Apparatus and method for layerwise formation of 3D objects from particulate build material

Abstract

Provided is an apparatus for the layerwise manufacture of 3D objects from particulate build material, wherein the apparatus comprises a distributor to distribute each layer of build material; a first fusing source and a second fusing source, each configured to provide fusing energy based on image slices defining each object cross section to be formed within a corresponding layer, wherein at least one of the first and second fusing sources is configured to provide selective fusing energy to selected object voxels of the layer within the object cross sections; and a controller configured to: control the distributor to distribute a layer; control in either order the first fusing source to fuse selected object voxels to form an object cross section and the second fusing source to further fuse the object voxels fused by the first fusing source; and repeat the distributing, fusing and further fusing to form the object from a sequence of object cross sections. A method of operation of the apparatus is also provided.

Description

[0001] APPARATUS AND METHOD FOR LAYERWISE FORMATION OF 3D OBJECTS FROM PARTICULATE BUILD MATERIAL

[0002] FIELD OF THE INVENTION

[0003] The present disclosure relates to the field of layerwise manufacture of 3D objects from particulate build material, and in particular to the control of fusion of object cross sections. Apparatus and methods of control therefor are disclosed, which may find particular benefits in applications requiring improved parts properties.

[0004] BACKGROUND

[0005] In powder bed fusion processes like print and fuse or laser sintering, an object is built up layer by layer from powder material by selectively fusing, or melting, the powder over successive object cross sections. This results in the object being built up within a supportive bed of unfused powder that can be removed at the end of the process by simply brushing and / or blowing the supportive powder off by pressurised gas streams. The object cross sections are typically defined as part of slice data that defines the voxels to be fused for each layer. Selective fusion may be achieved by for example a laser tracing the cross section, or by a printhead printing infrared absorber over the voxels to be fused followed by irradiation with an infrared light source, whereby the absorber causes selective heating. Recently, improvements to powder bed fusion processes have led to high yield through fast layer times and increasing build bed size. While laser sintering processes typically are more time consuming due to having to trace the object cross section along a route, print and fuse processes employ scanning printheads and lamps that can process each layer in a single pass and are thus faster and have high potential for industrial large volume manufacture. In conventional print and fuse processes, fusing energy is provided by non-selective heat sources. The non-selective heat source is typically an elongate infrared lamp that spans the width of the layer and that is operated to provide fusing energy to the object cross sections as it traverses the layer. Print and fuse processes rely on the deposition of absorber to create selectivity, which is typically provided by a printhead scanning over the layer before application of the fuse lamp heating of the printed cross section. Conventionally the absorber is carbon black, leading to dark grey to black parts in appearance that are challenging to colour, especially to to achieve vibrant colours. Meanwhile, absorbers of lighter or nonblack appearance may be used but these typically lead to drawbacks in mechanical properties. Thus, print and fuse processes retain limitations that may be addressed by the present invention disclosed herein.

[0006] SUMMARY

[0007] The invention is set out in the appended independent claims, while particular embodiments of the invention are set out in the appended dependent claims.

[0008] BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Reference is now directed to the drawings, in which:

[0010] Fig. 1 is a schematic cross section side view through a build bed of an apparatus for the layerwise manufacture of 3D objects according to the invention;

[0011] Fig. 2 is a flow chart of a method of using the apparatus according to the invention;

[0012] Fig. 3 is a flow chart of a further method of using the apparatus according to the invention;

[0013] Fig. 4 illustrates temperature deviations of a line of voxels along x;

[0014] Fig. 5 is a variant of Fig. 1 illustrating different fusing energy beams based on the temperature deviations of Fig. 4;

[0015] Fig. 6 is a variant of Fig. 1 illustrating adjusted fusing energy beams and non-fusing energy beams, each of different energies;

[0016] Figs. 7A to 7E illustrate different stages of fusing of a digital light projector (DLP) comprising a scrolling digital mirror device (DMD);

[0017] Fig. 8A illustrates a mask to be applied to a slice for a DMD;

[0018] Fig. 8B illustrates a slice comprising non-fusing heating data;

[0019] Fig. 8C illustrates a schematic top view of a layer heated according to a combination of the slice of Fig. 8B modified by the mask of Fig 8A;

[0020] Fig. 9 illustrates a series of mask weights used to modify the pixel data of a non-fusing slice; Figs. 10A to 10D illustrate a sequence of slices modified by a mask for a scrolling DMD; Fig. 11A illustrates the example intensity profile obtained by Figs. 10A to 10D;

[0021] Fig. 1 IB illustrates a variant of the profile of Fig. 1 IB; and

[0022] Fig. 12 illustrates intensity profiles obtained by conventional powder bed fusion apparatus.

[0023] In the drawings, like elements are indicated by like reference numerals throughout. It should be noted that the drawings are not to scale and that certain features may be exaggerated in size to be more clearly visible. DETAILED DESCRIPTION

[0024] In the following, a “fusing energy beam source” is operable to ‘fuse’ build material by being operated at an energy output and / or wavelength capable of raising the temperature of the build material to or above the melting temperature. In this way, fusion of the build material over object cross sections may be achieved. By contrast, a “non-fusing energy beam source” is operated at a comparatively lower energy output and / or at a different wavelength, capable of raising the temperature of the build material without it reaching or exceeding the melting temperature. Herein, the term ‘non-fusing’ is not intended to define any particular order in the steps of a thermal event sequence. A non-fusing step may be applied at any suitable instance of the process.

[0025] A “selective” heat source herein is a heat source able to address the layer per voxel or unit control area so as to selectively heat to fuse or warm specific areas of the layer. A “non- selective” heat source herein is a heat source that applies heat over groups of voxels at a time without being configured to selectively heat or fuse specific area of the layer in absence of other means to enhance the absorption of radiation of the non-selective heat source at specific voxels of the layer. Such means may be a printhead selectively applying radiation absorber to specific voxels that are to fuse. Examples of non-selective heat sources are scanning infrared bar heaters conventionally used in print and fuse apparatus, or the overhead heaters fixedly arranged above the layer to help maintain a uniform temperature over the layer.

[0026] To overcome present shortcomings of conventional powder bed fusion apparatus, embodiments of the invention and their variants will now be described in more detail with reference to Fig. 1 to Fig. 12.

[0027] Turning first to Fig.l, which illustrates a schematic cross section side view of a build bed formed from a sequence of layers 160, provided is an apparatus 10 for the layerwise manufacture of 3D objects from particulate build material, wherein the apparatus 10 comprises two fusing light sources which in combination provide a solution to address or at least reduce the present shortcomings. The apparatus comprises a first fusing source and a second fusing source, at least one of which is a digital light projector (DLP) for providing selective fusing energy. The two fusing sources in combination provide selective application of energy and prolonged heating of the fused cross sections to ensure full consolidation and improved quality of the object and unfused material. Applying a combination of such first and second fusing sources allows reducing or omitting the presence of absorber to create selectivity and presents a solution to achieving white or light-coloured objects with improved mechanical properties.

[0028] The DLP is arranged to provide fusing energy beams to the layer 160 and comprises a digital mirror device (DMD) 240 and a light source 250, such as an infrared light source. The digital mirror device 240 comprises an array of individually addressable mirrors, wherein the light source directs a light beam 450 towards the array and each mirror is configured to receive and redirect an energy beam received from the light source 250 either away from or towards the layer, based on the image slice for a corresponding layer. When a mirror is in an operative state, the mirror reflects a beamlet of the energy beam towards a corresponding voxel of the layer 160. When a mirror is in an inoperative state, the mirror reflects the beamlet of the energy beam away from the layer. Such operation of a DMD is well known. The location on the layer and thus the voxel addressed may be altered by altering the position of the array, by moving it across the layer or operating it in scrolling mode to sweep the energy beams of the array across the layer, by altering the angle of the array with respect to the layer surface, and / or by altering the angle of the light source with respect to the array.

