Method for generating data for an embedded pattern for use in powder bed fusion

Abstract

Provided is a method of generating object model data comprising embedded pattern data e.g. a QR code, to cause an embedded pattern to be applied to a 3D object formed by layerwise fusion of particulate build material, the method comprising generating object model data in the form of initial slices, each slice defining one or more object pixels to be processed in a corresponding layer to form a cross section of the 3D object and each object pixel defining an amount of absorber to be deposited over a corresponding object voxel within a respective layer. Modified object model slices are generated from the initial object model slices to include embedded contrast pattern pixels by defining at least one ultimate group of object pixels comprised within the object model surface to define corresponding at least one group of encapsulating object voxels and at least one penultimate group of object pixels overlapping with the ultimate group of pixels, wherein the penultimate group of object pixels comprises at least a first subgroup and second subgroup of object pixels that together define the embedded contrast pattern. The amount of absorber of the first group is set to at least a first amount of absorber and the amount of absorber of the second subgroup is set to either to zero or to a second amount lower than the first amount. Further provided is a processor configured to carry out the method and an object formed from the object model data generated according to the method.

Description

FIELD OF THE INVENTION The present disclosure relates to methods of generating bitmap data for applying a readable pattern to 3D printed objects formed by layerwise fusion of particulate build material. BACKGROUND Additive manufacturing is reaching industrial capabilities and the volume of parts that can be manufactured in a single build is increasing. Consequently, there is a desire to apply information to the manufactured parts, in the form of, for example, labels or graphics, to allow efficient identification, traceability or other management of the individual parts following their extraction from the apparatus. Labels or graphics applied to parts made by powder bed fusion, in which objects are built layerwise from particulate material, are typically embossed or debossed onto the part. This changes the shape and / or surface quality of the part. For print and fuse applications in which a dark colour absorber, such as carbon black, is applied to create selectivity for location specific fusion, the resulting dark and non-reflective appearance of the surface of the part makes an optical scan of e g. 3D QR codes formed on the surface of the parts challenging. Attempts have been made to create a surface pattern on the part by varying the dark surface with a portion over which no absorber is applied. While this creates contrast without changing the surface topography significantly, the durability of the pattern during handling is not guaranteed without subsequent surface finish. Another approach is to create an invisible code or pattern buried within the part by changing density, or by translucency in the case of thin parts, that is otherwise invisible to the eye but may be read with ultrasound or optical transmission. Improvements are therefore needed to apply labels and patterns to the surface of a 3D printed object that are visible and durable without changing the surface quality or mechanical properties of the object. SUMMARY The invention is set out in the appended independent claims, while particular embodiments of the invention are set out in the appended dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS Reference is now directed to the drawings, in which: Fig. 1 is a flow chart of a method according to the invention of generating modified slice data to form an embedded pattern; Fig. 2 is an illustration of a schematic virtual build volume comprising an object model; Fig. 3 is an illustration of an initially generated slice of the virtual build volume of Fig. 2; Fig. 4A is a side view section through the object model boundary pixels of Fig. 3; Fig. 4B shows modified object model boundary pixels compared to those in Fig. 4A; Fig. 5A is a plan view of a group of penultimate pattern pixels defining a QR code; Fig. 5B is a modified penultimate object model slice comprising the QR code of Fig. 5A; Fig. 5C is a modified ultimate object model slice comprising a group of encapsulating pixels; Fig. 6 is a variant of Fig. 1; Fig. 7 is a variant of Fig. 4B; Fig. 8 illustrates a modified object model slice comprising encapsulating pixels and contrast pattern pixels defined within the ultimate and penultimate surface pixels of a vertical wall of the object model of Fig. 2; and Fig. 9 illustrates a schematic plan view of an encapsulated and embedded pattern formed according to modified slice data. In the drawings, like elements are indicated by like reference numerals throughout. DETAILED DESCRIPTION The above problems may be overcome by the present invention, in which the inventors have found that, surprisingly, contrast patterns may be embedded in a sub-surface volume and durably encapsulated by a surface volume of encapsulating voxels, even when the selectively printed agent, which in the case of powder bed fusion is often carbon black absorber, generally has high opacity. The encapsulating voxels form an ultimate surface volume that covers the penultimate sub-surface pattern, and is at least partially transparent to visible radiation, and thus visible to the human eye, or detectable by an optical scanning device. The embedded contrast pattern and the encapsulating surface volume may be formed by changing the agent or absorber amount deposited to define the pattern, and optionally over the encapsulating volume, compared to the remaining object surface. The embedded contrast pattern may be defined within bitmaps (herein also referred to as “slices”, each corresponding to a layer of powder defining an object cross section) generated according to a method of the invention that will be described herein. Herein, the “amount of absorber” refers to the amount per pixel defined in the bitmap data that a voxel of the powder layer is to receive. The final concentration of absorber in the voxel depends on the voxel volume. For example, the same amount of absorber defined for two pixels will lead to different concentrations if the two receiving voxels are in different layers of different thickness. A “boundary amount of absorber” may be taken to