BACKGROUND

[0001] Metal injection molding (MIM) is a metalworking process useful in creating a variety of metal objects. A mixture of powdered metal and binder (e.g., a polymer such as polypropylene) forms a“feedstock” capable of being molded, at a high temperature, into the shape of a desired object. The initial molded part, also referred to as a“green part,” then undergoes a debinding process to remove the binder, followed by a sintering process. During sintering, the part is brought to a temperature near the melting point of the powdered metal, which evaporates any remaining binder and forming the metal powder into a solid mass, thereby producing the desired object.

[0002] Additive manufacturing, also referred to as 3D printing, includes a variety of techniques for manufacturing a three-dimensional object via an automated process of forming successive layers of the object. 3D printers may utilize a feedstock comparable to that used in MIM, thereby creating a green part without the need for a mold. The green part may then undergo comparable debinding and sintering processes to produce the object.

SUMMARY

[0003] Example embodiments include a method of printing an object, including printing a shell and an infill structure. The shell may define an exterior of an object and include at least one aperture enabling flow of a debinder solvent therethrough. The infill structure may occupy a volume encompassed by the shell, and may define a network of interconnected channels, a cubic subset of the infill structure enabling percolation of the debinder solvent from any one face of the cubic subset to any other face of the cubic subset via the network of interconnected channels.

[0004] In further embodiments, the infill structure can enable percolation of the debinder solvent from any one volume internal to the infill structure to any other volume internal to the infill structure via the network of interconnected channels. The any one volume and any other volume may be internal to the network of interconnected channels. Alternatively, the any one volume and any other volume may be substantially equiaxed and have a

characteristic dimension equal to a width of two channels of the network of interconnected channels. The infill structure may enables percolation of the debinder solvent from any one channel segment adjacent to the shell to any other channel segment adjacent to the shell via the network of interconnected channels.

[0005] In further embodiments, the cubic subset of the infill structure may define a geometry that is isotropic. Printing the shell may also include printing the at least one aperture at an upper surface of the shell and at a bottom surface of the shell. The method may further include debinding the object, the debinding including enabling the debinder solution to flow through a first one of the at least one aperture, through the infill structure, and through a second one of the at least one aperture. The infill structure may be printed using a feedstock including a metal powder and a binder.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The foregoing will be apparent from the following more particular description of example embodiments, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments.

[0007] Fig. 1 is a block diagram of an additive manufacturing system.

[0008] Fig. 2 is a flow chart of a method for printing with composites.

[0009] Fig. 3 illustrates an additive manufacturing system for use with metal injection molding materials.

[0010] Fig. 4 illustrates a typical object printed by an additive manufacturing system.

[0011] Figs. 5A-B illustrate a printed object in an example embodiment.

[0012] Figs. 6A-C illustrate an infill structure in a further embodiment.

[0013] Figs. 7A-B illustrate an infill structure in a further embodiment.

[0014] Figs. 8A-B illustrate an infill structure in a further embodiment.

DETAILED DESCRIPTION

[0015] A description of example embodiments follows.

[0016] Fig. 1 is a block diagram of an additive manufacturing system for use with composites. The additive manufacturing system may include a three-dimensional printer 100 (or simply printer 100) that deposits metal using fused filament fabrication. Fused filament fabrication is well known in the art, and may be usefully employed for additive

manufacturing with suitable adaptations to accommodate the forces, temperatures and other environmental requirements typical of the metallic injection molding materials described herein. In general, the printer 100 may include a build material 102 that is propelled by a drive train 104 and heated to a workable state by a liquefaction system 106, and then dispensed through one or more nozzles 110. By concurrently controlling robotic system 108 to position the nozzle(s) along an extrusion path, an object 112 (also referred to as a part) may be fabricated on a build plate 114 within a build chamber 116. In general, a control system 118 manages operation of the printer 100 to fabricate the object 112 according to a three-dimensional model using a fused filament fabrication process or the like.

