Patent Publication Number: US-2022234287-A1

Title: Printing and curing binder agent

Description:
BACKGROUND 
     There exist a multitude of kinds of three-dimensional (3D) printing techniques that allow the generation of 3D objects through selective solidification of a build material based on a 3D object model. 
     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. 
    
    
     
       BRIEF DESCRIPTION 
       Examples will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: 
         FIG. 1  shows a cross-section of simplified illustration of a 3D printing system  100  according to one example; 
         FIG. 2  is a flow diagram outlining an example method of operating a 3D printing system according to one example; 
         FIG. 3  shows a cross-section of simplified illustration of a 3D printing system  100  according to one example; 
         FIG. 4  shows a cross-section of simplified illustration of a 3D printing system  100  according to one example; and 
         FIG. 5  shows a cross-section of simplified illustration of a 3D printing system  100  according to one example. 
     
    
    
     DETAILED DESCRIPTION 
     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 100 degrees Celsius, for example above 120, or above 150 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 100 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. 1  there is shown a cross-section of simplified illustration of a 3D printing system  100  according to one example. The printing system  100  comprises a build unit  102  which is an integral part of the printing system  100 . In another example, the build unit  102  is a removable module that may be inserted into a suitable interface (not shown) in the printing system  100 . 
     The build unit  102  comprises sidewalls  104  which form a build chamber  106  in which 3D objects may be generated by the printing system  100 . In one example the build chamber  106  has a generally open-top cuboidal shape. The base of the build unit  102  is provided by a movable build platform  108  on which successive layers of build material may be formed and have binder agent selectively applied, for example by printing, thereon. The build platform  108  is movable in a generally vertical axis, or z-axis, ( 110 ) by a controllable drive module (not shown). The build platform  108  may initially be positioned just below the top of the build chamber  106  at a distance corresponding to the height of the first layer of build material to formed thereon. The build platform  108  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  108 , or on previously formed layers, by a layer formation device  112 . In one example, the layer formation device  112  is a translatable recoater roller or wiper blade, although in other examples the layer formation device  112  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  114 , 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  114 . Both the layer formation device  112  and the agent deposition device  114  are translatable over the build platform  108  in an axis  116 . 
     A controllable heating element  118 , such as a resistive heater, is provided to apply heat to a portion of the build chamber  118 . As illustrated in  FIG. 1  the heating element  118  is positioned a predetermined distance below the top of the build chamber  106 . In one example the heating element  118  is disposed around all, or substantially all, of the periphery of a portion of the build chamber  106 . In one example the heating element  118  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  118  is positioned may be, in one example, between about 5 and 20 cm below the top of the build unit, although in other examples the distance may be a greater or lesser distance. The heating element  118  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  118  has a height of between about 10 to 30 cm, 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  100  is generally controlled by a controller  120 , as will be described in greater detail below. The controller  120  may comprise a processor, such as a microprocessor, microcontroller, or the like. The controller  120  is coupled to a memory in which are stored processor executable printer control instructions  122 , and processor executable heater control instructions. 
     When executed by the controller  120 , the printer control instructions  122  cause the controller to control the height of the build platform  108 , to control the layer formation device  112  to form a layer of build material on the build platform, and control the agent deposition device  114  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  120 , the heater control instructions  124  cause the controller to control the heating element  118 , as described below, to apply heat to a portion of the build chamber  106  to cure binder agent in a portion of build chamber  106  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  100 , 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  122  and the heater control instructions  124  may be executed in parallel. 
     An example of operating the system  100  will now be described with reference to the flow diagram of  FIG. 2 , and  FIG. 3 . 
     At block  202 , the controller  120  executes the printer control instructions  122  to control elements of the printing system  100  to selectively form layers of build material on the build platform  108  and selectively print binder agent  304  on each formed layer. The selective printing of binder agent  304  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  204 , the controller  120  executes the heater control instructions  124  to control the heating element  118  to apply heat to a portion of the contents of the build chamber  106  in a curing zone  306  delimited by dotted lines  308  and  310 . The design of the build unit  100  and the position of the heating element  118  provide the following general conditions within the build unit, as illustrated in  FIG. 3 :
         a. Layers of build material above the curing zone  306  are maintained at a temperature below a first predetermined printing temperature. In one example the printing temperature is a temperature below the boiling point of carrier fluids within the binder agent. This ensures good printing conditions on the upper layers of build material. In one example the printing temperature is about 10, or about 20, or about 30, or 40 degrees Celsius below the boiling point of carrier fluids within the binder agent.   b. Layers within the curing zone  306  are maintained at a temperature at or above the curing temperature of the binder agent. Thus, any binder agent in the curing zone  306  will be thermally cured. In one example the curing temperature is about 100, or about 120, or about 140, or about 160 degrees Celsius depending on the type of binder agent used.   c. Layers below the curing zone  306  are allowed to cool below the curing temperature of the binder agent.       

