具体实施方式：
[0010]Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements. The figures are not necessarily to scale, and the size of some parts may be exaggerated to more clearly illustrate the example shown. Moreover, the drawings provide examples and/or implementations consistent with the description; however, the description is not limited to the examples and/or implementations provided in the drawings.
DETAILED DESCRIPTION
[0011]Additive manufacturing systems form a three-dimensional (3D) object through the solidification of layers of build material. Additive manufacturing systems make objects based on data in a 3D model of the object generated, for example, with a computer-aided drafting (CAD) computer program product. The model data is processed into slices, each slice defining portions of a layer of build material that are to be solidified.
[0012]In one particular example, a build material is deposited as dry powder and a binding agent is selectively applied to the layer of build material. The binding agent is deposited in a pattern of a slice of a 3D object to be printed. This process is repeated per layer until the 3D object is formed.
[0013]In some examples, the build material is deposited as part of a slurry. That is, the slurry may include build material particles, which build material particles may be metallic, ceramic, or polymer. In this example, the slurry is deposited, spread and allowed to dry. A binding agent may be selectively applied to the slurry material in a pattern of a slice of a 3D object to be printed.
[0014]In either case, i.e., build material deposited as dry powder or as part of a slurry, the binding agent is cured to form a “green” 3D object. Cured binding agent holds the build material of the green 3D object together. The binding agent may include binding component particles which are dispersed throughout a liquid carrier. The binding component particles of the binding agent move into the vacant spaces between the build material particles. The binding component particles are activated or cured by heating the binding agent to the melting point of the binding component particles. The binding component particles then flow to form adhesive bridges between build material particles. This process is repeated in a layer-wise fashion to generate a green 3D object. When activated or cured, the binding component particles glue the build material particles into the cured green object shape. The cured green object has enough mechanical strength such that it is able to withstand extraction from the build area without being deleteriously affected (e.g., the shape is not lost).
[0015]In another example, the binding agent may include a binding component that is soluble in a liquid carrier. In this example, adhesive connections between build material particles are created upon evaporation of the liquid carrier of the binding agent. Additional heating beyond evaporation of the liquid carrier may be used to cure the binding component. Soluble binding components may include polymers or inorganic materials, such as metal salts. Thermal curing of the binding agent may be done in a layer-wise manner during printing or by a post-print treatment in an oven.
[0016]In another example, the binding agent may include an ultraviolet (UV) curable component that is activated or cured by exposure to UV radiation in a layer-wise manner. The radiation cross-links the binding molecules to form a rigid polymer that links together build material particles to form a green body. The green 3D object may then be placed in a furnace to expose the green 3D object to electromagnetic radiation and/or heat to sinter the build material in the green 3D object to form the finished 3D object. Specifically, the binding agent is removed and the temperature is further raised such that sintering of the powder particles occurs to form a 3D object. It is to be understood that the term “green” does not connote color, but rather indicates that the part is not yet fully processed.
[0017]In another example, referred to as selective laser melting (SLM), portions of the build material are selectively melted to form a slice of a 3D printed object. In SLM, no binding agent is used and no subsequent oven-sintering process is performed. That is, in this case, the metal 3D object is formed as a high power-density laser melts and fuses metallic powder particles together. Similar to binding agent-based systems, the SLM additive operation may be performed in a layer-wise fashion where a layer of powder build material is laid down as part of a slurry and select portions are fused. The process is repeated until a complete metal 3D object is formed.
[0018]While such additive manufacturing operations have greatly expanded manufacturing and development possibilities, further development may make 3D printing a part of even more industries. For example, in systems where a liquid is used, either in the form of a binding agent or a slurry in which the build material is dispersed, the liquid may indirectly affect the mechanical, thermal, and/or electrical properties of the finished 3D object.
[0019]That is, liquid introduced onto the surface of a layer of build material is drawn into underlying layers via capillary forces. Air displaced by the encroaching liquid may become trapped before it escapes to the surface, creating voids or porosity in the layer. Slurry coating and agent-based printing operations both apply liquid to the surface of the build material and may give rise to air entrapment. That is, voids are formed as displaced air is trapped as a liquid agent percolates into the build material.
