Patent Description:
Examples will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:.

Some 3D printing systems apply fusing energy, such as heat, generally uniformly over each formed layer of build material. The build material used in such systems is generally light in colour (such as white, or a colour close to white) and is a poorer absorber of fusing energy than portions of the build material on which a fusing agent is applied. This energy absorption selectivity allows, upon application of fusing energy, those portions of a layer on which fusing agent is applied to heat up sufficiently to coalesce or melt, whilst portions of the layer on which no fusing agent is applied do not heat up sufficiently to coalesce or melt.

Different build materials have different chemical and physical properties. It has been observed that some build materials on which no fusing agent is applied remain largely free-flowing after being subjected to fusing energy, whereas other types of build material exhibit what is generally known as 'caking'. Caking is where individual particles of a particulate build material become bound together, or agglomerate, under the application of fusing energy (and in the absence of fusing agent). Some build materials exhibit relatively weak caking bonds, whereas other materials exhibit relatively stronger caking bonds.

Build material caking complicates the process of separating non-solidified build material from 3D objects formed from solidified build material. For example, a build material cake may need to be mechanically, or manually, cracked open to allow access to 3D printed objects formed in the cake, and caked build material may have to be mechanically processed, for example using sieves, before it can be reused in the generation of further 3D objects.

Herein, the term 'solidified build material' is used to refer to build material that has coalesced and has solidified to form an intended part of a 3D object generated by the printer <NUM>. It is not used to refer to build material that has undergone caking and which does not form an intended part of a 3D object generated by the printer <NUM>.

Examples described herein provide a method and apparatus to reduce build material caking during the generation of 3D objects.

Referring to <FIG>, there is shown a side view of a simplified 3D printer according to an example. The example printer <NUM> comprises a carriage <NUM> on which is mounted a recoater <NUM>, such as a wiper or a roller, to form a layer of build material in a print zone <NUM>, an agent distributor, such as a printhead, <NUM> to print patterns of a fusing agent on a layer of build material formed in the print zone <NUM>, an agent distributor, or printhead, <NUM> to print patterns of a detailing agent on a layer of build material formed in the print zone <NUM>, and a fusing energy module <NUM> to apply fusing energy to a layer of build material formed in the print zone <NUM>. The example printer <NUM> shown shows only one possible arrangement of the elements <NUM> to <NUM>. In other examples the elements <NUM> to <NUM> may arranged differently, for example be arranged on one or multiple carriages, be provided multiple times (e.g. provide two or more fusing modules, etc.).

According to one example, a suitable fusing agent may be an ink-type formulation comprising carbon black, such as, for example, the fusing agent formulation commercially known as V1Q60A "HP fusing agent" available from HP Inc. In one example such a fusing agent may additionally comprise an infra-red light absorber. In one example such an ink may additionally comprise a near infra-red light absorber. In one example such a fusing agent may additionally comprise a visible light absorber. In one example such an ink may additionally comprise a UV light absorber. Examples of inks comprising visible light enhancers are dye based colored ink and pigment based colored ink, such as inks commercially known as CE039A and CE042A available from HP Inc. According to one example, a suitable detailing agent may be a formulation commercially known as V1Q61A "HP detailing agent" available from HP Inc. According to one example, a suitable build material may be PA12 build material commercially known as V1R10A "HP PA12" available from HP Inc.

The printer <NUM> may comprise a build unit <NUM>. In one example the build unit <NUM> may be an integral part of the printer <NUM>. In another example the build unit <NUM> may be a removal element of the printer <NUM>, for example to allow the build unit to be transported between the printer <NUM> and one or multiple other processing stations. A processing station may comprise, for example, a de-caking, or unpacking, station, where 3D objects generated by the printer <NUM> are separated from non-solidified build material.

The build unit <NUM> comprises a build chamber formed by a housing <NUM> within which is located a vertically moveable build platform118. The build chamber provides the volume within which 3D objects may be generated by the printer <NUM>. The build platform <NUM> is moved by a drive mechanism, such as a piston or a lead screw, <NUM>.

