GB2601736A - Powder deposition in additive layer manufacturing apparatus - Google Patents

Abstract

A powder depositing apparatus for additive layer manufacture comprising a build platform 10, powder depositor 14, spreader 15 and powder supply unit 19. The platform supports successively deposited layers of powder that are to be fused by thermal energy in layer-by-layer manufacture. The depositor travels over the top of the support in the y direction, progressively forming a line of deposited material 17. The spreader is movable along a path above the support in a direction transverse to the depositor (the x direction) and spreads the deposited powder into a layer. If desired, the depositor may be moved along the x direction before depositing in the y direction, hence forming a multitude of lines. The width and height of the bead of deposited powder is determined by the width of the outlet on the depositor and the height of the depositor above the build platform, both of which may be adjusted. In a predetermined position, the depositor may be refilled with powder by the supply unit.

Description

POWDER DEPOSITION IN ADDITIVE LAYER MANUFACTURING APPARATUS

The present invention relates to additive layer manufacturing apparatus and has particular reference to powder deposition in such apparatus.

Additive layer manufacturing is a well-established procedure for manufacture of three-dimensional articles. In this procedure, an article is produced by selectively melting fusible powder material by irradiation with an energy beam, such as an electron or laser beam. Irradiation and fusion are carried out in relation to successively laid layers of the powder material so that material in each layer is melted in accordance with a predefined pattern and fused not only to itself, but also to any previously fused material of an underlying layer, whereby the article shape is created on a layer-by-layer or additive basis. A key element of apparatus for performing this procedure is a powder material feed and distribution system, which typically comprises a supply arrangement for powder material and a spreader, particularly a bar or roller, for spreading successive layers of powder material on a powder bed in a build space as the bed descends. For product quality reasons it is important that the powder material forming each layer is evenly spread over the area of the build space and that the layer thickness is well-defined and corresponds with a predetermined value. In addition, it is desirable for each layer to be applied quickly so as to keep the manufacturing rate as high as possible.

Various feed and distribution systems for this purpose exist or have been proposed. In the case of a system described in European patent specification 2398611 B1 a spreader bar is driven into a heap of powder, such that a portion of the powder in the heap is displaced to one side of the bar and can then be moved by the bar across the build area. The effectiveness of this procedure is dependent on powder flow, but the flowability of a particular powder material can change over time and varies from one powder material to another. The effectiveness also depends on the heap being an even height in the length direction of the bar. These factors impose constraints on achieving consistent results.

In another system, described in United States patent specification 252264 A, two pistons on either side of a volume of powder material are raised so as to present a quantity of the material to a spreader bar. The quantity of powder to be spread can be varied by moving the pistons a variable distance. A particular issue with this system is the space taken up by the pistons in the immediate vicinity of the build area. In a further system disclosed in United States patent specification 9126167 B2 a rotary hopper dispenses powder in an amount determined by the geometry of plates within the hopper. This obliges use of a very large size of hopper in order to accommodate sufficient powder to carry out a manufacturing cycle, which again consumes space in the associated additive layer machine.

A more effective approach is taken in powder supply and distribution equipment disclosed in United States patent specification 10059058 B2, in which a filling container is movable in the longitudinal direction of a distribution container to fill the latter with powder. The filled distribution container can then move parallel to a build surface to deposit a layer of powder. A disadvantage of this concept is that the distribution container simultaneously dispenses the powder and spreads it to form a layer, with the result that the quality of the powder layer can be affected by powder flow properties. Moreover, spreading in this case obliges use of a rigid spreader bar without the flexible elements usually employed to avoid equipment damage due to unintended swelling of part of a manufactured article as a consequence of a local excess of powder material.

It is therefore the principal object of the present invention to address the limitations of existing powder material deposition procedures by providing more precise deposition to assist with subsequent even spreading of the deposited material.

A subsidiary object is to provide a system configuration which may allow scope for convenient control and variation of the volume of deposited powder before and optionally during spreading.

Other objects and advantages of the invention will be apparent from the following description.

According to a first aspect of the present invention there is provided additive layer manufacturing apparatus comprising a support for supporting successively deposited layers of powder material to be fused by thermal energy of thermal energy generating means of the apparatus for layer-by-layer manufacture of an article, a depositor for depositing the powder material for the layers and a spreader movable along a path above the support for spreading deposited powder material to form each of the layers, the depositor being movable across the support transversely to the spreader path to progressively form a line of deposited powder material for subsequent spreading by the spreader.

