirculation loop or a hopper (140) external to the powder recirculation loop (1).)

1. An additive manufacturing powder recycling apparatus, the powder recycling apparatus comprising:

a powder recirculation loop having:

an inlet for receiving powder from an additive manufacturing apparatus;

an outlet for supplying powder to the additive manufacturing apparatus; and

a powder flow path extending between the inlet and the outlet;

a diverter valve in the powder flow path configured to selectively place the powder flow in fluid communication with a downstream powder recirculation loop or a hopper external to the powder recirculation loop.

2. An additive manufacturing powder recirculation apparatus according to claim 1, wherein the diverter valve comprises a valve element movably disposed within a valve body and movable by an actuator between a first position when an outlet of the diverter valve is in fluid communication with the downstream powder recirculation circuit, and a second position when the outlet is in fluid communication with a hopper external to the powder recirculation circuit.

3. The additive manufacturing powder recirculation apparatus of claim 2, wherein the diverter valve body includes spaced apart inner and outer sidewalls defining an outlet gap therebetween.

4. The additive manufacturing powder recirculation apparatus of claim 3, wherein the outlet void is divided into a plurality of outlet ports, each outlet port having an inlet orifice in communication with an interior of the valve body and an outlet orifice in communication with an exterior of the valve body.

5. The additive manufacturing powder recirculation apparatus of any one of claims 2 to 4, wherein the diverter valve further comprises a rotary valve member disposed within the valve body and rotatable by an actuator to selectively place one of the outlet ports in fluid communication with an interior of the valve body.

6. Additive manufacturing powder recycling apparatus according to claim 5, wherein the inner wall of the housing and the valve element have complementary cylindrical cross-sections and the valve element is a truncated cylinder.

7. The additive manufacturing powder recirculation apparatus of any one of the preceding claims, further comprising a separator disposed within the powder flow path.

8. The additive manufacturing powder recirculation apparatus of claim 7, wherein the separator comprises an oversized particle separator, and wherein an inlet of the oversized particle separator is downstream of the diverter valve.

9. The additive manufacturing powder recirculation apparatus of claim 8, wherein an outlet of the diverter valve is in fluid communication with an inlet of the oversized particle separator.

10. Additive manufacturing powder recycling apparatus according to claim 7, 8 or 9, wherein the separator comprises a powder separator to separate powder from a gas recirculation loop, the gas separator being upstream of the diverter valve.

11. The additive manufacturing powder recirculation apparatus of claim 10, wherein an outlet of the powder separator is in fluid communication with an inlet of the diverter valve.

12. Additive manufacturing powder recycling apparatus according to claim 10 or 11, wherein the powder separator is a dynamic separator, such as a cyclone.

13. Additive manufacturing powder recirculation device according to any one of claims 10 to 12, wherein the diverter valve comprises a valve body coaxial with an outlet of the powder separator.

14. An additive manufacturing system comprising an additive manufacturing powder recycling apparatus as claimed in any preceding claim, and a powder bed additive manufacturing apparatus.

15. The additive manufacturing system of claim 14, wherein the powder recycling apparatus comprises a module adapted to be removably attached to the additive manufacturing apparatus.

Technical Field

The present invention relates to powder-based additive manufacturing and a powder processing apparatus for powder-based additive manufacturing.

Background

Additive manufacturing processes (which may be referred to as "3D printing" in some cases) typically form three-dimensional articles by accumulating material in a layer-by-layer manner. Additive manufacturing has several advantages over traditional manufacturing techniques, such as: additive manufacturing hardly limits the geometry of the part; additive manufacturing can reduce material waste (because even complex geometries can be made to their final net shape or close to their final net shape); and, additive manufacturing does not require special tools, thus enabling flexible manufacturing of small batches or individually customized products.

