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

Abstract

Provided is an apparatus for the layerwise manufacture of objects from particulate material, comprising a distribution device to distribute each layer; a digital light projector operable in scrolling mode over the layer and comprising a digital mirror device and a light source, wherein the digital mirror device comprises an array of individually addressable micromirrors each of which is arranged to redirect a light beam received from the light source away from or towards the layer based on image slices, wherein some or all of the image slices comprise a fusing slice defining a fusing energy level for each of a plurality of object voxels to be fused to form an object cross section within the layer; and a controller configured to: control the distribution device to distribute the layer; and control the digital light projector to scroll redirected light beams across the layer while controlling each micromirror to selectively direct fusing energy beams towards each object voxel within the object cross section based on modifying each fusing slice by a fusing mask; wherein the fusing mask is associated with the reference frame of the micromirror array and defines modification data for each micromirror to modify the initial energy level of each fusing slice, such that each object voxel is provided with a predefined fusing intensity profile over an irradiation duration. A method of providing energy beam control data for the digital mirror device in form of modified fusing slices 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 thermal control during powder bed fusing. Apparatus and methods of control and the provision of image slices therefor are also disclosed. The apparatus and its operation may find particular benefits in applications requiring improved part uniformity.

[0004] BACKGROUND

[0005] In powder bed fusion processes like print and fuse or laser sintering processes, an object is built up layer by layer from particulate build 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 brushing and / or blowing the supportive powder off by pressurised gas streams. 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 infrared lamps that can process each layer in a single, rapid pass. The coalescence between particles of the build material should preferably occur at temperatures that are above the melt temperature but below temperatures at which the polymer may degrade. Laser sintering involves a relatively small spot laser passing relatively quickly across the surface of the layer, leading to a high intensity of radiation being applied to particles of a unit volume (e.g. over a “voxel”). This high intensity over a very short time period of typically milliseconds or less ensures that the material stays in a low viscosity state to achieve full coalescence. It is thus an aggressive approach that results in the polymer material being heated to high peak temperatures, often to far above the melt temperature, which can lead to degradation of the polymer by for example chain scission and / or oxidation. Print and fuse approaches apply heat over a longer period of time, of typically around a second, so that the excessive peak temperatures incurred by laser sintering can be avoided. A drawback of print and fuse approaches however is that the scanning infrared fuse lamp that is typically used to fuse the powder delivers heat, per voxel, with an intensity profile that starts at a low intensity as the lamp approaches, and then rises as it passes above the powder to be fused. Peak intensity is achieved when the lamp is directly above the voxel to be fused, before decreasing again as the lamp moves away from the voxel. The intensity profile experienced over time by the voxel is typically Gaussian. As a result, powder particles are heated very slowly at first, which delays the time at which particle to particle coalescence can commence, and thus reduces the overall time for coalescence to occur. Therefore, powder bed fusion 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 flow chart of a method according to the invention for generating modified fusing slice data for a digital mirror device (DMD);

[0011] Fig. 2 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 and comprising a DMD; Fig. 3A illustrates a fusing slice;

[0012] Fig. 3B illustrates a fusing mask;

[0013] Fig. 3C illustrates a modified fusing slice based on the fusing slice of Fig. 3A and the fusing mask of Fig. 3B;

[0014] Fig. 4 illustrates a series of mask weights used to modify the pixel data of a fusing slice;

[0015] Figs. 5 A to 5D illustrate different stages of a scrolling DMD, in a variant of the apparatus of Fig. 2, applying modified slice data according to a variant of Fig. 4 to temporally control the fusing profile for each fusing voxel;

[0016] Fig. 6 illustrates a variant of the method of Fig. 1;

[0017] Fig. 7 illustrates a dynamic correction routine for modified slice data during a build process; Fig. 8 is a variant of the method of Fig. 1 comprising non-fusing slice data;

[0018] Figs. 9A to 9E illustrate different stages of a scrolling DMD, in a variant of the apparatus of Fig. 2, applying modified adjusted fusing slice data to spatially control fusing beam energies as well as applying modified non-fusing data; Figs. 10A to 10D illustrate different stages of a scrolling DMD, in a variant of Figs. 9A to 9E, applying modified adjusted non-fusing data to spatially control non-fusing beam energies;

[0019] Fig. 11 is a variant of the dynamic correction routine illustrated by Fig. 7;

[0020] Fig. 12A illustrates the example intensity profile obtained by Figs. 5 A to 5D;

[0021] Fig. 12B illustrates a variant of the profile of Fig. 12B; and

[0022] Fig. 13 illustrates intensity profiles obtained by conventional powder bed fusion apparatus. 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.

[0023] DETAILED DESCRIPTION

[0024] In the following, reference to “fusing energy” indicates an energy output of a level that is capable of ‘fusing’ build material by raising the temperature of the build material to or above the melting temperature. This energy output may be different for different wavelengths as provided by the radiative energy source providing the fusing energy to the build material. In this way, fusion of the build material over an object cross section may be achieved. On the other hand, reference to “non-fusing energy” indicates a comparatively lower energy output and / or a different wavelength, capable of raising the temperature of the build material without it reaching or exceeding the melting temperature, or the onset of melting. Herein, the melting temperature may in variants be defined by the onset of melting. Similarly, a “fusing energy beam source” as described herein 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 as described herein herein is a heat source configured 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] Herein, a reference to pixel is to the 2D data space of slices, wherein a voxel is the three dimensional control area that is addressed by at least one of the mirrors based on the 2D slice data. Due to the relatively small thickness of a layer, any heat energy applied by the heat sources above the layer is assumed to heat the entire voxel. Depending on the mode of operation, one or more mirrors may address each voxel. The voxel may extend over a rectangular area having sides measuring several 10s to 100s of micrometers. The layer may be several 10s to 100s of micrometers, or even several millimeters, thick.