[0020]Air entrapment arises in slurry coating processes, where particles suspended in a carrier fluid are applied as a coating onto a bed. In a slurry-based process, volume between powder particles initially is filled with slurry vehicle, which is evaporated prior to patterning and coating the subsequent layer. Consequently, in a slurry-based binder jet process, liquid penetrates into underlying dry powder twice each print cycle: 1) when jetting the pattern, and 2) when slurry coating.
[0021]Porosity in 3D printed objects degrades mechanical performance by providing nucleation sites for crack propagation. Reduced tensile strength and premature fatigue failure may result. In the case of metal build material, electrical and thermal conductivities may also be degraded due to electron scattering at pore boundaries.
[0022]Accordingly, the present specification reduces the presence of trapped air in the body of the 3D printed object. Specifically, the present specification includes a source that applies a vacuum to the underside of a porous substrate platen. The vacuum permeates through the pores of the platen. A pressure gradient across the build material drives displaced air downward where it is exhausted through the porous platen into a vacuum pump. Applying a vacuum to the bottom side of the build area through a porous substrate platen gives air displaced by wicked liquid an escape route. Removal of displaced air prevents voids from forming in green parts leading to higher density parts that have better mechanical and electrical performance.
[0023]Applying vacuum to the build bed may also provide a path for removal of vapor phase components of the binding agent and/or slurry liquid carriers as they evaporate from the build layer. An increased evaporation rate may reduce layer cycle time, thereby increasing throughput of the additive manufacturing system.
[0024]Specifically, the present specification describes an additive manufacturing stage. The additive manufacturing stage includes a bed to define a volume where a three-dimensional object is to be formed. The additive manufacturing stage also includes an air-permeable platform on which build material is deposited. A vacuum system draws air resident in the build material down through the air-permeable platform.
[0025]The present specification also describes a method. According to the method, a layer of slurry build material is deposited across a build area. Air resident in the slurry build material is drawn down through a thickness of the layer via a vacuum from underneath an air-permeable platform on which the layer is deposited. A binding agent is applied onto the layer in a pattern of a slice of a three-dimensional part to be formed.
[0026]The present specification also describes an additive manufacturing device. The additive manufacturing device includes a build material distributor to successively deposit layers of a slurry build material into a build area. The additive manufacturing device also includes a bed to define a volume where a three-dimensional (3D) object is to be formed, an air-permeable platform on which build material is deposited, and a vacuum system to draw air resident in the build material down through the air-permeable platform. The additive manufacturing device also includes a controller to control the vacuum pressure of the vacuum system.
[0027]Such systems and methods 1) reduce the presence of voids caused by trapped air in slurry coatings and/or agent-based additive manufacturing operations; 2) is a low-cost solution; 3) broaden the range of slurry chemistries that may be used; 4) increases throughput by increasing carrier evaporation rates; and 5) allow for higher coverage of a binding agent.
[0028]As used in the present specification and in the appended claims, the term “binding agent” may refer to any agent that includes a binding compound to join build material particles together. In some examples, the binding agent may include a latex binder. In other examples, the binding agent may include metal salts or metal nanoparticles that sinter the build material particles together when energy is applied to the build bed. In yet another example, the binding agent may be a UV-curable material.
[0029]FIG. 1 is a block diagram of an air-permeable platform (104) for additive manufacturing, according to an example of the principles described herein. As described above, additive manufacturing refers to a process where a raw material such as dry powder build material or powder build material dispersed throughout a slurry is deposited as a layer. A binding system, such as an agent-based system, operates to form a slice of a 3D object to be printed. These processes are repeated per layer until a 3D printed object is formed. These operations occur in a bed (102) of an additive manufacturing stage (100). That is, the bed (102) may define the volume where the three-dimensional (3D) object is to be formed.
[0030]The additive manufacturing stage (100) also includes an air-permeable platform (104) on which build material is deposited. That is, the air-permeable platform (104) provides a rigid base on which the build material is deposited and where the build material is hardened. As described above, the build material may take a variety of forms. For example, the build material may be a dry powder. In this example, the binding system may include a liquid binding agent that is deposited on the dry powder.
[0031]In another example the build material is a slurry that includes build material particles, a liquid carrier, and a viscosity increasing agent. The viscosity increasing agent may be a hydrocolloid. In an example, the build material particles may be metallic, ceramic, or polymeric. The hydrocolloid may be present in an amount ranging from 0.05 percent by volume to less than 2 percent by volume of the slurry. The liquid carrier may be any variety of liquids including water.