[0017] A variety of commercially available compositions have been engineered for metal injection molding (“MIM”). These highly engineered materials can also be adapted for use as a build material 102 in printing techniques such as fused filament fabrication. For example, MIM feedstock materials, when suitably shaped, may be usefully extruded through nozzles typical of commercially available FFF machines, and are generally flowable or extrudable within typical operating temperatures (e.g., 160-250 degrees Celsius) of such machines. This temperature range may depend on the binder— e.g., some binders achieve appropriate viscosities at about 205 degrees Celsius, while others achieve appropriate viscosities at lower temperatures such as about 160-180 C degrees Celsius. One of ordinary skill will recognize that these ranges (and all ranges listed herein) are provided by way of example and not of limitation. Further, while there are no formal limits on the dimensions for powder metallurgy materials, parts with dimensions of around 100 millimeters on each side have been demonstrated to perform well for FFF fabrication of net shape green bodies. Any smaller dimensions may be usefully employed, and larger dimensions may also be employed provided they are consistent with processing dimensions such as the print resolution and the extrusion orifice diameter. For example, implementations target about a 0.300pm diameter extrusion, and the MIM metal powder may typically be about l~22pm diameter, although nano sized powders can be used. The term metal injection molding material, as used herein, may include any such engineered materials, as well as other fine powder bases such as ceramics in a similar binder suitable for injection molding. Thus, where the term metal injection molding or the commonly used abbreviation, MIM, is used, the term may include injection molding materials using powders other than, or in addition to, metals and, thus, may include ceramics. Also, any reference to“MIM materials,”“powder metallurgy materials,” “MIM feedstocks,” or the like may generally refer to metal powder and/or ceramic powder mixed with one or more binding materials, e.g., a backbone binder that holds everything together and a bulk binder that carries the metal and backbone into position within a mold or print. Other material systems may be suitable for fabricating metal parts using fabrication techniques such as stereolithography or binder jetting, some of which are discussed in greater detail below. Such fabrication techniques may, in some applications, be identical to techniques for fabricating parts from ceramic material.

[0018] In general, fabrication of such materials may proceed as with a conventional FFF process, except that after the net shape is created, the green part may be optionally machined or finished while in a more easily workable state, and then debound and sintered into a final, dense object using any of the methods common in the art for MIM materials. The final object, as described above, may include a metal, a metal alloy, a ceramic, or another suitable combination of materials.

[0019] The build material 102 may be fed from a carrier 103 configured to dispense the build material to the three-dimensional printer either in a continuous (e.g., wire) or discrete (e.g., billet) form. The build material 102 may for example be supplied in discrete units one by one as billets or the like into an intermediate chamber for delivery into the build chamber 118 and subsequent melt and deposition. The carrier 103 may include a spool or cartridge containing the build material 102 in a wire form. Where a vacuum or other controlled environment is desired, the wire may be fed through a vacuum gasket into the build chamber 118 in a continuous fashion, however, typical MIM materials can be heated to a workable plastic state under normal atmospheric conditions, except perhaps for filtering or the like to remove particles from the build chamber 116. Thus, a MIM build material may be formed into a wire, the build material including an engineered composite of metal powder and a polymeric binder or the like, wherein the carrier 103 is configured to dispense the build material in a continuous feed to a three-dimensional printer. For environmentally sensitive materials, the carrier 103 may provide a vacuum environment for the build material 102 that can be directly or indirectly coupled to the vacuum environment of the build chamber 118. More generally, the build chamber 118 (and the carrier 103) may maintain any suitably inert environment for handling of the build material 102, such as a vacuum, and oxygen-depleted environment, an inert gas environment, or some gas or combination of gasses that are not reactive with the build material 102 where such conditions are necessary or beneficial during three-dimensional fabrication.

[0020] A drive train 104 may include any suitable gears, compression pistons, or the like for continuous or indexed feeding of the build material 116 into the liquefaction system 106. The drive train 104 may include gear shaped to mesh with corresponding features in the build material such as ridges, notches, or other positive or negative detents. The drive train 104 may use heated gears or screw mechanisms to deform and engage with the build material. Thus, a printer for a fused filament fabrication process can heats a build material to a working temperature, and that heats a gear that engages with, deforms, and drives the composite in a feed path. A screw feed may also or instead be used.