One type of additive manufacturing is powder bed melting, which is particularly useful for high strength materials such as metal alloys (but can also be used for ceramic or polymer based materials). In powder bed fusion, a thin layer of powder is provided on a base and selectively exposed to an energy source to fuse portions of the layer. Another layer of powder is provided on the solidified layer, typically by lowering a platform supporting the powder, and selectively fusing the subsequent layer. This causes the powder in the new layer to melt and fuse to the melted region of the previous layer. This process is repeated to build the complete part on a layer-by-layer basis. Powder bed melting includes, for example, selective laser melting (where the energy source is a laser) and electron beam melting (where the energy source is an electron beam).

In order to obtain the full benefits of an additive manufacturing process, the powders used in additive manufacturing must be very fine and of high quality (both chemically and physically). Powder characteristics such as particle size, particle shape, and particle shape distribution can directly affect powder flow and layer agglomeration and thus can directly affect the quality and consistency of the final part. The metal powder particles used in powder bed additive manufacturing may for example have a particle size (for selective laser melting) in the range of 15 to 45 μm.

Powders for additive manufacturing must be handled with care for both process and safety reasons. For example, fine metal powders pose a threat to human health by skin contact or inhalation, and present a fire or explosion risk. Further, exposure of the metal powder to moisture and/or oxygen may result in degradation of the powder. For example, some materials, such as titanium alloys, are particularly reactive and readily absorb atmospheric impurities, such as oxygen and nitrogen. Therefore, it is best to keep the powder for additive manufacturing in an inert atmosphere, for example by using a sealed powder flask and powder loading arrangement.

The amount of unfused powder in a typical layer-by-layer build may be relatively high, making most of the powder in the powder bed available for reuse. In order to maintain process quality and consistency, any reclaimed unfused powder typically requires some degree of processing before being reintroduced into the additive manufacturing process to ensure that the reclaimed powder is chemically and/or physically consistent with the original powder to provide consistent results. For example, the recovered powder may be passed through a screen or filter to remove oversize particles (which may have formed by the heating of an additive process — for example, particles that have sintered together or formed irregularly shaped agglomerates).

Despite these advantages, powder recycling devices have not been commonly employed. For example, in some applications of additive manufacturing, stringent performance or regulatory requirements may currently limit the use of recycling. Such limitations may require the use of the original powder or may require testing and certification of the unfused powder prior to reuse. Accordingly, it is desirable to provide additive manufacturing methods and apparatus that can increase the use of recycled powder.

Disclosure of Invention

According to a first aspect of the present invention, there is provided an additive manufacturing powder recycling apparatus comprising:

a powder recirculation loop having:

an inlet for receiving powder from an additive manufacturing apparatus;

an outlet for supplying powder to the additive manufacturing apparatus; and

a powder flow path extending between the inlet and the outlet;

a diverter valve in the powder flow path configured to selectively place the powder flow in fluid communication with a downstream powder recirculation loop or a hopper external to the powder recirculation loop.

By placing the outlet in fluid communication with a downstream powder recirculation loop or a hopper external to the loop, embodiments of the invention enable a flow of powder from the powder recirculation loop to be maintained within the powder recirculation loop for recirculation through an additive manufacturing process, or to be accumulated outside the loop for later removal. It will be appreciated that the "hopper" external to the powder recirculation loop is not limited to any particular arrangement and may be any suitable container or conduit for guiding or receiving powder, for example it may be a removable container. Typically, the downstream powder recirculation loop will comprise a powder supply hopper for the additive manufacturing process. Thus, the diverter valve may direct the powder to one of: a first hopper external to the powder recirculation loop; or a second hopper within the powder recirculation loop.

The diverter valve body may include spaced apart inner and outer sidewalls. Thus, the valve body may define an outlet gap between the wall portions. For example, where the valve body is cylindrical, the outlet void may comprise an annular space formed between concentric cylindrical walls. The outlet gap may be at least partially divided into a plurality of outlet ports, for example a series of segments (such as circumferential segments separated by partitions or longitudinal wall segments). Each outlet void may have an inlet port in communication with the interior of the valve body and an outlet port in communication with the exterior of the valve body. The inlet orifices may be formed in an inner wall of the housing and the outlet orifices may be formed in an outer surface of the housing, for example in a base of the valve body.