[0027] In conventional print and fuse processes, fusing energy is provided by non-selective heat sources. Selectivity is achieved by printing absorber over each object cross section with one or more printheads which causes enhanced heating of the cross section compared to the surrounding area. 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. The direction of traverse is perpendicular to the direction of extension of the lamp. The intensity profile of the fusing radiation at which each voxel is addressed cannot be controlled finely - it relies on a gaussian shape over time, typically of the order of seconds, as determined by the speed and duty cycle of the lamp. Such a fusing intensity profile is illustrated in Fig. 13 which schematically shows a dashed curve of a fusing intensity profile as may be delivered by a print and fuse apparatus. Fine control in intensity is not possible along the direction over which the elongate lamp extends - all voxels extending along that direction are heated in the same way. During the fusion step, the surrounding powder is heated also by the non-selective lamp which may be an unwanted heating step. 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 the 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 effectively needs to be overheated to ensure it remains in a sufficiently low viscous state for long enough. A comparative intensity profile for laser sintering is illustrated by the solid line in Fig. 13. 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. Neither laser sintering nor print and fuse processes allows control over the irradiation intensity profile at which each voxel is heated over time.

[0028] To overcome present shortcomings of conventional powder bed fusion apparatus, provided is an improved power bed fusion apparatus according to the invention comprising a digital light projector (DLP) as primary selective fusing energy source, operated according to modified image slices (or “slice data”) based on original image slices modified by one or more masks. The masks modify the energy levels defined for the operation of the mirrors for each voxel. A method for generating modified slice data for the control of the DLP is also provided and will be described with reference to Fig. 1. The DLP may be provided instead of the conventional primary fusing lamp or in addition to a conventional fusing lamp. The DLP is arranged to provide fusing energy beams 410 to the layer, and comprises a digital mirror device (DMD) 240 and a light source 250, such as an infrared light source. The array of micromirrors of the DMD is arranged to receive the energy beam 450 from the light source. When a mirror is in an operative state, the mirror reflects a beamlet of the energy beam towards a corresponding voxel of the layer. 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 altering its angle with respect to the layer surface, or by altering the angle of the light source with respect to the array. The inventors have found that masks may be applied to slice data according to the method of the invention to generate modified slice data based on which the improved apparatus for the layerwise manufacture of objects from particulate material may be controlled to provide improved control over the fusing profile of the object voxels.

[0029] The apparatus 10 according to the invention is illustrated in a schematic cross section side view in Fig. 2. The apparatus comprises: a distribution device 290 arranged to distribute each layer over a build area; and the digital light projector (DLP) comprising a digital mirror device (DMD) 240, and a light source 250. The distributor may be a roller or a blade configured to spread an amount of build material over the preceding layer to form a new layer. The so-formed layer stack is supported on a platform that is lowered before each new layer is distributed by the distributor 290. The DMD 240 comprises an array of individually addressable micromirrors, each of which is arranged to redirect a light beam 450 received from the light source 250 either away from or towards the layer 160 based on slice data (or “slices”) comprising fusing slice data (or “fusing slices”), the fusing slices each defining an object cross section 150 for a corresponding layer 160. The controller 300 controls each micromirror by switching it between the operative and the inoperative state at a switching rate, or duty cycle as defined in the fusing slice data. The DLP is configured to redirect the light beam in a “scrolling mode” over the layer. The scrolling mode may be provided by the DMD 240 being configured to traverse over the layer, wherein the speed of the traverse is controlled to provide an irradiation duration for each voxel. Alternatively, the DLP may be configured to be adjustable in the angle of one or both of the micromirror array and the light source to sweep the redirect light beam across the layer, and wherein the speed of sweeping is controlled to provide the irradiation duration. Each object voxel may in this way be sequentially heated by a subgroup of micromirrors of the array extending along the fusing direction, which is defined by the direction of traverse or sweep of the DMD. The light beam 250 may be an infrared light beam, for example, or a light beam of a spectrum that is absorbed by the build material so as to heat it.

[0030] The apparatus comprises a controller 300 that is configured to: control the distribution device, or distributor, 290 to distribute each layer; and control the micromirror array to scroll across the layer 160 while controlling each micromirror based on modified fusing slices to selectively direct fusing energy beams 410 towards each object voxel within the object cross section 150. The modified fusing slices are based on the fusing slices and a fusing mask. The fusing slices comprise data defining an initial energy level for each object voxel; and the fusing mask defines modification data for each micromirror to modify the initial energy level of the fusing slices, wherein the modification data does not change for a given mirror while the DMD 240 traverses the layer. The fusing mask is associated with the reference frame of the micromirror array, and the modification data is defined such that the DMD 240 provides a predefined fusing intensity profile of variable fusing intensity to each obj ect voxel over an irradiation duration as defined by the fusing mask. The distribution of layers and selective fusing of object cross sections 150 in each layer is repeated to form a plurality of layers forming one or more objects. According to the fusing mask design that modifies the energy levels provided by the DMD 240, the apparatus and methods according to the invention disclosed herein enable fine control over the period over which coalescence can occur, without risking damage to the fused material. Furthermore, the melt temperature may be reached without delay compared to a print and fuse process. In addition, the intensity profile may be individually tailored to the material, and / or to the geometry of the object cross section 150 for example, to keep the material at each object voxel at a sufficiently high temperature to remain in a low viscosity state that ensures full coalescence between particles. This improves for example the mechanical strength of the object.

[0031] Masks to generate modified slice data

[0032] Typically, in static DLP projection, the energy applied to each voxel, which is the area associated with a single pixel of the image or slice being projected, 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 and / 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:

[0033] (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.

[0034] (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.

[0035] A method 100 for providing modified slice data for temporal control of the intensity provided to each object voxel for a DLP operable in a scrolling mode, is illustrated in Fig. 1, and further by Figs. 3A to 3C. Scrolling in this variant is provided by the DMD 240 being configured to traverse over the layer so as to sweep energy beams over at least an object cross section 150 within the layer.

[0036] At a block 110, image slice data (or image slices) is provided which defines a two- dimensional matrix of pixels and associated information. The pixels of a slice map correspond to a voxel of a corresponding layer to be processed. A slice comprises at least data to identify those pixels corresponding to object voxels that are to be fused to form an object cross section. Furthermore, each such “fusing pixel” may be associated with data for the operation of a corresponding mirror of the DMD 240. Such data may comprise an energy level, e.g in the form of a duty cycle, at which each mirror is to be operated. In a simplest variant, the duty cycle may be initially the same for all fusing pixels and is to be modified by applying a fusing mask.