Additive layer manufacturing apparatus embodying the invention, in particular apparatus with a travelling depositor capable of depositing powder material progressively along a line transverse to the direction of spreader movement, has the advantage that powder material can be laid down on the support and subsequently on a powder material bed already on the support in a relatively precisely definable volume so as to create preconditions for even spreading of the material by the spreader, in particular spreading to create a layer of consistent thickness. The deposited line of powder material may generally adopt the form of a ridge which extends across the entire width of the support, that is to say a dimension transverse to the direction of spreader movement, and which has an approximately frustoconical cross-section of substantially consistent height and area. The ridge can be engaged by the spreader, for example a bar, and spread across an article build area of the support to a desired depth which, in view of the substantially consistent starting dimensions of the ridge, will similarly have a substantially constant value. Achieving the desired consistent ridge dimensions is promoted by formation of the line of deposited material in a progressive manner, which provides scope for control of the rate of material flow and allows differing degrees of flowability of powder to be taken into account. The end result is a deposition system more finely tuneable or adaptable to the powder material concerned and the requirements for even spreading.

For preference the depositor comprises a powder material storage receptacle with an outlet for discharge of powder material, in a static state of the depositor, onto a confined area of the support with respect to the dimension thereof transversely to the spreader path, thus an outlet which has only a small dimension in a direction transverse to the spreader path so that in the absence of movement of the depositor the powder material can drop onto only a small part of the intended line of material. A line, as such, is formable only by the movement of the depositor. The outlet can be, for example, substantially circular so that material deposited in a stationary state of the depositor is generally cone-shaped and can be progressively drawn out into a ridge of frusto-conical section by travel of the depositor transversely over the support. In that connection, the outlet of the depositor can be of such a size and height above the deposit area, thus a zone of the top of the support or a powder material bed thereon, as to form a powder material line of predetermined width and height for a given rate of movement of the depositor. The cross-sectional area and hence the volume of the formed line can thus be simply and conveniently defined by appropriately matching the parameters of size of the outlet, height of the outlet above the deposit area and rate of movement of the depositor across the support to provide a desired volume of powder material per unit of travel of the depositor.

Further, the apparatus may advantageously comprise means for adjusting the size of the outlet in order to vary the width and height of the powder material line. Such an adjustment will result in discharge from the depositor of a greater or lesser amount of powder material for a given rate of travel of the depositor. As an alternative to physical adjustment of the outlet size, the depositor as a whole or a relevant part could be replaced by another with a differently sized outlet. Additionally or alternatively the apparatus may comprise means for adjusting the height of the outlet relative to the support, whether by displacing the depositor in height or displacing the support in height, in order to vary the width and height of the powder material line and/or means for adjusting the rate of movement of the depositor again in order to vary the width and height of the powder material line. Adjustment of the height of the outlet relative to the support could also be carried out in such a way that adjustment is made at specific depositor positions so that a line of variable height is formed if this should be needed, for example to make up for a local loss of powder material. It thus possible by one or more of these measures to adapt the volume of the deposited line of powder material to the requirements of a manufacturing cycle or a number of such cycles, which may include changes required in connection with individual layers during manufacture of one and the same article.

In a preferred construction the depositor can be mounted on linear guide means for movement across the support transversely to the spreader path. Linear guide means for that purpose may be of any suitable form, for example a low-friction sliding guide system, capable of guiding the depositor on a rectilinear path and resisting any tendency of the depositor to tilt or tip, in particular rotate about an axis parallel with that path. Guide means appropriate to that task could comprise, for example, two parallel bars extending through guide passages in a body of the depositor.

For preference the apparatus comprises drive means for reciprocating movement of the depositor across the support transversely to the spreader path, such drive means providing departure from and return to a starting point for powder material discharge or movement between starting points at opposite ends of a range of depositor travel. In a preferred embodiment the apparatus also includes a supply unit for supplying the powder material to the depositor, especially in a volume greater than that consumed in a single traverse of the support by the depositor. The volume of supplied powder, assuming a corresponding storage capacity of the depositor, can if desired be sufficient for multiple traverses. The powder material supply unit is preferably arranged to supply the depositor at a predetermined position of the depositor, so that the depositor can travel to a specific location for replenishing with powder material to be deposited as a line. In that case, the predetermined position of the depositor is a position thereof in the region of a limit of its travel across the support transversely to the spreader path. The supply unit can then be located in a specific region suitable for providing a flow of powder material to the supply unit or for access to refill or replace a powder material reserve.