The diverter valve may further comprise a rotary valve member. The rotary valve member may be disposed within the valve body and rotatable by the actuator to selectively place one of the outlet ports in fluid communication with the interior of the valve body. The inner wall of the housing and the valve element have complementary cylindrical cross-sections. The valve element may comprise a substantially cylindrical member. The valve element may be a frusto-cylindrical body. The frustum may have upper and lower surfaces that are not parallel. The base surface may be perpendicular to the axis of the valve and the upper surface may be at an oblique angle to the axis. The valve element may be a cylindrical wedge (i.e. the bevelled surface may intersect the base plane of the cylinder). During operation of the valve, the circumferential position of the lowermost portion of the cylinder may be aligned with the opening of the valve and the upper portion may close the opening. Thus, the frusto-cylindrical body may provide an inclined surface to direct the flow of powder towards the open outlet.

The diverter valve may include a valve body. For example, the valve body may be a cylindrical housing. The diverter valve may include a valve element movably positioned within the valve body. The valve element may be moved by an actuator. The valve element is movable between a first position when the outlet of the diverter is in fluid communication with a downstream powder recirculation circuit and a second position when the outlet is in fluid communication with a hopper external to the powder recirculation circuit.

The apparatus may further comprise a separator in the powder flow path. The separator may comprise a plurality of separator stages. For example, the separator may include a first stage for separating the smaller particles and a second stage for separating the larger particles. The diverter valve may be between stages of the separator.

The separator may comprise an oversized particle separator. The oversized particle separator may be a screen or a filter, such as an ultrasonic screen. The oversized particle separator may be downstream of the diverter valve. The oversized particle separator may have an inlet. The inlet of the oversized particle separator may be in fluid communication with the outlet of the diverter valve (it being understood that this diverter valve outlet would be the outlet in fluid communication with the downstream powder recirculation loop).

The separator may comprise a powder separator to separate powder from the gas recirculation loop. The gas recirculation loop may convey gas from the powder separator back to a location where powder received by the inlet is entrained into the gas stream. The powder separator may also be a small particle separator. The powder separator may be upstream of the diverter valve. The powder separator may have an outlet. The powder separator outlet may be in fluid communication with the inlet of the diverter valve.

The powder separator may be a dynamic separator, such as various types of inertial separators known to those skilled in the art. In particular, the powder separator may be a cyclone separator.

It may be beneficial to provide a diverter valve between the powder separator and the oversized particle separator, as the oversized particle separator is only used to handle powder that will continue to be used in the additive manufacturing system. The powder discharged from the powder recirculation loop will typically be processed outside the system and then reused.

The diverter valve body may be coaxial with the outlet of the separator, e.g. the powder separator. In particular, the valve body may be coaxial with the operating axis of the cyclonic separator. The valve body may form a passage adjacent the outlet of the separator.

Another aspect of the invention may provide an additive manufacturing system comprising an additive manufacturing powder recycling apparatus according to an embodiment, and an additive manufacturing apparatus. The additive manufacturing apparatus may be a powder bed additive manufacturing apparatus.

The powder processing apparatus may comprise a module adapted to be removably attached to an additive manufacturing system. An advantage of such a modular system is that when powder replacement is required, the powder system can be removed from the machine and replaced with a new module (so that only the process chamber part of the additive manufacturing system needs cleaning).

Although the invention has been described above, it extends to any inventive combination of the features set out in the foregoing or following description or drawings.