[0037] A simple example of a slice 600 comprising a plurality of fusing pixels defining an object model cross section 650 is shown in Fig. 3A. The area surrounding the object model cross section 650 is anon-fusing slice area 630 defining “non-fusing areas” over which voxels in the layer are not to fuse. The slices are prepared typically from object 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 intensity profile to each fusing voxel using the DMD, at block 130 of Fig. 1 a fusing mask 500 as illustrated by way of example in Fig. 3B is provided and applied as follows. The fusing mask comprises data to modify that of the fusing pixels of each slice. In Fig. 3B, 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 700 of Fig. 3C illustrates the modified model cross section 750 over which the modified energy level now gradually increases from left to right. This is further illustrated further in Fig. 4, 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 500R 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 240, 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%.

[0038] Masks and DLP scrolling mode for intensity profile control

[0039] Instead of using the mask to modify the slice data such that it is associated with the reference frame of the layer, the mask may be associated with the reference frame of the DMD. The weights may therefore map onto a corresponding micromirror instead. The row Pn of Fig. 4 may instead indicate 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 scanning or scrolling mode, such that it heats a successive band of voxels along the x-direction, this achieves spatial control over the fusing intensity profile provided to each voxel.

[0040] This is illustrated in more detail in Figs. 5A to 5D, 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. 2. During its traverse at a constant speed v, the DMD addresses an object cross section 150 progressively with six mirrors of the 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 500 defining the weights a inset at the top left of Fig. 5A, 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 240, 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 700 corresponding to the position at which the DMD is operated above the layer. The modified slice 700 is identified by the time step at which is it applied, where the time difference between successive mirrors providing fusing energy beams 410 to a given voxel remains constant since the speed of the DMD 240 is constant. The scale of the x-axis of the layer 160 is not the same as that of the modified slice 700 since modified slice 700 shows only an extract to focus on the modified pixels. The unmodified fusing slice data 600 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 modifed 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.

[0041] In Fig. 5A, 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 240 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 700(tl).

[0042] 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 700(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.

[0043] 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 as defined by the modified fusing slice 700(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.

[0044] 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, such that the first voxel is now heated by the sixth mirror modified at a=0.4 at the trailing edge of the array, and operated at 40% duty cycle. The fusing energies are defined by the modified fusing slice 700(t6). 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.

[0045] A linear intensity 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. 12A 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 5A to 5D are also included in this Fig. 12A. This is a simple illustration of how a linear intensity profile may be achieved applying the method according to the invention.

[0046] 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 240 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. Instead of moving the DMD over the layer, the angle of a fixedly arranged DMD and / or the angle of light source 250 may be varied to achieve a similar scrolling effect. While Figs. 5A to 5D illustrate, by way of example, six different duty cycles defined by a linearly varying fusing mask fixed to the DMD reference frame, any fusing intensity profile shape may be defined to control the fusing energy for each fusing voxel over the duration of irradiation when applied in combination with a scrolling DMD. This provides a simple process that allows full control over the fusing intensity profile for each fusing voxel of the layer. By tuning the temporal fusing intensity profile for the fusing voxels, improved mechanical properties may be provided that are not achieveable with conventional powder bed fusion apparatus.

[0047] An improved intensity profile is shown in Fig. 12B, which may be achieved by designing a suitable mask 500 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. 5A to 5D. 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.

[0048] Returning to Fig. 1, at block 130 therefore, modification data for each micromirror of the digital mirror device may be defined, such that when the fusing mask is applied to each fusing slice to generate modified fusing slices, each object voxel, upon irradiation by the digital mirror device, receives over the irradiation duration a predefined fusing intensity profile from a subgroup of the plurality of micromirrors during the traverse of the scrolling DMD. The irradiation duration is defined by the length of the array along x and the speed of the traverse over the layer. Modified fusing slices are generated by combining the fusing mask, for example associated with the reference frame of the DMD 240 or micromirror array, with the initial fusing slice data. The modified data may be generated within the apparatus upon provision of the mask and slice data, or it may be generated during the image processing stage and provided in modified form to the apparatus. At block 190, the apparatus is controlled to apply the modified slice data to manufacture the obj ect based on the modified slice data.

[0049] The fusing mask may be designed to provide a predefined fusing intensity profile with an asymmetric shape for each object voxel of the layer. The irradiation duration may comprise an initial irradiation interval to increase the temperature of each object voxel to above the melting point, and a second irradiation interval to maintain the temperature of each object voxel above the melting temperature. Optionally, the second irradiation interval may be longer than the initial irradiation interval, wherein the intensity of the energy beam provided to each object voxel over the second irradiation interval may be sufficient to allow the build material to fully coalesce.

[0050] In addition to a “temporal” fusing mask (a fusing mask achieving temporal intensity control for each object voxel by being associated with the reference frame of the DMD), the controller may control the DMD 240 further based on a further, “spatial”, fusing mask (a fusing mask achieving spatial intensity control across the layer by being associated with the reference frame of the layer). This allows spatial compensations to be applied to the fusing voxels depending on their location within the layer, to compensate for spatial variations in temperature of the object cross sections to be irradiated. Fusing masks defining spatial compensations over the layer, and which are associated with the reference frame of the slice (or layer), may be provided such that varying energy levels can be individually assigned along 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, or layers, to ensure that the variation of energy applied is optimal for each individual object cross section, and / or non-fusing area.

[0051] Thus, multiple masks may be defined that are associated with different reference frames. In addition, or instead, multiple fusing masks, for example defining different fusing intensity profiles and associated with the reference frame of the micromirror array, may be provided that modify the energy level provided by each mirror of the DMD differently for different layers. A different fusing mask may be selected from the plurality of fusing masks to modify different groups of fusing slices. For example, the fusing intensity profile may be varied depending on the geometry of the object cross section. Object voxels within a narrow object cross section may receive a relatively higher intensity over the second interval of the fusing profile intensity, compared to object voxels within a large blocky object cross section to compensate for a more rapid heat dissipation from the narrow object cross section.

