Patent Description:
One technique forms successive layers of a powdered or granular build material on a build platform in a build chamber, and selectively applies a thermally curable binder agent on regions of each layer that are to form part of the 3D object being generated. The thermally curable binder agent has to be thermally cured to form a sufficiently strong green part that may be removed from the build chamber, cleaned up, and then sintered in a sintering furnace to form the final 3D object.

<CIT> discloses a build receptacle that comprises a housing comprising a sidewall at least partially enclosing a build chamber. A build platform positioned within the build chamber. A position of the build platform being slidably adjustable within the build chamber in a vertical direction from a lower position to one of a plurality of upper positions and from the one of the plurality of upper positions to the lower position. The build receptacle further comprises a plurality of heating elements disposed around the build chamber. This document is prior art under Article <NUM>(<NUM>) EPC.

<CIT> discloses an additive manufacturing apparatus that includes a recoat head for distributing build material in a build area, a print head for depositing material in the build area, one or more actuators for moving the recoat head and the print head relative to the build area, and a cleaning station for cleaning the print head. This document is prior art under Article <NUM>(<NUM>) EPC.

<CIT> discloses an actuator assembly for distributing build material and depositing binder material in an additive manufacturing apparatus. This document is prior art under Article <NUM>(<NUM>) EPC.

<CIT> discloses an additive manufacturing apparatus that comprises a support chassis including a print bay, a build bay, and a material supply bay. This document is prior art under Article <NUM>(<NUM>) EPC.

<CIT> discloses a binder system and a material system for producing components using layering technology, wherein the temperature in the building space and/or in the applied material is set to at least <NUM>° C. and maintained for at least <NUM> hours. Areas on which binder has been selectively applied, solidify, and form the component.

<CIT> discloses a method and an apparatus for producing three-dimensional models by layering in a high-speed sintering process.

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

Some powder-based 3D printing techniques use a binder agent to form a so-called green part by selectively applying a liquid binder agent on successively formed layers of a build material, such as a metal, ceramic, or plastic powder, and subsequently curing the binder agent. Curing of the binder agent creates a relatively weak matrix of build material particles bound together by the cured binder. When a 3D object is generated in this manner, the 3D object is commonly referred to as a green part. A green part generated with powdered metal or ceramic build material, for example, has to be sintered in a sintering furnace to transform the green part into a highly dense final object.

If a thermally curable binder agent, such as a latex-based binder agent, is used to generate a green part, the build material on which the binder agent is applied has to be heated to a suitable temperature to cure the binder agent. For example, if a latex-based binder agent is used, the build material on which the binder agent is applied may have to be heated to a temperature above <NUM> degrees Celsius, for example above <NUM>, or above <NUM> degrees Celsius. However, if the binder curing temperature is close to, or is higher than, the boiling point of carrier liquids in the binder agent this makes it unsuitable to thermally cure each layer after application of binder agent. This is because printing binder agent on a layer of build material at a temperature above the boiling point of carrier fluids in the binder agent would cause, upon printing, rapid evaporation or boiling of liquid carriers, which can disturb the formed layer of build material. This can, for example, cause build material to become airborne which may contaminate printheads and other parts of a 3D printer and may also cause defects in the powder layer and ultimately in the green part. In one example the boiling point of carrier fluids of a water-based binder agent may be around <NUM> degrees Celsius.

Current techniques separate printing of thermally curable binder agent and thermal curing of thermally curable binder agent into separate and sequentially performed processes, whereby binder agent is selectively printed in successive layers of build material based on a 3D object model, and all of the layers are subsequently heated up to the binder curing temperature during a single curing process.

The present disclosure describes examples of a 3D printing system in which the process of printing a thermally curable binder agent on successively formed layers of a build material and the process of curing the binder agent may be performed, at least partially, in parallel. Such a system may substantially reduce the amount of time it takes to generate green parts ready for sintering.

Referring now to <FIG> there is shown a cross-section of simplified illustration of a 3D printing system <NUM> according to one example. The printing system <NUM> comprises a build unit <NUM> which is an integral part of the printing system <NUM>. In another example, the build unit <NUM> is a removable module that may be inserted into a suitable interface (not shown) in the printing system <NUM>.

The build unit <NUM> comprises sidewalls <NUM> which form a build chamber <NUM> in which 3D objects may be generated by the printing system <NUM>. In one example the build chamber <NUM> has a generally open-top cuboidal shape. The base of the build unit <NUM> is provided by a movable build platform <NUM> on which successive layers of build material may be formed and have binder agent selectively applied, for example by printing, thereon. The build platform <NUM> is movable in a generally vertical axis, or z-axis, (<NUM>) by a controllable drive module (not shown). The build platform <NUM> may initially be positioned just below the top of the build chamber <NUM> at a distance corresponding to the height of the first layer of build material to formed thereon. The build platform <NUM> may be successively lowered by a height corresponding to the height of each subsequent layer of build material to be formed to allow successive layers of build material to be formed thereon.