Drawings

Embodiments of the invention may be carried out in various ways and will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 illustrates an existing commercially available additive manufacturing system;

fig. 2a and 2b are schematic views of an additive manufacturing powder processing apparatus;

FIGS. 3a and 3b are three-dimensional views of a diverter valve arrangement according to an embodiment of the present invention;

FIGS. 4a and 4b are cross-sectional views of the diverter valve of FIGS. 3a and 3 b;

FIG. 5 is a three-dimensional ghost view of a diverter valve according to a second embodiment of the present invention;

FIG. 6 is the diverter valve shown in FIG. 5 in a first position; and

fig. 7 is the diverter valve shown in fig. 5 in a second position.

Detailed Description

It will be appreciated that references herein to vertical or horizontal refer to an axis of the additive manufacturing process. In particular, since powder bed melting is a layer-by-layer process, the horizontal axis corresponds to the plane of the layers (which plane is in turn defined by the powder bed and the support) and the vertical axis is perpendicular to the powder bed.

A commercially available additive manufacturing system 100, the RenAM 500 series of the present applicant, is shown in fig. 1. Additive manufacturing system 100 includes both additive manufacturing apparatus 30 and integrated powder processing apparatus 1. The additive manufacturing apparatus comprises a process chamber 2 accessible via a chamber door 3 in which a laser is used to melt selective regions of a powder bed in a layer-by-layer process. The additive manufacturing process is typically computer controlled and may have a touch screen interface 4 for operator interaction. The powder processing apparatus 1 is arranged in an integrated cabinet 6 and is accessible through an access door. The powder handling apparatus comprises a hopper 12 for storing powder for use by the additive manufacturing apparatus 30. The hopper 12 can be filled with powder via a filling point 15 provided with an isolation valve 14. The powder handling apparatus comprises an inlet in the form of a return pipe 13 for returning unused powder from the process chamber 2 to the hopper 12. Below the hopper 12 is a powder metering screw 10 which feeds the ultrasonic screen 7 via isolation valves 8 and 9. Ultrasonic sieves are used to remove oversized particles from the powder so that they can be collected via a metal flask (e.g., flask 18) and removed from the machine. Different sized screens can be used for different materials. The powder handling equipment maintains the powder loaded into the hopper and passed through the recovery system under an inert atmosphere. The powder processing apparatus may also comprise filtering for inert gases used in the process chamber and/or the powder processing apparatus (although the skilled person will also appreciate that such filtering may alternatively be provided in the additive manufacturing apparatus 30). The example system of fig. 1 includes both first and second filters 17 that capture process emissions from an inert gas atmosphere.

Another configuration of an additive manufacturing system has been proposed in US patent application US 2019/0001413. The system described in this patent application has a powder supply device and a powder recovery device which are combined to form a subassembly which is designed as an interchangeable module.

An additive manufacturing powder processing apparatus 1 according to an embodiment of the invention is shown in fig. 2a and 2b (which are alternative views of the same apparatus). The embodiment shown in fig. 2 is suitable for use with a self-contained powder processing module and it may be noted that it is mounted on a frame 101 with castors to enable easy movement into and out of an associated additive manufacturing apparatus. It should be understood that the powder processing apparatus 100 according to embodiments of the present invention may be used in systems with integrated or interchangeable powder processing apparatuses and is not limited accordingly.

It may be noted that some parts of the powder handling device 100 are omitted from fig. 2 for clarity. These features, such as the pipe sections, will be considered standard by those skilled in the art. Furthermore, the skilled person will appreciate that the present invention is not limited to any particular additive manufacturing apparatus, or in particular any particular build chamber thereof, for example an additive manufacturing apparatus may be substantially similar to the RenAM 500 series described above (and as shown in figure 1), requiring only routine modifications to operate with the powder processing apparatus of figure 2.

The powder processing apparatus 100 includes a powder bin 110 that can receive powder from a fresh powder inlet 112 and/or from a process chamber (not shown) of an additive manufacturing system via a powder inlet 114. The silo 110 has a powder feeder 114 at its lowermost end and tapers towards the powder feeder to direct the powder contained therein. The powder silo is located on the support frame 101 at a level below the level of the process chamber (and then in the space directly above the inlet 114) so that it can be gravity fed when receiving powder. The powder feeder 114 is arranged to pass powder through a valve into the gas recirculation loop 120.