[0052] Adjusted fusing slices

[0053] The fusing slice data may not define a constant initial fusing energy level for all pixels. Instead, it may comprise data defining a spatial distribution of fusing energy levels, e.g. in terms of duty cycle, to be provided to each object voxel. The fusing slices may be adjusted fusing slices defining two or more energy levels for fusing energy beams 410 to be provided to each object voxel, and wherein the fusing mask is configured to modify the adjusted fusing slices. For example, individual duty cycles for each fusing pixel may be determined based on one or more of: the shape and / or area of the object model cross section; the shape and / or volume of one or more preceding object model cross sections; the shape and / or volume of one or more subsequent object model cross sections; and the distance to a neighbouring object model cross section and / or to a boundary wall of the envelope defining the slice stack, for example. In this way, any 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 volumes deposited per unit area. The more absorber is deposited over a certain voxel, the more energy is absorbed by the voxel from the heat source, and the hotter the voxel gets during fusing. Similarly, the duty cycle per pixel 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.

[0054] With reference to Fig. 6, which is a variant of the method 100 illustrated by the flowchart of Fig. 1, the fusing slice data at block 110 may be adjusted in a further step at block 120 to define spatially varying energy levels per slice for each pixel of an object model cross section. This may be done using a simulation routine that models thermal impacts on each object cross section to be built based on its environment, determines deviations of the modelled solution from a predefined target temperature, and adjusts the duty cycle for some or all object pixels to reduce such unwanted deviations. This results in adjusted fusing slice data that is modified by the fusing mask provided at block 130 to generate the modified adjusted fusing slice data at block 140. Following the generation of modified slice data, the modified slice data may be provided to the apparatus to build the object. Masks associated with the reference frame of the layer

[0055] In addition to any of the variants described, a further mask may be defined that is associated with the reference frame of the layer. In this way, the mask may be defined to compensate for local and persistent variations from an intended thermal profile of the fused object cross sections and / or non-fusing area. For example, the design or operation of the apparatus may generate inhomogeneous heating or cooling effects on the layer that create non uniform effects on the fusing temperature where the layer comprises one or more object cross sections. As an example there may be a process gas flow along the negative x-direction that induces an increasing colder thermal profile along the direction of distribution of each new layer. A mask similar to mask 500 of Fig. 3B may be applied to compensate at least in part for this effect in the modified fused sliced data when applied with respect to the reference frame of the layer. The overall fusing energy may thus be increased from left to right in analogy to the modification of pixel energies described in Fig. 4, where in this case the mask modifies the energy levels from left to right on the layer. A similar mask may also be provided to non-fusing energy beams 420 applied to warm the layer without causing fusing as will now be described. In any of the above, a validation routine may be applied to determine whether the minimum fusing energy level required to fuse a voxel is still achieved, following application of the fusing mask, and optionally of the correction mask. If the validation routine determines that the lowest modified energy level is too low to achieve fusion, the fusing mask and / or adjusted fusing slice data may require adjustment to raise the lowest modified energy level to a fusing energy level.

[0056] Non-fusing energy beams provided by the DLP

[0057] In addition to defining different energy levels for the object cross sections, different nonfusing energy levels may be defined for the areas of the layer that are not to fuse (herein referred to as “non-fusing areas”). The different non-fusing energy levels may be defined by applying a mask fixed to the reference frame of the layer. This may be used to compensate for process or equipment dependent variations in temperature of the non-fusing areas. For example, it may be determined for a certain process that the layer temperature generally increases from left to right. A predetermined variation may be compensated for by applying a mask defining weights for each pixel location that are to be applied to a corresponding non-fusing pixel. The compensation mask may be the same throughout the build process, or it may be varied according to thermal measurements similar to the dynamic corrections described for the fusing slice data with reference to Fig. 7. The image slice data may therefore further comprise non-fusing slice data as part of or derived from the image slice data. The non-fusing slice data defines non-fusing areas outside the object cross section, such as the location and initial non-fusing energy level of non-fusing pixels outside the object model cross section. The controller 300 may be configured to control the DMD 240 to provide non-fusing energy beams 420 to non-fusing voxels of non-fusing areas of the layer according to the non-fusing slices. The non-fusing energy levels may be defined so as to heat some or all voxels of the non-fusing area of the layer. Where all non-fusing voxels are to be heated, the energy levels defined by the non-fusing slice data may be such that a predefined uniform non-fusing target temperature, i.e. below the melting temperature, is achieved across the non-fusing area of the layer.

[0058] To compensate for temperature deviations that persist over significant areas of the unfused area, layer after layer, for example due to the left to right thermal variations or thermal nonuniformities in the apparatus, the non-fusing mask may be associated with the reference frame of the layer. The non-fusing mask defines per-pixel energy level modification data for non-fusing pixels for one or more layers, such as by defining per-pixel non-fusing weights. The mask may be defined for example in terms of a matrix of factors to be multiplied with the corresponding energy levels of the non-fusing slice data, e.g. from 0 to 1 as described for the fusing mask. The digital mirror device may thus be controlled based on modified non-fusing slices generated from the non-fusing slices and the non-fusing mask.

[0059] Fig. 4 may be used to illustrate a mask factor varying over a rows of pixels as shown, which in this case may represent non-fusing pixels. If the initial non-fusing energy (for example at a different wavelength or at a generally lower intensity to that of the fusing energy) is defined at 100% duty cycle in the non-fusing slice data, the non-fusing mask modifies the initial non-fusing energies according to the mask factor at corresponding pixel locations, wherein the non-fusing mask is fixed to the reference frame of the layer. Each modified non-fusing slice therefore defines modified non-fusing energy beams 420 that increase in energy from left to right, for example to compensate for an apparatus inhomogeneity that induces higher temperature towards the left side of the layer.