be the amount of absorber typically defined by pixels at the periphery of the object model. The object “upskin” or “downskin” typically refers to the ultimate first or last few layers of the object, and may extend over more or fewer layers of the object upper or lower surfaces than the embedded pattern and the encapsulating layers. Considerations regarding a sequence of layers defining the “upskin” of a horizontal surface of the object herein may be analogous to any location on the object surface, where such a layer sequence is represented instead by a sequence of conformal groups of voxels extending from the surface of the object to the object interior. Embodiments of the method of the invention and variants will now be described in detail with reference to Figs. 1 to 9. Turning first to Fig. 1, which is a flowchart illustrating a method 500 of generating object model data comprising sub-surface pattern data, and as will be illustrated further with reference to Figs. 2 to 9 below, the method comprises: At block 510, object model data is generated in the form of initial slice data. The slice data is typically generated after placing a digital model of the object within a virtual build volume based on the physical build volume of the additive manufacturing apparatus, which may first be modified to apply geometric compensations. After this the virtual build volume is divided into a stack of slices corresponding to the layers over which the object is to be manufactured. Each slice defines one or more object pixels to be processed in a corresponding layer of voxels to form a cross section of the 3D object, and wherein each object pixel is a 2-dimensional representation of a voxel, with the layer thickness proving the third dimension. Each object pixel further defines the amount and / or type of agent, or fluid comprising an agent, that is to be deposited over each voxel within a respective layer. From the initial slices, modified object model slices are generated that include embedded contrast pattern pixels, by: At block 520, defining at least one ultimate group of object pixels comprised within the object model surface. The at least one ultimate group corresponds to at least one group of encapsulating object voxels. Furthermore, at least one penultimate group of object pixels, overlapping with the ultimate group of pixels, is defined, wherein the penultimate group of object pixels comprises at least a first subgroup and second subgroup of object pixels that in combination define the embedded contrast pattern. The at least one ultimate group and the at least one penultimate group of object pixels may be defined according to or based on a user-selectable object model boundary region. The user may determine to which part of the object model surface the embedded pattern is to be applied. The selected object model boundary region defines a surface area that in turn may determine the location of the overlapping ultimate and penultimate groups of pixels, such that an outermost ultimate group of pixels represents a portion of the object model surface. The remaining ultimate groups (if any) and the at least one penultimate group are stacked sequentially from the outermost ultimate group to the interior of the object model in an overlapping sequence. The ultimate and penultimate groups of pixels correspond to a boundary volume of the 3D object that extends over at least two object voxels from the object surface into the object interior; At optional block 530, selecting the embedded contrast pattern to be applied from a plurality of available patterns. In variants, the embedded pattern may be autogenerated, for example to apply a unique identifier to each of a plurality of objects, for identification and process traceability; At block 550, setting, replacing or modifying the amount of absorber of the first subgroup to at least a first amount of absorber, and setting the amount of absorber of the second subgroup either to zero or to a second amount lower than the first amount at sub-block 550A. At optional sub-block 550B, setting, replacing or modifying the amount of absorber defined by the ultimate group of object pixels by or to an encapsulating amount that is lower than a boundary amount of absorber defined by object surface pixels surrounding the ultimate group of object pixels, of the first subgroup of the embedded pattern pixels may for example define a positive part of the contrast pattern by at least a first amount of absorber, or by a first fluid. The negative part of the contrast pattern is defined by the second subgroup of pixels. If the particulate material is of white or light appearance and the absorber is a black pigment, the first subgroup of corresponding object voxels is caused to fuse by application of fusing radiation, and gives a dark appearance. The second subgroup of corresponding object voxels remains unfused if the absorber amount is set to zero and is of light or white appearance. Thus a two-tone contrast pattern may be achieved. Additionally, or instead, the negative part may be defined by a second fluid that contains absorber of a different colour or instead of absorber contains a dye or other pigment that does not cause fusion. The difference in amounts and / or fluids is chosen to provide sufficient visual contrast. The ultimate one or more groups of pixel defines an encapsulating amount. The amount of absorber defined by the ultimate group(s) of object pixels may be the same as a boundary amount of absorber defined by the object surface pixels that surround the ultimate group of object pixels. The encapsulating amount is chosen such that the encapsulating voxels fuse at least partially to provide surface stability, and based on the required transparency of the encapsulating voxels and the final contrast between the negative and positive part of the pattern when viewed through the encapsulating voxels. The transparency is further determined by the number of groups of encapsulating voxels and their depth into the object. The amount defined by the ultimate group(s) may therefore remain the same value as the original boundary amount, as it may not require modification to form the encapsulating voxels. Alternatively, the amount of absorber defined by the ultimate group of object pixels may be set to an encapsulating amount that is lower than the original boundary amount of absorber, and thus lower than that defined by object surface pixels surrounding the ultimate group of object pixels. The amount of absorber defined by the ultimate group(s) of object pixels, herein also called the “encapsulating amount”, may be lower than the first amount of absorber, and may be intermediate to the first and second amounts. Suitable amounts can be readily determined by the skilled person by routine experimentation. Where different fluids are used, the first and second fluids may be applied to the first and second subgroups to form the contrast pattern, and one of the fluids may be defined by the encapsulating pixels to also form the encapsulating voxels. Use of different fluids will be discussed further below. The apparatus may be a “print and fuse” powder bed fusion apparatus in which the object is formed from particulate material and defined by a selectively deposited fluid. The layers are processed at voxel resolution as defined by the resolution of the printhead depositing the fluid, and by the layer thickness. In apparatus in which a single absorber fluid is used, such as a fluid comprising carbon black pigment, an embedded visible pattern may therefore easily be generated by replacing an “upskin” amount of absorber at a flat upper surface of the object model, which is usually a relatively high amount of absorber, with an encapsulating amount of absorber lower than the upskin amount of absorber, and encapsulating pixels over one or more adjacent slices. Turning now to Figs. 2 to 9, the method according to the invention of generating the slice data comprising the embedded contrast pattern data will now be described in detail. Fig. 2 shows a schematic virtual build volume 10 as may be provided by a computer program configured to receive and position an object model 20 in the virtual build volume 10. A simple cylindrical object model is shown, placed in the centre of the virtual build volume. The cylinder has an upwards facing upper surface and a downwards facing lower surface parallel to the floor of the virtual build volume 10 in the x-y plane. The cylinder walls extend vertically upwards along the z-direction. The user may select a boundary region within the upper surface of the cylinder within which an embedded pattern is to be applied. Next, the virtual build volume is divided into a sequence of slices. Fig. 3 shows in plan view the top-most (or upper-most) initial slice 100 of the cylinder at block 510. The upper boundary of the cylinder model is defined by object pixels 200. An optional selected boundary region 300 is shown in dashed outline. Its position may be used to define, as described at block 520, at least two overlapping groups of object pixels: at least one ultimate group of object pixels in the ultimate, uppermost object slice, forming part of the object surface, and at least one penultimate group of object pixels in the, penultimate object slice underlying the ultimate slice. At least these two groups of object pixels are to be modified to define the embedded pattern in the penultimate group(s) and the encapsulating surface in the ultimate group(s) of pixels. Fig. 4A illustrates a cross section A-A' through the two groups of pixels of the initial ultimate slice of Fig. 3 and its underlying, penultimate, initial slice. Both slices may be considered “upskin” slices, and the boundary region may be represented by an ultimate “upskin” group of pixels 300A, defining the ultimate object boundary volume of voxels within the ultimate object layer, and by a penultimate “upskin” group of pixels 300B within the underlying object slice. In other words, the upskin pixels 300A and 300B may each comprise several overlapping groups of pixels from several adjacent slices. The penultimate upskin group of pixels 300B defines an interior volume of corresponding voxels just below the surface of the object. Compared to the amount defined over the interior of the object, these at least two groups of “upskin” pixels 300A and 300B may typically define a relatively higher amount of absorber for a high quality surface finish. Next, embedded pattern and encapsulating pixels are defined according to block 550, by replacing, modifying or setting at block 550A the pixels within at least the penultimate group of object pixels 300B to at least the first and second subgroups of contrast pattern pixels, and optionally also by setting the ultimate group of object pixels 300A to encapsulating pixels. In some cases the ultimate group of pixels may remain unchanged compared to the initial slice(s). Fig. 4B illustrates an ultimate group of encapsulating pixels 400A replacing the ultimate upskin group of pixels 300A, and a penultimate group of contrast pattern pixels 400B replacing the penultimate upskin group of pixels 300B. In the Figures herein, shading from light to dark signifies a low to high amount of absorber. The group of penultimate, contrast pattern, pixels 400B defines at least a first subgroup of pixels 400B1, defining a positive part of the contrast pattern (for example dark in appearance), and a second subgroup of pixels, defining a negative part of the contrast pattern 400B2 (for example light in appearance). The pixels of the first subgroup 400B1 may define the same or a higher amount of absorber compared to the pixels of the ultimate group of pixels. Preferably, the amount is chosen so that the corresponding voxels fuse fully, or at least partially. The amounts are furthermore selected to provide sufficient contrast. The pixels of the second subgroup 400B2 may set the absorber amount to zero, so that a corresponding second subgroup of embedded pattern voxels are to be void of absorber. Where the particulate build material is white in appearance, the corresponding second subgroup of embedded pattern voxels will be white. Alternatively the second subgroup may define a second amount of absorber that is lower than the first. This may cause the second subgroup of embedded pattern voxels to at least fuse partially, thereby providing a mechanically stronger embedded pattern. The ultimate group of pixels 400A may define an amount of absorber different to that of the original “upskin”, or boundary, pixels 300A. As described above, the relative amounts will be determined by the opacity of the absorber, its ability to cause fusion of the particulate material, and the layer thickness of the individual ultimate and penultimate layers to which they are to be applied. In combination, the group of voxels of the embedded pattern to be formed therefore comprises the first subgroup of partially fused, or fully fused, embedded pattern voxels of dark visual appearance and the second subgroup of embedded voxels consisting of unfused, or optionally at least partially fused, embedded pattern voxels of lighter