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1. WO2020221996 - ADDITIVE MANUFACTURE

Note: Text based on automatic Optical Character Recognition processes. Please use the PDF version for legal matters

[ EN ]

ADDITIVE MANUFACTURE

Field of Invention

The present invention relates to powder based additive manufacturing and powder handling systems for powder based additive manufacture.

Background

Additive manufacturing methods (which in some cases may be referred to as“3D printing”) typically form three-dimensional articles by building up material in a layer-by-layer manner. Additive manufacture has several benefits over traditional manufacturing techniques, for example: additive manufacture has very few limitations on component geometry; additive manufacturing may reduce material waste (as even complex geometries can be produced at or near to their final net-shape); and additive manufacture does not require dedicated tooling so can enable flexible manufacture of small batches or individually tailored products.

One type of additive manufacture is powder bed fusion, which is particularly applicable to high strength materials such as metal alloys (but may also be used for ceramic or polymer based materials). In powder bed fusion a thin layer of powder is provided on a base and is selectively exposed to an energy source to fuse sections of the layer. A further layer of powder is provided over the solidified layer, generally by lowering a platform supporting the powder, and the subsequent layer is selectively fused. This fuses the powder both within the new layer and to the fused regions of the previous layer. The process is repeated to build the full component on a layer-by-layer basis. Powder bed fusion includes, for example, Selective Laser Melting (in which the energy source is a Laser) and Electron Beam Melting (in which the energy source is an Electron Beam).

In order to gain the full benefits of the additive manufacture process, the powder used in additive manufacture must be extremely fine and of high quality (both chemically and physically). Characteristics of the powder such as the particle size, particle shape and particle shape distribution can, for example, directly impact powder flow and layer build up such that they may have a direct impact upon the final component quality and consistency. Metallic powder particles for use in powder bed additive manufacture may, for example, have a particle size in the range of 15 to 45 pm (for Selective Laser Melting).

For both process and safety reasons the powders used in additive manufacture must be handled with caution. For example, fine metal powders are a health risk to humans through skin contact or inhalation and are a fire or explosion risk. Further, exposure of metal powders to moisture and/or oxygen can cause powder degradation. For example, some materials such as titanium alloys are particularly reactive and prone to absorption of atmospheric impurities such as oxygen and nitrogen. It is therefore, best practice to keep powders for additive manufacture in an inert atmosphere for example by using sealed powder flasks and powder loading arrangements.

Commercially available additive manufacturing systems, for example the Applicant’s Renishaw AM systems, are available with powder recirculation arrangements. Such systems allow powder to be sieved and recirculated through the additive manufacture system without the need to remove powder from the system. Such arrangements provide clear benefits in reducing handling of powder and can enable the powder to be held within an inert system loop once loaded into the system.

Whilst in some applications it may be practical to maintain an additive manufacture system for a single powder type, the apparatus is generally suitable for use with different powders and, therefore, a user may wish to use a single system for different materials (possibly even when producing a single article). This introduces a need to change over the powder in the apparatus. In addition to necessitating further handling of powder a changeover must be done with care to ensure that no traces of the previous powder are able to contaminate the new powder to be processed (which could impact the quality of the resulting component produced). As a result, it has been proposed, for example in US Published Patent Application US2019/0001413, to provide an additive manufacturing system which has a powder supply and powder recovery apparatus which are combined in a subassembly to provide an interchangeable module for connection to the additive manufacture apparatus. This allows the module to be used for a dedicated material and reduces the components which will require cleaning during a changeover (i.e. primarily to the components associated with the process chamber rather than those of the powder supply).

Embodiments of the invention seek to provide alternative arrangements which improve the powder handling and/or powder changeover in additive manufacturing.

Summary of Invention

According to a first aspect of the invention, there is provided an additive manufacturing system comprising

an additive manufacture apparatus having an energy source and a process chamber in which the energy beam is used to selectively solidify powder on a layer-by-layer basis; and

at least one powder supply module removably attachable to the additive manufacture apparatus, the powder supply module having a powder supply and a powder dispenser;

wherein the process chamber of the additive manufacturing apparatus is provided with an opening extending from the exterior of the apparatus into the chamber and the powder supply module is provided with a complementary connection interface which closes the opening and wherein the powder dispenser delivers powder into the process chamber beyond the connection interface. For example, the powder dispenser may extend through the opening to deliver powder into the process chamber.