Layers of a suitable build material, such as a powdered metal, plastic, or ceramic, build material, may be formed on the build platform <NUM>, or on previously formed layers, by a layer formation device <NUM>. In one example, the layer formation device <NUM> is a translatable recoater roller or wiper blade, although in other examples the layer formation device <NUM> may comprise a build material deposition device, such as a hopper, a sprinkler, or the like. A binder agent, such as a thermally curable binder agent, may be selectively applied to each formed layer of build material by a controllable agent deposition device <NUM>, such as a thermal or a piezo printhead. Binder agent may be stored in a binder agent storage container (not shown) that is fluidically coupled to the agent deposition device <NUM>. Both the layer formation device <NUM> and the agent deposition device <NUM> are translatable over the build platform <NUM> in an axis <NUM>.

A controllable heating element <NUM>, such as a resistive heater, is provided to apply heat to a portion of the build chamber <NUM>. As illustrated in <FIG> the heating element <NUM> is positioned a predetermined distance below the top of the build chamber <NUM>. In one example the heating element <NUM> is disposed around all, or substantially all, of the periphery of a portion of the build chamber <NUM>. In one example the heating element <NUM> may comprise multiple heating elements arranged and controllable to act in one example as a single heating element, and in another example to act as multiple independently controllable heating elements. The predetermined distance at which the heating element <NUM> is positioned may be, in one example, between about <NUM> and <NUM> below the top of the build unit, although in other examples the distance may be a greater or lesser distance. The heating element <NUM> may, in one example, be a thermal blanket, and may, comprise one or multiple heating elements, coils, or the like, that are to generate heat when electrically powered. In one example, the heating element <NUM> has a height of between about <NUM> to <NUM>, although in other examples it may have a higher or lower height. In one example, the heating element is configured to apply a substantially uniform amount of heat around the portion of the periphery of the build chamber to which the heating element is in thermal contact with.

The operation of the printing system <NUM> is generally controlled by a controller <NUM>, as will be described in greater detail below. The controller <NUM> may comprise a processor, such as a microprocessor, microcontroller, or the like. The controller <NUM> is coupled to a memory in which are stored processor executable printer control instructions <NUM>, and processor executable heater control instructions.

When executed by the controller <NUM>, the printer control instructions <NUM> cause the controller to control the height of the build platform <NUM>, to control the layer formation device <NUM> to form a layer of build material on the build platform, and control the agent deposition device <NUM> to selectively apply binder agent to the formed layer of build material in accordance with data derived from a 3D object model of the object to be generated.

When executed by the controller <NUM>, the heater control instructions <NUM> cause the controller to control the heating element <NUM>, as described below, to apply heat to a portion of the build chamber <NUM> to cure binder agent in a portion of build chamber <NUM> whilst other layers of build material may be formed and have binder agent printed thereon. In this way, curing of binder agent may be performed within the build unit <NUM>, which may help significantly speed up the generated of green parts, compared to performing curing as a separate process after the printing of binder agent.

In one example, the printer control instructions <NUM> and the heater control instructions <NUM> may be executed in parallel.

An example of operating the system <NUM> will now be described with reference to the flow diagram of <FIG>, and <FIG>.

At block <NUM>, the controller <NUM> executes the printer control instructions <NUM> to control elements of the printing system <NUM> to selectively form layers of build material on the build platform <NUM> and selectively print binder agent <NUM> on each formed layer. The selective printing of binder agent <NUM> may be performed based on data derived from a 3D object model, for example based on a layer of a 3D object to be generated. For example, a 3D object model may be sliced, and each slice may define portions of each layer of build material that is to receive binder agent such that they ultimately form a solid portion the 3D object to be generated.

At block <NUM>, the controller <NUM> executes the heater control instructions <NUM> to control the heating element <NUM> to apply heat to a portion of the contents of the build chamber <NUM> in a curing zone <NUM> delimited by dotted lines <NUM> and <NUM>. The design of the build unit <NUM> and the position of the heating element <NUM> provide the following general conditions within the build unit, as illustrated in <FIG>:.

The number of upper layers that are to be maintained at or below the printing temperature may be chosen to take into account the penetration of binder agent into previously formed layers. For example, if the binder agent is susceptible of penetrating into two previously formed layers, then the temperature of all of these layers should be maintained below the first predetermined temperature. However, due to difficulties in precisely determining and/or controlling the temperature of layers above the curing zone <NUM>, in one example the number of layers that are to be maintained below the printing temperature may incorporate a suitable number of buffer layers, for example <NUM>, <NUM>, <NUM>, or <NUM> buffer layers.

