[0001 ] Powder-based three-dimensional (3D) printing systems typically form successive layers of a powder, or powder-like, build material on a build platform and selectively solidify portions of each formed layer to generate a 3D object on a iayer-by-layer basis. Various selective solidification techniques exist that include, for example, selective laser sintering (SLS), binder deposition, and fusing agent and thermal fusing systems. Build materials may include suitable plastic, metal, and ceramic build materials.

BRIEF DESCRIPTION

[0002] Examples will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

[0003] Figures 1A and 1 B are schematic diagrams of an apparatus according to one example;

[0004] Figure 2 is a schematic section view of an apparatus according to one example;

[0005] Figure 3 is a schematic section view of an apparatus according to one example;

[0006] Figure 4 is a schematic side view of a spreading roller according to one example;

[0007] Figure 5 is a block diagram of a 3D printer controller according to one example; [0008] Figure 8 is a flow diagram outlining a method of operating a 3D printing system according to one example; and

[0009] Figures 7A and 7B are schematic illustrations of an apparatus according to further examples.

[00010] One technique for forming a layer of a powder or powder-like build material is to use a roller mounted on a moveable carriage to spread a pile of build material over a build platform. The roller may be powered to rotate as the roller is moved over the build platform. In such systems, a pile or volume of build material may be initially deposited on a platform to one side of the build platform using, for example, a hopper or other build material deposition device, before being spread by the roller.

[00011] Referring now to Figure 1 there is shown a schematic diagram of an improved build material layer forming system according to one example.

[00012] Figure 1 A shows a simplified plan view illustration of a portion of a 3D printing system according to one example. Figure 1 B shows a corresponding side view.

[00013] The 3D printing system 100 comprises a build platform 102 on which successive layers of build material, such as a powder or powder-like build material, may be formed. Suitable build materials may include, for example, suitable plastic, metal, ceramic, and the like build materials. The build platform 102 is movable in the vertical, or z-axis, such that after each formed layer of build material has been processed by the 3D printing system the build platform 102 may be lowered by a small amount to allow a subsequent layer of build material to be formed thereon. In one example, each layer of build material formed may be in the region of about 80 to 120 microns thick, although in other examples thinner or thicker layers of build material may be formed. [00014] The build platform 102 has a rectangular shape and fits within a housing 104 that collectively forms an open-topped cuboidai-shaped buiid chamber 106 in which 3D objects may be generated on a layer-by-iayer basis.

[00015] Two lateral auxiliary platforms 108 and 1 10 are provided, one on either side of the build platform 102. One of the auxiliary platforms 108 is to receive a pile of buiid material to be spread over the build platform 102, and the other auxiliary platform 1 10 is to receive any excess buiid material leftover after a layer of build material has been formed.

[00016] A roller 1 12 is provided to spread buiid material from the first auxiliary platform 108, over the build platform 102. Any excess build material may be spread onto the second auxiliary platform 1 10 by the roller 1 12. The roller 1 12 may be mounted, for example, on a carriage (not shown) that is moveable over the auxiliary platforms 108 and 1 10 and over the build platform 102. The roller 1 12 may be powered to rotate about its central axis in a direction based on the direction in which the roller carriage is being moved.

[00017] As shown in Figure 1 B, a pair of build material retaining walls 1 14 are provided that extend at least partially up the height of the roller 1 12. in one example the retaining wails extend along at least the length of the build platform 102 in an axis parallel to the axis of movement of the roller 1 12. The roller 1 12 is moveable between the retaining wails, as is described further below with additional reference to Figures 2 and 3. in one example the retainer wails are rigid, or substantially rigid, retaining walls, although in some examples the retaining walls may have a degree of flexibility.

[00018] Although not shown, the 3D printing system 100 may additionally comprise a selective solidification module to enable portions of each formed layer of build material to be selectively solidified in accordance with a 3D object model. The selective solidification module may use any suitable selective solidification technology including: selective laser sintering, fusing agent and fusing energy; and chemical binders. [00019] Figure 2 is a schematic section view of Figure 1 B taken through the plane A~A, in accordance with a first example. Figure 2 shows a build unit 200 for use in a 3D printer, according to one example. The build unit 200 is to be removably installed in a 3D printing build unit receiver housing 202 shown in dotted lines. The build unit may, for example, be installed into a 3D printer, for example in the build unit receiver housing 202, in order for a 3D print job to be performed therein. After completion, the build unit 200 may be removed from the 3D printer build unit receiver housing 202 and, for example, be installed in a post-processing station where non-solidified build material may be removed and any 3D printed objects may be recovered. In this example, the roller 1 12 is part of the 3D printer, and as such is also shown in dotted lines.

[00020] The build unit 200 comprises the housing 104 within which the build platform 102 is vertically moveable. For example, the build platform 102 is moveable downwards to create a void or space between the build platform 102 and the base of the roller 1 12 into which a layer of build material may be formed. An initial layer of build material is formed directly on the surface of the build platform 102, whereas subsequently formed layers of build material are formed on previously formed layers.

[00021] As is shown in Figure 2, the build unit housing 104 comprises a pair of retaining walls 208 which extend vertically above the top of the housing 104 and at least partially up the height of the roller 1 12. In the example shown the retaining walls 208 extend around 1/3 of the height of the roller, although in other examples the retaining walls 208 may extend to a greater or lesser extent, in general, however, the retaining wails should extend up the height of the roller by at least the height of the pile of build material that is to be spread by the roller.