The powder supply module may for example be a powder supply module in accordance with the embodiments of the invention. Accordingly, a further aspect of the invention provides an additive manufacturing powder supply module configured for removable attachment to an additive manufacturing apparatus, the module comprising:

a powder supply; and

a powder dispenser to dispense powder from the powder supply into the process chamber of the additive manufacturing apparatus, the powder dispenser comprising:

a powder outlet; and

a connection interface for sealingly engaging a corresponding interface of the additive manufacturing apparatus;

wherein the powder outlet dispenses powder within the process chamber of the additive manufacturing apparatus.

The additive manufacturing system of embodiments of the invention may comprise a modular system having at least one additive manufacturing apparatus module and at least one, removable powder supply module. Typically, a plurality of interchangeable powder supply modules may be used with one or more additive manufacturing apparatus modules. The, or each, powder supply module may be dedicated to a pre-selected powder type.

Embodiments of the invention advantageously provide a dispenser which is arranged to deliver powder to where it is needed within the interior of the process chamber. The dispenser may be part of the powder supply module but is configured to deliver powder (for example a metered quantity of powder) directly into the process chamber during use. Thus, the powder dispenser of embodiments may deliver the powder to the working plane of the process chamber (from where it may be spread across the powder bed). This may for example, decrease the powder contaminated parts of the additive manufacturing system which would require changing or cleaning in a powder changeover. For example, no powder lines may remain in the additive manufacturing apparatus when the powder supply is removed. Further, embodiments may allow a simplified sealing arrangement between the additive manufacture apparatus and powder supply and therefore enable a more reliable seal. Embodiments of the invention may provide an interface on the powder module which is more readily sealed when removal from the additive manufacture system is required. Specifically, embodiments enable the powder supply module to be fully sealed from the external atmosphere before it is detached from the additive manufacturing machine. This may, for example, be directly contrasted with the arrangement shown in US2019/0001413 in which the connection interfaces are formed on connection pipes and, in the case of the powder supply pipe, around a screw conveyor apparatus; this arrangement leads to powder filled components remaining in the additive manufacture apparatus.

Several types of powder dispenser may be used in embodiments of the invention, provided the outlet of the of the powder dispenser (which is a component of the removable power supply module) in embodiments of the invention may be within the process chamber at least when powder is dispensed. In other words, a dispenser for use in embodiments of the invention may be arranged to transfer powder from a powder supply external to the process chamber and deliver powder from an outlet located within the process chamber (at least at the time of dispensing). For example, suitable powder dispenser arrangements for use in an embodiment could include any of: a rotating auger, a bottom feeder (such as an angled piston) or a trap-door type dispenser. The provision of a dispenser which feeds powder from above the working plane of the process chamber may be particularly advantageous. Such arrangements enable the powder to be at least in part gravity fed.

The powder dispenser may deliver a predetermined metered volume of powder. The powder dispenser may comprise a delivery member having at least one metering void extending from an inlet at a first surface to an outlet at a second opposing surface, the void(s) defining a metered volume for receiving powder. An actuator may be provided to reciprocate the metering member between a first position, in which the metering member is retracted and powder from the powder supply can pass through the inlet to be retained by the at least one metering void and a second position, in which the metering member is advanced and powder retained in the at least one metering void is dispensed through the outlet. In the second position, the outlet of the powder dispenser may be within the process chamber.

Thus, it may be appreciated that in effect embodiments of the invention provide an arrangement in which the metering void moves between a first position in which it is outside of the process chamber (and within the powder supply module) and a second position in which the metering void is within the interior of the process chamber. In other words, in the first position the inlet of the void is in communication with the powder supply. In the second position the outlet of the void is in communication with the process chamber.

The connection interface may surround the metering member. The connection interface may comprise complementary profiled surfaces on the metering member and the process chamber (for example on the exterior wall of the process chamber) which are configured to sealingly engage. The complementary surfaces may be planar surfaces. The connection interface may therefore, close an opening in the process chamber. Thus, the metering member may be positioned within the opening so as to move into the interior of the process chamber in use. The connection interface may include a seal, such as a resilient member. The seal may surround the metering member.

The powder dispenser may comprise a body, defining a slot for slidably receiving the metering member. The body may have a forward face which includes the connection interface. In this context it will be understood that forward would mean the face which when assembled is proximal to the process chamber. A seal may be provided between the body and the metering member to seal the powder dispenser when the metering member is in a retracted position.