In a development of the apparatus the depositor is additionally movable parallel to the spreader path so as to be able to form a line of deposited powder material at a plurality of discrete locations. The depositor thus has the capability of moving in two mutually perpendicular directions such that a line of powder material for spreading can be deposited at, for example, a starting point of spreader movement along its path and a further line deposited and at one or more further points along that path. This allows the depositor to, for example, deposit an initial quantity of powder material sufficient for spreading by the spreader over a given proportion of the area of the support and then, by travel of the depositor parallel to the spreader path to a position spaced from the initial deposit and by movement of the depositor at the spaced position transversely to the spreader path, to deposit a further quantity of powder material for spreading over a further proportion of the area. A layer of powder material can in that case be formed from a plurality of deposits. Since the spreader then has to spread smaller amounts of powder material at a time, this may assist spreader movement and attainment of an even distribution of the material, particularly in cases where a larger volume of powder material has to be spread over the support area.

According to a second aspect of the invention there is provided a method of depositing powder material in apparatus according to the first aspect of the invention, comprising the step of moving the depositor across the support transversely to the path of the spreader to progressively form a line of deposited powder material for subsequent spreading by the spreader.

According to a third aspect of the invention there is provided a method of forming a layer of powder material in apparatus according to the first aspect of the invention, comprising the step of depositing powder material in the apparatus by the method according to the second aspect of the invention and then spreading the deposited material of the line by movement of the spreader along the spreader path to form a layer of the material on the support or on a previously formed layer on the support.

A preferred embodiment of the present invention will now be more particularly described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a schematic perspective view of components of apparatus embodying the invention, showing a stage of deposit of powder material to form part of a line; Fig. 2 is a view similar to Fig. 1, showing a stage of deposit with formation of a full line; Fig. 3 is a view similar to Fig. 1, showing spreading of the line of powder material; and Fig. 4 is a diagram showing dimensional parameters of a cross-section of a powder material line formed by apparatus embodying the invention.

Referring now to the drawings there is shown, in diagrammatic form, components of additive layer manufacturing apparatus for manufacturing three-dimensional articles of predefinable shapes by selective melting and fusion of powder material, particularly a metallic powder, in successively deposited layers in a build zone. The apparatus provides melting by the action of a high-energy beam, such as a laser beam or electron beam, directed downwardly along a vertical neutral axis from the top of the apparatus. The beam can be deflected relative to the axis in X and Y directions to provide movement of the point of incidence of the beam on a powder material layer so as to scan and melt an area of predetermined shape corresponding with an individual cross-sectional layer of an article undergoing manufacture. Apparatus constructed and operating on this principle are well-known. Accordingly, more detailed illustration and explanation of parts not essential to an understanding of the present invention are unnecessary.

The apparatus comprises a raisable and lowerable powder material support 10, thus a support movable in Z direction, for a bed formed from the successively deposited layers of the selected powder material. The support 10 has the form of a vertical stage consisting of a rectangular-plan table 11 which is mounted on a post or posts (not shown) and guided for vertical movement in a shaft bounded by an enclosure 12 roller-mounted -in the case of electron beam melting -in a vacuum chamber within a casing (not shown). The walls of the enclosure generally confine the material bed, when present, to the area projection of the table. Above the walls, the table 11 is surrounded by a surround 13 with a planar surface with which the top of the table or top of a material bed on the table is alignable, in the latter case by lowering of the table in steps, to generally lie in a common plane. The area of the top of the table 11 represents a build zone for production of an article from a molten and solidified body within the powder material bed.

