WO2015108556A1

WO2015108556A1 - Generating three-dimensional objects

Info

Publication number: WO2015108556A1
Application number: PCT/US2014/036001
Authority: WO
Inventors: Krzysztof Nauka; Lihua Zhao; Howard S. Tom; Sivapackia Ganapathiappan; Yan Zhao; Hou T. Ng
Original assignee: Hewlett-Packard Development Company, L.P.
Priority date: 2014-01-16
Filing date: 2014-04-30
Publication date: 2015-07-23

Abstract

According to one example, there is provided a method of processing data representing at least a portion of a three-dimensional object to be generated by an additive manufacturing system. The method comprises adding shell data to the data, to generate modified data, to cause at least the portion of the object, when generated by the additive manufacturing system, to be generated within a shell.

Description

GENERATING THREE-DIMENSIONAL OBJECTS

BACKGROUND

[0001] Additive manufacturing systems that generate three-dimensional objects on a layer-by-layer basis have been proposed as a potentially convenient way to produce three-dimensional objects.

[0002] Some additive manufacturing systems generate three-dimensional objects through the selective solidification of successive layers of a build material, such as a powdered build material.

BRIEF DESCRIPTION

[0003] Examples will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

[0004] Figures 1 a-1 g show a series of cross-sections of a layer or layers of build material according to one example;

[0005] Figure 2 is a flow diagram outlining a method of generating a three- dimensional object according to one example;

[0006] Figure 3 is a simplified isometric illustration of an additive manufacturing system according to one example;

[0007] Figure 4 is a cut-away perspective view illustrating a three-dimensional object generated according to one example;

[0008] Figure 5 is a cut-away perspective view illustrating a three-dimensional object and shell generated according to one example;

[0009] Figure 6 is a block diagram of a processing system according to one example;

[00010] Figure 7 is a flow diagram outlining a method of adding a shell according to one example;

[00011] Figure 8 is a block diagram of a processing system according to one example;

[00012] Figure 9 is a flow diagram outlining a method of adding a shell according to one example; [00013] Figure 10 is an illustration of an image slice according to one example; and

[00014] Figure 1 1 is an illustration of an image slice according to one example. DETAILED DESCRIPTION

[00015] Some additive manufacturing systems generate three-dimensional objects through the selective solidification of successive layers of a build material, such as a powdered build material. Some such systems may solidify portions of a build material by selectively delivering an agent to a layer of build material. Some systems, for example, may use a liquid binder agent to chemically solidify build material. Other systems, for example, may use liquid energy absorbing agents, or coalescing agents, that cause build material to solidify when suitable energy, such as infra-red energy, is applied. [00016] Repetition of these processes enables a three-dimensional object to be generated layer-by-layer, through selective solidification of portions of successive layers of build material.

[00017] PCT patent application PCT/EP2014/050841 , filed by Hewlett-Packard Development Company on 16 January 2014, the contents and teachings of which are hereby incorporated herein in their entirety, and for which priority is claimed, describes an additive manufacturing system to generate a three- dimensional object. The described system enables the generation of a three- dimensional object through the selective solidification of portions of successive layers of a build material through selective delivery of multiple agents to layers of a build material. In one example a coalescing agent and a coalescence modifier agent may be selectively delivered to layers of build material. A coalescing agent is a material that, when a suitable amount of energy is applied to a combination of build material and coalescing agent, may cause the build material to coalesce and solidify. A coalescence modifier agent is a material that serves to modify the degree of coalescence of a portion of build material on which the coalescence modifier agent has been delivered or has penetrated. PROCESS OVERVIEW

[00018] A process of generating a tangible three-dimensional object according to an example will now be described with reference to Figures 1a-1g and 2. Figures 1a-1g show a series of cross-sections of a layer or layers of build material according to one example. Figure 2 is a flow diagram outlining a method of generating a three-dimensional object according to one example.