The metering member may extend from a rearward end proximal to the actuator to a forward end which includes the leading edge advanced in use (and proximal to the process chamber). The metering member may be provided with a faceplate at its forward end. The faceplate may include a flange extending around the exterior of the metering member forward end. The faceplate may provide a sealing surface for sealing engaging the body surrounding the forward end of the slot. For example, the rear surface of the faceplate flange may provide the sealing surface. The rear surface of the flange may carry a seal, for example a resilient seal. Alternatively, the seal may be carried on the face of the body opposing the flange. When the metering member is in a retracted position a seal may be compressed between the faceplate and the body of the powder dispenser. Thus, the seal may form an airtight seal between the body and the metering member of the powder dispenser. By

sealing the body and metering member the slot through which the metering member reciprocates is sealed and the powder supply module may be isolated.

During normal, powder dispensing, movement of the powder dispenser it may not be necessary to form a fully airtight seal; in fact, it may reduce the life of the seal elements to do so. Accordingly, in embodiments the actuator is configured to further move the metering member to a third position. The third position may be a “sealed” closed position. In the third position the metering member may be retracted beyond the second position. In the third position the seal between the body and metering member may be under compression.

The powder dispenser may be lockable in the closed position. The actuator may include an actuator mechanism having at least one linkage for positioning the metering member. The actuator may further comprise a motor. The actuator mechanism may comprise a crank which engages a slot to provide a linear action. The actuator mechanism may include an over centre linkage to lock the metering member into the third position. Advantageously, the actuator may retain the seal of the closed powder dispenser even when no power is provided to the actuator.

The powder supply module may also have a powder recirculation loop for returning unused powder to the powder source. The powder supply module may have a powder inlet for connecting to the additive manufacturing apparatus for receiving unused powder. The inlet may be provided with a closure or valve to seal the inlet when not connected to the additive manufacturing apparatus. The powder recirculation loop may include a separator, for example at least one of: a cyclonic separator, a sieve (for example an ultrasonic sieve) or a filter.

The powder supply module may further comprise a supply of inert gas for maintaining the powder in an inert atmosphere. The supply may for example comprise a pump and a canister of gas. The gas may be argon or nitrogen. The inert gas may be supplied to the powder supply via the powder recirculation loop, for example by being used as a motive gas for the powder recirculation loop. The powder supply module may also comprise a filter system for removing contaminants from the inert gas. Advantageously, when the powder supply is connected to the additive manufacturing system the powder is maintained under an inert atmosphere continuously throughout the supply, processing and recirculation. The inert atmosphere is also maintained over the powder when the powder supply is removed by sealing the inlet and the powder dispenser.

The opening in the additive manufacture apparatus process chamber may be a slot. The opening may be in a sidewall of the process chamber. The opening may be configured to receive the metering member. By receiving the metering member in the opening, in the assembled state, the metering member may extend through the slot in its second, advanced, position. The connection interface may include a sealing face which surrounds the metering member and abuts the external surface of the process chamber when the powder supply module is attached to the additive manufacturing apparatus. Thus, the powder supply apparatus may close the opening in the process chamber when the modules of the additive manufacture system are assembled.

The additive manufacture apparatus may further comprise a recoater. The recoater may travel across a powder bed in the process chamber to distributing a layer of powder thereon. The recoater may include a linear actuator. The powder dispenser may deliver powder to the recoater, for example to a recess within the recoater. The recess may be positioned between first and second recoater blades for distributing and levelling the powder. Advantageously, delivering the powder from the

dispenser into the recoater further simplifies the powder transfer and changeover. It may be appreciated that the recoater may be easily exchanged during a powder changeover. The recess or trough provided in the recoater may, in some embodiments, be sized to control the amount of powder to be dispensed across each layer.

The powder supply may comprise a powder hopper. The hopper may be part of a powder recirculation system or may be a stand-alone hopper. The powder hopper may be part of the powder supply module and is external to the additive manufacturing apparatus. The powder supply (or powder hopper) may be located above the working plane of the additive manufacturing apparatus when the powder supply module is connected to the additive manufacturing apparatus.

According to a further aspect of the invention there is provided a method of additive manufacture, comprising:

connecting a powder supply apparatus to an additive manufacturing apparatus;

sealingly engaging a delivery portion of the powder supply apparatus to an opening in the process chamber of the additive manufacturing apparatus;

extending at least a portion of a powder dispenser through the opening in the process chamber;

dispensing powder from the powder supply apparatus via the powder dispenser; and

releasing the powder into the process chamber

The method may be carried out using an apparatus in accordance with an embodiment of the invention

The method may further comprise providing an inert atmosphere in the powder supply apparatus and process chamber.