A gas recirculation loop 120 circulates inert gas around the powder system. The gas recirculation loop also carries the output gas (including the effluent from the process) from the process chamber to a filtration system before returning the inert gas to the process chamber. One skilled in the art will appreciate that there may be multiple flow paths for gases through the chamber to optimize effluent removal and maintain a clean and optically transparent process chamber. For example, the RenAM 500 series includes both high volumetric flow rates horizontally across the powder bed and a cascading gas flow from a showerhead arrangement at the top of the process chamber.

The flow of inert gas in the gas recirculation loop 120 provides a motive gas flow for entraining the powder. The powder is fed into the circuit 120 by the powder feeder 114 and entrained in the inert gas so that it is transported from the lowermost part to the uppermost part of the powder handling apparatus 1. Advantageously, the powder can move under the action of gravity once in the upper part of the powder handling device 1. At the top of the frame 101 is located a separator 130 comprising both a powder separator 132 in the form of a cyclone separator and an oversize particle separator 134 in the form of an ultrasonic sieve. Those skilled in the art will appreciate that both the cyclone separator 132 and the ultrasonic screen 135 may be of any convenient design and of the type well known in the art. It is further understood that other separator arrangements are possible and may be used with embodiments of the present invention. The powder separator of the illustrated embodiment includes an inlet 131 through which the inert gas and powder are introduced and an outlet 133 through which the gas exits the cyclone 132. It will be appreciated that the conduits to/from the ports 131 and 133 have been omitted from fig. 2 for clarity, but in practice, for example, a simple conduit would continue the recirculation circuit 120 by extending from the coupling 121 to the inlet port 131.

The gas separated from the powder in the cyclone 132 may be directed from the outlet port 133 to at least one filter to further remove emissions or contaminants before the gas is returned for use in the process chamber. A pump (not shown) is located after the cyclone 132 and filter but before the powder feeder 114 to force the gas around the gas recirculation loop 120. The powder is separated from the gas by the cyclone 132 and falls under gravity into the next stage of the separator 130. The cyclone 132 can be considered to define a minimum particle size because particles smaller than the size separated by the cyclone will be entrained by the gas stream and will not remain in the recirculation loop. For example, particles smaller than 10 microns may be removed with the airflow exiting the outlet port 133 of the cyclone 132. It will be appreciated that if the powder transport is by means other than pneumatic action, the cyclone 132 may be replaced by another form of separator for removing only the undersized particles (without also separating the gas and powder).

After exiting the cyclone 132, the oversized particles are then separated from the powder. For example, the powder may be passed through an ultrasonic screen 134 to remove oversized particles from the powder. The screen thus defines the maximum particle size remaining in the recirculation loop.

As will be explained further below, the embodiment of fig. 2 includes two hoppers 140 and 150 to which powder exiting the separator 130 can be selectively directed and accumulated. The first hopper 140 is a "full loss" hopper arranged to collect powder that is not recycled by the powder handling apparatus. Thus, the loss-in-total hopper is used to accumulate unused powder so that the unused powder can be removed from the system via an outlet valve 142 provided at the bottom of the loss-in-total hopper. Thus, it should be understood that the full loss hopper 140 is not typically part of the powder recirculation loop. Although not immediately recycled, it is desirable that the powder held in the full loss hopper 140 be under an inert gas. This ensures that the powder can be used subsequently, for example after testing or processing, or for subsequent use by an additive manufacturing system, for example for components with less restrictive material requirements. In this regard, it may be noted that the outlet 142 is positioned relatively close to and above the inlet 112 in the upper sidewall of the cartridge 110. This enables powder from the full loss hopper 140 to be reintroduced into the powder recirculation loop when required (by simply attaching a suitable hose) without the need to remove or leave the inert atmosphere therein from the powder handling apparatus.