[00019] In the method of Figure 2, at 202 a first layer 102a of build material may be provided, as shown in Figure 1a. The first layer of build material is provided on a suitable support member (not shown). In one example the thickness of the layer of build material provided is in the range of about 90 to 110 microns, although in other examples thinner or thicker layers of build material may be provided. Using thinner layers may enable higher resolution objects to be generated but may increase the time taken to generate an object.

[00020] In the method of Figure 2, at 204, a coalescing agent 104 and a coalescence modifier agent 106 are selectively delivered to one or more portions of the surface of the layer 102a of build material. The selective delivery of the agents 104 and 106 is performed in accordance with data derived from a model of a three-dimensional object to be created.

[00021] By selective delivery is meant that both coalescing agent and coalescence modifier agent may be delivered to selected portions of the surface layer of the build material in respective independent patterns. The patterns are defined by data derived from a model of a three-dimensional object to be created. In some examples, coalescing agent 104 may be selectively delivered to a portion of build material according to a first pattern, and coalescence modifier agent 106 may be selectively delivered to a portion of build material according to a second pattern. In one example the patterns define a bitmap. [00022] In one example the coalescing agent 104 and coalescence modifier agent 106 are fluids that may be delivered using any appropriate fluid delivery mechanism, as will be described in greater detail below. In one example the agents are delivered in droplet form. It should be noted, however, that Figures 1 a to 1 g show the delivery of the agents in schematic form.

[00023] Figure 1 b shows that the agents 104 and 106 delivered to the surface of the build material penetrate into the layer 102a of build material. The degree to which the agents penetrate may differ between the two different agents, or may be substantially the same in different examples. The degree of penetration may depend, for example, on the quantity of agent delivered, on the nature of the build material, on the nature of the agent, etc. In the examples shown in Figures 1 a-1 g the agent is shown to penetrate substantially completely into the layer 102a of build material, although it will be appreciated that this is purely for the purposes of illustration and is in no way limiting. In other examples, one or both of the agents may penetrate less than 100% into the layer 102a. In some examples, one or both of the agents may penetrate completely into the layer 102a of build material. In some examples one or both of the agents may penetrate completely into the layer 102a of build material and may further penetrate into an underlying layer of build material.

[00024] Once coalescing agent and coalescence modifier agent have been delivered in the method of Figure 2, at 206, a predetermined level of energy is temporarily applied to the layer 102a of build material. In one example the energy applied is infra-red or near infra-red energy, although in other examples other types of energy may be applied, such as microwave energy, ultra-violet (UV) light, halogen light, ultra-sonic energy or the like. The length of time the energy is applied for, or energy exposure time, may be dependent, for example, on one or more of: characteristics of the energy source; characteristics of the build material; characteristics of the coalescing agent; and characteristics of the coalescence modifier agent. The type of energy source used may depend on one or more of: characteristics of the build material; characteristics of the coalescing agent; and characteristics of the coalescence modifier agent. In one example energy may be applied for predetermined length of time.

[00025] The temporary application of energy may cause portions of the build material on which coalescing agent has been delivered or has penetrated to heat up above the melting point of the build material and to coalesce. Upon cooling, the portions which have coalesced become solid and form part of the three-dimensional object being generated. One such portion is shown as portion 108a in Figure 1c.

[00026] Energy absorbed by build material on which coalescing agent has been delivered or has penetrated may also propagate into surrounding build material and may be sufficient to cause surrounding build material to heat up. This may cause, for example, heating of build material beyond its melting point, or may cause, for example, heating of build material below its melting point but to a temperature suitable to cause softening and bonding of build material. This may result in the subsequent solidification of portions of the build material that were not intended to be solidified and this effect is referred to herein as coalescence bleed. Coalescence bleed may result, for example, in a reduction in the overall accuracy of generated three-dimensional objects.

[00027] The effects of coalescence bleed may be managed by delivering coalescence modifier agent on appropriate portions of build material. In the present example the coalescence modifier agent serves to reduce the degree of coalescence of a portion of build material on which the coalescence modifier agent has been delivered or has penetrated.