[0021] For more brittle MIM materials, a fine-toothed drive gear of a material such as a hard resin or plastic may be used to grip the material without excessive cutting or stress concentrations that might otherwise crack, strip, or otherwise compromise the build material.

[0022] The drive train 104 may use bellows, or any other collapsible or telescoping press to drive rods, billets, or similar units of build material into the liquefaction system 106.

Similarly, a piezoelectric or linear stepper drive may be used to advance a unit of build media in a non-continuous, stepped method with discrete, high-powered mechanical increments. Further, the drive train 104 may include multiple stages. In a first stage, the drive train 104 may heat the composite material and form threads or other features that can supply positive gripping traction into the material. In the next stage, a gear or the like matching these features can be used to advance the build material along the feed path. A collet feed may be used (e.g., similar to those on a mechanical pencil). A soft wheel or belt drive may also or instead be used. A shape forming wheel drive may be used to ensure accuracy of size and thus the build. More generally, the drive train 104 may include any mechanism or combination of mechanisms used to advance build material 102 for deposition in a three-dimensional fabrication process.

[0023] The liquefaction system 106 may be any liquefaction system configured to heat the composite to a working temperature in a range suitable for extrusion in a fused filament fabrication process. Any number of heating techniques may be used. Electrical techniques such as inductive or resistive heating may be usefully applied to liquefy the build material 102. This may, for example include inductively or resistively heating a chamber around the build material 102 to a temperature at or near the glass transition temperature of the build material 102, or some other temperature where the binder or other matrix becomes workable, extrudable, or flowable for deposition as described herein. Where the contemplated build materials are sufficiently conductive, they may be directly heated through contact methods (e.g., resistive heating with applied current) or non-contact methods (e.g., induction heating using an external electromagnet to drive eddy currents within the material). The choice of additives may further be advantageously selected to provide bulk electrical characteristics (e.g., conductance/resistivity) to improve heating. When directly heating the build material 102, it may be useful to model the shape and size of the build material 102 in order to better control electrically -induced heating. This may include estimates or actual measurements of shape, size, mass, etc. [0024] In the above context,“liquefaction” does not require complete liquefaction. That is, the media to be used in printing may be in a multi-phase state, and/or form a paste or the like having highly viscous and/or non-Newtonian fluid properties. Thus the liquefaction system 106 may include, more generally, any system that places a build material 102 in condition for use in fabrication.

[0025] In order to facilitate resistive heating of the build material 102, one or more contact pads, probes or the like may be positioned within the feed path for the material in order to provide locations for forming a circuit through the material at the appropriate location(s). In order to facilitate induction heating, one or more electromagnets may be positioned at suitable locations adjacent to the feed path and operated, e.g., by the control system 118, to heat the build material internally through the creation of eddy currents. Both of these techniques may be used concurrently to achieve a more tightly controlled or more evenly distributed electrical heating within the build material. The printer 100 may also be instrumented to monitor the resulting heating in a variety of ways. For example, the printer 100 may monitor power delivered to the inductive or resistive circuits. The printer 100 may also or instead measure temperature of the build material 102 or surrounding environment at any number of locations. The temperature of the build material 102 may be inferred by measuring, e.g., the amount of force required to drive the build material 102 through a nozzle 110 or other portion of the feed path, which may be used as a proxy for the viscosity of the build material 102. More generally, any techniques suitable for measuring temperature or viscosity of the build material 102 and responsively controlling applied electrical energy may be used to control liquefaction for a fabrication process using composites as described herein.

[0026] The liquefaction system 106 may also or instead include any other heating systems suitable for applying heat to the build material 102 to a suitable temperature for extrusion. This may, for example include techniques for locally or globally augmenting heating using, e.g., chemical heating, combustion, ultrasound heating, laser heating, electron beam heating or other optical or mechanical heating techniques and so forth.