[0060] Fig. 8 illustrates a variant of the method 100 of Fig. 2, the blocks of which have been given the suffix “A” in Fig 8, and therefore are labelled blocks 110A to 140A. The analogous blocks for the provision of the modified non-fusing slices are given the suffix “B” and are similar in concept and application. Additional non-fusing slices are provided at block HOB. At block 130B, a non-fusing mask is provided, defining a spatial non-fusing energy profile across the layer and fixed to the reference frame of the layer. Modified non-fusing slice data is generated at block 140B from the non-fusing slice data and the non-fusing mask. At common block 190, the DMD may be controlled to provide both fusing energy beams 410 and non-fusing energy beams 420 based on the non-fusing slice data and the spatial nonfusing mask. When the modified non-fusing slice data is used to control the DMD, a nonfusing energy profile is applied over voxels of some or all of the non-fusing area, so as to control the spatial temperature profile of the non-fusing area. The provision of blocks HOB to 140B may be sequential or applied in parallel to blocks 110A to 140A. The fusing slices and non-fusing slices may be handled separately throughout the image processing method 100, and they may be applied separately to the DMD 240 during the build process 190, or they may be combined and provided to the DMD as combined slice data. The DMD 240 may be controlled based on the combined or separate slice data to provide fusing and nonfusing energy beams concurrently to each layer. The modified slice data may be generated in a single step to modify the slice data based on the fusing mask and based on the nonfusing mask. For example, the pixels of each slice may comprise an identifier, or address, that identifies them as a fusing or non-fusing pixel. To generate the modified slice, the fusing mask may be applied to the fusing energy levels defined by corresponding fusing pixels and the non-fusing mask may be applied to non-fusing energy levels defined by corresponding non-fusing pixels. Any calibrations relating for example to the brightness and / or duty cycle “grey level” for individual mirrors may be performed on a combined data slice. This may further save memory and lead to faster image processing operations.

[0061] Dynamic corrections to modified slice data

[0062] The modified slice data may require dynamic corrections as may be determined from time to time during the build process 190. The apparatus 10 may comprise a thermal sensor such as thermal sensor 280 shown in Fig. 2, 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 data slice, 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. Alternatively, a second fuse step may be determined to be carried out by the DMD 240 to fine tune the temperature across the object cross section. The step of processing the modified slice data at block 190 may further comprise a decision flow involving dynamic corrections based on in-process thermal measurements.

[0063] A dynamic correction routine 1700 is illustrated in Fig. 7, which is a further variant of Fig. 1 and may be applied in addition to or instead of the variant of Fig. 6. The dynamic correction routine 1700 may be applied for some or all layers, for example after the step of fusing of the previous layer. The routine comprises at block 1710 a step of measuring temperature of the layer and generating a thermal profile for the object cross section based on the measured temperatures. By comparison of the generated thermal profile against a target thermal profile for the object cross section, it may be determined whether the measured thermal profile deviates by more than a predefined threshold. If the threshold is not exceeded, the modified slice data is selected, or provided, at block 1720 to fuse the next layer at block 1900. If the threshold is exceeded, the modified slice data is not used, and a correction is determined at block 1730 instead, for example in the form of a correction mask. The correction is applied to the modified fusing slice data in at least one of the next layers at the step of fusing at block 1900 to compensate for the determined deviation. The correction may be determined for a subsequent layer that is not the immediate next layer, and applied to the relevant modified slice data for that subsequent layer. The correction determination may only be triggered once a deviation has been determined repeatedly and with a consistent trend for a predefined number of successive layers. Before it is triggered, the modified slice data may be applied. In other words, block 1710 may comprise measuring layer temperature and determining over a predefined number of layers that the measured layer temperature repeatedly exceeds a predefined threshold in the same or similar manner, e.g. over the same region and of similar or growing magnitude in temperature, for the predefined number of layers. Upon determining that the measured layer temperature repeatedly exceeded the predefined threshold over the predefined number of layers, the routine proceeds to block 1730 and a correction for the modified slice data for a subsequent layer is determined.

[0064] A correction routine 1700 may be defined for correcting modified non-fusing slices, similarly to that for the modified fusing slices. The thermal sensor 280 may be arranged to measure temperatures of the layer, and the controller 300 may be configured to: receive dynamic correction data based on the measured temperatures to correct the modified nonfusing and / or fusing slice data, and control the DMD based on the corrected modified nonfusing and / or fusing slice data. This may be done for the modified non-fusing slice data at subroutine 1700 of Fig. 7, where the image slice data may be fusing slice data and / or nonfusing slice data. The dynamic corrections to the non-fusing energies may be defined based on the per-voxel temperature map of the distributed layer, measured before the step of fusing is applied by the DMD. The method 100 may thus further comprise: providing the nonfusing slice data and the non-fusing mask, optionally in the form of the modified non-fusing slice data, to the apparatus to commence forming the object at block 190, by processing one or more layers 160 from modified non-fusing slice data. At block 1710, temperatures of the layer are measured using the thermal sensor, for example after the step of fusing is applied by the DMD 240. The measured temperatures may be used to determine a temperature profile for the non-fusing area for one or more layers. From the measured temperature profile(s), a deviation from a predefined temperature profile for the non-fusing area is determined. Upon determining that the deviation exceeds a predefined range and / or size, for example consistently over a predefined number of layers, the method proceeds to block 1730. Before such determination, the method proceeds to block 1720 and the modified nonfusing slice is applied. At block 1730, dynamic correction data is provided for one or more of the modified non-fusing slices, for example in the form of a correction mask. The dynamic correction data may be provided for the modified slice of a subsequent layer which may not be the next layer. As described for the correction for the modified fusing slice data, the correction to the modified non-fusing slice data may be determined and applied similarly, albeit compared to different non-fusing target temperatures. Next, modified non-fusing slice data is generated based on the correction mask, and the modified non-fusing slice data. The correction mask may be associated with or fixed to the reference frame of the layer. For example, it may be applied to fine tune the spatial energy profile to be provided by the nonfusing energy beams 410 in a subsequent layer.