[0029] The apparatus 10 comprises a distributor 290 to distribute each layer of build material; and a first fusing source and a second fusing source, each configured to provide fusing energy based on image slices defining each object cross section to be formed within a corresponding layer, wherein at least one of the first and second fusing sources is configured to provide selective fusing energy to selected object voxels of the layer within the object cross sections. Such a fusing source is herein described in the form of a digital light projector. The apparatus comprises a controller configured to control the distributor 290 to distribute each layer; to control the first fusing source to fuse selected object voxels to form an object cross section; and to control the second fusing source to further fuse the object voxels fused by the first fusing source. The steps of distributing and fusing by the first and second fusing source are repeated to form the object from a sequence of object cross sections.

[0030] A method 600 for controlling the apparatus 10 is further illustrated in Fig. 2, which is a flow chart showing at block 610 the step of distributing a layer; at block 650 the step of fusing selected object voxels within the cross section and at block 680 the step of further fusing the object voxels using the second fusing source. This defines the layer sequence that is repeated until the object is complete, by repeating the steps of distributing, fusing and further fusing to form the object from a sequence of object cross sections. As will now be described, the two fusing steps at blocks 650 and 680 may be applied in either order to improve the provision of fusing energy compared to conventional apparatus:

[0031] (i) In a first variant of providing the two fusing sources, Fig. 1 illustrates the DLP as the first fusing source applying the fusing step at voxel resolution at block 620. The further fusing step may be provided by the second fusing source, which is shown in the form of a scanning infrared lamp or non-selective energy source 220 configured to heat the object cross section fused in the first fusing step. Selectivity is achieved by the higher absorber weight per volume within the densified build material of the object cross section after fusing by the DLP. The higher density of the object cross section 150 results in an absorptivity to radiation from the non-selective second fusing source, such as an infrared scanning lamp, that is higher than that of the surrounding undensified build material. Therefore, the further fuse step results in selective fusing of the object cross section without the need of applying absorber with an inkjet printhead. The first fusing source may therefore be the DLP; and the second fusing source may be a non-selective fusing source controlled to further fuse the object cross section after fusing by the DLP. The step of fusing selected object voxels is provided using the DLP at block 650, and before the step of further fusing at block 680, wherein the fusing of selected object voxels causes a densification of build material within the object cross section; wherein the second light source is a non-selective light source and wherein the densification provides selective absorption by the object cross section. In this way, further fusing is provided to the object cross section only and not to non-fusing voxels outside the fused object cross section. Optionally, the build material may comprise an absorber, wherein the amount of the absorber per unit volume in the build material is sufficient to cause fusing within the object cross section by the second fusing source following fusing by the first fusing source, and insufficient to cause fusing within the nonfusing voxels by the second fusing source. The amount of absorber may be less than required in conventional print and fuse apparatus and thus lead to objects lighter in colour or lighter grey. The sequence of steps may be represented by the flow chart of Fig. 2 in which block 650 of selective fusing is applied, by the first fusing source, before a second fusing step at block 680. At block 680, non-selective heating is applied by a second fusing source that causes further fusing of the cross section and warming without fusing of the unfused build material within the non-fusing area.

[0032] (ii) In a second variant, the first fusing source may be a non-selective fusing lamp, and the second fusing source a digital light projector comprising a digital mirror device of individually addressable mirrors and a light source, wherein the light source directs light towards the digital mirror device and each mirror redirects an energy beam of the light either away from or towards the layer based on the image slice of a corresponding layer. In this case the apparatus further comprises an absorber deposition device configured to deposit absorber over the object cross section before the step of fusing by the first light source. The controller may be configured to control the absorber deposition device to deposit absorber over the object cross section; and to control the first fusing source to fuse the object voxels of the object cross section and next to control the second fusing source to further fuse some or all object voxels of the object cross section. The amount of absorber or its absorptive properties may be lower than in conventional print and fuse process and may allow nonblack absorbers. The first fusing step in combination with the second, selective fusing step by the DLP may be applied in a gentler manner. In addition, the DLP may be operated at a relatively lower power as a result of providing the further fusing step and / or as a result of the object cross section comprising absorber that may enhance the absorption of radiation by the DLP. Allowing the DLP to be operated at lower power reduces the risk of overheating the DMD and prolongs its lifetime. The sequence of steps may be represented by the flow chart of Fig. 2 in which block 650 of selective fusing is applied after block 680 of non- selective further fusing.

[0033] The provision of the DLP as one of the first and second fusing sources may further improve object quality by allowing provision of different fusing energy beams to different object voxels. The different energy levels may be defined by slice data where the DLP is applied as first fusing source. Additionally, or instead, the different energy levels may be provided or corrected based on thermal feedback from temperature measurements of the fused cross sections, for example following the first fuse step, where the first fusing source is the non- selective fusing source. Different fusing energies based on thermal feedback

[0034] As indicated in Fig. 1, the apparatus 10 may comprise a thermal sensor 280 configured to measure temperature of the layer 160, after the provision of fusing energy from the first light source (in the form of the non-selective fusing source) and before the provision of fusing energy by the second light source (in the form of the DLP). From the measured temperatures, a fusing thermal profile of the object cross section(s) of that layer defined by the first fusing step may be determined, and compared to a predefined fusing thermal profile for the cross section(s), for example as predetermined from a thermal model. Based on the comparison, deviations from the predefined fusing thermal profile may be determined and used to define a correction to the fusing energy levels to be provided by some or all of the mirrors of the digital mirror device 240 to corresponding object voxels in the further step of fusing at block 680 of Fig. 2. The controller may be configured to control the thermal sensor 280 and to make the comparisons and determinations; and to control the digital mirror device to provide the corrected different energy levels so as to fine tune the fusing thermal profile of the cross section. In this way, the fusing process may be fine-tuned by the step of further fusing at block 680. Fig. 3 is a flow chart further illustrating this variant method 700 of the method 600 of Fig. 2. Fig. 3 shows at block 710 the step of distributing a new layer, and at block 750 the combined step of depositing absorber over the object cross sectional area of each layer as defined by one or more image slices, and then fusing the object voxels defined by absorber with the first, non-selective fusing source. Next, at block 760, temperatures of the object cross section are measured using the thermal sensor 280 to determine a fusing thermal profile of the object cross section; which is compared to a predefined fusing thermal profile. Based on the comparison, corrections to the fusing energy levels are determined that are to be applied to the fusing energy beams at a subsequent fusing step by the digital mirror device. At block 770 for example, the further fusing step carried out by the DLP applies the corrected different fusing energy beams to some or all of the object voxels of the object cross section. The determined different energies are to achieve an improved fusing profile that is closer to or substantially corresponds to the predefined fusing thermal profile.

[0035] The object cross sections are typically defined as part of slice data that defines the voxels to be fused in each layer comprising one or more object cross sections. During a fusing step applied by the DLP, the controller 300 controls each micromirror by switching it between the operative and the inoperative state at a switching rate, or duty cycle. In Fig. 1, the different fusing energy beams 410 provided by the DMD 240 are indicated by arrows. The differences in fusing energy are indicated by the relative thickness of the arrows, wherein the thicker the arrow, the higher the fusing energy of the fusing energy beam provided by a corresponding mirror of the DMD. In Fig. 1, as an example, different thicknesses are indicated for three of the fusing energy beams, fusing energy beams 410 1, 410_2, 410_3. The outer fusing energy beam 410 3 with respect to the array of mirrors may for example be of higher energy, for example as defined by a higher duty cycle or “ON” time of the mirror, than an inner fusing energy beam 410_l, with respect to the array. A fusing energy beam 410_2 between the outer and inner fusing energy beams 410 1 and 410_3 may provide an energy level intermediate to that of the outer and central fusing energy beams.