[0032]In the case of a metallic build material, the slurry may include metallic particles that may be any particulate metallic material. The particulate metallic material may be in powder form prior to being incorporated into the slurry. As an example, metal injection molding (MIM) powders may be used as the metallic particles. In an example, the metallic particles may be a single-phase metallic material composed of one element. In another example, the metallic particles are composed of two or more elements, which may be in the form of a single-phase metallic alloy or a multiple-phase metallic alloy. Some examples of the metallic particles include steels, stainless steel, bronzes, titanium (Ti) and alloys thereof, aluminum (Al) and alloys thereof, nickel (Ni) and alloys thereof, cobalt (Co) and alloys thereof, iron (Fe) and alloys thereof, nickel cobalt (NiCo) alloys, gold (Au) and alloys thereof, silver (Ag) and alloys thereof, platinum (Pt) and alloys thereof, and copper (Cu) and alloys thereof.
[0033]In another example, the build material particles may be ceramic. The ceramic particles may be any particulate ceramic material. Examples of suitable ceramic materials include metal oxides, inorganic glasses, carbides, nitrides, and borides. Some specific examples include alumina (Al2O3), Na2O/CaO/SiO2 glass (soda-lime glass), silicon nitride (Si3N4), silicon dioxide (SiO2), zirconia (ZrO2), titanium dioxide (TiO2), or combinations thereof. As an example of one suitable combination, 30 wt % glass may be mixed with 70 wt % alumina. In yet another example, the build material particles may be polymer particles, such as PA-12.
[0034]As described above, the slurry may include a viscosity increasing agent such as a hydrocolloid. In some examples, the hydrocolloid is a high molecular weight polymer (e.g., ranging from about 0.5×106 g/mol to about 50×106 g/mol) that can undergo physical crosslinking when little or no shear is applied (e.g., a shear rate of 0.1 s−1 or less). The physical crosslinks create an entangled polymer network, e.g., through Van der Waals forces, hydrogen bonding, etc. The metallic or ceramic particles of the build material slurry may be held within this entangled polymer network, which helps decrease particle settling. In an example, the hydrocolloid is selected from the group consisting of xanthan gum, scleroglucan, carboxymethyl cellulose, guar gum, locust bean gum, tara gum, cassia gum, gum tragacanth, agar, welan gum, diutan gum, rhamsan gum, carrageenan, flaxseed gum, tamarind gum, konjac maanan, agarose, gellan gum, and combinations thereof.
[0035]The platform (104) is air-permeable such that air resident in the build material may be drawn down when a vacuum is applied. That is, as additional layers of slurry or binding agent are applied, the liquid therein seeps into underlying layers and in so doing displaces air between particles of these underlying layers. This displaced air may not be able to escape and if entrapped in the layer, may form performance-impacting voids in the 3D object. Accordingly, an air-permeable platform (104) provides a path through which the air may be drawn. In some examples, the air-permeable platform (104) may be a porous metal or ceramic that has pores ranging from 5 to 150 microns. The pores may allow passage of air, but may be small enough that build material particles may not pass through. As used in the present specification the pore size may refer to an average of a distribution of pore sizes.
[0036]In another example, the air-permeable platform (104) is a perforated plate with holes dispersed throughout. Similar to the porous plate, the perforated plate also allows air to pass through. That is, as a layer of build material is deposited, the liquid, whether as a subsequent layer of slurry or as the binding agent which may be deposited on the build material, wicks into underlying layers. The liquid causes air in those underlying layers to aggregate in pockets. While some air may escape, other air may remain in the layer throughout the additive manufacturing operation up to, and including sintering. These air pockets may result in voids that adversely impact the mechanical, electrical, and thermal properties of the formed part. In yet a further example, the air-permeable platform (104) includes both a porous material and a perforated plate. For example, a porous material plate may be placed on top of the perforated plate.
[0037]Accordingly, the additive manufacturing stage (100) includes a vacuum system (106) to draw the air pockets resident in the build material out of the build material. Specifically, the vacuum system (106) may be disposed under the air-permeable platform (104). As the platform (104) is air-permeable, the vacuum system (106) draws air pockets down out of the layer, and through the air-permeable platform (104).