[0065] The modified slice data to be dynamically corrected may be the modified fusing slice data and / or the modified non-fusing slice data. The slice data applied at block 1900 for a subsequent layer is the corrected modified slice data. This is illustrated in Fig. 11 which is a variant of Fig. 7 showing the two dynamic correction processes in parallel, wherein the correction routine 1700 comprises a first subroutine to select or correct the modified fusing slice data based on thermal measurements of the fused cross sections in blocks 1710A to 1730A, and a second subroutine to select or correct the modified non-fusing slice data based on thermal measurements of the non-fused area in blocks 1710B to 1730B. The description of Fig. 7 and the considerations made with regard to corrections to modified fusing slice data apply in the same way to dynamic corrections determined and applied to the modified non-fusing slice data, wherein the respective target temperature profiles are non-fusing target profiles for the non-fusing areas of the layer. Blocks 1710A and 1710B comprise the steps of block 1710 in Fig. 7, as described herein, of measuring and determining a temperature map for the fused cross sections and / or the non-fusing areas to determine if either deviates by more than a predefined threshold. The temperature measurements at blocks 1710A and 1710B may be made at different and / or multiple times of the layer process. The measured temperature at block 1710 may comprise the temperature of the layer following the provision of fusing energy to at least part of the layer, and the different nonfusing energies are corrected based on the per-voxel temperature map of the layer following the provision of fusing energy.

[0066] Adjusting non-fusing slice data before applying a mask

[0067] In addition to defining a non-fusing mask to alter the non-fusing energy level of the nonfusing slice data, variable non-fusing energy levels for some or all of the non-fusing pixels may be defined in the non-fusing slice data similarly to “greyscaling” of the fusing energy slice data. Different non-fusing energies may be determined based on one or more of the fusing slice data, the shape of the object model cross section, the area of the object model cross section, a closest distance to an adjacent model cross section, and a location of the object model cross section with respect to a boundary of the slice stack. For example, the variable non-fusing energy levels may be based on one or more of, with respect to the build volume rather than the slice stack: (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;

[0068] (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

[0069] (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. When the three options are applied in combination, this may result in a fused object enveloped in a powder support that is heated to a higher level near the object surface than elsewhere in the build volume. This helps to slow down heat loss from the object and reduces warping and distortions during cooling.

[0070] Spatially and temporally varying fusing and non-fusing energy slices may be applied in parallel, or concurrently, as the DMD 240 is controlled to apply energy beams to the layer.

[0071] Figs. 9A to 9E illustrate different stages during the co-application of fusing energy beams 410 and non-fusing energy beams 420 by the DMD 240 as it traverses a layer to fuse an object cross section 150. The modified, combined slice data may be applied to operate the DMD. For example, in Fig. 9A, the DMD is controlled based on the non-fused modified slice data. The modified slice data for the non-fusing pixels may be based on a mask fixed to the reference frame of the layer and applying weights to each pixel to achieve a predefined spatial distribution of energies in the modified non-fusing energy beams 420. The non-fusing energy beams 420 may be modified to provide an decreasing level of energy from left to right to the layer to compensate for a left to right temperature decrease that may have been determined empirically.

[0072] Fig. 9B shows the DMD beginning to provide fusing energy beams 410 to the defined cross section 150, by mirrors at or neat the leading edge of the DMD 240. The remaining mirrors may be operated to provide non-fusing energy beams according to the modified non-fusing pixel data for that layer. The modified fusing slice data may have been generated according to the method described with reference to Figs. 5 A to 5D, such that a temporal fusing energy profile of a predetermined shape is provided to each object voxel.

[0073] In addition, before applying the fusing mask, the fusing slice data may have been adjusted to comprise varying levels of “greyscale” energy, in terms of varying duty cycles, for example. This is indicated in Fig. 9C, in which the DMD addresses the entire cross section 150 with variable fusing beam energies 410 as indicated by the arrow thicknesses. The central fusing energy beam is shown to be thinner, and of lower energy, compared to the thicker fusing energy beams applied to the inner peripheral area of the cross section. In addition, the fusing energy profile is controlled according to for example the fusing energy profile shown in Fig. 12, by providing a suitable mask fixed to the reference frame of the DMD to modify the adjusted fusing slice data.

[0074] Fig. 9D shows the last two fusing energy beams provided to the far side of the object cross section 150, while the remaining mirrors downstream are operating in non-fusing mode according to the mask defined and described with reference to Fig. 9A. Fig. 9E completes the traverse of the layer and is similar in operation to Fig. 9A. Instead of combined slice data, it may be preferable to provide the modified fusing data slices and the modified nonfusing data slices separately. This may allow simpler operations to apply any dynamic corrections as described herein to one or both of the modified slices.

[0075] The process of fusing with the DMD in Fig. 9A to 9E may be controlled to follow the distributing device (not shown in Fig. 9A to 9E). The controller 300 may be arranged to control respective delays at each object voxel of the layer between the steps of: distribution and fusing; fusing and distribution; and, where present, heating by providing non-fusing energy beams and distribution; such that the delays are the same as experienced by each voxel of the layer.

[0076] In a further example of providing both fusing and non-fusing energy beams by the DMD, Figs. 10A to 10D illustrate a variant of Fig. 9A to 9E in which non-fusing energy beams 420 of different energies are provided along with fusing energy beams 410. The non-fusing energies are defined by adjusted non-fusing slice data so as to provide non-fusing energy beams 420H of higher non-fusing energy to a peripheral region 130P surrounding the object cross section 150 compared to lower non-fusing energy of non-fusing energy beams 420L provided to voxels of the remaining non-fusing area 13 ON outside the peripheral region I 30P. To avoid complexity in these Figures, the fusing energy beams are not specifically shown apart from being labelled as fusing energy beams 410 as part of a section of the DMD, and may be applied as described for fusing energy beams 410 applied to the object cross section 150 in Fig. 9A to 9E.