[0036] The mirrors and their operation define the resolution of voxels that may be selectively addressed on the layer. Each mirror of the DMD may be assumed to heat a control area (based on the footprint of the energy beam projected onto the layer by each mirror) and at least the volume of the layer underneath the control area, due to the relatively small layer thickness of the layer. Thus each energy beam is referred to heat a corresponding voxel of at least the top most layer. Herein, a “voxel” is referred to as the smallest area selectively addressable by the DMD extending through a layer, wherein the layer may be from 10pm to several 100pm thick or even of the order of microns thick. The build area may be referred to synonymously as the layer 160 due to its relatively small thickness. The reference to “resolution” herein is intended to compare a relatively high resolution, at or exceeding the voxel resolution as may be defined by conventional print heads, and as may be achieved by the DMD, and relatively low resolution as may be provided by an elongate infrared lamp that extends along the width of the layer (along y) or a conventional overhead heater.

[0037] Optional non-fusing energy beams

[0038] In powder bed fusion it is desirable to control the temperature uniformity across the unfused areas in order to achieve dimensional accuracy and high quality part properties reliably. In addition, it may be desirable to apply different non-fusing energy to different non-fusing areas within the layer, outside of the object cross section, for example to reduce the rate of heat leakage from object cross sections. Use of a DLP in a powder bed fusion apparatus advantageously enables fine control of heating with non-fusing energy at voxel-level, by selectively providing different non-fusing energies to different voxels of the layer. This improves thermal control across the build area that cannot currently be achieved by conventional non-fusing devices. Furthermore, selectively controlling the level of nonfusing depending on the location of object cross sections, the risk of curling, and ultimately failure of the build process, may be reliably avoided and unfused powder may be kept in a relatively cooler state compared to currently possible, thus reducing ageing and allowing the unfused powder to be reused for longer in further build processes (herein also referred to as reuse rate).

[0039] A non-fusing step may be applied by the DLP when provided as either the first or second fusing source. The first and / or second fusing source, when configured to apply selective fusing energy to selected obj ect voxels, may be configured to also apply selective non-fusing energy to selected non-fusing voxels within a non-fusing area outside of the object cross section. Different non-fusing energy levels may be defined based on one or more of: one or more image slices, for example by the shape of the object cross section as defined by each image slice, the area of the object cross section as defined by each image slice, a closest distance to an adjacent cross section as defined within an image slice or across several image slices, and a location of the object cross section with respect to a boundary of the slice or build volume. Control of the non-fusing energy beams may be based on image slices, which may optionally define a non-fusing energy level for each non-fusing pixel. The image slices may define the non-fusing area for each layer; wherein the energy levels of the non-fusing energy beams are configured to raise the temperature of the non-fusing voxels to a temperature below the onset of melting. The image slices may define different energy levels for some or all non-fusing voxels so as to define a spatial variation in non-fusing energy level, wherein the spatial variation may be based on one or more of, with respect to the layer stack rather than the slice stack:

[0040] (a) a peripheral non-fusing area surrounding the periphery of the object model cross section compared to a remaining non-fusing area outside the peripheral non-fusing area and the object cross section. The energy levels within the peripheral non-fusing area may be different to, such as higher than, the energy levels of the remaining non-fusing area, which may be zero;

[0041] (b) an overlap region that is to overlap with an object model cross section of a subsequent slice or layer. By heating non-fusing voxels in the overlap region and not elsewhere, using the DLP, or heating the overlap region to a higher level compared to elsewhere, the relatively higher heating of the boundary or peripheral volume around the object surface reduces heat dissipation from the fused cross section; and

[0042] (c) a covering region that overlaps with an object model cross section of a previous slice or layer. Similarly as for (b), a higher level of heating by non-fusing energy above the final cross section of an object may reduce heat dissipation.

[0043] When the three options (a)-(c) are applied in combination, this results in a fused object enveloped by a surface cover of powder heated to a higher level near the object surface than elsewhere in the build volume. For example, heat dissipation may be reduced or delayed so as to ensure that the cross section remains above a certain fusing temperature for a desired duration of time. In this way, a sufficient degree of fusion may be reliably achieved within each cross section and between overlapping cross sections. Additionally, or instead, it may be desirable to compensate for heat leakage through the build volume boundaries. Typically, the side walls and support supporting the build bed present heat leakage paths through which energy provided to each layer escapes. This can cause dimensional inaccuracies particularly near the side walls and edges, radially outward from the centre of the build bed. Provision of a DMD to selectively supply heating where needed, for example to non-fusing voxels closest to the layer boundaries compared to the centre of the layer, compensates for at least some of the heat leakage through the side walls.

[0044] The different energies of the non-fusing energy beams applied by the DLP may be optional to any embodiment and its variants described herein, and enables additional control over unwanted heat dissipation from cross sectional areas that received fusing energy.

[0045] The control of the DLP to provide non-fusing energy beams may be based on modified image slices that have been modified by one or more masks, as will be described in more detail below. Applying selective fusing energy to selected object voxels during the step of fusing and / or further fusing at blocks 650 or 680 of Fig. 2 or at blocks 780 of Fig. 3 may comprise applying non-fusing energy beams to some or all non-fusing voxels; wherein an energy level of the non-fusing energy beams is configured to raise the temperature of the non-fusing voxels to a temperature below the melting temperature of the build material. The melting point may in some cases be defined by the onset of melting of the build material, at which the material particles become sufficiently viscous to stick to one another. Additionally, or instead, the DLP may be operated based on thermal feedback from the thermal sensor 280 to fine tune the non-fusing energy profile determined using the thermal sensor 280. The thermal sensor 280 may be configured to measure temperature of the nonfusing area before the provision of non-fusing energy beams by the digital light projector; wherein the controller is configured to determine from the measured temperature a nonfusing thermal profile of the non-fusing area; compare the determined non-fusing profile to a predefined non-fusing profile for that layer; and, based on the comparison, determine an energy level to be provided by the digital mirror device to some or all of the non-fusing voxels by corresponding non-fusing energy beams. This will be described further with reference to Figs. 5 and 6. The non-fusing step applied by the DLP may be concurrent or following the step of fusion or further fusion.

[0046] Optionally, the apparatus may comprise a further heat source configured to provide nonfusing energy. The heat source may be provided by one or more of the digital light projector; a further digital light projector or a non-selective heat source, configured to provide nonfusing energy to the layer. A non-selective heat source may be mounted behind the distributor to immediately preheat the freshly distributed layer. The further heat source may be operated to heat the non-fusing area towards or to a predefined target layer temperature that may be close to the melting temperature to prevent curling due to excessive temperature differentials during fusing, and sufficiently below the melting temperature of the build material so as to avoid accidental fusion. For example, the target layer temperature may be chosen to be within 10 °C or 20 °C below the melting temperature.

[0047] Defining different fusing energy beams, greyscale

[0048] The first or second light source when configured to provide fusing energy beams selectively to individual object voxels, e.g. the digital light projector, may be configured to provide fusing energy beams of different energy levels to the object cross section. This is indicated by the different fusing beams 410 in Fig. 1 for example.