The method may further comprise dispensing and releasing a metered quantity of powder. The method may comprise releasing the powder into a recoater for delivering powder across a powder bed of the process chamber.

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

Description of the Drawings

Embodiments of the invention may be performed in various ways, and embodiments thereof will now be described by way of example only, reference being made to the accompanying drawings, in which:

Figure 1 shows an existing commercially available additive manufacturing system;

Figures 2a and 2b are schematic representations of an additive manufacture powder handling apparatus in accordance with an embodiment;

Figure 3 is an isolated three-dimensional view of the powder dispenser of an embodiment;

Figure 4 is a cross sectional view of the powder dispenser of Figure 3;

Figure 5 is a schematic representation of the actuator mechanism; and

Figures 6a and 6b are cross sections of the powder handing apparatus of figures 2 to 4.

Detail Description of Embodiments

As noted above, references made in the following description to forward or rearward are with relative to the movement of the dispenser; thus, forward is the direction toward the process chamber and rearward is the direction away from the process chamber. It may also be appreciated that references herein to vertical or horizontal are with reference to the axis of the additive manufacture process. In particular, as powder bed fusion is a layer by layer process the horizontal axis corresponds to the plane of the layers (which is in turn defined by the powder bed and support) and the vertical axis is perpendicular to the powder bed.

A commercially available additive manufacturing system 100, the Applicant’s RenAM 500 Series, is shown in Figure 1. The additive manufacturing system 100 includes both an additive manufacture apparatus 30 and an integrated powder handling apparatus 1. The additive manufacturing apparatus includes a process chamber 2 (which may alternatively be referred to as a build chamber), accessible via a chamber door 3, in which a laser is used to melt selective regions of a bed of powder on a layer-by-layer process. The additive manufacture process is generally controlled by a computer and may have a touchscreen interface 4 for operator interaction. The powder handling apparatus 1 is provided in an integrated cabinet 6 and accessible through a service door. The powder handling apparatus includes a hopper 12 for storing powder for use by the additive manufacturing apparatus 30. The hopper 12 may be filled with powder via a filling point 15 which is provided with an isolation valve 14. The powder handling apparatus includes 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 there is provided a powder metering screw 10 which feeds, via isolation valves 8 and 9, an ultrasonic sieve 7. The ultrasonic sieve is used to remove oversized particles from the powder so that they can be collected and removed from the machine via a metal flask, such as flask 18. Different size sieve meshes may be use for different materials. The powder handling apparatus maintains the powder loaded into the hopper and passing

through the recycling system under an inert atmosphere. The powder handling apparatus may also include filtering for the inert gas used in the process chamber and/or powder handling 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 Figure 1 includes both first and second filters 17 capture process emissions from the inert gas atmosphere.

Another configuration for an additive manufacturing system has been proposed in US Patent Application US2019/0001413. The system described in this patent application has a powder supply apparatus and a powder recovery apparatus which are combined to form a subassembly that is designed as an interchangeable module.

An additive manufacturing powder handling apparatus 1 in accordance with an embodiment of the invention is shown in figures 2a and 2b (which are alternate views of the same apparatus). The embodiment shown in figure 2 is adapted to be a self-contained powder handling module and it may be noted that it is mounted on a frame 101 with casters to enable ease of removal to and from the associated additive manufacturing apparatus. It will be appreciated that powder handling apparatus 1 in accordance with embodiments of the invention may be utilised in systems having either an integrated or an interchangeable powder handling apparatus and are not limited accordingly.

It may be noted that, for clarity purposes, some parts of the powder handling apparatus 1 are omitted in figure 2. Such features, for example ducting sections would be considered standard by those skilled in the art. Additionally, the skilled person would understand that the invention is not limited to any specific additive manufacturing apparatus, or particularly any specific process chamber thereof, for example the additive manufacturing apparatus may be substantially similar to the RenAM 500 Series described above (and shown in figure 1) with only routine modification required to operate with the powder handling apparatus of figure 2.

The powder handling apparatus 1, comprises a powder silo 110 which may receive powder from either a fresh powder inlet 112 and/or from the process chamber (not shown) of the additive manufacturing system via powder inlets 114. The silo 110 has a powder feed 114 at its lowermost end and tapers towards the powder feed to direct powder contained therein. The powder silo is located on the supporting frame 101 at a level below the position of the process chamber (which would be in the space immediately above the inlets 114) such that it may be gravity fed when receiving powder. The powder feed 114 is arranged to pass powder through a valve into a recirculation loop 120.