<FIG> shows a simplified schematic illustration of the curing zone <NUM> having a clearly delimited upper and lower horizontal boundaries. However, it will be appreciated that, in use, heat will radiate and/or conduct form one layer to another leading to a more complex thermal pattern. However, by positioning the heating element <NUM> at a suitable position within the build unit <NUM> the above-mentioned temperature zones can be obtained at least for a portion of the layers of build material therein. Consequently, in use there may be an intermediate zone (not shown) between the curing zone <NUM> and below the upper layer(s) of build material within which the temperature of build material may be below the curing temperature but above the boiling point of binder agent carrier fluids. In one example the intermediate zone may not be heated directly from a heating element but may, for example, be heated due to radiative and/or conductive heating from heated build material. Binder agent in the intermediate zone may start to dry without curing, for example as elements of binder agent carrier fluids evaporate.

For example, layers of build material above the curing zone <NUM> may be maintained at a temperature below the printing temperature when then heating element <NUM> is applying heat due to ambient radiant cooling of the upper layers of build material. Similarly, layers of build material below the curing zone <NUM> may be allowed to cool below the curing temperature of the binder agent due to cooling through the build unit walls <NUM>.

As successive layers of build material are formed and as binder agent is selectively printed on each layer, layers of build material will move into the curing zone <NUM> causing the layers to be heated to a temperature above the curing temperature of the binder agent, thereby causing any binder agent present to be thermally cured. These layers will then move out of the curing zone <NUM> causing these layers to cool to a temperature below the curing temperature of the binder agent.

The speed at which layers are moved through the curing zone <NUM> will depend on the time it takes to process (i.e. to form and selectively print binder agent) on each layer. In one example, a layer processing time may be between <NUM> and <NUM> seconds, although in other examples the layer processing time may be faster or slower. In one example, the build platform may be controlled to be lowered to allow the formation of build material layers in the range of about <NUM> to <NUM> micrometers, although in other examples other layer thicknesses may be used. The time which build material layers spend in the curing zone <NUM> may depend on factors such as the height of the curing zone <NUM>, the layer processing time, and the layer thickness.

In one example, to ensure that all layers of build material on which binder agent is printed are thermally cured in the build unit the controller <NUM> controls the printer <NUM> to make all layers of build material on which binder agent is printed move through the curing zone <NUM>. For example, the controller <NUM> may control the printer <NUM> to, when no more binder agent is to be printed, continue to form successive layers of build material until all layers on which binder agent have been printed enter into the curing zone <NUM>. In one example, the controller <NUM> continues to form successive layers of build material until all layers on which binder agent have been printed enter and leave the curing zone <NUM>. In this way, all layers on which binder agent are printed spend the substantially the same length of time in the curing zone <NUM>.

In a further example, the controller <NUM> may control the build platform to move the last printed layers into the curing zone <NUM> without forming any additional layers of build material thereon, for example by controlling the build platform <NUM> to lower at a predetermined speed. In one example the predetermined speed may be a speed substantially the same as the speed in which the build platform <NUM> is lowered during formation of build material layers and selective printing of binder agent thereon.

In another example, the controller <NUM> may control the build platform <NUM> to move the last layer on which binder agent was printed into the curing zone and may control the heating element <NUM> to stop applying heat at a suitable time such that all layers on which binder agent is printed are heating for substantially the same length of time.

In one example, the controller <NUM> controls the heating element <NUM> to start applying heat when the build platform <NUM> is moved in proximity to the heating element <NUM>. In this way, the heating element may not be used during the formation and printing of a set of first layers.

In a further example, shown in <FIG>, a supplementary heating element <NUM> may be provided to selectively apply heat to upper layers of build material in the build unit <NUM>. In this example, the controller <NUM> may control the energy source <NUM> to apply heat to upper layers of build material once all binder agent has been printed to heat up a number of the upper layers of build material to at or above the binder agent curing temperature without having to move those layers of build material into the curing zone <NUM>. In one example, the energy source <NUM> may be a fixed energy source located above the build chamber <NUM>. In another example, the energy source <NUM> may be a translatable energy source that may be scanned one or multiple times over the build chamber.

In a yet further example, the build platform <NUM> may be provided with, or may incorporate, a heating element to apply heat, under control from the controller <NUM>, to build material layers in proximity thereto. In this way, curing of lower layers of build material may be performed when a suitable number of build material layers have been formed thereon.

In a still further example, as illustrated in <FIG>, the build unit <NUM> may be provided with a plurality of horizontally arranged heating elements 118A to 118N. The controller <NUM> may, in accordance with the heater control instructions <NUM>, control each of the plurality of heating elements <NUM> to apply different amounts of heat to generate a plurality of zones at different temperatures. For example, the controller <NUM> may control the temperature of a first drying zone <NUM> to be at a drying temperature between the boiling point of carrier liquids in the binder agent and the curing temperature of the binder agent, and may control the temperature of a second curing zone <NUM> to be at or above the curing temperature of the binder agent.