[0049] The initial image slices may define a constant fusing energy level for all object voxels. A model based simulation routine may be applied to determine adjusted slice data based on the initial slices that may determine different fusing energy levels to be applied to some of the object voxels, e.g. in terms of duty cycle, to be provided to each object voxel. The routine may determine thermal impacts on each object cross section to be built based on its environment, determine deviations of the modelled solution from a predefined target temperature and adjust the duty cycle for some or all object pixels to reduce such unwanted deviations. For example, individual duty cycles to be applied for each object voxel of a layer may be determined based on one or more of: the shape and / or area of the object cross section; the shape and / or volume of one or more preceding object cross sections; the shape and / or volume of one or more subsequent object cross sections; and the distance to a neighbouring object cross section and / or to a boundary wall of the build volume as defined by the layer stack, for example. In this way, thermal build up may be reduced. This allows control over undesirable effects such as wall growth, caused by unfused powder sticking to an overly hot object wall, adjacent objects affecting one another due to thermal interference, or even joining of adjacent objects. In print and fuse applications, an analogous approach would be to define different “greyscales” in terms of absorber amounts deposited per unit area on the object cross section. The higher the amount, the more energy is absorbed from the heat source, and the hotter the per unit area gets during fusing. Similarly, the duty cycle of the fusing energy beam per voxel may be defined for similar effects. For large objects for example, the duty cycle may be low at and near the centre of the object volume and increased gradually from the centre outward, with the highest duty cycle defined at the object surface periphery. Narrow sections meanwhile may receive a constant high level of energy (defined in terms of duty cycle, for example) since they are more affected by heat dissipation due to their high surface to volume ratio.

[0050] Following the generation of adjusted image slices different fusing energy levels, they may be provided to the apparatus to process each layer. Fig. 1 illustrates the DMD 240 applying different fusing energy beams 410 to the object cross section wherein the energy is varied from the centre of the object cross section 150 outwards, so that near the centre the energy of inner fusing energy beams 410 1 is lower compared to outer fusing energy beams 410_3 at the periphery of the object cross section 150.

[0051] Adjusted energy levels including optionally non-fusing energy levels

[0052] Fig. 6 shows adjusted energy beams of different energy levels in more detail in a simplified variant of Fig. 1 and in which only the energy beams provided by the DMD, and not the DMD itself, area shown. All features shown and described in relation to Fig. l may equally be present and be applied to Fig. 6. The controller 300 may be configured to control each mirror of the DMD based on adjusted slices to provide the different fusing energy beams 410 to the object cross section 150 and optionally non-fusing energy beams 420 to the nonfusing areas within the layer 160 and outside the object cross section 150. Adjusted levels of energy for the non-fusing and fusing voxels may have been determined based on the considerations described herein, so as to achieve a thermal profile for each layer as determined from simulations of heat impact considering the location, size and shape of the object cross sections for example. The desired temperature level and profile may be different for different object cross section, and may be different for voxels within the non-fusing areas. The optional provision of the non-fusing energy beams 420 is again indicated by dashed arrows. The non-fusing energy beams 420 may be provided by the DMD by operating corresponding mirrors with a shorter operative phase, thus reducing the energy to non-fusing energy levels. Alternatively, a second light source configured to irradiate the DMD at a different wavelength not as readily absorbed as that provided by the fusing light source 250, or a second DLP may be provided that is operable at a different wavelength. Selective and differential non-fusing may be provided by the DMD as is illustrated by selective non-fusing energy beams 420 such that energy beams 420H closest to the periphery of the object cross section 150 are of higher energy compared to energy beams 420L further away from the periphery. Thus, non-fusing energy beams 420 may reduce in energy outwards from the object cross section 150 over a peripheral region surrounding the object cross section. Selective non-fusing by the DMD may be applied concurrently with or following fusing of the object cross section. Meanwhile, fusing energy beams 41 OH are highest for obj ect voxels near the periphery of the obj ect cross section 150 and fusing energy beam 410L is the lowest for object voxels at the centre of the object cross section 150.

[0053] Thermal feedback optionally including non-fusing energy beams

[0054] In addition or instead, the different fusing energies may be defined based on thermal feedback following the fuse step at block 750 of Fig. 3. The apparatus 10 may comprise a thermal sensor such as thermal sensor 280 shown in Fig. 1, arranged above the layer and configured to measure temperatures of the layer. During the build process, the thermal sensor may be used to measure the temperature of, for example, the fused cross sections and / or non-fusing area. A thermal map may be generated to represent the thermal profile over the fused cross sections which may be compared to a target fusing thermal map. The target fusing map may comprise a defined thermal profile map based on “greyscale” information of the adjusted image data, in other words it may comprise more than an average fusing temperature and the threshold range of deviation from the average. If, in comparing the thermal map of the fused cross sections to the target thermal map, any deviations are found to exceed a predefined threshold, a correction mask may be determined that is applied to the modified fusing slice data of a subsequent layer so as to compensate for such excessive deviations in fusing temperature and to converge towards the intended fusing topography.

[0055] Fig. 4 and Fig. 5 illustrate the method of Fig. 3, in which the DLP is the second fuse source used to provide a further, fine tuning fuse step. After the first fuse step at block 750 by the first fuse source, the post-fuse temperature profile is measured at block 760. An example profile for illustrative purposes only of a determined deviation as determined at block 760 of Fig. 3 for example, of the measured thermal profile from a predefined thermal profile, is shown in Fig. 4. The deviations correspond to a line of temperatures measured along the x- direction measured over a 2-dimensional section along x of the layer 160 of Fig. 5. The layer 160 comprises fused cross sections 150A, 150B and 150C. The predefined fusing thermal profile may be different for different object cross sections 150A, 150B and 150C. For example, for the relatively narrow cross section 150C the predefined fusing thermal profile may be higher than for the largest cross section 150B, for example to compensate for the higher heat leakage from the narrow cross section 150C. The non-fusing areas 130 A, 13 OB may be fine tuned in non-fusing temperature based on a voxel- specific desired temperature determined according to the considerations described herein.

[0056] Fig. 5 is a simplified variant of a cross section side view through layers 160 of Fig. 1 irradiated with energy beams by a digital mirror device 240, with fusing energy beams 410 and optionally concurrently with non-fusing energy beams 420. For example, in correspondence to the object cross sections 150A, 150B and 150C being fused in Fig. 5, measured deviations from a predefined fusing profile are shown in Fig. 4. Optionally, deviations of a measured non-fusing profile across non-fusing areas 130A and 130B from a predefined non-fusing profile may be determined, and 150C being fused in Fig. 5, measured deviations from a predefined fusing profile are shown in Fig. 4. In Fig. 4, the determined deviations are all below the predefined respective profile temperature for all voxels measured. This is indicated by the x-axis crossing the y-axis above the measured deviations; i.e. the measured temperature deviations are all negative in the example shown, although this is not essential. This means that the step of further fusing by the digital mirror device 240 is to apply additional heat to both object voxels and non-fusing voxels by applying energy beams as determined based on the measured deviations. Here it should further be noted that the predefined fusing profile will be at an overall higher temperature level than the predefined non-fusing profile. Therefore even where the determined deviation is of the same value for a fusing object voxel and a non-fusing voxel, the determined energy level to be applied by the DMD to the fusing voxel may be expected to be higher than that to be applied to the non-fusing voxel.

[0057] Fig. 5 illustrates the determined fusing energy beams 410 to achieve a temperature within a range of the predefined temperature target of a respective predefined target profile based on the deviations in Fig. 4. As before, the arrow line thickness indicates relative energy levels for fusing energy beams 410 and non-fusing energy beams 420, such that thin to thick arrows indicate low to high energy. The first cross section 150A is determined to deviate the most from its fusing target temperature and receives the highest energy fusing energy beams 410A. The narrow cross section 150C was determined to deviate the least from its desired fusing target temperature and receives the lowest fusing energy beam 410C. The temperature of object cross section 150B deviates from the desired fusing target over the largest range compared to the other two object cross sections, and the fine tuning fusing energy beams 410B differ the most from one another compared to fine tuning fusing energy beams 410A and 410C. It should be noted that Figs. 4 and 5 are merely for illustration and do not suggest actual ranges or energy differences. Shown in dashed lines are optional non-fusing energy beams 420 that may be applied concurrently or sequentially by the DMD to the non-fusing areas such as 130A, 130B, based on the deviations determined in Fig. 4. Thus non-fusing energy beams may also be of different energies so as to fine tune the temperature of the nonfusing areas to achieve improved object properties.