In one example, a build material dosing mechanism (not shown) provides a volume of build material at one side of the build platform <NUM>, and is spread by the recoater <NUM> to form a layer <NUM> of build material having a thickness defined by the height at which the build platform <NUM> is positioned below the top of the housing <NUM>.

Operation of the printer <NUM> is controlled by a printer controller <NUM>, as described in further detail below with reference to the flow diagram of <FIG>. The controller <NUM> comprises a memory <NUM> on which are stored object print data generation instructions <NUM>, and anti-caking print data instructions <NUM>. These instructions may be, for example, computer or controller understandable instructions that, when executed by the controller <NUM>, cause the controller <NUM> to control the operation of the printer <NUM> as described below. Further reference is made to <FIG>, which illustrates a plan view of a layer of build material formed on the build platform <NUM>.

At block <NUM>, the controller <NUM> obtains data corresponding to an object or set of objects to be generated by the printer <NUM>. The data may be in any suitable format, such as in a 3MF data file, data representing a mesh of triangles, data representing a set of two-dimensional slices of an object or set of objects.

At block <NUM>, the controller <NUM> processes the obtained object data and determines, for each of a set of layers of build material that are to be processed, a pattern of one or multiple agents to be applied to addressable locations on each layer of build material formed in the print zone <NUM> to generate a layer of the object.

The controller <NUM> determines, based on the obtained object data, a pattern of both fusing agent and detailing agent to generate a layer of the object.

The controller <NUM> determines, based on the obtained object data, a pattern of fusing agent <NUM> that is to be applied to each layer of build material in the print zone <NUM> based on portions of each layer of build material that are to be solidified to form a layer of a 3D object. The controller <NUM> also additionally determines a pattern of detailing agent <NUM> to be selectively applied to each layer of build material in the print zone <NUM> to control thermal bleed from a portion of build material on which fusing agent is applied. Control of thermal bleed may include, for example, a level of reduction of thermal bleed that ranges from substantial prevention of thermal bleed to an acceptable reduction in thermal bleed. Thermal bleed may, for example, occur when, after application of fusing energy, a portion of build material on which fusing agent is applied heats up, and a portion of this heat migrates, or bleeds, onto a portion of build material on which no fusing agent was applied. Thermal bleed may cause build material on which no fusing agent was applied to heat and coalesce, or at least partially coalesce, and adhere to the portion on which fusing agent was applied. Thermal bleed may, therefore, cause dimensional inaccuracy of the formed object, or may cause the surface finish of the object to have other quality defects.

The pattern <NUM> of fusing agent and the pattern <NUM> of detailing agent may be independent from each other.

In one example, the controller <NUM> may determine that the pattern of detailing agent <NUM> is to be applied immediately adjacent to the pattern of fusing agent <NUM>. In another example, the controller <NUM> may determine that the pattern of detailing agent <NUM> is to be applied in close proximity to the pattern of fusing agent <NUM>. In one example, detailing agent may be applied within a distance of about from <NUM> to <NUM> from the portion of detailing agent <NUM>.

It should be noted that the controller <NUM> may determine that a pattern of fusing agent is only to be applied to layers of build material that have portions which are to form a layer of the 3D object to be generated. The controller <NUM> may, however, determine that a pattern of detailing agent <NUM> is to be applied above or below a layer of build material on which fusing agent is to be applied, for example to prevent or control thermal bleed between adjacent layers of build material.