The second hopper 150 is a powder dispensing hopper and is part of a recirculation loop. The powder distribution hopper has an inlet 152 at its upper end that receives powder from the sieve 134 of the separator 130. The lower end of the powder distribution hopper 150 tapers towards an outlet 154 for providing powder to the additive manufacturing apparatus. The outlet 154 may be an interface for connection to a build chamber of an additive manufacturing apparatus and may comprise or be connected to a powder dispensing arrangement. For example, an additive manufacturing system may have a drawer-type powder dispensing arrangement similar to the type shown, for example, in published patent application WO 2010/007396.

Referring to fig. 3 and 4, a diverter valve arrangement 200 is used to selectively direct powder in the powder recirculation loop to either the full loss hopper 140 or the powder distribution hopper 150. The valve 200 is formed of a valve body 210, a valve element 240 rotatable within the valve body 210, and an actuator 230 for rotating the valve element 240.

The upper end 210 of the diverter valve 200 forms an inlet 214 and is attached to the outlet of the cyclone 132. The lower end of the diverter valve has a first outlet 260 in communication with the ultrasonic screen 134 and a second outlet 270 that passes through the screen twice and is in communication with the surge bin 140. Since the full loss hopper (in normal operation) is outside the powder recirculation loop, the outlet 270 can be considered to direct the powder flowing from the cyclone 132 out of the powder recirculation loop. It should be appreciated that the ultrasonic screen 134 is located at the inlet of the powder dispensing hopper 150. Thus, powder directed through the outlet 260 remains in the powder recirculation loop.

The valve body 210 is formed of two concentric cylindrical walls. Both the inner wall 215 and the outer wall 219 are aligned with the axis of the valve, which is also aligned with the axis of the cyclone 132. The upper end 210 of the valve body is closed by a top plate 212 which includes a valve inlet port 214. Similarly, a bottom plate 222 closes the lower end 220 of the valve body. The base plate includes a pair of apertures 226 and 227 leading to outlets 260 and 270, respectively. A central bore 223 is also provided so that the main shaft 231 of the actuator 230 can enter the interior of the valve body 210.

An annular outlet gap 218 is formed between the inner wall 215 and the outer wall 219. The apertures 226 and 227 of the bottom plate 220 are aligned with the outlet gap 218 at circumferentially spaced (and opposing) portions of the valve body 210. The inner wall 215 is provided with a first opening 216 and a second opening 217 aligned with the apertures 226 and 227 at the lower end 220 of the valve body, which provide an inlet aperture to the outlet void 218. The outlet gap 218 may be divided into a plurality of separate ports by including a longitudinal baffle 218a between the inner wall 215 and the outer wall 219. Such an arrangement would provide a physical barrier to ensure that powder exiting one of the openings 216 or 217 in the inner wall can only exit the respective aligned aperture 226 or 227. Alternatively, it may be sufficient to rely on flow under gravity in the aligned openings to direct the powder through the aligned openings.

The valve element 240 is a cylindrical wedge that is positioned within the inner wall 215 of the valve body 210 and is rotatable therein. The base 242 of the valve element 240 is circular and engages the spindle 231 of the actuator 230. The upper surface 244 of the valve element 240 extends from a first side 245 adjacent the base 242 at an oblique angle to the axis of the cylinder to an opposite side 246 where the valve element has its full height. As can be seen in fig. 4, when the first side 244 is aligned with the opening 216 of the inner wall 215, the opening is in communication with the interior of the valve 220. The opposite side 246 of the valve element 240 blocks the other opening 217. Thus, a 180 degree rotation of the valve element will open and close the respective orifice and place the interior of the valve in fluid communication with the respective outlet via the outlet void 218. The upper surface 244 will also provide a ramped surface for directing the flow of powder toward the open outlet.