[0058] The predefined fusing profile may be based on similar or the same considerations as those made to determined adjusted image slices as described with reference to Fig. 6, for example, and as further described herein. Similarly, the predefined non-fusing profile may be based on similar or the same considerations as those described herein to arrive at adjusted nonfusing energy levels to generate adjusted image slices. Such predefined profile may be determined using model based simulations that were developed empirically for a given build material, for example. Directionality, scrolling and delays between thermal events

[0059] The build material at the point of distribution is at a lower temperature than the top most processed layer it is to cover. The step of distributing may therefore be referred to as a ‘cold’ thermal event. The steps of non-fusing and fusing meanwhile are ‘hot’ thermal events after each of which the layer starts to cool again. As described above, it may be advantageous to control the directionality of, and preferably also the timing between, thermal events on the build area. This improves repeatability and uniformity of the parts resulting from a build, by ensuring that each voxel experiences the same thermal history in the same or similar manner within the build area, and for each layer. Distribution of each new layer preferably occurs before the fused, hot, object cross sections of the (previous) layer cool to become too viscous to allow adequate adhesion with the new layer. The timing of the distribution step may thus be carefully controlled with respect to the temperature of the fused object cross section. As a result, the step of distributing a new layer is initiated before the step of fusing (e.g. the traverse of a fusing energy source over the layer) is complete.

[0060] At least one of the first and second fusing source may be configured to scroll across the layer while providing fusing energy and / or further fusing energy to object voxels, and optionally while providing non-fusing energy to non-fusing voxels. In processes applied by the apparatus in which each layer is distributed along a first direction, the first and second fusing source may be configured to scroll across the layer in the first direction while providing fusing energy and / or further fusing energy to object voxels, and optionally while providing non-fusing energy to non-fusing voxels. The steps of fusing and further fusing may be provided by the first and second light source while traversing the layer so as to progressively fuse and further fuse the object cross section during their respective traverse; wherein the steps of distributing, fusing and further fusing are applied along the same direction. The DMD and the non-selective fusing source may be arranged on the same carriage as and behind the distributor 290, for example.

[0061] Image slice data may be used to determine locations at which an underlying object cross section contributes to the heating of a subsequent layer. Additionally, it may be used to determine how best to condition a new layer for optimal fusing of object cross sections in a subsequent layer. This is illustrated in Figs. 7A to 7E, which illustrate different stages of a traverse of the DMD over the layer in a simplified variant of the apparatus of Fig. 1, and in which the DLP may be controlled to additionally provide selective non-fusing energy beams ahead of or behind the distributor and / or the other fusing energy source. The DMD 240 is arranged to scroll over the layer 160 along the x-direction from left to right, as indicated by the open arrow, so as to progressively provide fusing energy beams 410 to corresponding voxels within object cross section 150 along the x-direction. Optionally, the scrolling DMD or a further DLP comprising a scrolling DMD is arranged to provide non-fusing energy beams 420 to regions of the layer that are not to fuse.

[0062] Fig. 7A illustrates the scrolling DMD as it begins its traverse over the layer 160, and optionally being controlled by controller 300 to provide non-fusing energy beams 420 of different non-fusing energies. A lower energy may be provided in the form of non-fusing energy beams 420L up to a region 130P, which is a peripheral region just outside an object cross section 150 that is to be formed. Non-fusing energy beams 420H may be provided to the peripheral region 130P. which may also vary such that the closer a voxel is located within the peripheral region 130P to the object cross section to be formed, the higher the energy. In this example this is indicated by an inward non-fusing energy beam 420H2 which is of higher non-fusing energy than outward non-fusing energy beam 420H1.

[0063] Next, in Fig. 7B, based on slice data, the DMD provides fusing energy beams 410 to begin fusing the object cross section 150 while non-fusing energy beams 420 are provided concurrently to voxels upstream of the object cross section 150, with respect to the scrolling direction.

[0064] Fig. 7C illustrates how the scrolling DMD may apply fusing energy beams 410 of different energies to corresponding voxels within the object cross section 150, for example the fusing energy may be reduced towards the centre of the cross section 150. This is not specifically labelled in this Figure but may for example be as indicated by the thickness of the fusing energy beams 410 in Fig. 6.

[0065] Figs. 7D and 7E further illustrate stages of the DMD during its traverse and the different fusing energy beams provided depending on its position over the layer, and the optional different non-fusing energy beams 420.

[0066] The micromirror array may thus be controlled to provide non-fusing energy beams are provided and fusing energy beams to the layer 160. It can be seen how different mirrors of the array are operated to address the same voxel during the traverse of the DMD. In this way, each non-fusing voxel may receive a selective non-fusing boost concurrent with the provision of fusing energy beams following for example a preheating step by a non-selective preheating lamp. The DMD(s) may begin their traverse before the distributor has completed its traverse over the layer along the same direction (e.g. along x). The velocities at which each traverse the layer may be the same, constant velocity. The application of the DMD is however not limited to a unidirectional thermal cycle; improved results may be obtained in which thermal events are applied in an opposing or orthogonal direction to other thermal events. In multi-directional processes, which may generate larger thermal deviations from the predefined thermal profile across each layer, fine tuning of fusing energy and / or nonfusing energy may be advantageously provided by the DMD.

[0067] DLP operation with masks

[0068] Typically, in static DLP projection, the energy applied to each voxel, which is defined by the area of a single pixel of the two dimensional image or slice being projected onto the layer, is controlled through a combination of projection duration (global), light source intensity (global), and greyscale value (local to pixel). To locally vary the amount of energy applied (duration * intensity * greyscale %) a combination of exposing sequential images or adjusting greyscale value may be carried out. For a scrolling DMD, each voxel is instead associated with an entire row (along x) of mirrors, where the time each control area is associated with an individual pixel is equal to the projected pixel length divided by the scrolling speed. In the case where the DMD is scrolling along the x direction, the resolution of each voxel is defined by the projected pixel width (along y) and a combination of scroll speed, image projection frequency, and projected pixel length (along x). Typically, the images or slices being projected are synchronized with the scroll speed such that while the projection moves along the x axis, the projected object geometry remains in a fixed position on the layer for the duration over which the projected area overlaps with the voxel. Thus, when generating a series of slices and compensations that control the object forming process, they may largely be fixed to one of two reference frames:

[0069] (i) The reference frame fixed to the voxel: the position of features within the projected image are synchronized with the scrolling to maintain a consistent position on the layer surface. In this way, object geometries and compensations related to object geometry can be controlled. (ii) The reference frame fixed to the micromirror array: the position of features within the slice are fixed to the pixel location regardless of scrolling. This allows application of e.g. calibrations such as uniformity masks, scaling, etc. which are independent of object geometry.

[0070] Image slices for a digital mirror device may be modified by one or more masks for the temporal control for a DMD in scrolling mode, and / or spatial control of irradiation intensity, at each object voxel and / or non-fusing voxel. The image slices may be modified based on one or more masks applied to the different energy levels, wherein each mask is associated with one of the frame of the layer and the frame of the digital mirror device, and wherein modified different energy levels are provided to the object cross section during the step of fusing selected object voxels. The modification may be made to adjusted slice data comprising adjusted fusing energy levels and optionally adjusted non-fusing energy levels as described herein. This will now be described with reference to Figs. 8A to 12 and may be applied to any of the embodiments and their variants described herein. A simple example of an image slice 6000 comprising a plurality of fusing pixels defining an object model cross section 6500 is shown in Fig. 8A. A fusing energy level defined for each fusing pixel may in this simple example be the same for each object pixel. The area surrounding the object model cross section 6500 is a non-fusing slice area 6300 defining “non-fusing areas” over which voxels in the layer are not to fuse. The slices are prepared typically from obj ect models that have been placed or nested within a virtual build volume which is based on the real build volume of the apparatus. The virtual build volume is a 3D volume within which certain compensations may be applied to account for shrinkage and deformation during processing of the object. After this, the virtual build volume is divided into a sequence of slices, which are the 2D images defined to map to a corresponding layer of the build process. To provide a predetermined fusing profile to each fusing voxel using a scrolling DMD, a fusing mask 5000 as illustrated by way of example in Fig. 8B is provided.