The present invention is particularly directed to the supply of powder material to the build zone, thus the top of the table 11 and thereafter the top of the powder material bed progressively formed on the table, and to distribution of the supplied material to form successive layers in the build zone. For this purpose, the apparatus comprises a depositor 14 for depositing a selected powder material for the layers and a spreader 15 for evenly spreading the deposited material to form each of the layers, the spreader being capable of reciprocating movement along a spreader path 16 in the direction of an X axis above the support 10 to provide a spreading effect in a forward direction of travel. The spreader 15 need not, in fact, be constructed any differently from a conventional spreader, thus typically a bar 15a which is driven to execute a reciprocating motion along the path 16 and to entrain, in the forward direction of travel, a heap of deposited powder material and distribute the entrained material in a layer over the build zone. The depth of such a formed layer is in that case determined by a spacing, which can be settable, of the bar 15a above the top of the table 11 or top of a powder material bed on the table to define a gap. The forward travel of the bar 15a allows escape of entrained material through the gap to progressively form the layer behind the bar. Between each cycle of forward and then return travel of the bar 15a the table 11 is lowered to the extent of a predetermined powder material layer thickness so that the build zone remains approximately in the plane of the surround 13. However, the spreader 15 can be constructed to be capable of operation to perform more elaborate spreading routines, including entrainment of residual and/or fresh powder material in the zone of its reversal of travel direction so that material can be spread during both forward and return travel.

The focus of the present invention is on the construction and operation of the depositor 14. This comprises a powder storage receptacle 14a for a reserve of the powder material of suitable volume. The receptacle 14a is provided with an outlet funnel 14b having an outlet 14c (Fig. 4) positioned or positionable closely adjacent to the plane of the surround 13, in particular at a small spacing from the top surface of the table 11 of the support 10 or from the top surface of a powder material bed, thus one or more layers of powder material, on the table. Discharge of powder material from the reserve through the outlet 14c can be controlled by valve means (not shown) in the depositor 14. The outlet 14c can be round, oval, square, oblong or any other shape appropriate to the intended pattern of discharge. The outlet is, however, of such a size that discharge when permitted is directed solely to a confined or local area on the surface of the table 11 or bed so as to form a small heap, unlike conventional depositors in which discharge is generally over the entire width of the table or bed at once.

In order to create a body of powder material suitable for spreading to fill the area of the build zone, thus all or a designated part of the top surface of the table 11 or of a powder material bed -whether a single layer or several layers -on the table, the depositor 14 is movable across the support transversely to the spreader path 16, i.e. in the direction of a Y axis and parallel to the plane of the top of the table 11, to progressively form a ridge-like line 17 of deposited powder material for subsequent spreading by the spreader 15. In effect, a small heap of powder material that would be discharged by the depositor 14 in a static state is drawn out to form the line 17 as the depositor moves across the support 10. For that purpose the depositor 14 is controllable to move from a starting point at one side of the support 10 on a rectilinear track to a terminating point at the opposite side of the support, as is evident from a comparison of Figs. 1 and 2. During this travel, discharge of powder material from the outlet 14c takes place so that there is progressive formation of the line 17 to extend transversely across the entire width of the support. The extent of spread of the discharged powder in the direction of the X axis under gravitational force, i.e. the width of the formed line 17, and the height of that line are both a function of the dimension of the outlet 14c in that direction, the height of the outlet 14c above the receiving surface represented by the top of the table 11 or powder bed thereon, and the rate of movement of the depositor 14 across the support 10.

The depositor 14 is also controllable to execute a return travel from the terminating point, thus a travel reversal point, to the starting point, during which further powder material can be discharged to top up the line 17 if this should be required. In that case, the height and width of the initially deposited line are augmented so that the line has an increased total volume; this may be appropriate depending on various factors such as, for example, the size of powder layer to be formed from the line 17 by spreading and the viscosity, as it were, of an aggregation of the powder material concerned. Further, if the spreader bar 15a carries any residual powder material back to the zone of action of the depositor 14 the latter can be operated to form a line of powder material of desired total volume from both the returned residual powder and further material supplied from the reserve held in the depositor receptacle 14a.

Mounting of the depositor 14 and linear guidance of its travel can be achieved in various ways, one preferred solution as shown in the figures being linear guide means in the form of parallel bars 18 along which the depositor is movable under maintenance of a tilt-free orientation. Motion can be imparted to the depositor 14 by any suitable reversible drive means, including a mechanical spindle drive utilising one of the bars 18 as a rotary threaded spindle engaged in a thread in a body forming the powder material receptacle 14a. Other forms of linear actuator are equally usable. In that connection it is advantageous if the drive speed of the drive means is controllable so that the rate of travel of the depositor 14 can be varied if required.