appearance. The second subgroup of negative pattern pixels 400B2 may define a relatively low amount of absorber that causes a low level of fusion or sintering, so that the resulting voxels will be near-white in appearance to provide sufficient contrast for the dark pixels. Fig. 5A illustrates a plan view of a penultimate group of contrast pattern pixels 400B defining a QR code by a first subgroup of pixels 400B1 representing the positive, in this case dark, part of the contrast pattern and a second subgroup of pixels 400B2 (e.g. no absorber is to be applied to corresponding embedded voxels) representing the negative (in this case lighter) part of the contrast pattern. In this way, a contrast pattern code may be provided as an embedded code by modifying the initial slice data 100, to arrive at a modified penultimate slice 1000B, for example as shown in Fig. 5B. The penultimate slice of the object model defines the group of embedded contrast pattern pixels 400B. The remaining object model pixels 200 are shown in a grey shading that is unmodified compared to the original slice 100 in Fig. 3. Fig. 5C illustrates the modified ultimate slice 1000A in which the ultimate group of pixels 400A defines an encapsulating amount lower than the absorber amount of the surrounding object pixels 200, as seen by the lighter shading. Fig. 9 schematically illustrates the combined visual appearance of an embedded contrast pattern forming a visible QR code 4 manufactured by a print and fuse apparatus resulting from the modified penultimate slice 1000B of Fig. 5 A and the modified ultimate slice 1000A of Fig. 5C. A corresponding penultimate group of contrast pattern voxels 40B is embedded below a corresponding ultimate group of encapsulating voxels. The QR code is defined by a first subgroup of dark pattern voxels 40B1 of the positive part of the QR code 4 and a second subgroup of white pattern voxels 40B2 of the negative part of the QR code 4. The encapsulating voxels are transparent enough to allow the QR code to be seen, despite lowering the contrast slightly. The absorber that causes consolidation of the build material may be an infrared absorber, for example carbon black pigment suspended in a fluid, and the particulate material may be a polymeric powder substantially white in visual appearance. A sample comprising such an embedded QR code was formed using PAI 1 powder, which is white in appearance, and carbon black absorber within HAF™ oil-based infrared absorption fluid by Stratasys. The absorber fluid was deposited using a Xaar 1003 GS6U printhead able to deposit up to 7 drops of absorber fluid per voxel, which defines “100% greyscale” or the maximum amount of absorber that may be deposited per pixel, where 0% greyscale means no drops are deposited. The resolution of the Xaar 1003 GS6U printhead is 360 dots per inch and defines the pixel size and drop volume. The absorber amount per voxel is defined by the drop volume and the number of drops deposited per voxel. The embedded group of voxels was provided with HAF fluid printed at 70% greyscale over a first subgroup of pattern voxels forming the positive, dark part of the pattern. No fluid was printed over the second subgroup forming the light voxels. The encapsulating layer was formed with HAF fluid printed at 25% “greyscale” printed over two successive groups of encapsulating voxels (i.e. over two “upskin” layers, each 100 microns thick). The test sample provided an embedded QR code that was mechanically robust, easily visible to the eye and could be readily recognised by a QR code scanner of a mobile phone. As the skilled person will recognise, the transparency of the one or more groups of encapsulating voxels is material dependent, and suitable thicknesses of the upskin layers and absorber amounts to form the embedded contrast pattern and the encapsulating volume may be found by routine experimentation. The amount of absorber may be comprised within a fluid deposited in the form of a number of drops per voxel. The amount of fluid may range from 0 to 7 drops per pixel. The first subgroup may for example define a drop number of 2 to 7 drops per pixel, and the ultimate group of pixels may define a drop number of 2 to 7 drops per pixel. In variants, in which a single absorber is used, the amount of absorber provides a visual tone, wherein the first subgroup of pixels may define an amount of absorber that causes at least partial fusion of corresponding voxels and provides the darkest tone; wherein the second subgroup may define no absorber, or an amount of absorber that causes at least partial fusion of corresponding voxels, and provides a lightest tone. The at least one ultimate group of pixels may define an amount of absorber that causes at least partial fusion in corresponding voxels while providing at least partial visual transparency to the at least one penultimate group of voxels forming the contrast pattern. More than one ultimate and / or penultimate pixel groups: Figs. 6&7 The depth over which the one or more groups of encapsulating voxels and / or the one or more groups of embedded pattern voxels are to be formed may be defined for example in terms of the number of ultimate and / or penultimate groups of pixels defining the embedded contrast pattern. For example, the embedded pattern may be formed over an embedded volume extending over more than one voxel group into the interior of the object. Similarly, the encapsulating volume may extend over more than one ultimate group of voxels from the boundary of the object into the object interior, in other words there may be more than one overlapping groups of ultimate pixels conformal with and representing the surface of the object model. Fig. 7 illustrates a variant of Fig. 4B, in which the embedded volume is to be formed over three voxel group depths, (in this case over three horizontal layers) below a single ultimate group of encapsulating voxels that forms the top-most one of the upskin layers. In other words, the penultimate “upskin” pixels that are to be modified are selected to lie within three overlapping penultimate pixel groups. The selected three penultimate “upskin” pixel groups are set to three identical overlapping penultimate pixel groups 400B of contrast pattern pixels as shown in Fig. 7. The same description for the first and second subgroups defining the positive and negative parts of the pattern applies as before. The ultimate, encapsulating group of pixels 400A in the example of Fig. 7 is the same as before. Together with selecting or determining the number of ultimate and / or penultimate groups of