     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  306 , 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 10, 50, 100, or 200 buffer layers. 
       FIG. 3  shows a simplified schematic illustration of the curing zone  306  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  118  at a suitable position within the build unit  102  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  306  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  306  may be maintained at a temperature below the printing temperature when then heating element  118  is applying heat due to ambient radiant cooling of the upper layers of build material. Similarly, layers of build material below the curing zone  306  may be allowed to cool below the curing temperature of the binder agent due to cooling through the build unit walls  104 . 
     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  306  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  306  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  118  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 5 and 10 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 50 to 150 micrometers, although in other examples other layer thicknesses may be used. The time which build material layers spend in the curing zone  118  may depend on factors such as the height of the curing zone  306 , 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  120  controls the printer  100  to make all layers of build material on which binder agent is printed move through the curing zone  306 . For example, the controller  120  may control the printer  100  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  306 . In one example, the controller  120  continues to form successive layers of build material until all layers on which binder agent have been printed enter and leave the curing zone  306 . In this way, all layers on which binder agent are printed spend the substantially the same length of time in the curing zone  306 . 
     In a further example, the controller  120  may control the build platform to move the last printed layers into the curing zone  106  without forming any additional layers of build material thereon, for example by controlling the build platform  108  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  108  is lowered during formation of build material layers and selective printing of binder agent thereon. 
     In another example, the controller  120  may control the build platform  108  to move the last layer on which binder agent was printed into the curing zone and may control the heating element  118  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  120  controls the heating element  118  to start applying heat when the build platform  108  is moved in proximity to the heating element  118 . 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. 4 , a supplementary heating element  402  may be provided to selectively apply heat to upper layers of build material in the build unit  102 . In this example, the controller  120  may control the energy source  402  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  306 . In one example, the energy source  402  may be a fixed energy source located above the build chamber  106 . In another example, the energy source  402  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  108  may be provided with, or may incorporate, a heating element to apply heat, under control from the controller  120 , 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. 5 , the build unit  102  may be provided with a plurality of horizontally arranged heating elements  118 A to  118 N. The controller  120  may, in accordance with the heater control instructions  124 , control each of the plurality of heating elements  118  to apply different amounts of heat to generate a plurality of zones at different temperatures. For example, the controller  120  may control the temperature of a first drying zone  502  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  504  to be at or above the curing temperature of the binder agent. 
     In another example, the heating element  118  may be provided such that the curing zone  306  extends to, or in proximity to, the base of the build unit  102 . In this way, build material layers above the curing zone would be maintained at a temperature below the boiling point of binder agent, and build material within the curing zone would be maintained 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  120  may control the heat output of the heating element(s)  118  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 1 wt % to about 50 wt %, from about 2 wt % to about 30 wt %, from about 5 wt % to about 25 wt %, from about 10 wt % to about 20 wt %, from about 7.5 wt % to about 15 wt %, from about 15 wt % to about 30 wt %, from about 20 wt % to about 30 wt %, or from about 2 wt % to about 12 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 100 nm to about 1 μm. In other examples, the polymer particles can have an average particle size that can range from about 150 nm to about 300 nm, from about 200 nm to about 500 nm, or from about 250 nm to 750 nm. 
     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, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl acrylate, 2-hydroxypropyl methacrylate, methyl methacrylate, hexyl acrylate, hexyl methacrylate, butyl acrylate, butyl methacrylate, ethyl acrylate, ethyl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, propyl acrylate, propyl methacrylate, octadecyl acrylate, octadecyl methacrylate, stearyl methacrylate, vinylbenzyl chloride, isobornyl acrylate, tetrahydrofurfuryl acrylate, 2-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 2-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. 
     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 such as, for example, a CD, DVD, magnetic disk or magnetic tape. 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 described herein may be conveyed electronically via any medium such as a communication signal carried over a wired or wireless connection. 
     All 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. 
     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.