[0077] Fig. 10A shows the DMD in a first position at the start of its traverse, addressing the remaining non-fusing area 130N with low energy non-fusing beams 420L. In Fig. 10B, the DMD is in a second position and nearing the object cross section and begins to heat the peripheral area 13 OP with higher non-fusing energy beams 420H while applying lower nonfusing energy beams 420L to a portion of the remaining non-fusing area 13 ON upstream of the DMD 240. The higher non-fusing energy beams 420H may gradually increase in energy towards the periphery of the object cross section 150 that is being fused as indicated by the black infill, as indicated by the thickness of the arrows representing a first higher non-fusing energy beam 420H1 and a second higher non-fusing energy beam 420H2 higher than the first. In Fig. 10C, the DMD is nearing the end of its traverse over the object cross section. It has applied fusing energy beams to all of the object cross section 150, which is fused as indicated by the black infill. The leading edge of the DMD is now applying non-fusing energy beams 420H of higher non-fusing energy to the peripheral area 130P, and non-fusing energy beams 420L of lower non-fusing energy to the remaining non-fusing area 130N. The non-fusing energy beams 420H of higher non-fusing energy increase in energy towards the periphery of the cross section 150, as before. Therefore, the thickness of the arrows represent a first higher non-fusing energy beam 420H1 and a second higher non-fusing energy beam 420H2 higher than the first.

[0078] The adjusted non-fusing slice data defining the variable energy levels of the non-fusing energy beams may be further modified by a non-fusing mask, for example by a mask associated with the reference frame of the layer to compensate for spatial variations throughout the build. The controller 300 may be arranged to control the scrolling mode such that the DLP progressively provides fusing energy beams, and optionally non-fusing energy beams, to the layer along a fusing direction in synchronisation with the motion of the distribution device, wherein the fusing direction is along the direction of distribution. As has been described herein, masks may be defined to generate temporal and optionally spatial variations in the heating intensity per voxel. This may be applied to either or both fusing or non-fusing pixels, wherein the mask is associated with the reference frame of a DMD arranged to operate in scrolling mode over the layer. In addition, or instead, masks may be defined to generate spatial variations in the heating energy per voxel. This may be applied to either or both fusing or non-fusing pixels, wherein the mask is associated with the reference frame of the layer.

[0079] General considerations

[0080] The infrared digital light projector may comprise a first energy beam source to provide fusing energy beams and a second energy source to provide non-fusing energy beams. The two energy beam sources may operate at different wavelengths. The energy source(s) may be an infrared laser, for example. The DMD 240 may comprise a single 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 herein. Alternatively, the DMD and / or the energy beam source 250 may be mounted to a galvanometer for example, and the controller may be configured to vary the angle of the galvanometer with respect to the layer surface. The digital light projector may comprise a plurality of DMD arrays, 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 sub-slices 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. In variants, the infrared digital light projector may comprise a first micromirror array arranged to preheat the build area at block 520B, behind the distribution device with respect to the direction of distribution, and a second DMD array arranged to fuse at block 550, and / or to preheat at block 520C, the build area ahead of the next step of distribution at block 510. Each DMD may be controlled based on modified slice data or modified adjusted slice data as described herein.

[0081] Fig. 2 shows an apparatus 10 that may optionally comprise an overhead heater 270 arranged above the build area and configured to maintain the layer at a predefined target layer temperature. This may be different to local target temperatures defined for the non-fusing area. Such an overheat 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 for example comprise four or more heater elements, 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 Fig. 7 and Fig. 11, the thermal sensor may measure temperatures of the layer at block 1710 after the distribution of the new layer. The overhead heater 290 may be controlled continuously to maintain the layer temperature at the average layer target temperature throughout the layer cycle. The micromirror array of the DMD may be controlled to provide non-fusing energy beams to the distributed layer based on temperature measurements made after the step of distributing each new layer, such that corrections to the modified non-fusing slices are determined as described for block 1730B.

[0082] In variants, the apparatus 10 may comprise a preheating energy source arranged to preheat each layer immediately following distribution, by providing a non-selective preheating energy to the layer, and wherein the measured temperature comprises the temperature of the layer following the provision of the non-selective preheating energy. Such a preheating energy source may be a near infrared lamp arranged to follow behind the distribution device 290 while preheating the fresh layer. This reduces the power at which the DMD is operated to apply fusing and optionally non-fusing energy beams subsequently.

[0083] 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 300 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 objects from particulate material, the apparatus comprising: a distribution device arranged to distribute a layer over a build area; a digital light projector operable in scrolling mode over the layer and comprising a digital mirror device and a light source, wherein the digital mirror device comprises an array of individually addressable micromirrors each of which is arranged to redirect a light beam received from the light source away from or towards the layer based on image slices, wherein some or all of the image slices comprise a fusing slice defining a fusing energy level for each of a plurality of object voxels to be fused to form an object cross section within the layer; and a controller configured to:- control the distribution device to distribute the layer; and- control the digital light projector to scroll redirected light beams across the layer while controlling each micromirror to selectively direct fusing energy beams towards each object voxel within the object cross section based on modifying each fusing slice by a fusing mask; wherein the fusing mask is associated with the reference frame of the micromirror array and defines modification data for each micromirror to modify the initial energy level of each fusing slice, such that each object voxel is provided with a predefined fusing intensity profile over an irradiation duration.

2. The apparatus of claim 1, wherein, to provide the scrolling mode, the digital mirror device is configured to either: traverse over the layer, wherein the speed of the traverse is controlled to provide the irradiation duration; or, to adjust the angle of one or both of the micromirror array and the light source to sweep the redirect light beam across the layer, and wherein the speed of sweeping is controlled to provide the irradiation duration.

3. The apparatus of claim 1 or claim 2, wherein the predefined fusing intensity profile is asymmetric and comprises an initial irradiation interval to increase the temperature of each object voxel to above the melting point, and a second irradiation interval to maintain the temperature of each object voxel above the melting temperature.

4. The apparatus of claim 3, wherein the second irradiation interval is longer than the initial irradiation interval and an energy level provided to each object voxel over the second irradiation interval is sufficient to allow the build material to fully coalesce.

5. The apparatus of any preceding claim, wherein one or more of the fusing slices are modified by a second fusing mask associated with a reference frame of the layer.