[0071] The fusing mask comprises data to modify that of the fusing pixels of each slice. In Fig. 8B, the shading from light to dark, from left to right, along the x-direction indicates a linear variation of weights increasing from low to high. The weights may be factors varying from 0 to 1 as the shading changes from white to black. Therefore, a constant energy level defined for the fusing pixels of the model cross section 650 would be modified by the mask by multiplying each energy level with a corresponding weight. The modified slice 7000 of Fig. 3C illustrates the modified model cross section 7500 over which the modified energy level now gradually increases from left to right. This is further illustrated in Fig. 9, by a linearly increasing behaviour of weights a from left to right from a = 0.4 to 1.0 along the x-direction, for example along a mask row 5000R defining a sequence of weights for a corresponding pixel row Pn along the object cross section 650. The pixels Pn are mapped below the graph, separated by a gap for visual clarity, and their solid black colour indicates data for each energy level, for example a constant 100% duty cycle. Mapped underneath are the modified pixel energy levels of modified slice 700. For example, if pixels Pi, P2 and P3 of the model cross section define a duty cycle dci = dc2 = des of 100% beam energy output from the DMD, and the weights for a sequence of pixels is a.i = 0.5, a.2 = 0.6, a.3 = 0.4, the modified duty cycles for the pixels would be ai x dci = 50%; a.2 x dc2 = 60%; a.2 x dc3 = 40%.

[0072] Turning now to option (ii) above, instead of using a mask associated with the reference frame of the layer to modify the slice data, the mask may be associated with the reference frame of the DMD. The weights of the mask map onto a corresponding micromirror. To illustrate this, the row Pn of Fig. 9 may instead be thought of as representing a row of n mirrors. For a given mirror, the mask data value remains the same irrespective of the slice data. When the DMD is operated in a scrolling mode to sweep the layer along the x-direction, such that it heats successive bands of voxels extending along the y-direction, the mask may be defined to provide control over the fusing energy profile provided to each voxel.

[0073] This is illustrated in more detail in Figs. 10A to 10D, which shows four different stages of a traverse of the DMD 240 over the layer 160 in a variant of the apparatus 10 of Fig. 1. During its traverse at a constant speed v, the DMD 240 addresses an object cross section 150 progressively with six mirrors of its array, aligned one after the other in the direction of the traverse. For a row of different mirrors extending along the x-direction, the mask 5000 defining the weights a inset at the top left of Fig. 10A, each mirror along x is operated with a specified weight for that mirror. This weight does not change for that mirror as the DMD scrolls along the layer since the fusing mask is fixed to reference frame of the DMD, or micromirror array. As a result, the right-most mirror at the leading edge of the array may be operated at the highest modified duty cycle, for example at 90% (a=0.9), and the left-most mirror at the trailing edge is operated at the most reduced duty cycle, for example at 40% (a=0.4). Each Figure further shows modified fusing pixel data of the modified slice 7000 corresponding to the position at which the DMD is operated above the layer. The modified slice 7000 is identified by the time step at which is it applied, where the time difference between successive mirrors providing fusing energy beams to a given voxel remains constant since the speed of the DMD is constant. The scale of the x-axis of the layer 160 is not the same as that of the modified slice 7000 since modified slice 7000 shows only an extract to focus on the modified pixels. The unmodified fusing slice data 6000 may in this example have specified a constant duty cycle of 100% for each pixel. To illustrate the effect of the weights on each pixel in each slice, the variation in weight is superimposed on the modified slice, in which the y-axis of the slice is parallel to the vertical axis of the weight value. The modified energy level for each pixel of the cross section 150, which in this example is formed from four successive pixels Pl to P4, is indicated by its shading at each stage. As before, white indicates the lowest energy level of a modified pixel and black the highest energy level. The six mirrors may for example have been assigned weights 0.9, 0.8, 0.7, 0.6, 0.5 and 0.4, from right to left. The original pixel values may have been a constant 100% and are modified the least, by a=0.9, and thus the modified pixel values are the highest at the leading edge of the DMD. The energy level provided by the left most (sixth) mirror at the trailing edge of the DMD is the most modified, by a=0.4 and provides the lowest modified pixel values.

[0074] In Fig. 10A, the DMD 240 has just started at starting time tl to address the first voxel of the object cross section 150, with the first mirror. The first voxel is shown in black to indicate fusing, and the first mirror at the leading edge of the DMD provides a fusing energy beam 410 according to the first modified pixel Pl. The first voxel receives an energy beam at a duty cycle of 90% (a=0.9) as defined by the modified fusing slice 7000(tl).

[0075] After a first time interval, at time t2 illustrated by Fig. 5B, the DMD 240 is addressing the first and the second voxel at the same time. The modified fusing slice 7000(t2) now defines by the first modified pixel energy Pl the energy level the second mirror provides for the first voxel, provided by the second mirror operated at 80% duty cycle (a=0.8). The second modified pixel P2 defines the location and energy level of the second voxel to the right of the first voxel, and is fused by the first mirror at a duty cycle of 90%. The shading of the first modified pixel Pl is slightly lighter than that of the second modified pixel P2. After a further two time intervals, at time t4 illustrated by Fig. 5C, the DMD 240 is addressing all four voxels at the same time based on the modified fusing slice 7000(t4). The first voxel is now heated at a 60% duty cycle by the fourth mirror, according to the first modified pixel Pl, and for which a=0.6. The second and third voxels are addressed by the second and third mirror, for which a= 0.7 and 0.8, respectively, which are thus operated at duty cycles of 70% and 80%. The fourth voxel at right edge of the cross section 150 is heated by the first mirror, at 90% duty cycle.

[0076] After a further two time intervals, at time t6 illustrated by Fig. 5D, the DMD 240 is addressing all four voxels with a different set of mirrors, mirrors six to three from left to right, based on the modified fusing slice 7000(t6). The first voxel is now heated by the sixth mirror at the trailing edge of the array. The sixth mirror is operated at 40% duty cycle due to being modified by a=0.4. The fourth voxel is now heated by the third mirror operated at a duty cycle of 70%. The second and third voxels are heated by respective fifth mirror at 50% and fourth mirror at 60% duty cycle.

[0077] A linear power profile may be provided to each voxel in this way during the traverse of the DMD over it, as illustrated by the graph of Fig. 11A for the first voxel for example. Assuming the DMD travels at a constant speed v, the first voxel is heated at a modified duty cycle of 90% at a starting time tl (a=0.9), at a modified duty cycle of 80% at t2 (a=0.8), at a modified duty cycle of 60% at t4 (a=0.6), and at a modified duty cycle of 40% at t6 (a=0.4). For completeness, the times t3 and t5 not illustrated in Figs 10A to 10D are also included in this Fig. 11 A. This is a simple illustration of how a linear power profile may be achieved applying the method according to the invention.

[0078] It should be noted that while the pixel numbering has been kept the same for clarity, the position of each pixel within the modified slice data is different to reflect the changing position of the DMD with respect to the layer. In other words, the slice and modified slice is fixed to the reference frame of the layer so that object cross section remains in the same position for each position of the DMD. Thus, the location of the first, second, third and fourth pixel changes in each slice when the DMD is provided in a scrolling mode.