The controller <NUM> may determine a contone level at which the pattern of detailing agent <NUM> is be applied. The purpose of the pattern of detailing agent <NUM> is to prevent, or at least to reduce, the effect of thermal bleed from energy absorbed by the pattern of fusing agent <NUM> once fusing energy is applied thereto. The contone level at which the pattern of detailing agent is applied may vary depending on the cooling effect of the detailing agent, the size of detailing agent drops ejected by the printhead <NUM>, and other factors. Thermal bleed can cause build material that is not intended to form part of a 3D object to heat up sufficiently to coalesce, or at least partially coalesce, and to adhere to the surface of a 3D object. Detailing agent helps prevent build material from heating up sufficiently to coalesce, and thus helps improve the dimensional accuracy and surface finish of generated 3D objects. To prevent or reduce thermal bleed, the controller <NUM> may determine that the pattern of detailing agent <NUM> be applied at a relatively high density or contone level. After application of fusing energy, the portion of build material to which the pattern of detailing agent <NUM> is applied may remain generally non-solidified.

In some examples, the controller <NUM> may also determine a pattern (not shown) of detailing agent to be applied at the same locations as locations at which fusing agent may be applied. This may be done, for example, to prevent portions of build material on which fusing agent is applied from overheating.

As described above, the pattern <NUM> of fusing agent, or the pattern <NUM> of fusing agent and the pattern <NUM> of detailing agent, are used in the generation of the object.

At <NUM>, the controller <NUM> determines a pattern <NUM> of detailing agent to be applied to control caking on build material that is not intended to form part of the 3D object (or objects) being generated. In one example, the pattern is to control caking on all build material in a layer that is not intended to form part of the 3D object (or objects) being generated. In another example, the pattern is to control caking on at least a portion of build material in a layer that is not intended to form part of the 3D object (or objects) being generated. In one example the portion may comprise greater than <NUM>%, or greater than <NUM>%, or greater than <NUM>%, or greater than <NUM>%, or greater than <NUM>%, or greater than <NUM>% of build material that is not intended to form part of the 3D object (or objects).

Control of caking may include, for example, substantial prevention of caking, or at least the reduction of caking to an acceptable degree of caking. An acceptable level of caking control, or a desired degree of reduction of caking may, for example, be based on techniques used to de-cake a print job. This pattern <NUM> of detailing agent is not used in the generation of the object. The controller <NUM> may determine a contone level at which the pattern of detailing agent <NUM> is to be applied. In one example, the contone level of the pattern of detailing agent <NUM> may be less than the determined contone level for the pattern of detailing agent <NUM>. A higher contone level for the detailing agent pattern <NUM> may result in a reduced degree of caking compared to a lower contone level, at the expense of increased usage of detailing agent. Depending on particular needs, the contone level for the detailing agent pattern <NUM> may be chosen as a comprise between efficient use of detailing agent and ease of de-caking. The amount of detailing agent to be used in the pattern <NUM> may be based, for example, on properties of the type of build material used in the process, and may obtained, for example, through experimentation. In one example, the controller <NUM> may be obtain a type of build material to be used to generate the object, and may determine the pattern <NUM> of detailing agent to be used accordingly.

In one example, a contone level may be a density of detailing agent, for example, x picolitres of detailing agent per cm<NUM>.

The controller <NUM> determines that a first pattern <NUM> of detailing agent is to be applied to a first layer of build material, and a second different pattern <NUM> of detailing agent is to be applied to a second layer of build material, in such a way that the first and second patterns prevent, or at least reduce, caking on both the first and second layer. According to the appended claims, a first pattern comprises a first checkered pattern, and a second different pattern comprises a complementary checkered pattern. In this way, the amount of detailing agent that is to be applied to prevent or reduce caking may be reduced compared to if detailing agent is applied in a regular pattern. In other examples the controller <NUM> may determine other suitable patterns.

In a yet further example, the controller <NUM> may determine that detailing agent is to be applied to a successive layers of build material in a semi-random uniform distribution at a predetermined density, for example, at a <NUM>% density, or a <NUM>% density, or a <NUM>% density, or a <NUM>% density, or a <NUM>% density, or a <NUM>% density, or a <NUM>% density, or an <NUM>% density, or a <NUM>% density.

At block <NUM>, the controller <NUM> controls elements of the printer <NUM>, as described above, to form a layer of build material <NUM> on the build platform <NUM>.