pixels, one or more embedded contrast pattern pixel groups 400B and / or encapsulating pixel groups may be generated. The embedded contrast pattern pixel groups 400B may be identical to one another although it is possible to define non-identical groups, for example for enhanced contrast or resolution. Fig. 6 is a variant of the flow diagram of Fig. 1 in which, at a further optional block 540, the number of ultimate and / or penultimate groups may be determined or defined corresponding to a depth over which the embedded contrast pattern and / or the encapsulating volume are to extend in the object. The number of groups may be based on a predefined set of options. An upper limit of the number of ultimate groups may be predefined based on its required transparency for a given amount of absorber and / or the first and optionally the second amount of absorber defined by the first and second subgroups. Alternatively, the depths may be determined based on for example user input or process data, such as the type of absorber and / or build material that is to be used. Such data may be used to determine a transparency level that may be achieved based on the type of absorber, the encapsulating depth and / or a contrast level by the embedded pattern. An algorithm implemented by the processor may be configured to determine an optimal amount of first and / or second absorber and / or encapsulating amount and / or to optimise the number of ultimate groups based on a predefined absorber amount and voxel dimension. At optional block 600, which is equally applicable to the method described for Fig, 1, the resulting modified slice data comprising the embedded contrast pattern data may be provided to an additive manufacturing apparatus to form the object with the embedded and encapsulated pattern. In variants, blocks 530 to 550 may be carried out by a processor onboard the additive manufacturing process. This may be advantageous where codes or labels that are to be applied to an object are to include apparatus-specific process information, and in events where this information is not available to the slicing processor. In variants of any of the embodiments herein therefore, the boundary volume, which may for example be an upskin or downskin volume, of the object may extend over two or more conformal and overlapping ultimate voxel groups over an encapsulating depth from the object surface to the object interior. A plurality of ultimate groups of object pixels may be defined at block 540 that overlap conformally with one another to extend over a corresponding number of pixels from the object model surface into the object model interior, and define a plurality of corresponding ultimate voxel groups stacked from the object surface into the object interior. As an example, the number of ultimate groups may be from 2 to 5 groups and / or correspond to a depth from the object surface of 50 microns to 500 microns. Additionally or instead, a plurality of penultimate groups of object pixels may be determined or defined at block 540 that overlap conformally with one another to extend over a corresponding number of pixels into the object model interior, to define a plurality of corresponding penultimate voxel groups stacked from the ultimate voxel groups into the object interior. The penultimate groups may be identical to, and coincident with, one another. For a horizontal surface region, this means that the pattern is repeated in an identical manner over several layers. The number of penultimate groups may be from 2 to 5 groups and / or correspond to a depth from the object surface of 50 microns to 500 microns. In variants, the absorber amount defined by the first and / or the second subgroup of pixels 400B2 may be increased or decreased over successive penultimate groups of pixels (and thus over the contrast pattern depth) to enhance contrast and / or to improve mechanical integrity. Optionally, absorber amount defined by the ultimate groups of pixels may be increased or decreased over successive ultimate groups of pixels (and thus over the contrast pattern depth) to enhance transparency while optimising mechanical stability. The ultimate group(s), especially when applied over a horizontal surface of the object, may be formed at a lower thickness than other object layers. For example the ultimate layer may be formed at 50-80 microns and the remaining, underlying layers at 80-100 microns thickness. Alternatively, the encapsulating layer may be printed with a greater thickness, for example 150 microns, and with lower absorber concentration per unit volume to reduce the concentration of pigment per unit volume to compensate against loss of transparency. In variants of the method therefore, at least one of the first amount of absorber, the second amount of absorber, and the amount of absorber of the at least one ultimate group of pixels may be defined or determined based on at least one of: the number of ultimate groups of pixels; the number of penultimate groups of pixels; the absorber properties; the build material properties; and one or more voxel dimensions and / or the layer thickness. Embedded pattern formed over non-horizontal object surfaces: Fig, 8 .................................................................................................................*........................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................... The embedded and encapsulated pattern may be applied over some or all of the object surface. For example, the ultimate group(s) of object pixels selected may represent some, the majority of, or all of, the object model boundary. A boundary region may be chosen on the vertical cylinder wall surface of the object model to define the location of an embedded contrast pattern. In this variant, the groups of pixels to be modified or replaced may be defined by a group of boundary pixels that are located at the periphery of object model pixels over a sequence of object slices. An initial slice may be similar to that of Fig. 3 but representing a mid-section of the object model slices of the cylinder 20 of Fig. 2. Fig. 8 illustrates a modified slice 1000 of a sequence of modified slices As before, the different level of shading represents different amounts of absorber defined by the pixel groups, resulting in different visual tones, and when exposed to fusing irradiation, results in different temperatures. Fig. 8 shows two concentric groups of boundary pixels 220, a section of which has been set to an ultimate group of encapsulating pixels 400A and a penultimate group of pixels