[00028] The coalescence modifier agent may be used for a variety of purposes. In one example, as shown in Figure 1 , coalescence modifier agent 106 may be delivered adjacent to where coalescing agent 104 is delivered, as shown in Figure 1a, to help reduce the effects of lateral coalescence bleed. This may be used, for example, to improve the definition or accuracy of object edges or surfaces, and/or to reduce surface roughness. In another example, coalescence modifier agent may be delivered interspersed with coalescing agent (as will be described further below) which may be used to enable object properties to be modified, as mentioned previously.

[00029] The combination of the energy supplied, the build material, and the coalescing and coalescence modifier agent may be selected such that, excluding the effects of any coalescence bleed: i) portions of the build material on which no coalescing agent have been delivered do not coalesce when energy is temporarily applied thereto; ii) portions of the build material on which only coalescing agent has been delivered or has penetrated do coalesce when energy is temporarily applied thereto; and iii) portions of the build material on which only coalescence modifier agent has been delivered or has penetrated do not coalesce when energy is temporarily applied thereto.

[00030] Portions of the build material on which both coalescing agent and coalescence modifier agent have been delivered or have penetrated may undergo a modified degree of coalescence when energy is applied thereto. The degree of modification may depend, for example, on any one or more of:

the proportions of the coalescing agent and the coalescence modifier agent at any portion of build material;

the pattern in which coalescing agent is delivered to build material;

the pattern in which coalescence modifier agent is delivered to build material;

the chemical properties of the coalescing agent;

the chemical properties of the coalescence modifier agent;

the chemical properties of the build material;

the chemical interaction between the build material and the agents; and the interactions between the build material and agents whilst energy is applied. [00031 ] After one layer of build material has been processed as described above, a new layer of build material 102b is provided on top of the previously processed layer of build material 102a, as shown in Figure 1 d. This is illustrated in block 202 of Figure 2. In this way, the previously processed layer of build material acts as a support for a subsequent layer of build material.

[00032] The process of blocks 204 and 206 of Figure 2 may then be repeated to generate a three-dimensional object layer by layer. For example, Figure 1 e illustrates additional coalescing agent 104 and coalescence modifier agent 106 being selectively delivered to the newly provided layer of build material, in accordance with block 204 of Figure 2. For example, Figure 1f illustrates penetration of the agents 104 and 106 into the build material 102b. For example, Figure 1 g illustrates coalescence and solidification of portions of build material 102b where coalescing agent 104 has been delivered or has penetrated, upon the application of energy in accordance with block 206 of Figure 2.

[00033] Heat absorbed during the application of energy from a portion of build material on which coalescing agent 104 has been delivered or has penetrated may propagate to a previously solidified portion, such as portion 108a, causing a portion of that portion to heat up above its melting point. This effect helps creates a portion 1 10 that has strong interlayer bonding between adjacent layers of solidified build material, as shown in Figure 1 g. [00034] The particular manner in which coalescing agent 104 and coalescence modifier agent 106 are delivered to the layers of build material that are used to generate an object may enable the object to have different object properties.

SYSTEM OVERVIEW

[00035] Referring now to Figure 3 there is shown a simplified isometric illustration of an additive manufacturing system 300 according to one example. [00036] The system 300 may be operated, as described herein, for example with reference to the flow diagram of Figure 2, to generate a tangible three- dimensional object by causing the selective solidification of portions of successive layers of a build material.

[00037] In one example the build material is a powder-based build material. As used herein the term powder-based materials is intended to encompass both dry and wet powder-based materials, particulate materials, and granular materials.

[00038] It should be understood, however, that the examples described herein are not limited to powder-based materials, and may be used, with suitable modification if appropriate, with other suitable build materials. In other examples the build material may be a paste or a gel, or any other suitable form of build material, for instance.

EXAMPLE SYSTEM CONFIGURATION

[00039] The system 300 comprises a system controller 302 that controls the general operation of the additive manufacturing system 300. In the example shown in Figure 3 the controller 302 is a microprocessor-based controller that is coupled to a memory 304, for example via a communications bus (not shown). The memory stores processor executable instructions 306. The controller 302 may execute the instructions 306 and hence control operation of the system 300 in accordance with those instructions.