At block <NUM>, the controller <NUM> controls the printer <NUM> to apply the determined patterns of fusing agent <NUM>, of detailing agent <NUM>, and of detailing agent <NUM> to the layer of build material <NUM>.

At block <NUM>, the controller <NUM> controls the printer <NUM> to apply fusing energy to the layer of build material <NUM> to cause portions of build material on which fusing energy is applied to heat up sufficiently to coalesce, and then solidify upon cooling. Blocks <NUM>, <NUM>, and <NUM>, and then repeated until all of the layers of the 3D object being generated have been formed.

By applying a pattern of detailing agent <NUM> to each layer of build material in a suitable pattern prevents, or at least reduces, the amount of caking experienced by build material which is not intended to form part of a 3D object being generated. Accordingly, after processing of all of the layers, the 3D object may be more easily separated from non-solidified build material. This may help reduce the amount of effort or time needed to separate generated 3D objects, which may in turn help improve the throughput of 3D printing systems. Furthermore, by reducing or eliminating caking allows for simpler build material post-processing operations, which may in turn help reduce the cost of generating 3D objects.

In a further example, a printable fluid other than a detailing agent may be used to apply the pattern <NUM> to a layer of build material. For example, an anti-caking agent different from a detailing agent may be used. In this example, a third agent distributor, or printhead, may be provided in the printer <NUM> to print patterns of a de-caking agent on a layer of build material formed in a print zone <NUM>. The de-caking agent may, for example, comprise a high percentage of water, such as greater than <NUM>% of water, such that, upon the application of fusing energy, the anti-caking agent substantially evaporates from build material on which it is applied without unduly affecting build material on which it is applied. In this way, the anti-caking agent does not unduly affect the ability to reuse build material on which anti-caking agent was applied to generate further 3D objects.

It will be appreciated that example described herein can be realized in the form of hardware, software or a combination of hardware and software. Any such software may be stored in the form of volatile or non-volatile storage such as, for example, a storage device like a ROM, whether erasable or rewritable or not, or in the form of memory such as, for example, RAM, memory chips, device or integrated circuits or on an optically or magnetically readable medium. It will be appreciated that the storage devices and storage media are examples of machine-readable storage that are suitable for storing a program or programs that, when executed, implement examples described herein. Accordingly, some examples provide a program comprising code for implementing a system or method as claimed in any preceding claim and a machine-readable storage storing such a program. Still further, some examples may be conveyed electronically via any medium such as a communication signal carried over a wired or wireless connection.

All of the features disclosed in this specification and/or all of the steps of any method or process so disclosed, may be combined in any combination, within the scope as defined by the appended claims, except combinations where at least some of such features and/or steps are mutually exclusive.

Claim 1:
A three-dimensional printer (<NUM>), comprising:
a recoater (<NUM>) for forming a layer of build material;
a first agent distributor to selectively distribute a pattern of fusing agent on a formed layer of build material;
a second agent distributor to selectively distribute a pattern of detailing agent on a formed layer of build material;
an energy module (<NUM>) to apply fusing energy to a formed layer of build material; and
a controller (<NUM>) configured to:
obtain data corresponding to an object to be generated;
control the recoater (<NUM>) to form a layer of build material;
determine a pattern of fusing agent to be applied to the formed layer of build material to generate a layer of the object;
determine a first pattern (<NUM>) of detailing agent to be applied to the formed layer of build material to reduce or prevent caking of build material that is not to form part of the object;
control the agent distributors to apply the determined patterns of fusing agent and detailing agent to the formed layer of build material formed;
control the energy source to apply fusing energy to the formed layer of build material;
characterised in that the controller is configured to determine, for a first layer of build material, the first pattern (<NUM>) of detailing agent to have a first checkered pattern, and for a subsequent layer of build material, the first pattern (<NUM>) of detailing agent to have a complementary checkered pattern.