[0027] The liquefaction system 106 may include a shearing engine. The shearing engine may create shear within the composite as it is heated in order to maintain a mixture of the metallic base and a binder or other matrix, or to maintain a mixture of various materials in a paste or other build material. A variety of techniques may be employed by the shearing engine. The bulk media may be axially rotated as it is fed along the feed path into the liquefaction system 106. Further, one or more ultrasonic transducers may be used to introduce shear within the heated material. Similarly, a screw, post, arm, or other physical element may be placed within the heated media and rotated or otherwise actuated to mix the heated material. Bulk build material may include individual pellets, rods, or coils (e.g., of consistent size) and fed into a screw, a plunger, a rod extruder, or the like. For example, a coiled build material can be uncoiled with a heater system including a heated box, heated tube, or heater from the printer head. Also, a direct feed with no heat that feeds right into the print head is also possible.

[0028] The robotic system 108 may include a robotic system configured to three- dimensionally position the nozzle 110 within the working volume 115 of the build chamber 116. This may, for example, include any robotic components or systems suitable for positioning the nozzle 110 relative to the build plate 114 while depositing the composite in a pattern to fabricate the object 112. A variety of robotics systems are known in the art and suitable for use as the robotic system 108 described herein. For example, the robotics may include a Cartesian or xy-z robotics system employing a number of linear controls to move independently in the x-axis, the y-axis, and the z-axis within the build chamber 116. Delta robots may also or instead be usefully employed, which can, if properly configured, provide significant advantages in terms of speed and stiffness, as well as offering the design convenience of fixed motors or drive elements. Other configurations such as double or triple delta robots can increase range of motion using multiple linkages. More generally, any robotics suitable for controlled positioning of the nozzle 110 relative to the build plate 114, especially within a vacuum or similar environment, may be usefully employed including any mechanism or combination of mechanisms suitable for actuation, manipulation, locomotion and the like within the build chamber 116.

[0029] The nozzle(s) 110 may include one or more nozzles for dispensing the build material 102 that has been propelled with the drive train 104 and heated with the liquefaction system 106 to a suitable working temperature. In a multiphase extrusion this may include a working temperature above the melting temperature of the metallic base of the composite, or more specifically between a first temperature at which the metallic base melts and the second temperature (above the first temperature) at which a second phase of the composite remains inert.

[0030] The nozzles 110 may, for example, be used to dispense different types of material so that, for example, one nozzle 110 dispenses a composite build material while another nozzle 110 dispenses a support material in order to support bridges, overhangs, and other structural features of the object 112 that would otherwise violate design rules for fabrication with the composite build material. Further, one of the nozzles 110 may deposit a different type of material, such as a thermally compatible polymer or a metal or polymer loaded with fibers of one or more materials to increase tensile strength or otherwise improve mechanical properties of the resulting object 112. Two types of supports may be used— (1) build supports and (2) sinter supports— e.g., using different materials printed into the same part to achieve these supports, or to create a distinguishing junction between these supports and the part.

[0031] The nozzle 110 may preferably be formed of a material or combination of materials with suitable mechanical and thermal properties. For example, the nozzle 110 will preferably not degrade at the temperatures wherein the composite material is to be dispensed, or due to the passage of metallic particles through a dispensing orifice therein. While nozzles for traditional polymer-based fused filament fabrication may be made from brass or aluminum alloys, a nozzle that dispenses metal particles may be formed of harder materials, or materials compatible with more elevated working temperatures such as a high carbon steel that is hardened and tempered. Other materials such as a refractory metal (e.g. molybdenum, tungsten) or refractory ceramic (e.g. mullite, corundum, magnesia) may also or instead be employed. In some instances, aluminum nozzles may instead be used for MIM extrusion of certain MIM materials. Further, a softer thermally conductive material with a hard, wear- resistant coating may be used, such as copper with a hard nickel plating.