comprising first and second subgroups 400B1 and 400B2 of contrast pattern pixels. This is analogous to the “upskin” example of Figs. 3 to 4C for which two depths of boundary pixels 220 are shown in the form of the ultimate “upskin” group of pixels 300A, and the penultimate “upskin” group of pixels 300B. Analogous to the “upskin” amount of absorber, the boundary amount in dark grey defines the outermost voxels not comprising the embedded code. In the example of Fig. 8, the ultimate group of pixels of the object model pixels is set to an encapsulating amount of absorber that is lower than the boundary amount, as indicated by the relatively lighter shading in this example. . The first subgroup of pattern pixels 400B1 indicate a first, relatively high, amount of absorber compared to the other object pixels, and may be the highest amount defined by the slice, and higher than the object boundary amount of absorber, as shown by the black shading. As before, the second subgroup of pixels 400B2 may define an absence of absorber or a second amount of absorber lower than the first, as indicated by their white colour. In variants, the second amount may be greater than zero but lower than the first amount, and optionally lower than the encapsulating amount. The interior of the cylinder may receive a relatively lower amount, for example an amount between the encapsulating amount and the boundary amount of absorber, as indicated by the hashed shading of interior object pixels 230. This may be defined so as to control the temperature within each object cross section. The method may be applied over complex surfaces, and may comprise determining the location of ultimate and penultimate groups of pixels for a selected boundary region that comprises curvature and / or angles, and / or other features like recesses or protrusions, to achieve a suitable visual appearance, for example based on one or more angles to the boundary surface that may be selected. The processor may be configured to adapt the desired pattern by for example stretching it to fit over a complex surface. In the Figures described herein, in the simplest implementation a single absorber may be defined. The encapsulating absorber amount defined by the ultimate group(s) of pixels 400A may be the same as or lower than the first amount of absorber. In variants, the encapsulating amount may be the same amount per unit volume of the encapsulating layer (or effective concentration of absorber per voxel at a given layer thickness) as the first amount while still providing visibility in certain materials, such as polypropylene. The negative pattern part may be defined by an absence of absorber, such that resulting embedded voxels comprise unfused embedded voxels and fused embedded voxels defined by the first amount. In any of the variants described, the boundary amount of absorber may be the same as the first amount of absorber, and in some variants may also be the same as the encapsulating amount of absorber. The different amounts of absorber may be provided by depositing the absorber at different droplet sizes per unit area or volume, or at the same droplet size but at different spacings. The encapsulating volume may be formed by applying an absorber encapsulating amount from 10% to 75% greyscale, or from 10% to 50%, or from 10% to 25% greyscale. The first and second amount may be comparatively higher. For example the encapsulating layer may be formed at 25% greyscale. The first amount of absorber may be selected from 25% to 100% greyscale and the second amount of absorber is smaller than the first amount and selected from a range of from >0% to 75% greyscale, for example. In a variant the negative part of the pattern may be void of absorber for maximum contrast in combination with a first amount of greater than 50%, for example. Depending on pattern fine features this may be offset against a required visual resolution, for example due to thermal bleed. The second amount and optionally the encapsulating amount of absorber may be selected to be just high enough so as to cause necking or partial fusion. In variants, a white build material may be dyed with a liquid dye, and / or a suspension comprising reflecting particles (e.g. TiO2, alumina, ceramic) to enhance the contrast of the pattern, for example of the visually lighter part of the pattern. Different absorbers, additional patterning fluids It is not essential that the absorber is a single type of absorber, as will now be described in further variants. Some apparatus may be configured to apply more than one absorber. In variants, the first or second amount may be provided by a second type of absorber, or by one or more of a colouring pigment or dye, or a fluorescent or phosphorescent pigment or dye, that may be applied to form part of the pattern to enhance the contrast with respect to the other part. The absorber, pigment or dye may be provided comprised within a fluid. Two different absorbers may for example be co-deposited over the same layer by using a Xaar 2001 printhead having two separate ink channels. The encapsulating layer or layers do not require to be fused fully as long as they are strong enough to retain and / or encapsulate the unfused and / or partially fused material in the embedded pattern volume. Thus non-carbon black absorbers may be used and / or ultraviolet or microwave radiation instead of infrared. In this way, the pattern may be formed without affecting the remaining process. The fluid used for the first or second amount to form the pattern may have beneficial properties such as phosphorescence, fluorescence or ESD for example, to enhance optical contrast. The encapsulating amount of absorber may define a further absorber different to the object boundary absorber. The further absorber may comprise a non-black pigment and thus may offer an improved transparency over a black absorber applied to form the remaining object surface or upskin volume. The further absorber may be applied at a relatively higher amount than a black absorber and this improve the durability of the encapsulating volume. The at least one ultimate group of pixels 400A may thus further define a different type of absorber, or a different fluid altogether compared to the pixels of the first subgroup of pixels, and that is capable of causing consolidation of the particulate matter. In variants, the first subgroup(s) of pixels of the penultimate group(s) of pixels 400B may define an absorber different to that defined by the second subgroup(s), and / or different to that defined by the ultimate group(s) of pixels. One of