6. The apparatus of any preceding claim, wherein the fusing slices are adjusted fusing slices defining two or more energy levels to be provided to each object voxel, and wherein the fusing mask is configured to modify the adjusted fusing slices.

7. The apparatus of any preceding claim, wherein the slices further comprise non-fusing slices defining non-fusing areas outside the object cross section, and wherein the controller is configured to control the digital mirror device to provide non-fusing energy beams to the non-fusing areas so as to heat some or all of the non-fusing area to a predefined temperature below the melting temperature.

8. The apparatus of claim 7, wherein the controller controls the digital mirror device further based on a non-fusing mask for each layer, wherein the non-fusing mask is associated with the reference frame of the layer and defines non-fusing modification data for each nonfusing slice pixel, such that the digital mirror device is controlled based on modified nonfusing slices.

9. The apparatus of claim 7 or claim 8, wherein the non-fusing slices further comprise non-fusing energy level data for some or all non-fusing slice pixels so as to define a spatial variation in initial non-fusing energy levels, wherein the non-fusing energy levels are defined within the non-fusing area to be different over one or more of:(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;(b) an overlap region that is to overlap with an object model cross section of a subsequent slice; and(c) a covering region that overlaps with an object model cross section of a previous slice.

10. The apparatus of any preceding claim, further comprising a thermal sensor arranged to measure temperature of the layer, wherein the controller is configured to: receive dynamic correction data based on the measured temperatures to correct one or more of the modified slices, and to control the digital mirror device based on the one or more corrected modified slices.

11. The apparatus of any preceding claim, wherein the controller is arranged to control respective delays at each object voxel between the steps of: distribution and fusing; fusing and distribution; and, where present, heating and distribution; such that the delays are the same for each voxel.

12. The apparatus of claim 11, wherein the controller is arranged to control the scrolling mode such that the digital light projector progressively provides fusing energy beams, and optionally non-fusing energy beams, to the layer along a fusing direction in synchronisation with the motion of the distribution device, wherein the fusing direction is along the direction of distribution.

13. The apparatus of any preceding claim, wherein the digital light projector comprises a first light source to provide a fusing light beam and a second light source to provide a nonfusing light beam.

14. A method of providing energy beam control data for a digital light projector of an apparatus for the layerwise manufacture of 3D objects from particulate build material, wherein the digital light projector comprises a digital mirror device and a light source and is operable in a scrolling mode, the method comprising: providing slices comprising fusing slices, each fusing slice defining object pixels within an object model cross section for each layer of the object to be formed; and providing a fusing mask defining modification data for each micromirror of the digital mirror device, such that when the fusing mask is applied to the digital mirror device, each fusing slice is modified by the fusing mask to generate modified fusing slices, such that each object voxel, upon irradiation by the digital light projector operated in scrolling mode, receives a predefined fusing intensity profile from a subgroup of the plurality of micromirrors.

15. The method of claim 14, comprising a step of adjusting the fusing slices to define variable fusing energy levels within each object model cross section; wherein the modified fusing slices are based on the adjusted fusing slices being modified by the fusing mask.

16. The method of claim 14 or claim 15, wherein the fusing mask comprises a plurality of fusing weights, wherein each weight is associated with a corresponding mirror of the digital mirror device.

17. The method any one of claims 14 to 16, wherein a plurality of fusing masks are provided and wherein the method comprises selecting a different fusing mask from the plurality of fusing masks to modify different fusing slices.

18. The method of any one of claims 14 to 17, further comprising providing non-fusing slices based on the slices, wherein the non-fusing slices define non-fusing pixels outside of the object model cross sections and anon-fusing energy level for each non-fusing pixel.

19. The method of claim 18, comprising providing a non-fusing mask to be associated with the reference frame of the layer, wherein the non-fusing mask modifies one or more of the non-fusing slices so as to define spatial modifications to the non-fusing levels to be applied by the digital mirror device to a corresponding one or more layers.

20. The method of claim 18 or claim 19, comprising generating modified non-fusing slices based on the non-fusing mask and the non-fusing slices, optionally before the modified nonfusing slices are provided to the apparatus.

21. The method of any one of claims 18 to 20, wherein the modified non-fusing slices are combined with the modified fused slices to generate modified slices, optionally before the modified slices are provided to the apparatus.

22. The method of any one of claims 18 to 20, wherein the slices comprise pixel identification data to define the fusing pixels and the non-fusing pixels, and wherein the fusing mask is used to modify the fusing energy level defined by the fused pixels, and the non-fusing mask is used to modify the non-fusing energy level defined by the non-fusing pixels based on the pixel identification data.

23. A method of controlling an apparatus for the layerwise formation of 3D object from particulate material, the method comprising: processing with a digital light projector in scrolling mode one or more layers from modified slices, the modified slices comprising modified fusing slices as provided by any one of claims 14 to 23, and optionally modified non-fusing slices as provided by any one of claims 15 to 23; measuring object cross section temperatures of the layer; generating a thermal profile for the object cross section based on the measured temperatures; determining that a deviation between the generated thermal profile deviates from a target thermal profile is larger than a threshold deviation; providing dynamic correction data based on the determined deviation; generating a corrected modified fusing slice based on the dynamic correction data and the modified fusing slices for a subsequent layer; and processing the subsequent layer based on the corrected modified fusing slice.

24. The method of claim 22 or claim 23, comprising the modified non-fusing slices, wherein the method comprises: measuring non-fusing area temperatures of the layer; determining a non-fusing temperature profile for the non-fusing area for one or more layers; determining a non-fusing deviation of the measured temperature profile from a nonfusing target temperature profile; determining that the non-fusing deviation exceeds a predefined non-fusing threshold; determining dynamic non-fusing correction data, optionally in the form of a spatial correction non-fusing mask; generating a corrected modified non-fusing slice based on the dynamic non-fusing correction data and the modified non- fusing slices for a subsequent layer; and processing the subsequent layer based on the corrected modified non-fusing slice.

25. The method of claim 23 or claim 24, wherein the digital light projector is controlled based on the modified fusing slices and the modified non-fusing slices to provide fusing and non-fusing energy beams concurrently to each layer.

Images