The recirculation loop 120 circulates inert gas around the powder system. The recirculation loop also takes output gas, including emissions from the process, from the process chamber to a filter system before returning inert gas to the process chamber. The skilled person may appreciate that there may be multiple flow routes for gas through the chamber to optimise emissions removal and maintain a clean and optically clear process chamber. For example, the RenAM 500 series includes both a high volume and flow horizontally across the powder bed and a cascading flow of gas from a showerhead type arrangement in the ceiling of the process chamber.

The inert gas flow in the recirculation loop 120 provides a motive flow for carrying powder. The powder is fed into the loop 120 by the powder feed 114 and is entrained in the inert gas such that it is carried from the lower most portion of the powder handling apparatus 1 to the upper most portion. Advantageously, once at the upper part of the powder handling apparatus 1, the powder can move under

gravity. At the top of the frame 101 is positioned a separator 130 comprising both a cyclonic separator 132 and an ultrasonic sieve 134. The skilled person will appreciate that both the cyclonic separator 132 and ultrasonic sieve 135 may be of any convenient design and of a type well known in the art. Further it will be appreciated that other separator arrangements are also possible and may be used with embodiments of the invention. The separator of the illustrated embodiment includes an inlet port 131 through which inert gas and powder is introduced and an outlet port 133 through which gas leaves the cyclonic separator 132. It will be appreciated that the ducting to/from the ports 131 and 133 has been omitted from Figure 2 for clarity but that in practice, for example, a simple duct would continue the recirculation loop 120 by extending from the coupling 121 to the inlet port 131. The powder is separated from the gas by the cyclonic separator 132 and falls under gravity through to the next stage of the separator 130. For example, the powder may pass through an ultrasonic sieve 134 to remove oversize particles from the powder.

The embodiment of Figure 2 includes two hoppers 140 and 150 to which powder exiting the separator 130 may be selectively directed and accumulated. A diverter (not shown) is provided to allow selection of which of the hoppers is to collect the powder. The first hopper 140 is a“total loss” hopper which is arranged to collect powder which is not being recycled by the powder handling apparatus. The total loss hopper is, therefore, used to accumulate unused powder so that it may be removed from the system via an outlet valve 142 provided at the bottom of the total loss hopper. As such, it will be understood that the total loss hopper 140 is not normally part of the recirculation loop. Whilst not being immediately recycled it is still desirable that the powder held in the total loss hopper 140 is under an inert gas. This ensures that the powder can be removed subsequently used, for example after testing or processing or for later use by the additive manufacturing system, for example for a component having less restrictive material requirements. In this regard it may be noted that the outlet 142 is positioned relatively close to, and above, an inlet 112 in an upper sidewall of the silo 110. This enables powder from the total loss hopper to be reintroduced into the powder recirculation loop if required (by simply attaching a suitable hose line) without being removed from the powder handling apparatus or leaving the inert atmosphere therein.

The second hopper 150 is a powder dispensing hopper and is part of the powder recirculation loop. The powder dispensing hopper has an inlet 152 at its upper end which receives powder from the sieve 134 of the separator 130. The lower end of the powder dispensing hopper 150 tapers towards an outlet 154 for providing powder to the additive manufacturing apparatus. The outlet 154 is connected to a powder dispenser 200 (as discussed below) which provides an interface for connecting to the process chamber of the additive manufacturing apparatus.

The powder dispenser 200 will now be described in further detail with reference to figures 3 and 4. The powder dispenser 200 is formed of three primary components: the body 220; the metering member 210; and the actuator 230. It may be appreciated that the general operation of the powder dispenser 200 is generally similar to the powder dispenser arrangement shown in published Patent Application

W02010/007396.

The metering member 210 is slidably mounted within a slot 221 which extends through the body 220. The body 220 includes a powder inlet 222 in the upper surface of the body, which is provided with a seal 224 around its periphery for sealing the connection to the outlet of the powder dispensing hopper 150. The metering member 210 includes a linear array of metering voids 212 which extend fully through the depth of the metering member 210. Collectively the array of voids 212 provide a predetermined volume which defines the volume of powder that the metering member 210 will dispense with each dispensing action. The actuator 230

is arranged to move the metering member 210 between a rearward position in which the void array 212 is aligned with the inlet 222 of the body 220 and a forward position in which the void array 212 are forward of the body 220. Thus, in the rearward position the voids of the array 212 can receive powder through their upper openings and in the forward position powder is released from the lower openings of the voids 212. It may be noted that the array of voids 212 consist of oblique elongated voids such that the volume of powder may be further controlled by varying the extent to which the voids overlap the corresponding voids of the inlet 222 of the body 220 on each stroke of the actuator. It may be noted from the cross section of figure 4, that the body 220 further includes a spacing member 228 around the inlet 222 which act as a powder leak seal.