[00022] The pair of retaining walls are located parallel to the axis of movement of the roller 1 12, such that a base portion of the roller moves within the confines of the retaining walls. In one example the retaining walls 208 are spaced apart to allow the roller 1 12 to move without contact to the retaining wails 206. The spacing of the retaining wails may allow for a small gap, for example less than 5mm, to exist between the ends of the roller 1 12 and their respective retainer wall 206.

[00023] Figure 3 is a schematic section view of Figure 1 B taken through the plane A~A, in accordance with a further example. Figure 3 shows a portion of a 3D printer 300 according to one example. The 3D printer 300 comprises a 3D printer build unit receiver housing 202 to removably receive a 3D printer build unit, such as the build unit 200 shown in Figure 2. The 3D printer 300 additionally comprises a roller 1 12. According, in Figure 3, the elements of the build unit 200, namely the housing 104 and the build platform 102, are shown in dotted lines. The 3D printer build unit receiver 202 comprises retaining wails 302 as described above within which the roller 1 12 is to move.

[00024] The retainer walls may serve a number of purposes. A first purpose is to prevent, or at least reduce the amount of, build material that spreads out beyond the lateral edges of the spreading roller as a pile of build material is spread over a build platform. In some examples this may allow lateral vacuum systems that may be used to recover such laterally spreading build material to be removed from 3D printing systems.

[00025] Furthermore, the retaining wails may enable layers of build material from being formed with a higher degree of surface flatness. For example, the lateral retaining walls may help prevent layers of build material being formed from having a slightly convex form, especially at those portions of the build material layer formed by the extremities of the spreading roller. This is because the retaining walls prevent build material from laterally spreading beyond the retaining walls.

[00026] Referring now to Figure 4 there is shown a side view of a spreading roller 400 according to one example, in this example the roller 400 is formed with a pair of lateral channels 402. The channels 402, positioned towards each extremity of the roller 400, are to allow the roller 400 to be positioned over the retaining walls such that at least a portion of the retaining walls are within the channels 402 as the roller 400 is moved over the build platform. Provision of the channels 402 may further reduce the amount of build material that spreads laterally beyond the retaining walls during a build material spreading operation.

[00027] Operation of a 3D printer, such as the 3D printer 100, may be controlled by a 3D printer controller 500, as illustrated in Figure 5. The controller 500 comprises a processor, such as a microprocessor, 502 coupled to a memory 504 by an appropriate bus (not shown). The memory stores build layer formation control instructions 508 that are processor understandable instructions. When the instructions 506 are executed by the processor 502 the processor 502 controls the operation of elements of the 3D printer 100 as described below with reference to the flow diagram of Figure 6.

[00028] At block 602, the controller 500 controls an element of the 3D printer 100 to form a pile of build material that is to be spread on the build platform 102. For example, the controller 500 may control a build material deposition hopper (not shown) to form a pile of build material on the auxiliary platform 108.

[00029] At block 604, the controller 500 controls the roller 1 12 to move over the build platform within the retaining walls to spread the pile of build material to form a layer of build material on the build platform 102.

[00030] A further example of a portion of a rotary 3D printing system 400 is illustrated in Figures 7A and 7B. Figures 7A and 7B show a plan view of a cylindrical build unit 400 in which is located a vertically movable circular build platform 404. In Figure 7A a roller or spreader 406 is provided to form a layer of build material on the build platform 404. The roller 406 is rotatabie (as illustrated by the dotted lines), in a direction shown by arrow 410, around an axis central to the build platform 404 to form a layer of build material as the roller 406 is rotated. In this example, a build material deposition mechanism (not shown) may form a pile of build material on the build platform 404 to be spread by the roller 406 to form a layer of build material. The build unit 402 has a circular retaining wall 408 which, as previously described, extends to a height at least partially up the side of the roller 406. in the example shown in Figure 7A the roller 408 extends across the radius of the build platform 404 such that external end of the roller 406 remains in dose proximity to the retaining wall 408 as the roller 406 is rotated in the direction 410.

[00031] In a further example, illustrated in Figure 7B, a roller 412 is provided that extends across the diameter of the build platform 404 such that both ends of the roller remain in close proximity to the retaining wall 408 as the roller 412 is rotated in the direction 410.

[00032] The system 400 may additionally include a selective solidification module and other modules (not shown).

[00033] In these examples the rollers 406 and 412 may be additional rotated around their longitudinal axis are they are rotated in the direction 410.

[00034] From the examples described herein it can be generally understood that at least one end of the spreader remains a constant distance from a build material retainer wail as the spreader is moved over a buiid platform to form a layer of build material thereon. This is the case both where the spreader is moved along an axis parallel to a longitudinal retaining wail and where the spreader is axialiy rotated within a circular retaining wail. In both cases it can be seen that at least a portion of the spreader is positioned below the top of the retaining wall as the spreader is moved over the build platform.

[0003S] The retaining wail may, in use, be in proximity to at least one edge of the build platform, for example and may be provided either as part of the build unit, or as part of a build unit receiving housing of a 3D printing system.

[00036] Although the examples described herein refer generally to a pair of retaining wails, in one example only a single retaining wail may be provided. Additionally, although the examples described herein refer to a roller, in other examples other appropriate spreader apparatus may be used, such as, for example, a wiper blade. [00037] Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.