[00040] The system 300 further comprises a coalescing agent distributor 308 to selectively deliver coalescing agent to a layer of build material provided on a support member 314. In one example the support member has dimensions in the range of from about 10 cm by 10 cm up to 100 cm by 100 cm. In other examples the support member may have larger or smaller dimensions. [00041] The system 300 also comprises a coalescence modifier agent distributor 310 to selectively deliver coalescence modifier agent to a layer of build material provided on a support member 314. [00042] The controller 302 controls the selective delivery of coalescing agent and coalescence modifier agent to a layer of provided build material in accordance with agent delivery control data 316.

[00043] In the example shown in Figure 3 the agent distributors 308 and 310 are printheads, such as thermal printheads or piezo inkjet printheads. In one example printheads such as suitable printheads commonly used in commercially available inkjet printers may be used.

[00044] The printheads 308 and 310 may be used to selectively deliver coalescing agent and coalescence modifier agent when in the form of suitable fluids. In one example the printheads may be selected to deliver drops of agent at a resolution of between 300 to 1200 dots per inch (DPI). In other examples the printheads may be selected to be able to deliver drops of agent at a higher or lower resolution. In one example the printheads may have an array of nozzles through which the printhead is able to selectively eject drops of fluid. In one example, each drop may be in the order of about 10 pico liters (pi) per drop, although in other examples printheads that are able to deliver a higher or lower drop size may be used. In some examples printheads that are able to deliver variable size drops may be used.

[00045] In some examples the agent distributor 308 may be configured to deliver drops of coalescing agent that are larger than drops of coalescence modifier agent delivered from the agent distributor 310. [00046] In other examples the agent distributor 308 may be configured to deliver drops of coalescing agent that are the same size as drops of coalescence modifier agent delivered from the agent distributor 310. [00047] In other examples the agent distributor 308 may be configured to deliver drops of coalescing agent that are smaller than drops of coalescence modifier agent delivered from the agent distributor 310.

[00048] In some examples the first and second agents may comprise a liquid carrier, such as water or any other suitable solvent or dispersant, to enable them to be delivered via a printhead. [00049] In some examples the printheads may be drop-on-demand printheads. In other examples the printheads may be continuous drop printheads.

[00050] In some examples, the agent distributors 308 and 310 may be an integral part of the system 300. In some examples, the agent distributors 308 and 310 may be user replaceable, in which case they may be removably insertable into a suitable agent distributor receiver or interface module (not shown).

[00051 ] In some examples a single inkjet printhead may be used to selectively deliver both coalescing agent and coalescence modifier agent. For example, a first set of printhead nozzles of the printhead may be configured to deliver coalescing agent, and a second set of printhead nozzles of the printhead may be configured to deliver coalescence modifier agent. [00052] In the example illustrated in Figure 3, the agent distributors 308 and 310 have a length that enables them to span the whole width of the support member 314 in a so-called page-wide array configuration. In one example this may be achieved through a suitable arrangement of multiple printheads. In other examples a single printhead having an array of nozzles having a length to enable them to span the width of the support member 314 may be used. In other examples, the agent distributors 308 and 310 may have a shorter length that does not enable them to span the whole width of the support member 314. [00053] The agent distributors 308 and 310 are mounted on a moveable carriage (not shown) to enable them to move bi-directionally across the length of the support 314 along the illustrated y-axis. This enables selective delivery of coalescing agent and coalescence modifier agent across the whole width and length of the support 314 in a single pass. In other examples the agent distributors 308 and 310 may be fixed, and the support member 314 may move relative to the agent distributors 308 and 310. [00054] It should be noted that the term 'width' used herein is used to generally denote the shortest dimension in the plane parallel to the x and y axes illustrated in Figure 3, whilst the term 'length' used herein is used to generally denote the longest dimension in this plane. However, it will be understood that in other examples the term 'width' may be interchangeable with the term 'length'. For example, in other examples the agent distributors may have a length that enables them to span the whole length of the support member 314 whilst the moveable carriage may move bi-directionally across the width of the support 314. [00055] In another example the agent distributors 308 and 310 do not have a length that enables them to span the whole width of the support member but are additionally movable bi-directionally across the width of the support 314 in the illustrated x-axis. This configuration enables selective delivery of coalescing agent and coalescence modifier agent across the whole width and length of the support 314 using multiple passes. Other configurations, however, such as a page-wide array configuration, may enable three-dimensional objects to be created faster.