In use, the powder recirculation system may be used in a continuous recirculation mode or a "full loss" mode. In the continuous recirculation mode, the valve element 240 is positioned such that the underside 245 is aligned with the opening 216, and powder exiting the outlet of the cyclone separator 132 is directed via the ramp profile of the element 240 through the opening 216, out of the valve body 210 of the diverter valve via the outlet gap 218 and the orifice 226. The powder proceeds from the outlet 226 to the ultrasonic screen 133 to separate and/or remove any oversized powder particles. The screen 133 is located directly above a powder distribution hopper 150 that accumulates powder exiting the screen in preparation for reintroduction into the process chamber of the additive manufacturing apparatus.

To switch the device to "full loss" mode, it is only necessary to rotate the valve element (e.g. 180 degrees). For example, an all-loss mode may be used if the article manufactured in the additive manufacturing apparatus has particularly stringent material requirements or if material changes are imminent. Actuator 230 is activated to rotate valve element 240 until underside 245 is aligned with opening 217. When in this second position, the opposite side 246 of the valve element blocks the opening 216. Powder exiting the outlet of the cyclone 132 is directed via the ramp profile of the element 240 to exit the valve body 210 of the diverter valve through the opening 217 via the outlet gap 218 and the orifice 227. The aperture 227 is in fluid communication with the full loss hopper 140 and so in this configuration powder exiting the cyclone 132 is removed from the powder recirculation loop.

In another embodiment of the present invention, diverter valve 200 is replaced with diverter valve 400, as shown in fig. 5-7. Diverter valve 400 includes a valve body 419. The upper end of the valve body 419 forms the inlet 414 and is attached to the outlet of the cyclonic separator 132. The lower end of the valve body 419 has a first outlet 460 in communication with the ultrasonic screen 134 and a second outlet 470 that passes through the screen twice and is in communication with the full loss hopper 140. Since the full loss hopper (in normal operation) is outside the powder recirculation loop, the outlet 470 can be considered to direct the powder flowing from the cyclone 132 out of the powder recirculation loop. It should be appreciated that the ultrasonic screen 134 is located at the inlet of the powder dispensing hopper 150. Thus, powder directed through the outlet 460 remains in the powder recirculation loop.

Within the valve body 419 there is a closure or valve element in the form of a partition plate 415 which is mounted for rotation about axis a such that the partition plate 415 can be rotated from a first position shown in fig. 6 (closing the second outlet 470 to the powder flow but allowing powder to flow from the inlet 414 to the first outlet 460) to a second position shown in fig. 7 (closing the first outlet 460 to the powder flow but allowing powder to flow from the inlet 414 to the second outlet 470). The rotation of the partition plate 415 is driven by an actuator, in this embodiment in the form of a pneumatic or hydraulic piston 430. Thus, to switch the apparatus between the powder recirculation mode and the "full loss" mode, the valve element 415 is rotated. The diverter valve 400 according to this second embodiment may have advantages over the first embodiment of the diverter valve 200 in that it reduces the risk of powder being trapped in the valve mechanism (which may lead to valve sticking) and it is possible to manufacture a diverter valve 400 that is not as tall as the diverter valve 200, but still provides an equivalent function.

Although the invention has been described above with reference to preferred embodiments, it should be understood that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims. For example, it will be appreciated that although the system described above includes two hoppers and one diverter valve with two corresponding outlets, additional hoppers could be accommodated by adding additional valve outlets and valve element positions.

Further, while the above embodiments have been described primarily as using a diverting hopper as an "all-loss hopper", the skilled person will appreciate that it may have other uses. For example, a diverter valve may be used to temporarily divert powder from a powder recirculation loop for powder testing or sampling. Powder from the diverter valve may be fed to the sieve for continued use in the process most of the time during the build, but at some point during the build the diverter valve may switch the powder to a full loss hopper to sample the powder and then back. The user can then remove the powder sample from the full loss hopper. Such sampling may be reduced periodically or in response to user input.