In a further example, a thermal sensor (not shown), such as a thermal camera, may be used to monitor the temperature of the upper layer of build material. In this way, the controller <NUM> may control the heat output of the heating element(s) <NUM> to ensure that the temperature of the upper layer of build material remains below the boiling point of carrier fluids in the binder agent.

In a yet further example, a vacuum source may be provided to, draw air through the build platform and/or at least a portion of build chamber, to help remove water vapor and/or solvents formed during printing and curing process.

In one example, the binder agent can include a binder in a liquid carrier or vehicle for application to the particulate build material. For example, the binder can be present in the binding agent at from about <NUM> wt% to about <NUM> wt%, from about <NUM> wt% to about <NUM> wt%, from about <NUM> wt% to about <NUM> wt%, from about <NUM> wt% to about <NUM> wt%, from about <NUM> wt% to about <NUM> wt%, from about <NUM> wt% to about <NUM> wt%, from about <NUM> wt% to about <NUM> wt%, or from about <NUM> wt% to about <NUM> wt% in the binding agent.

In one example, the binder can include polymer particles, such as latex polymer particles. The polymer particles can have an average particle size that can range from about <NUM> to about <NUM>. In other examples, the polymer particles can have an average particle size that can range from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, or from about <NUM> to <NUM>.

In one example, the latex particles can include any of a number of copolymerized monomers, and may in some instances include a copolymerized surfactant, e.g., polyoxyethylene compound, polyoxyethylene alkylphenyl ether ammonium sulfate, sodium polyoxyethylene alkylether sulfuric ester, polyoxyethylene styrenated phenyl ether ammonium sulfate, etc. The copolymerized monomers can be from monomers, such as styrene, p-methyl styrene, α-methyl styrene, methacrylic acid, acrylic acid, acrylamide, methacrylamide, <NUM>-hydroxyethyl acrylate, <NUM>-hydroxyethyl methacrylate, <NUM>-hydroxypropyl acrylate, <NUM>-hydroxypropyl methacrylate, methyl methacrylate, hexyl acrylate, hexyl methacrylate, butyl acrylate, butyl methacrylate, ethyl acrylate, ethyl methacrylate, <NUM>-ethylhexyl acrylate, <NUM>-ethylhexyl methacrylate, propyl acrylate, propyl methacrylate, octadecyl acrylate, octadecyl methacrylate, stearyl methacrylate, vinylbenzyl chloride, isobornyl acrylate, tetrahydrofurfuryl acrylate, <NUM>-phenoxyethyl methacrylate, benzyl methacrylate, benzyl acrylate, ethoxylated nonyl phenol methacrylate, ethoxylated behenyl methacrylate, polypropyleneglycol monoacrylate, isobornyl methacrylate, cyclohexyl methacrylate, cyclohexyl acrylate, t-butyl methacrylate, n-octyl methacrylate, lauryl methacrylate, tridecyl methacrylate, alkoxylated tetrahydrofurfuryl acrylate, isodecyl acrylate, isobornyl methacrylate, isobornyl acrylate, dimethyl maleate, dioctyl maleate, acetoacetoxyethyl methacrylate, diacetone acrylamide, N-vinyl imidazole, N-vinylcarbazole, N-vinyl-caprolactam, or combinations thereof. In some examples, the latex particles can include an acrylic. In other examples, the latex particles can include <NUM>-phenoxyethyl methacrylate, cyclohexyl methacrylate, cyclohexyl acrylate, methacrylic acid, combinations thereof, derivatives thereof, or mixtures thereof. In another example, the latex particles can include styrene, methyl methacrylate, butyl acrylate, methacrylic acid, combinations thereof, derivatives thereof, or mixtures thereof.

Claim 1:
A 3D printing system (<NUM>) comprising:
a build material distributor (<NUM>);
a binder agent distributor (<NUM>);
a controllable heater element (<NUM>) to heat a portion of a build chamber (<NUM>); and
a controller (<NUM>) configured to:
control the build material distributor to form successive layers of build material on a build platform (<NUM>) in a build chamber (<NUM>) as the build platform is successively lowered;
control (<NUM>) the binder agent distributor to selectively print patterns of a thermally curable binder agent on each formed layer in patterns based on a layer of a 3D object to be generated; and
control (<NUM>) the heater element to heat a portion of the contents of the build chamber to or above a curing temperature suitable to cure any binder agent present therein whilst maintaining upper layers of the build chamber at or below a lower printing temperature suitable for printing binder agent thereon and allowing build material below the portion to cool below the curing temperature.