[0032] The nozzle 110 may include one or more ultrasound transducers 130 as described herein. Ultrasound may be usefully applied for a variety of purposes in this context. The ultrasound energy may facilitate extrusion by mitigating clogging by reducing adhesion of a build material to an interior surface of the nozzle 110. A variety of energy director techniques may be used to improve this general approach. For example, a deposited layer may include one or more ridges, which may be imposed by an exit shape of the nozzle 110, to present a focused area to receive ultrasound energy introduced into the interface between the deposited layer and an adjacent layer.

[0033] The nozzle 110 may include an induction heating element, resistive heating element, or similar components to directly control the temperature of the nozzle 110. This may be used to augment a more general liquefaction process along the feed path through the printer 100, e.g., to maintain a temperature of the build material 102 during fabrication, or this may be used for more specific functions, such as declogging a print head by heating the build material 102 substantially above the working range, e.g., to a temperature where the composite is liquid. While it may be difficult or impossible to control deposition in this liquid state, the heating can provide a convenient technique to reset the nozzle 110 without more severe physical intervention such as removing vacuum to disassemble, clean, and replace the affected components.

[0034] The nozzle 110 may include an inlet gas or fan, e.g., an inert gas, to cool media at the moment it exits the nozzle 110. The resulting gas jet may, for example, immediately stiffen the dispensed material to facilitate extended bridging, larger overhangs, or other structures that might otherwise require support structures underneath.

[0035] The object 112 may be any object suitable for fabrication using the techniques described herein. This may include functional objects such as machine parts, aesthetic objects such as sculptures, or any other type of objects, as well as combinations of objects that can be fit within the physical constraints of the build chamber 116 and build plate 114. Some structures such as large bridges and overhangs cannot be fabricated directly using fused filament fabrication or the like because there is no underlying physical surface onto which a material can be deposited. In these instances, a support structure 113 may be fabricated, preferably of a soluble or otherwise readily removable material, in order to support the corresponding feature.

[0036] Where multiple nozzles 110 are provided, a second nozzle may usefully provide any of a variety of additional build materials. This may, for example, include other composites, alloys, bulk metallic glass’s, thermally matched polymers and so forth to support fabrication of suitable support structures. One of the nozzles 110 may dispense a bulk metallic glass that is deposited at one temperature to fabricate a support structure 113, and a second, higher temperature at an interface to a printed object 112 where the bulk metallic glass can be crystallized at the interface to become more brittle and facilitate mechanical removal of the support structure 113 from the object 112. Conveniently, the bulk form of the support structure 113 can be left in the super-cooled state so that it can retain its bulk structure and be removed in a single piece. Thus, a printer may fabricate a portion of a support structure 113 with a bulk metallic glass in a super-cooled liquid region, and may fabricate a layer of the support structure adjacent to a printed object at a greater temperature in order to crystalize the build material 102 into a non-amorphous alloy. The bulk metallic glass particles may thus be loaded into a MIM feedstock binder system and may provide a support. Pure binding or polymer materials (e.g., without any loading) may also or instead provide a support. A similar metal MIM feedstock may be used for multi-material part creation. Ceramic or dissimilar metal MIM feedstock may be used for a support interface material. [0037] The build plate 114 within the working volume 115 of the build chamber 116 may include a rigid and substantially planar surface formed of any substance suitable for receiving deposited composite or other material (s)s from the nozzles 110. The build plate 114 may be heated, e.g., resistively or inductively, to control a temperature of the build chamber 116 or the surface upon which the object 112 is being fabricated. This may, for example, improve adhesion, prevent thermally induced deformation or failure, and facilitate relaxation of stresses within the fabricated object. Further, the build plate 114 may be a deformable build plate that can bend or otherwise physical deform in order to detach from the rigid object 112 formed thereon.

[0038] The build chamber 116 may be any chamber suitable for containing the build plate 114, an object 112, and any other components of the printer 100 used within the build chamber 116 to fabricate the object 112. The build chamber 116 may be an environmentally sealed chamber that can be evacuated with a vacuum pump 124 or similar device in order to provide a vacuum environment for fabrication. This may be particularly useful where oxygen causes a passivation layer that might weaken layer-to-layer bonds in a fused filament fabrication process as described herein, or where particles in the atmosphere might otherwise interfere with the integrity of a fabricated object, or where the build chamber 116 is the same as the sintering chamber. Alternatively, only oxygen may be removed from the build chamber 116.