[0038]In some examples, one or multiple layers of build material may be deposited before the vacuum system (106) is engaged to draw down resident air. Doing so may prevent the transport of metal particles through the porous material. In the case of just a perforated plate and no porous material, the delay in application of the vacuum may prevent the metal particles from clogging the perforations.
[0039]Accordingly, the additive manufacturing stage (100) of the present specification reduces the presence of air pockets in layers of build material, which air pockets may result in performance-reducing voids in the finished 3D printed object. That is, the vacuum system (106) provides a route by which entrapped air may escape so that it does not remain in the 3D object as a void. Moreover, as described above, application of vacuum may assist in the evaporation of liquid carrier in both the slurry and any binding agent.
[0040]FIG. 2 is a block diagram of an additive manufacturing device (208) with an air-permeable platform (104) for additive manufacturing, according to an example of the principles described herein. As described above, the additive manufacturing device (208) may include a bed (102), air-permeable platform (104), and vacuum system (106) to draw out the air that may be present in a layer of build material so as to reduce void formation in the final 3D printed object.
[0041]The additive manufacturing device (208) may include other components as well. For example, the additive manufacturing device (208) may include a build material distributor (210) to successively deposit layers of a build material, be it a slurry or dry powder, into a build area. The build material distributor (210) may acquire the build material from a hopper or other receptacle. In some examples, such as that depicted in FIG. 3, the build material distributor (210) may be coupled to a carriage that traverses across the bed (102) to distribute the build material. As described above, the build material may be of a variety of types including dry powder build material and powder build material dispersed throughout a slurry.
[0042]The additive manufacturing device also includes a controller (212) to control the vacuum pressure in the vacuum system (106) as well as other aspects of the additive manufacturing process. For example, as layers are added, the thickness of the build material in the bed (102) increases. With each successive layer that is deposited, the vacuum force at the top may reduce due to the increased distance between the vacuum source and the top layer. Accordingly, the controller (212) may increase the vacuum force as more layers of build material are deposited to ensure that air is adequately drawn from the top layer of build material. That is, to reduce a pressure differential, pressure on the vacuum side of the system is increased either by throttling the pump or bleeding gas into the vacuum side or both.
[0043]Other of the previously described physical elements may also be operatively connected to the controller (212). Specifically, the controller (212) may direct a build material distributor (210) and any associated scanning carriages to move to add a layer of build material. Further, the controller (212) may send instructions to direct a printhead of an agent distributor to selectively deposit the agent(s) onto the surface of a layer of the build material. The controller (212) may also direct the printhead to eject the agent(s) at specific locations to form a 3D printed object slice.
[0044]In the example, where the additive manufacturing device (208) is agent-less, such as with an SLM additive manufacturing system, the controller (212) may direct operation of those components. The controller (212) may further raise and lower the air-permeable platform (104). That is, as described below in connection with FIG. 3, the air-permeable platform (104) may rise and lower to accommodate the formation of subsequent layers of the 3D object to be printed.
[0045]The controller (212) may include various hardware components, which may include a processor and memory. The processor may include the hardware architecture to retrieve executable code from the memory and execute the executable code. As specific examples, the controller as described herein may include computer readable storage medium, computer readable storage medium and a processor, an application specific integrated circuit (ASIC), a semiconductor-based microprocessor, a central processing unit (CPU), and a field-programmable gate array (FPGA), and/or other hardware device.
[0046]The memory may include a computer-readable storage medium, which computer-readable storage medium may contain, or store computer usable program code for use by or in connection with an instruction execution system, apparatus, or device. The memory may take many types of memory including volatile and non-volatile memory. For example, the memory may include Random Access Memory (RAM), Read Only Memory (ROM), optical memory disks, and magnetic disks, among others. The executable code may, when executed by the controller (212) cause the controller (212) to implement at least the functionality of building a 3D printed object.
[0047]FIG. 3 is a side view of an additive manufacturing device (208) with an air-permeable platform (104) for additive manufacturing, according to an example of the principles described herein. In an example, the additive manufacturing device (208) includes a receptacle (314) to hold the build material. As described above, in some examples the build material may be a dry powder. In other examples, the build material may be a slurry of build material particles, a liquid carrier, and a viscosity increasing agent. The additive manufacturing device (208) may also include a build material distributor (316) to deposit the build material across the bed (FIG. 1, 102).