[00056] The coalescing agent distributor 308 may include a supply of coalescing agent or may be connectable to a separate supply of coalescing agent. The coalescence modifier agent distributor 310 may include a supply of coalescence modifier agent or may be connectable to a separate supply of coalescing agent. [00057] The system 300 further comprises a build material distributor 318 to provide the layer of build material 102 on the support 314. Suitable build material distributors may include, for example, a wiper blade and a roller. Build material may be supplied to the build material distributor 318 from a hopper or build material store (not shown). In the example shown the build material distributor 318 moves across the length (y-axis) of the support 314 to deposit a layer of build material. As previously described, a first layer of build material will be deposited on the support 314, whereas subsequent layers of build material will be deposited on a previously deposited layer of build material.

[00058] In the example shown the support 314 is moveable in the z-axis such that as new layers of build material are deposited a predetermined gap is maintained between the surface of the most recently deposited layer of build material and lower surface of the agent distributors 308 and 310. In other examples, however, the support 314 may not be movable in the z-axis and the agent distributors 308 and 310 may be movable in the z-axis.

[00059] The system 300 additionally comprises an energy source 320 to apply energy to build material to cause the solidification of portions of the build material according to where coalescing agent has been delivered or has penetrated. In one example the energy source 320 is an infra-red (IR) or near infra-red light source. In one example the energy source 320 may be a single energy source that is able to uniformly apply energy to build material deposited on the support 314. In some examples the energy source 320 may comprise an array of energy sources.

[00060] In some examples the energy source 320 is configured to apply energy in a substantially uniform manner to the whole surface of a layer of build material. In these examples the energy source 320 may be said to be an unfocused energy source. In these examples a whole layer may have energy applied thereto simultaneously, which may help increase the speed at which a three-dimensional object may be generated.

[00061 ] In other examples, the energy source 320 is configured to apply energy in a substantially uniform manner to a portion of the whole surface of a layer of build material. For example, the energy source 320 may be configured to apply energy to a strip of the whole surface of a layer of build material. In these examples the energy source may be moved or scanned across the layer of build material such that a substantially equal amount of energy is ultimately applied across the whole surface of a layer of build material.

[00062] In one example the energy source 320 may be mounted on the moveable carriage. [00063] In other examples the energy source may apply a variable amount of energy as it is moved across the layer of build material, for example in accordance with agent delivery control data. For example, the controller 302 may control the energy source only to apply energy to portions of build material on which coalescing agent has been applied.

[00064] In further examples, the energy source 320 may be a focused energy source, such as a laser beam. In this example the laser beam may be controlled to scan across the whole or a portion of a layer of build material. In these examples the laser beam may be controlled to scan across a layer of build material in accordance with agent delivery control data. For example, the laser beam may be controlled to apply energy to those portions of a layer on which coalescing agent is delivered.

[00065] Although not shown in Figure 3, in some examples the system 300 may additionally comprise a pre-heater to maintain build material deposited on the support 314 within a predetermined temperature range. Use of a pre-heater may help reduce the amount of energy that has to be applied by the energy source 320 to cause coalescence and subsequent solidification of build material on which coalescing agent has been delivered or has penetrated.

[00066] In some examples the support 214 may not be a fixed part of the system 100, but may, for example, be part of a removable module. In some examples both the support 114 and the build material distributor may not be a fixed part of the system 100, but may, for example, be part of a removable module. In other examples other elements of the system 100 may be part of a removable module.