As shown in figure 6, the forward end of the body 220 provides an interface surface 226. The interface surface 226 entirely surrounds the forward end of the slot 221 and is a planar surface extending perpendicular to the slot (specifically the slot extends in the horizontal axis and the interface surface is vertical). Figure 6A shows the cross section of the powder dispenser when not attached to the additive manufacturing system 100 and figure 6B shows the powder handling apparatus 1 connected to the additive manufacturing system 100. The interface surface 226 is configured and dimensioned to close an aperture 2b provided in the side wall 2a of the process chamber 2 of the additive manufacturing apparatus 30. In use, the powder handling apparatus 1 is aligned and fastened into position relative to the additive manufacturing apparatus 30 and in doing so the interface surface 226 closes the aperture 2b and provides a sealing surface for engaging the external surface 2c of the process chamber wall 2a around the periphery of the aperture 2b. It will be appreciated that one of either the external surface of the process chamber or the interface surface will generally be provided with a resilient seal to ensure an airtight configuration.

The forward end of the metering member 210 is provided with a face plate 214 which is perpendicular to the metering member 210. The face plate 214 is larger than the metering member 210 such that it overlaps and provides a flange 215 around the full periphery of the forward end of the metering member 210. A resilient seal 216 is mounted on the interface surface 224 for sealing against the rear face of the flange 215. The flange 215 and interface surface 224 provide opposing sealing surfaces that enable the powder dispenser 200 to be sealed when the metering member 210 is closed. Thus, by enabling the powder dispenser to seal when closed embodiments of the invention enable an inert atmosphere to be maintained in the powder delivery apparatus 1 when the additive manufacture process chamber 3 is opened or when the powder delivery apparatus 1 is removed from the additive manufacture apparatus 30. It will be appreciated that to fully seal the powder system, it will also be necessary to seal the inlet to the powder delivery system, but this requires only a simple closure such as a spring biased (normally closed) valve arrangement. For example, a valve 122 may be provided in the recirculation loop 120 to seal the inlet side of the recirculation loop. The valve 122 may close automatically whenever the door 3 to the build chamber 2 is opened or the powder supply module 1 is detached from the system. As such the powder system remains a sealed inert system. As the valve 122 is in the gas-borne stream of the powder, it is generally preferable that no part of the valve is exposed to the flow path when open (since the flow would be abrasive). As such the valve may, for example, be a sliding gate valve or a ball valve.

The actuator 230 comprises a motor 231 and an actuation mechanism 232 for converting the motors actuation movement to a reciprocal action of the metering member 210. The actuation mechanism includes a crank 233 which carries a pin 234. The pin 234 is received in a generally linear slot 235 to provide linear motion of a linkage arm 238 connected through the rear end of the metering member 210. It may be noted that the crank 233 is includes a bend in the crank arm the purpose of which will be explained below.

Operation of the actuator 230 is shown schematically in Figure 5a to 5c, in which a series of positions of the actuation mechanism are represented, In the position of Figure 5a, with the pin 234 of the crank 233 at a first (lower) end of the slot 235 the linkage arm 238 positions the metering member 210 in a forward position. By rotating the crank arm 233 anti -clockwise direction, to the position of figure 5b, the pin 234 travels along the slot 235 and causes the linkage arm 238 to withdraw the metering member in the rearward direction. As shown in figure 5c, when the pin 234 reaches the opposite (upper) end of the slot 235 the crank arm 233 is in an “over-centre” position. In this position the metering member 210 would be fully retracted to its closed position with the seal 216 (not shown in figure 5) closing the powder delivery system. It can be noted that any movement of the metering member 210 away from the closed position of figure 5c would urge the pin 234 of the crank 233 further upward in the slot (as it would be trying to urge the linkage arm forward). Accordingly, the engagement of the end of the slot 235 and the pin 234 will prevent unintended movement and will maintain the seal even if the actuator 230 is not receiving power to positively hold the crank 233.

Although the invention has been described above with reference to preferred embodiments, it will be appreciated that various changes or modification may be made without departing from the scope of the invention as defined in the appended claims. For example, other actuation arrangements will be known to the skilled person.