[0039] Similarly, one or more passive or active oxygen getters 126 or other similar oxygen absorbing material or system may usefully be employed within the build chamber 116 to take up free oxygen within the build chamber 116. The oxygen getter 126 may, for example, include a deposit of a reactive material coating an inside surface of the build chamber 116 or a separate object placed therein that completes and maintains the vacuum by combining with or adsorbing residual gas molecules. The oxygen getters 126, or more generally, gas getters, may be deposited as a support material using one of the nozzles 110, which facilitates replacement of the gas getter with each new fabrication run and can advantageously position the gas getter(s) near printed media in order to more locally remove passivating gasses where new material is being deposited onto the fabricated object. The oxygen getters 126 may include any of a variety of materials that preferentially react with oxygen including, e.g., materials based on titanium, aluminum, and so forth. Further, the oxygen getters 126 may include a chemical energy source such as a combustible gas, gas torch, catalytic heater, Bunsen burner, or other chemical and/or combustion source that reacts to extract oxygen from the environment. There are a variety of low-CO and NOx catalytic burners that may be suitably employed for this purpose without CO.

[0040] The oxygen getter 126 may be deposited as a separate material during a build process. Thus, a three-dimensional object may be fabricated from a metallic composite, including a physically adjacent structure (which may or may not directly contact the three- dimensional object) fabricated to contain an agent to remove passivating gasses around the three-dimensional object. Other techniques may be similarly employed to control reactivity of the environment within the build chamber 116, or within post-processing chambers or the like as described below. For example, the build chamber 116 may be filled with an inert gas or the like to prevent oxidation.

[0041] The control system 118 may include a processor and memory, as well as any other co-processors, signal processors, inputs and outputs, digital-to-analog or analog-to-digital converters and other processing circuitry useful for monitoring and controlling a fabrication process executing on the printer 100. The control system 118 may be coupled in a communicating relationship with a supply of the build material 102, the drive train 104, the liquefaction system 106, the nozzles 110, the build plate 114, the robotic system 108, and any other instrumentation or control components associated with the build process such as temperature sensors, pressure sensors, oxygen sensors, vacuum pumps, and so forth. The control system 118 may be operable to control the robotic system 108, the liquefaction system 106 and other components to fabricate an object 112 from the build material 102 in three dimensions within the working volume 115 of the build chamber 116.

[0042] The control system 118 may generate machine ready code for execution by the printer 100 to fabricate the object 112 from the three-dimensional model 122 stored to a database 120. The control system 118 may deploy a number of strategies to improve the resulting physical object structurally or aesthetically. For example, the control system 118 may use plowing, ironing, planing, or similar techniques where the nozzle 110 runs over existing layers of deposited material, e.g., to level the material, remove passivation layers, apply an energy director topography of peaks or ridges to improve layer-to-layer bonding, or otherwise prepare the current layer for a next layer of material. The nozzle 110 may include a low-friction or non-stick surface such as Teflon, TiN or the like to facilitate this plowing process, and the nozzle 110 may be heated and/or vibrated (e.g., using an ultrasound transducer) to improve the smoothing effect. This surface preparation may be incorporated into the initially-generated machine ready code. Alternatively, the printer 100 may dynamically monitor deposited layers and determine, on a layer-bylayer basis, whether additional surface preparation is necessary or helpful for successful completion of the object.

[0043] Fig. 2 shows a flow chart of a method for printing with composites, e.g., metal injection molding materials. As shown in step 202, the process 200 may include providing a build material including an injection molding material, or where a support interface is being fabricated, a MIM binder (e.g., a MIM binder with similar thermal characteristics). The material may include, for example, any of the MIM materials described herein. The material may be provided as a build material in a billet, a wire, or any other cast, drawn, extruded or otherwise shaped bulk form. As described above, the build material may be further packaged in a cartridge, spool, or other suitable carrier that can be attached to an additive

manufacturing system for use.