[0048]As depicted in FIG. 3, the receptacle (314) may be a container, bed, or other vessel or surface that is to deliver the build material to the build material dispenser (316). In the example shown, the receptacle (314) is a remote vessel that feeds the build material into the build material dispenser (316) through a tube or other fluid conduit. However, in other examples, the receptacle (314) may take other forms. For example, the receptacle (314) may include a mechanism (e.g., a delivery piston or pump) to provide, e.g., move, the build material from a storage location to a position to be spread onto the air-permeable platform (104). For example, the receptacle (314) may be a stationary container located at the side of the additive manufacturing device (208) and may include a piston or pump to push the build material into a position where it can be spread across the air-permeable platform (104) by the build material distributor (316). In some examples, the receptacle (314) may include a mixing device to ensure uniform distribution of slurry components throughout the slurry volume.
[0049]FIG. 3 clearly depicts the air-permeable platform (104). The air-permeable platform (104) may be a horizontal surface upon which the build material (317) is applied and patterned to define any desirable shape. The air-permeable platform (104) receives the build material (317) from the receptacle (314). The air-permeable platform (104) may be integrated with the additive manufacturing device (208) or may be a component that is separately insertable into the additive manufacturing device.
[0050]In the example depicted in FIG. 3, the air-permeable platform (104) is a porous material that includes pores through which air may travel. The porous material may have an open pore structure meaning that there is a continuous porosity path throughout the thickness of the platform as depicted in FIG. 3. By comparison, a closed pore structure would not allow air to travel through the porous material plate. As depicted in FIG. 3, the pore size and porosity path of the porous material is not necessarily drawn to scale and has been enlarged to show detail.
[0051]As described above, the porous material may have pores ranging from 5 microns to 150 microns in diameter such that air may pass through. Pores of this size ensure that build material (317) particles do not draw through the pores. Were metal particles from the slurry or from dry powder to pass through the pores of the porous air-permeable platform (104), these build material (317) particles may clog the pores thus reducing the effect of the vacuum system (106) to draw air out of a layer of build material. In some examples, the porous material may be a porous metal material such as a loosely-sintered steel or aluminum. In other examples, the porous material may be a porous ceramic material.
[0052]As described above, the air-permeable platform (104) allows air to pass through. That is, the vacuum system (106) may draw a vacuum. Due to the air-permeability of the platform (104), air that may reside in a deposited layer of build material (317) may draw down through the air-permeable platform (104) as indicated by the arrows. In this example, the air is removed from the build material (317) and therefore does not result in voids in the final 3D printed object.
[0053]In some examples, the air-permeable platform (104), and in some cases other surfaces of the bed (FIG. 1, 102) may be heated. A heated platform (104) and sidewalls increases the evaporation rate of carrier fluid from both the slurry and the binding agent. In some examples, the air-permeable platform (104) itself may be heated. In another example, the air-permeable platform (104) may be adhered to a resistive heater located just below the air-permeable platform (104).
[0054]In some examples, the air-permeable platform (104) may be moved in a direction as denoted by the arrow (318), e.g., along the Z-axis, so that the build material (317) may be delivered to air-permeable platform (104) or to a previously deposited layer of build material (317). That is, when a layer of build material (317) is to be delivered, the air-permeable platform (104) may be programmed to advance (e.g., downward) enough so that the build material dispenser (316) may deposit the build material (317) onto the air-permeable platform (104) to form a slurry layer of the build material thereon.
[0055]During operation, the build material dispenser (316), which may span a length of the build area receives the build material (317) from the receptacle (314) and delivers the build material (317) locally to the air-permeable platform (104). The build material dispenser (316) may be moved in a direction as denoted by the arrow (320) e.g., along the Y-axis, cross the air-permeable platform (104) to deliver the build material (317) to the air-permeable platform (104). In one example, the build material dispenser (316) is a slot die coater.
[0056]In some examples, the build material dispenser (316) may include an additional component to spread the build material (317). For example, the build material dispenser (316) may include a blade (e.g., a doctor blade), a roller, a combination of a roller and a blade, and/or any other device capable of spreading the build material (317) over the air-permeable platform (104). For instance, the build material dispenser (316) may include a counter-rotating roller.