CONTAMINATION OF BUILD MATERIAL

[00067] Turning now to Figure 4, there is illustrated a three-dimensional object 402 that has been generated in the manner described above. The object 402 is generated based on object model data. The object model data is transformed into control data to control an additive manufacturing system to generate the object. In one example the object model data is thus transformed into data which defines on which portions of a layer of build material are to be deposited one or more of: a coalescing agent; and a coalescence modifier agent. [00068] As can be seen, the object 402 is surrounded by volumes 404 and 406 of non-solidified build material. In the example shown, the build material in volume 404 may not be contaminated with coalescence modifier agent. However, coalescence modifier agent was delivered around the external surfaces of the object 402. This has created the volume 406 of non-solidified build material on which coalescence modifier agent has been delivered. This build material in volume 406 is hereinafter referred to as contaminated build material.

[00069] When the object 402 is removed from the volume 404 of non-solidified build material, the volume 406 of contaminated build material mixes with the volume 404 of non-solidified build material. Since coalescence modifier agent may be used to prevent coalescence of build material, the mixing of the volume 404 of build material with the volume 406 of contaminated build material causes contamination of the volume 404 of build material.

[00070] Contamination of the volume 404 of non-solidified build material is undesirable since it may affect the properties of the build material, especially where the percentage of contamination is above a predetermined threshold. Contamination of build material may thus reduce the number of times that non- solidified build material may be reused, as each time build material is reused its level of contamination may increase. Build material which is contaminated above a predetermined threshold may cause quality problems in generated objects. The reuse of build material may help reduce the cost of generating three-dimensional objects.

[00071] Furthermore, since the volume 406 of contaminated build material is in a non-solidified form and is contained within the volume 404 of non-solidified build material 404, there are no practical ways in which the volume 406 of contaminated build material may be separated from the volume 404 of non- solidified build material. CONFINEMENT OF CONTAMINATED BUILD MATERIAL

[00072] According to one example, a thin shell or skin 502 may be formed around both the object 402 and any contaminated powder 406 as the object 402 is being generated, as illustrated in Figure 5. In this way the shell 502, (herein after referred to as the confinement shell) internally confines a volume of contaminated build material between the shell and the object, preventing contaminated build material from contaminating the remainder of the volume 404 of build material. In one example a shell may be formed to confine both a volume of contaminated build material and a volume of non-contaminated build material, for example to allow a safety margin to ensure that all of the contaminated build material is confined within the confinement shell.

[00073] After the object 402 and confinement shell 502 have been generated, they may be removed from the volume of build material 404, without contaminating the volume 404 of the build material. The shell is generated to be strong enough to enable the object 402 and shell 502 to be removed from the volume 404 of build material without breaking, but is weak enough to be easily broken once removed from the volume 404 of build material. This enables the object 402 to be easily separated from the shell 502. In one example a shell 502 having a thickness of between about 1 to 5 mm may be used, although in other examples a higher or lower thickness of shell may be used. When determining the thickness of the confinement shell 502 it may be useful to reduce the amount of build material used for the shell to reduce unnecessary use of build material. The position of the confinement shell may also be chosen to be as close as possible to the object 402 whilst ensuring that as much contaminated build material is confined within the confinement shell 502. [00074] In one example, the confinement shell 502 may be formed by delivering a coalescing agent in an appropriate pattern on a layer of build material as the additive manufacturing system generates each layer of an object, such as the object 402. In other words, the object and the confinement shell are generated simultaneously.

[00075] In another example, the confinement shell 408 may be formed by applying a suitable binding agent, such as a chemical binding agent, an adhesive, or the like, in an appropriate pattern on a layer of build material. In one example the binding agent may be different from the coalescing agent used in the generation of the object 402. In one example a suitable binding agent may be delivered by a binding agent distributor (not shown).

[00076] In one example the confinement shell 502 may be formed without the use of a coalescence modifier agent.