[0079] The mask may be defined to achieve any desirable intensity profile for each non-fusing or fusing voxel over the duration of heating when applied in combination with a scrolling DMD. This provides a simple process that allows full control over the intensity profile for each voxel of the layer. Improved mechanical properties may be provided that are not achievable with conventional powder bed fusion apparatus. Different masks may be defined for different layers such that different energy levels can be individually assigned in any direction of the layer, per non-fusing area and / or per object cross section, either through varying the gradient between different y axis positions or by associating a specific mask to certain groups of the plurality of slices during the slicing operation to ensure that the variation of energy applied throughout the DMD pass is optimal for each individual object, or non-fusing area. An improved intensity profile is shown in Fig. 11B, which may be achieved by designing a suitable mask 5000 and associating it with the frame of reference of the DMD as it scrolls over the layer according to the method of operation of Figs. 10A to 10D. Such an intensity profile may rapidly achieve melting while maintaining the build material in a low viscosity state over a maximised duration without overheating of the fused material, therefore reducing the risk of build material degradation. By comparison, conventional powder bed fusion processes achieve intensity profiles illustrated in Fig. 12. For example, laser sintering may only achieve fusing by applying a very short and intense fusing intensity profile illustrated by the solid line in Fig. 12. The intensity delivered to each voxel is designed to overheat over a very short timeframe of the order of milliseconds or less. The particles typically reach full consolidation over a duration of of order of seconds. Since this process therefore needs to continue beyond the application of the laser beam for the particles to reach full consolidation, the material needs to effectively be overheated to ensure it remains long enough in a sufficiently low viscous state. The resulting high temperatures can lead to material degradation in the form of chain scission and / or oxidation within the fused cross section, leading to weaker parts. With conventional print and fuse apparatus, a relatively lower and broader intensity profile shown by the dashed line in Fig. 12 is typically provided, for example by a scanning infrared lamp, leading to a relatively slow heat up phase and relatively low fusing temperature. Application allows irradiation intensity control over the duration each voxel is heated.

[0080] General considerations

[0081] The fusing energy source 250 of the DLP may be an infrared laser, for example. The DMD 240 may comprise a single micromirror array arranged to simultaneously address each voxel of the entire layer surface from a static position. Alternatively, the DMD may comprise an array of mirrors fewer in number than the voxels on the layer surface, and may be arranged on a scanning (or “scrolling”) gantry above the layer to successively address rows of voxels along the y-direction as the DMD is moved (scrolled or scanned) above the layer surface, as is illustrated in Figs. 6A to 8C. Alternatively, the DMD may be mounted to a galvanometer for example, and the controller may be configured to vary the angle of the array with respect to the layer surface.

[0082] The digital light projector may comprise a group of DMD arrays 240, each receiving a portion of the slice, in the form of sub-slices, that are synchronised to project a reconstructed slice from the sub-slices. This mode of operation is also known as tiling. Typically the subslices partially overlap their nearest neighbours to avoid visible or measurable joints in the final object. Adjacent arrays may jointly irradiate the overlap section in any conventional manner known from “printhead stitching”, where neighbouring sections are addressed in any suitable pattern by both subarrays so as to obscure otherwise visible or measurable overlap joints.

[0083] Both the first light source and the second light source may be configured to apply selective fusing energy to the object voxels, wherein the method further comprises, between the steps of fusing and further fusing: measuring temperatures of the object cross section to determine a fusing thermal profile of the object cross section; comparing the measured fusing temperature profile to a predefined fusing temp thermal profile; determining different energy levels to be provided by the digital mirror display based on the comparison during the step of further fusing; wherein the step of further fusing is a step of further fusing selected object voxels based on the determined different energy levels. The thermal sensor 280 may be a thermal camera of sufficiently high resolution to capture the temperature of each layer at or near voxel resolution. The thermal sensor may be arranged stationary above the layer as shown in Fig 1. Alternatively, the thermal sensor may be a line scan camera having an elongate array of sensor pixels arranged to move above the layer, in a direction perpendicular to the direction of elongation of the array, to measure the temperature of the layer surface line by line. The line scan camera may for example follow the distributor and / or the fusing radiation source. A thermal map may be reconstructed from the multiple line scans taken to represent the temperature of the entire layer. The thermal map generated may be used to control the DMD to provide the different fusing energy beams to object cross sections of the layer.

[0084] Fig. 1 shows an apparatus 10 that may optionally comprise an overhead heater 270 arranged above the build area and configured to maintain the layer at the target preheat temperature. Such an overhead heater typically has an array of individually controllable heating elements, each element addressing a plurality (or “zone”) of voxels over a relatively large region on the build bed. The heating elements may be ceramic heater tiles and able to respond rapidly changes in control, thus the DMD enables a fast-acting correction at high resolution while the overhead heater is continuously operated to maintain the layer at the target layer temperature. The overhead heater 270 may comprise four or more heater elements, for example at least one above each comer of a rectangular build area, or one above and along each side of the build area. The controller 300 may be configured to control the heater elements of the overhead heater based on a temperature distribution of the layer as measured by the thermal sensor 280. For example, with reference to Figs. 3 or 4, the thermal sensor 280 may be controlled to measure temperatures of the layer after respective blocks 620 or 720 of non-fusing, for example, and before the respective steps of fusing the object cross section at block 650 or 750. The overhead heater 290 may be controlled continuously to maintain the layer temperature at the average layer target temperature throughout the layer cycle. As described before, the DLP may optionally provide fusing, and optionally nonfusing, energy beams to the layer based on temperature measurements made after the step of fusing at blocks 650, 750, such that the different energies of the fusing, and optionally non-fusing, energy beams are determined based on the measurements made following the steps of fusing at block 650, 750, for example at block 760 of Fig. 3. Any and any combination of variants as described herein may further apply. In any of the described variants, the DMD or a low resolution infrared non-fusing lamp may be applied to preheat or warm the layer immediately after a new layer is distributed, at the optional block 620 in Fig. 2 or optional block 720 in Fig. 3. In variants, the infrared digital light projector may comprise a first micromirror array arranged to preheat the build area at blocks 620 or 720, behind the distribution device with respect to the direction of distribution, and a second DMD array arranged to fuse at voxel resolution at block 650 or 750, and / or at block 770 of Fig. 3. Alternatively, a preheating step at block 620 of the method 600 of Fig. 2 or at block 720 of the method 700 of Fig. 3 may be provided by a scanning infrared lamp that follows the distributor to immediately heat the freshly distributed layer.

[0085] The controller 300 may be a computer or microprocessor provided with a program that, when executed, causes the steps of the methods disclosed herein. A computer program may be provided comprising instructions which, when the program is executed by the controller 300, e.g. a computer, cause the controller or computer to carry out the method and its variants as described herein. The controller may be configured to control any one or any combination of components to provide the method steps for controlling the apparatus 10 as described herein.

Claims

CLAIMS1. An apparatus for the layerwise manufacture of 3D objects from particulate build material, wherein the apparatus comprises: a distributor to distribute each layer of build material; a first fusing source and a second fusing source, each configured to provide fusing energy based on image slices defining each object cross section to be formed within a corresponding layer, wherein at least one of the first and second fusing sources is configured to provide selective fusing energy to selected object voxels of the layer within the object cross sections; and a controller configured to:- control the distributor to distribute a layer;- control the first fusing source to fuse selected object voxels to form an object cross section;- control the second fusing source to further fuse the object voxels fused by the first fusing source; and- repeat the distributing, fusing and further fusing to form the object from a sequence of object cross sections.

2. The apparatus of claim 1, wherein the first fusing source is a digital light projector; wherein the digital light projector comprises a digital mirror device of individually addressable mirrors, and a light source, wherein the light source is arranged to direct light towards the digital mirror device, and each mirror is arranged to redirect an energy beam of the light either away from or towards the layer based on the image slice of a corresponding layer; and wherein the second fusing source is a non-selective fusing lamp controlled to further fuse the object cross section after fusing by the digital light projector.