[0057]In some examples, the additive manufacturing device (208) includes an agent distributor (322) to selectively distribute a binding agent onto layers of the build material (317) in a pattern of a slice of the 3D object. The binding agent, when exposed to heat, may activate and selectively harden the underlying build material (317). The binding agent may take many forms. In one example, the binding agent includes metal salts or metal nanoparticles that bind the build material (317) particles together when energy is applied to the build bed.
[0058]In another example, the binding agent is a latex binder that temporarily adheres build material (317) particles together until they can be sintered together in a sintering operation. In this example, the latex binder may be a binding material that is inactive (non-binding) in a liquid vehicle, but becomes active (binding) upon exposure to heat or electromagnetic radiation. In some examples, the latex binder is a polymer.
[0059]A 3D object that has the latex binder dispensed thereon may be referred to as a patterned intermediate part which has a shape representative of the final 3D printed object and that includes metallic or ceramic particles patterned with a latex binder. In the patterned intermediate part, the metallic or ceramic particles may be weakly bound together by the hydrocolloid of the slurry, by components of the latex binder, and/or by attractive force(s) between the metallic or ceramic particles and the latex binder. In some instances, the mechanical strength of the patterned intermediate part is such that it cannot be handled or extracted from a bed (FIG. 1, 102).
[0060]Binding agent activation may include heating the binding agent to reach the minimum film formation temperature (MFFT) of the binding agent or to reach a reaction temperature that leads to a chemical change in the binding agent. In some examples, the binding agent may be exposed to ultraviolet (UV) radiation (e.g., wavelengths ranging from about 100 nm to about 400 nm) to initiate a chemical reaction in the binding agent.
[0061]In some examples, the binding agent may be a sacrificial intermediate agent in that it is present in various stages of the intermediate part that is formed, and then is ultimately removed (through thermal decomposition) in a de-binding operation and thus is not present in the final sintered 3D object. Examples of sacrificial intermediate agents that are activated by exposure to electromagnetic radiation in the UV range include epoxy acrylates, aliphatic urethane acrylates, aromatic urethane acrylates, polyester acrylates, and acrylic acrylates.
[0062]In another example, the binding agent is not sacrificial, but rather is a material that is not removed by thermal decomposition and is retained in the final, sintered 3D printed object. Metal nanoparticles and activated metal salts (e.g., metal salt decomposition products) are examples of agent materials that are not removed from the part by thermal decomposition. Examples of metal nanoparticle binders include silver (Ag), copper (Cu), gold (Au), nickel (Ni) and cobalt (Co) nanoparticles. Examples of metal salt binders include copper nitrate (Cu(NO3)2), iron nitrate (Fe(NO3)3), cobalt nitrate (Co(NO3)2), nickel nitrate (Ni(NO3)2), iron acetate (Fe(CH3COO)2,) magnesium acetate (Mg(CH3COO)2), copper sulfate (CuSO4), and manganese sulfate (MgSO4).
[0063]After exposure to elevated temperature, these metal salts can be activated (decomposed) to become water-insoluble decomposition products, such as metal oxides. While several example agents have been described, in an example of the binding agent, the binding component may be of a variety of types, such as acrylic latex, polyurethane, polyethylene, polypropylene, polyamide, UV curable monomers and oligomers, metal nanoparticles, metal salts and combinations thereof.
[0064]Note that in some examples, the additive manufacturing device (208) does not include an agent distributor (322). That is, the additive manufacturing device (208) may selectively harden portions of the build material (317) in other ways. For example, with selective laser melting (SLM), no binding agent is used and no subsequent oven-sintering process is performed. In these examples, the vacuum system (106) draws out the air from one layer of slurry build material (317) that is displaced as liquid from a second layer of slurry build material (317) seeps down.
[0065]In some examples, the agent distributor (322) is an inkjet agent distributor (322). The agent distributor (322) may include a reservoir containing the binding agent. The agent distributor (322) may also include nozzles, fluid slots, and/or fluidics for dispensing the binding agent. In some examples, the agent distributor (322) may be a thermal inkjet printhead or print bar, a piezoe