[00077] The creation of a confinement shell 502 may be performed at various stages of the creation of a three-dimensional object. [00078] In one example, the confinement shell 502 may be added to a model of a three-dimensional object, for example by a computer aided design (CAD) application, or other three-dimensional object processing system, such as a processing system 600 shown in Figure 6.

[00079] The system 600 comprises a processor 602, such as a microprocessor- based processor, that is coupled to a memory 604, for example via a communications bus (not shown). The memory 604 stores processor executable instructions 606. The controller 602 may execute the instructions 606 and hence control operation of the system 600 in accordance with those instructions.

[00080] Figure 7 shows a flow diagram outlining example processing operations defined by the instructions 606. [00081] At block 702, the system 600 obtains data defining an object model.

[00082] At block 704, the system 600 processes the object model data and adds geometrical features, or shell data, that define a shell suitable to contain the object and a volume of build material around the object. In one example the volume of build material around the object may be a volume in which a coalescence modifier agent may be delivered when the object 402 is generated by an additive manufacturing system, and which volume is not intended to form part of the generated object. [00083] In one example, the system 600 generates a shell around the whole of the object to ensure that any build material on which a coalescence modifier agent is delivered during generation of the object is confined within the shell and cannot contaminate any non-solidified portions of build material, such as the volume 404 shown in Figure 4.

[00084] At the object model level a confinement shell in the form of a suitable cuboid may be the easiest confinement shell shape to generate, since at the object model creation stage precise details about the type of additive manufacturing system on which the object is to be generated may not be known. A cuboid-shape shell, for example, may be easy to remove from a generated object. However, a cuboid-shape confinement shell may result in non-contaminated build material being confined within the shell in addition to contamination build material. Consequently, a cuboid-shape confinement shell may not be the most optimized shape in terms of minimizing the volume of confined build material. In one example a shell that follows at least some of the external contours of the object may be generated.

[00085] Generation of the confinement shell should therefore take into account various factors that may include, for example: the amount of build material to be confined within the shell; the ease of removing the shell; and stresses that may be applied to an object within a confinement shell upon removal of the shell. For example, for objects that have open internal structures or volumes (e.g. such as a torus type shape) removal of a shell from those open structures may prove difficult.

[00086] In one example a confinement shell may be generated to have variable thickness walls, for example to enable portions of the shell to be weaker than other portions of the shell to facilitate removal of the shell.

[00087] In other examples, however, more complex shells may be generated around more complex objects.

[00088] In another example, the shell 502 may be added by a slice processing system, such as a processing system 800 shown in Figure 8. In one example the slice processing system 800 may be incorporated into an additive manufacturing system.

[00089] The system 800 comprises a processor 802, such as a microprocessor- based processor, that is coupled to a memory 804, for example via a communications bus (not shown). The memory 804 stores processor executable instructions 806. The controller 802 may execute the instructions 806 and hence control operation of the system 800 in accordance with those instructions. [00090] Figure 9 shows a flow diagram outlining example processing operations defined by the instructions 806. [00091] At block 902, the system 800 obtains data defining slices of a three- dimensional object model to be generated. Each slice may be represented, for example, by an image, such as a vector or bitmap image. In one example each slice may define portions of a layer of build material onto which a coalescing agent may be delivered, and may additionally define portions of a layer of build material onto which a coalescence modifier agent may be delivered. Each slice may represent one layer of build material to be processed by an additive manufacturing system.

[00092] An example slice is illustrated in Figure 10. The slice 1000 defines a portion 1002 of a layer of build material 1006 on which a coalescing agent is to be deposited and defines a portion 1002 of a layer of build material on which a coalescence modifier agent is to be delivered.

[00093] At block 904, the system 800 modifies the slice data to generate a modified slice 1 100. The modified slice 1 100 comprises an additional portion 1 102 which defines a portion or portions of a layer of build material on which a coalescence agent is to be deposited to form a portion of a confinement shell, as described herein. The confinement shell may be defined by shell data. In one example the additional portion 1 102 may define a region on which a coalescing agent, or a binding agent, different from the coalescing agent used to generate object portion 1002 is to be deposited.