The apparatus further comprises a supply unit 19 for supplying the powder material to the depositor 14. The supply unit 19 is arranged to supply the depositor at a predetermined position of the latter, in particular a position in the region of a limit of travel of the depositor across the support 10 transversely to the spreader path 16. For the purpose of supply or replenishing, the supply unit 19 is positioned in the vicinity of the previously mentioned terminating or reversal point of the depositor travel. The supply unit 19 can be of any suitable form, such as the illustrated supply duct 19a, valve-controlled feed device 19b and tubular feed chute 19c. The duct 19a can be connected with a stock of powder material which may be outside the apparatus, for example outside a vacuum chamber if the apparatus employs an electron beam for scanning and selectively melting layered powder material on the support 10. When the reserve of powder material in the receptacle 14a of the depositor 14 requires replenishing the depositor can be moved to a position for receiving material from the supply unit 19, in particular as shown in Fig. 2 with the feed chute 19c of the supply unit communicating with the interior of the receptacle 14a of the depositor.

The utility of the depositor may be enhanced by provision of a capability for varying the extent of spread of the discharged powder in the direction of the X axis under gravitational force, thus the width of the formed line 17, and the height of the line 17. These line dimensions are a function of, collectively, the dimension of the outlet 14c in the direction of that axis, the height of the outlet 14c above the top of the table 11 or uppermost powder material layer thereon and the rate of movement of the depositor 14 across the table. Accordingly, the apparatus may be provided with means for adjusting the size of the outlet, such as a slide, diaphragm or other settable constrictor, or a replaceable insert for or part of the receptacle -even the entire receptacle -with an alternative outlet size. Further, the apparatus may be provided with means for adjusting the height of the outlet 14c above the table 11 of the support, for example a simple screw adjustment to increase or decrease the length of the outlet funnel 14b of the receptacle 14a. A similar result can, in fact, be achieved by adjusting the height of the table 11, such as by an addition to or subtraction from the extent to which the table is lowered for each new formation of a powder material layer. Finally, a capability of speed control of the drive means has already been mentioned and such a facility permits adjustment of the rate of movement of the depositor 14 in order to vary the amount of powder material able to be deposited per unit of time or unit of travel.

Fig. 4 diagrammatically depicts the substantially frusto-conical cross-section of a line 17 of powder material deposited by the outlet 14c of the outlet funnel 14b of the depositor receptacle 14a, in which H1 represents the height, above a receiving surface, of a line 17' formed by deposition to saturation level, that is to say to a maximum line height and base width permitted by the location and configuration of the outlet, and H2 the height of a line 17" resulting from onward movement of the depositor 14 before the saturation level is reached. The further height dimension H3 denotes the height, above the receiving surface or top of the table 11, of a lower end of the spreader bar 15a and thus a gap height substantially corresponding with the depth of a layer to be formed from the line 17, such as line 17' or 17", of powder material deposited by the depositor 14. The dimensions W1 and W2 represent the base width dimension of the powder material line in the direction of the X axis of the table 11 of the support 10 associated with, respectively, the line height H1 and the line height H2. The dimension W1 or W2 is a function of the angle of repose of the deposited powder material resulting from spread of the powder material due to gravity.

Deposition of the powder material in a line progressively formed by the depositor 14 transversely travelling across the support 10 has the result that a precisely determined volume of powder can be evenly deposited across the support, which assists even spreading by the spreader 15 and thus contributes to better layer quality, in particular consistency of layer depth and planarity. Optimisation of these parameters at the outset saves time and cost in correction of layer defects of the kind that commonly arise in additive layer manufacture. Moreover, the depositor 14 can be designed to be adaptable to different flow capabilities of different powder materials, whether metal/metallic powders or plastics material powders.

In a development of the apparatus, the depositor 14 is additionally movable parallel to the spreader path 16, thus in the direction of the X axis, so as to be able to form a line of deposited powder material at least one further discrete location, thus at a number of locations including the initial deposit. In that case for example, the parallel bars 18 supporting the depositor 14 and guiding its displacement in the direction of the Y axis would themselves be associated with a guidance system for guided movement in the direction of the spreader path 16. The apparatus would then also include suitable additional drive means for reciprocating movement of the parallel bars inclusive of the supported depositor. The further capability of movement of the depositor 14 along the spreader path 16 may be employed to, for example, deposit at least one further line of powder material at a suitable spacing from the initially deposited line so that, in the case of larger quantities of powder material required for a specific build, one or more supplementary volumes can be provided in order to reduce the overall volume of the initially deposited line and consequently the drag or resistance to travel to which the spreader 15 might otherwise be subjected.