the first and second subgroup of pixels may define an absorber, and the other one of the first and second subgroup of pixels may define a patterning fluid that enhances contrast, for example a fluid comprising a fluorescent or phosphorescent dye. Therefore, the method of generating modified object model data comprising embedded pattern data may comprise, in a variant: - generating object model data in the form of initial slice data, each initial slice defining a layer to be processed, wherein the object model data is defined by one or more object pixels in each initial slice, and wherein each object pixel defines an amount and / or type of absorber to be deposited over a corresponding voxel within a respective layer; - generating modified object model slices from the initial object model slices to include embedded contrast pattern pixels by: - defining at least one ultimate group of object pixels comprised within the object model surface to define corresponding at least one group of encapsulating object voxels, and at least one penultimate group of object pixels overlapping with the ultimate group of pixels, wherein the penultimate group of object pixels comprises at least a first subgroup and second subgroup of object pixels that together define the embedded contrast pattern; - modifying, replacing or setting the pixels of the first subgroup and / or the pixels of the second subgroup to define at least one patterning fluid and / or one or more patterning fluid amounts to define the embedded contrast pattern. The second subgroup may be set to define an absence of absorber or patterning fluid; - optionally modifying, replacing or setting the ultimate group of object pixels by or to an encapsulating fluid and / or an encapsulating amount of encapsulating fluid , optionally wherein the encapsulating fluid comprises a different absorber or a thermally reactive binder to the absorber provided to the remaining object voxels not forming the embedded and encapsulated contrast pattern. The patterning fluid may for example be defined for the second subgroup of pixels 400B2 forming the negative part of the contrast pattern for enhanced contrast / The absorber fluid may be applied to the first subgroup of pixels 400B1 such that the corresponding positive pattern voxels fuse or at least partially fuse during object formation. General Using the embodiments and their variants described herein, it is thus possible to generate embedded contrast patterns that are easily visible to the human eye by visual inspection, or may be detected easily when illuminated with a bright light. Where the pattern is formed by areas of white powder and areas of powder over which black absorber is applied, thereby providing high contrast in conventional print and fuse application, the encapsulating layers retain the unfused material underneath the object surface. The encapsulating fluid and / or an encapsulating amount of absorber fluid is selected to undergo at least partial fusion of the particulate build material, upon thermal irradiation during formation of the object, within the encapsulating voxels, while being at least partially transparent to the embedded pattern voxels. The embedded volume depth and / or the encapsulating depth may be determined at block 540 of Fig. 6, in terms of the number of ultimate and / or penultimate groups, based on one or more of: the encapsulating amount; the first amount of absorber; and the second amount of absorber; absorber properties; and build material properties. The absorber property may be for example be one or more of colour, transparency, reflectivity and absorptivity. Additionally, or instead, the method may further comprise, for example at block 550, determining one or more of: the encapsulating amount; the first amount of absorber; and optionally, where present, the second amount of absorber; based on one or more of: absorber properties; layer thickness; voxel dimensions and build material properties. The build material property may for example be reflectivity or colour. The method may comprise determining a resulting contrast between build material without absorber and build material with absorber, or with different amounts of absorber. Such determination may be from a predetermined look up table. The determination may comprise evaluating a plurality of available absorbers that may be chosen to form one or more of the encapsulating volume, the first absorber amount and the second absorber amount. The processor may be configured to optimise the visual appearance of the pattern and the durability of the encapsulating volume from the available options and based on the properties of the various materials. In this way, visible embedded code of enhanced contrast may be provided, that is encapsulated by a durable yet transparent surface layer. The encapsulating region seamlessly forms part of the surface of the object. The embedded pattern may comprise at least one of a unique object identifier, a part code, a recycling code, a recycling mark, a label, and a decorative pattern. The identifier or part code may be in the form of a bar code or QR code that is scannable and links to for example object information, process history, post processing instructions, assembly instructions, and / or use instructions stored on a database. For example, the embedded pattern may be autogenerated by the processor, based on a digital representation of the virtual build volume. A recycling mark may be applied to aid recyclability of the object, for example to allow it being sorted in automated waste identification systems, thus improving a circular economy and therefore environmental sustainability. The recycling mark may be automatically selected based on material type of the particulate build material and / or the absorber. The group of embedded pattern pixels may comprise more than two subgroups of dark pixels 400B1 and the light pixels 400B2. Further subgroups may define further amounts of absorber to create a multi-tone embedded pattern. Suitable amounts of absorber may be predefined to ensure visible contrast for multi-tone patterns. 5 Herein, the build material may be a polymeric material, such as for example selected from a group consisting of polyamide, polyurethane, aliphatic thermoplastic polyurethane, polyolefin, polyester, polyhydroxyalkanoate, polybutylene terephthalate, polycarbonate, or co-polymers thereof. Optionally, the particulate material comprises a pigment or a dye, for example to enhance contrast with respect to the first subgroup of pixels forming part of the 10 contrast pattern..