[00094] In one example, at block 904 the system 800 may add one or multiple additional slices, for example to provide a base or a top for a generated shell. For example, the system 800 may add one or multiple slices to be generated by an additive manufacturing system to form a base of a confinement shell, and on which the object to be generated may be formed. In one example the shell may be open, for example without a top. In another example the shell may be closed shell. If the confinement shell is generated to have an open shape, for example a cuboid without a top, an object generated within the confinement shell may be removed from the confinement shell without having to break the confinement shell. [00095] In other examples other suitable processing systems may be used to add a suitable confinement shell to an object model, to slice data, to additive manufacturing system control data, or the like.

[00096] It will be appreciated that examples of the present invention 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 such as, for example, a CD, DVD, magnetic disk or magnetic tape. It will , be appreciated that the storage devices and storage media are embodiments of machine-readable storage that are suitable for storing a program or programs that, when executed, implement examples described herein. Accordingly, some examples may 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.

[00097] All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

[00098] Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

Claims

1. A method of processing data representing at least a portion of a three- dimensional object to be generated by an additive manufacturing system, comprising:

adding shell data to the data, to generate modified data, to cause at least the portion of the object, when generated by the additive manufacturing system, to be generated within a shell.

2. The method of claim 1 , wherein the shell data defines the shell that encompasses the object and is to confine a volume of build material between the shell and the object.

3. The method of clam 1 , wherein the object data represents a three- dimensional model of the object, and wherein adding the shell data comprises adding geometrical features to the model to define the shell, the shell being suitable to contain the object and a volume of build material around the object.

4. The method of claim 1 , wherein the data represents a slice of a three- dimensional model of the object, and wherein the shell data defines a portion of a layer of build material on which a coalescing agent is to be deposited to form a portion of the shell, the shell being suitable to contain the object and a volume of build material around the object.

5. The method of claim 1 , wherein the object data corresponds to portions of a layer of build material on which one or more of: a coalescing agent; and a coalescence modifier agent are to be deposited by the additive manufacturing system.

6. The method of claim 5, wherein the shell data corresponds to portions of a layer of build material on which a coalescing agent is to be deposited by an additive manufacturing system.

7. The method of claim 6, wherein the shell data defines portions of a layer of build material on which a binding agent different from the coalescing agent is to be delivered.

8. The method of claim 1 , wherein the shell is a cuboid-shaped shell around the object.

9. The method of claim 1 , wherein the shell follows at least some of the external contours of the object.

10. The method of claim 1 , wherein the shell is a closed shell.

11. Apparatus for generating a three-dimensional object, comprising:

a first agent distributor to selectively deliver a coalescing agent onto portions of a layer of build material;

a second agent distributor to selectively deliver a coalescence modifier agent onto portions of a layer of build material; and

a controller to control the agent distributors to selectively deliver each of the agents onto a layer of build material in respective patterns derived from data representing a slice of a three-dimensional object to be generated, and wherein the controller is further to control the agent distributors to selectively deliver coalescing agent onto a layer of build material in respective patterns representing a slice of a shell within which the three-dimensional object is to be generated.

12. The apparatus of claim 11 , wherein the controller is to control the agent distributors to generate the object within a shell.

13. The apparatus of claim 12, wherein the controller is to control the agent distributors to confine between the shell and the object a volume of non- solidified build material on which coalescence modifier agent has been deposited.

14. The apparatus of claim 13, further comprising a third agent distributor to selectively deliver a second coalescing agent onto portions of a layer of build material, and wherein the controller is configured to generate the shell using the second coalescing agent.

15. A computer readable medium on which are stored processor understandable instructions that, when executed by a processor, control an additive manufacturing system to:

obtain data representing at least a portion of a three-dimensional object to be generated by the additive manufacturing system; and

generate the object within a shell, such that the shell confines a volume of non- solidified build material between the shell and the object.