Although the guidance system and drive means for depositor movement in the direction of the X axis can be individual to the depositor 14, it would be possible to integrate the depositor and spreader so that the former is mounted on or in association with the latter and can thereby utilise the guidance system and drive of the spreader.

The operation of the apparatus will be apparent from the preceding description. In a first step of additive layer manufacture by the apparatus, in which the depositor receptacle 14a contains a powder material reserve of desired quantity, the depositor 14 is displaced across the support 10 transversely to the spreader path 16 and thus in the direction of the Y axis to progressively form the line 17 of powder material on the top of the table 11 of the support 10, thus in the build zone. Fig. 1 shows formation of the line 17 at the point of approximately half its intended length and Fig. 2 formation of the line 17 to its full length. In a second step, this line 17 is then spread by the spreader 15 travelling along its path 16 in the direction of the X axis to form a layer over the area of the build zone, i.e. on top of the table 11, during which the depositor 14 is parked in a position clear of the spreader to one side of the build zone or is possibly elevated to a position well above that zone. Fig. 3 shows the spreader 15 in the course of movement along its path 16, with the depositor 14 parked to one side. After or before return of the spreader 15, the receptacle 14a of the depositor 14 can be refilled, in particular the powder material reserve topped up, which, depending on the capacity of the receptacle 14a, may be undertaken in each cycle of spreader reciprocating movement or periodically after a number of cycles, thus after several layers have been formed. Subsequent steps of activation of the high-energy beam to fuse powder material of each layer and progressive lowering of the table as successive layers accumulate are well-known aspects of additive layer manufacturing procedure. The deposit of powder material in lines by the transversely travelling depositor 14 offers an enhanced level of accuracy in the volume of deposited material to be spread and affords scope for adjusting the volume of the powder material line 17 to adapt to different conditions or requirements.

Claims (14)

1. CLAIMSAdditive layer manufacturing apparatus comprising a support for supporting successively deposited layers of powder material to be fused by thermal energy of thermal energy generating means of the apparatus for layer-by-layer manufacture of an article, a depositor for depositing the powder material for the layers and a spreader movable along a path above the support for spreading deposited powder material to form each of the layers, the depositor being movable across the support transversely to the spreader path to progressively form a line of deposited powder material for subsequent spreading by the spreader.
2. 2. Apparatus according to claim 1, wherein the depositor comprises a powder material storage receptacle with an outlet for discharge of powder material, as considered in a static state of the depositor, onto a confined area of the support with respect to the dimension thereof transversely to the spreader path.
3. 3. Apparatus according to claim 2, wherein the outlet of the depositor is of such a size and height above the area as to form a powder material line of predetermined width and height for a given rate of movement of the depositor.
4. 4. Apparatus according to claim 3, comprising means for adjusting the size of the outlet in order to vary the width and height of the powder material line.
5. 5. Apparatus according to claim 3 or claim 4, comprising means for adjusting the height of the outlet relative to the support in order to vary the width and height of the powder material line
6. 6. Apparatus according to any one of claims 3 to 5, comprising means for adjusting the rate of movement of the depositor in order to vary the width and height of the powder material line.
7. 7. Apparatus according to any one of the preceding claims, wherein the depositor is mounted on linear guide means for movement across the support transversely to the spreader path.
8. 8. Apparatus according to any one of the preceding claims, comprising drive means for reciprocating movement of the depositor across the support transversely to the spreader path.
9. 9. Apparatus according to any one of the preceding claims, comprising a supply unit for supplying the powder material to the depositor.
10. 10. Apparatus according to claim 9, wherein the supply unit is arranged to supply the depositor at a predetermined position of the depositor.
11. 11. Apparatus according to claim 10, wherein the predetermined position of the depositor is a position thereof in the region of a limit of its travel across the support transversely to the spreader path.
12. 12. Apparatus according to any one of the preceding claims, wherein the depositor is additionally movable parallel to the spreader path so as to be able to form a line of deposited powder material at a plurality of discrete locations.
13. 13. A method of depositing powder material in apparatus according to any one of the preceding claims, comprising the step of moving the depositor across the support transversely to the path of the spreader to progressively form a line of deposited powder material for subsequent spreading by the spreader.
14. 14. A method of forming a layer of powder material in apparatus according to any one of claims 1 to 12, comprising the step of depositing powder material in the apparatus by the method according to claim 13 and then spreading the deposited material of the line by movement of the spreader along the spreader path to form a layer of the material on the support or on a previously formed layer on the support.