Claims

1. A method of generating object model data comprising embedded pattern data, to cause an embedded pattern to be applied to a 3D object formed layerwise by a print and fuse process from particulate build material, the method comprising:- generating object model data in the form of initial slices, wherein each slice defines one or more object pixels to be processed in a corresponding layer to form a cross section of the 3D object, and wherein each object pixel defines an amount of absorber to be deposited over a corresponding object voxel within a respective layer;- generating modified object model slices from the initial object model slices to include embedded contrast pattern pixels by:- defining at least one ultimate group of object pixels comprised within the object model surface to define corresponding at least one group of encapsulating object voxels, and at least one penultimate group of object pixels overlapping with the ultimate group of pixels, wherein the penultimate group of object pixels comprises at least a first subgroup and second subgroup of object pixels that together define the embedded contrast pattern; and- setting the amount of absorber of the first subgroup to at least a first amount of absorber, and setting the amount of absorber of the second subgroup either to zero or to a second amount lower than the first amount.

2. The method of claim 1, wherein the amount of absorber defined by the ultimate group of object pixels is set to an encapsulating amount that is lower than a boundary amount of absorber defined by object surface pixels surrounding the ultimate group of object pixels.

3. The method of claim 1, wherein the amount of absorber defined by the ultimate group of object pixels is the same as a boundary amount of absorber defined by object surface pixels surrounding the ultimate group of object pixels.

4. The method of any preceding claim, wherein amount of absorber defined by the ultimate group of object pixels is the same or lower than the first amount of absorber, and optionally higher than the second amount of absorber.

5. The method of any preceding claim, wherein the first amount is the same or lower than a boundary amount of absorber defined by object surface pixels surrounding the at least one ultimate group of object pixels.

6. The method of any preceding claim, wherein the at least one ultimate group of pixels further defines a different type of absorber compared to the pixels of the first subgroup of pixels.

7. The method of any preceding claim, wherein the first subgroup of pixels defines an absorber different to that defined by the second subgroup and / or different to that defined by the ultimate group of pixels.

8. The method of any one of claim 1 to claim 6, wherein one of the first and second subgroup of pixels defines an absorber and the other one of the first and second subgroup of pixels defines a fluorescent or phosphorescent dye.

9. The method of any preceding claim, comprising a plurality of ultimate groups of object pixels that overlap conformally with one another to extend over a corresponding number of pixels from the object model surface into the object model interior, to define a plurality of corresponding ultimate voxel groups stacked from the object surface into the object interior.

10. The method of claim 9, wherein the number of ultimate groups is from 2 to 5 and / or correspond to a depth from the object surface of 50 microns to 500 microns.

11. The method of any preceding claim, comprising a plurality of penultimate groups of object pixels that overlap conformally with one another to extend over a corresponding number of pixels into the object model interior, to define a plurality of corresponding penultimate voxel groups stacked from the ultimate voxel groups into the object interior.

12. The method of claim 11, wherein the penultimate groups are identical to one another and coincident with one another.

13. The method of claim 11 or claim 12, wherein the number of penultimate groups is from 2 to 5 and / or correspond to a depth from the object surface of 50 microns to 500 microns.

14. The method of any preceding claim, comprising determining at least one of the first amount of absorber, the second amount of absorber, and the amount of absorber defined by the at least one ultimate group of pixels, based on at least one of: the number of ultimate groups of pixels; the number of penultimate groups of pixels; the absorber properties; the build material properties; and one or more voxel dimensions.

15. The method of any preceding claim, wherein the absorber is carbon black.

16. The method of any preceding claim, wherein the particulate material comprises a polymeric material; optionally wherein the polymeric material is selected from a group consisting of polyamide, polyurethane, aliphatic thermoplastic polyurethane, polyolefin, polyester, polyhydroxyalkanoate, polybutylene terephthalate, polycarbonate, or copolymers thereof; optionally wherein the particulate material further comprises a pigment or a dye.

17. The method of any preceding claim, wherein the at least one group of ultimate object pixels and the at least one group of penultimate object pixels is defined to extend over most or all of the ultimate and penultimate object model surface.

18. The method of any one of claims 1 to 16, wherein an outermost group of the at least one group of ultimate object pixels lies within the same slice.

19. The method of any preceding claim, wherein the at least one penultimate group of object pixels defines a contrast pattern of one or more of a unique object identifier, a part code, a recycling code, a recycling mark, a label, or a decorative pattern.

20. The method of any preceding claim, wherein the at least one group of ultimate object pixels and the at least one group of penultimate object pixels are selected based on a digital representation of the object model and / or the virtual build volume.

21. The method of any preceding claim, wherein the amount of absorber is comprised within a fluid deposited in the form of a number of drops per voxel, and wherein the amount of fluid ranges from 0 to 7 drops per pixel.

22. The method of claim 21, wherein the amount of absorber provides a visual tone, wherein the first subgroup defines an amount of absorber so as to cause at least partial fusion and to provide a darkest tone; wherein the second subgroup provides a lightest tone; and wherein the at least one ultimate group of pixels defines an amount of absorber so as to cause at least partial fusion while providing at least partial visual transparency to the at least one penultimate group of pixels.

23. The method of claim 22, wherein the first subgroup defines a drop number of 2 to 7 drops per pixel, and wherein the ultimate group of pixels defines a drop number of 2 to 7 drops per pixel.

24. A processor configured to carry out the method of any one of claims 1 to 23.

25. A 3D object comprising an embedded pattern manufactured from slice data generated according to the method of any one of claims 1 to 23.

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