3. The apparatus of claim 2, wherein the build material comprises an absorber, and wherein an amount of the absorber per unit volume is sufficient to cause fusing within the object cross section by the second fusing source following fusing by the first fusing source, and is insufficient to cause fusing within the non-fusing voxels by the second fusing source.

4. The apparatus of claim 1, wherein the apparatus comprises an absorber deposition device configured to deposit absorber over the object cross section; wherein the first fusing source is a non-selective fusing lamp, and wherein the second fusing source is a digital light projector comprising a digital mirror device of individually addressable mirrors and a light source, wherein the light source is arranged to direct light towards the digital mirror device and each mirror is arranged to redirect an energy beam of the light either away from or towards the layer based on the image slice of a corresponding layer; wherein the controller is configured to:- control the absorber deposition device to deposit absorber over the object cross section; and- control the first fusing source to fuse the object voxels of the object cross section before controlling the second fusing source to further fuse some or all object voxels of the object cross section.

5. The apparatus of claim 4 comprising a thermal sensor configured to measure temperature of the layer after the provision of fusing energy from the first light source and before the provision of fusing energy by the second light source; wherein the controller is configured to: determine from the measured temperature a fusing thermal profile of the object cross section; compare the determined fusing thermal profile to a predefined fusing thermal profile for that cross section; based on the comparison, to determine an energy level to be provided by the digital mirror device to some or all of the object voxels by corresponding fusing energy beams; and control the digital mirror device to provide the determined energy levels so as to fine tune the fusing thermal profile of the cross section.

6. The apparatus of any preceding claim, wherein the digital light projector is configured to provide fusing energy beams of different energy levels to the object cross section.

7. The apparatus of claim 6, wherein the different energy levels are defined within adjusted image slices based on the image slices.

8. The apparatus of claim 6 or claim 7, wherein the different energy levels are modified based on one or more masks, wherein each mask is associated with one of the frame of the layer and the frame of the digital mirror device, and wherein the one or more masks modify the different energy levels such that modified different energy levels are provided to the object cross section by the digital mirror device.

9. The apparatus of any preceding claim, wherein the digital light projector is configured to provide non-fusing energy beams to non-fusing voxels of a non-fusing area outside the object cross section of the layer, wherein the non-fusing area is defined by the image slices; wherein the non-fusing energy beams are configured to raise the temperature of the nonfusing voxels to a temperature below the onset of melting.

10. The apparatus of claim 9, comprising a thermal sensor configured to measure temperature of the non-fusing area before the provision of non-fusing energy beams; wherein the controller is configured to: determine from the measured temperature a non-fusing thermal profile of the nonfusing area; compare the determined non-fusing thermal profile to a predefined non-fusing thermal profile for that layer; and based on the comparison, determine an energy level to be provided by the digital mirror device to some or all of the non-fusing voxels by corresponding non-fusing energy beams.

11. The apparatus of claim 9 or claim 10, wherein image slices further define an energy level for some or all non-fusing voxels so as to define a spatial variation in non-fusing energy level, wherein the spatial variation is defined over one or more of:(a) a peripheral non-fusing area surrounding the object cross section compared to a remaining area outside the peripheral area and the object cross section;(b) a non-fusing overlap region that is to overlap with an object cross section of a subsequent layer; and(c) a non-fusing covering region that overlaps with an object cross section of a previous layer, optionally wherein the different non-fusing energy levels are defined based on one or more of the image slices, the shape of the object cross section, the area of the object cross section, a closest distance to an adjacent cross section, and a location of the object cross section with respect to a boundary of the build area.

12. The apparatus of any one of claims 1 to 11, wherein at least one of the first and second fusing sources is configured to scroll across the layer while providing fusing energy and / or further fusing energy to object voxels, and optionally while providing non-fusing energy to non-fusing voxels.

13. The apparatus of claim 12, wherein each layer is distributed along a first direction, and wherein the first and second fusing sources are configured to scroll across the layer in the first direction while providing fusing energy and / or further fusing energy to object voxels, and optionally while providing non-fusing energy to non-fusing voxels.

14. A method for controlling an apparatus for the layerwise manufacture of 3D objects from particulate build material, wherein the apparatus comprises a distributor to distribute each layer of build material; and a first fusing source and a second fusing source configured to provide fusing energy based on image slices defining each object cross section to be formed within a corresponding layer, wherein at least one of the first and second fusing sources is configured to apply selective fusing energy to selected object voxels of the layer within the object cross sections; wherein the method comprises: distributing a layer; fusing selected object voxels within the cross section using the first fusing source; further fusing the object voxels using the second fusing source; and repeating the steps of distributing, fusing and further fusing to form the object from a sequence of object cross sections.

15. The method of claim 14, wherein the first light source is configured to apply selective fusing energy to selected object voxels and the step of fusing selected object voxels is provided before the step of further fusing, wherein the fusing of selected object voxels causes a densification of build material within the object cross section; wherein the secondlight source is a non-selective light source and wherein the densification provides selective absorption by the object cross section such that the further fusing is provided to the object cross section only and not to non-fusing voxels outside the fused object cross section.

16. The method of claim 14, wherein the second light source is configured to apply selective fusing energy to the object voxels during the further step of fusing, wherein the method further comprises, between the steps of distributing and fusing, a step of depositing absorber over the object cross sectional area of each layer as defined by one or more corresponding image slices; and between the steps of fusing and further fusing: measuring temperatures of the object cross section to determine a fusing thermal profile of the object cross section; comparing the measured fusing temperature profile to a predefined fusing temp thermal profile; and determining different energy levels to be provided by the digital mirror display based on the comparison during the step of further fusing; wherein the step of further fusing is applied based on the determined different energy levels.

17. The method of claim 14, wherein the first light source and the second light source are configured to apply selective fusing energy to the object voxels, wherein the method further comprises, between the steps of fusing and further fusing: measuring temperatures of the object cross section to determine a fusing thermal profile of the object cross section; comparing the measured fusing thermal profile to a predefined fusing thermal profile; and determining different energy levels to be provided by the digital mirror display based on the comparison during the step of further fusing; wherein the step of further fusing is a step of further fusing of selected object voxels based on the determined different energy levels.

18. The method of any one of claims 14 to 17, wherein the images slices are modified based on one or more masks applied to the different energy levels, wherein each mask isassociated with one of the frame of the layer and the frame of the digital mirror device, and wherein modified different energy levels are provided to the object cross section during the step of fusing selected object voxels.

19. The method of any one of claims 14 to 18, wherein the first and / or second fusing source when configured to apply selective fusing energy to selected object voxels, is further configured to apply selective non-fusing energy to selected non-fusing voxels within a nonfusing area outside of the object cross section; wherein applying selective fusing energy to selected object voxels during the step of fusing and / or further fusing comprises applying non-fusing energy beams to some or all non-fusing voxels; wherein an energy level of the non-fusing energy beams is configured to raise the temperature of the non-fusing voxels to a temperature below the onset of melting of the build material.

20. The method of claim 19, further comprising, before applying non-fusing energy beams to some or all non-fusing voxels: measuring temperatures of the non-fusing area to determine a non-fusing thermal profile of the non-fusing area; comparing the measured non-fusing thermal profile to a predefined non-fusing thermal profile; based on the comparison, determining different energy levels to be provided by the non-fusing energy beams.

21. The method of any one of claims 14 to 20, wherein the steps of fusing and further fusing are provided by the first and second light sources while traversing the layer so as to progressively fuse and further fuse the object cross section during their respective traverse; wherein the steps of distributing, fusing and further fusing are applied along the same direction.

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