[0044] As shown in step 204, the process may include fabricating a layer of an object. This may include any techniques that can be adapted for use with MIM materials. For example, this may include fused filament fabrication, jet printing or any other techniques for forming a net shape from a MIM material (and more specifically for techniques used for forming a net shape from a polymeric material loaded with a second phase powder).

[0045] As shown in step 211, this process may be continued and repeated as necessary to fabricate an object within the working volume. While the process may vary according to the underlying fabrication technology, an object can generally be fabricated layer by layer based on a three-dimensional model of the desired object. As shown in step 212, the process 200 may include shaping the net shape object after the additive process is complete. Before debinding or sintering, the green body form of the object is usefully in a soft, workable state where defects and printing artifacts can be easily removed, either manually or automatically. Thus the process 200 may take advantage of this workable, intermediate state to facilitate quality control or other process-related steps, such as removal of supports that are required for previous printing steps, but not for debinding or sintering.

[0046] As shown in step 214, the process 200 may include debinding the printed object.

In general debinding may be performed chemically or thermally to remove a binder that retains a metal (or ceramic or other) powder in a net shape. Contemporary injection molding materials are often engineered for thermal debinding, which advantageously permits debinding and sintering to be performed in a single baking operation, or in two similar baking operations. In general, the debinding process functions to remove binder from the net shape green object, thus leaving a very dense structure of metal (or ceramic or other) particles that can be sintered into the final form. [0047] As shown in step 216, the process 200 may include sintering the printed and debound object into a final form. In general, sintering may be any process of compacting and forming a solid mass of material by heating without liquefaction. During a sintering process, atoms can diffuse across particle boundaries to fuse into a solid piece. Because sintering can be performed at temperatures below the melting temperature, this advantageously permits fabrication with very high melting point materials such as tungsten and molybdenum.

[0048] Numerous sintering techniques are known in the art, and the selection of a particular technique may depend upon the build material used, and the desired structural, functional or aesthetic result for the fabricated object. For example, in solid-state (non- activated) sintering, metal powder particles are heated to form connections (or“necks”) where they are in contact. Over time, these necks thicken and create a dense part, leaving small, interstitial voids that can be closed, e.g., by hot isostatic pressing (HIP) or similar processes. Other techniques may also or instead be employed. For example, solid state activated sintering uses a film between powder particles to improve mobility of atoms between particles and accelerate the formation and thickening of necks. As another example, liquid phase sintering may be used, in which a liquid forms around metal particles. This can improve diffusion and joining between particles, but also may leave lower-melting phase within the sintered object that impairs structural integrity. Other advanced techniques such as nano-phase separation sintering may be used, for example to form a high-diffusivity solid at the necks to improve the transport of metal atoms at the contact point

[0049] Debinding and sintering may result in material loss and compaction, and the resulting object may be significantly smaller than the printed object. However, these effects are generally linear in the aggregate, and net shape objects can be usefully scaled up when printing to create a corresponding shape after debinding and sintering.

[0050] Fig. 3 shows an additive manufacturing system for use with metal injection molding materials. The system 300 may include a printer 302, a conveyor 304, and a postprocessing station 306. In general, the printer 302 may be any of the printers described above including, for example a fused filament fabrication system, a stereolithography system, a selective laser sintering system, or any other system that can be usefully adapted to form a net shape object under computer control using injection molding build materials. The output of the printer 302 may be an object 303 that is a green body including any suitable powder (e.g., metal, metal alloy, ceramic, and so forth, as well as combinations of the foregoing), along with a binder that retains the powder in a net shape produced by the printer 302. [0051] The conveyor 304 may be used to transport the object 303 from the printer 302 to a post-processing station 306 where debinding and sintering can be performed. The conveyor 304 may be any suitable device or combination of devices suitable for physically transporting the object 303. This may, for example, include robotics and a machine vision system or the like on the printer side for detaching the object 303 from a build platform or the like, as well as robotics and a machine vision system or the like on the post-processing side to accurately place the object 303 within the post-processing station 306. Further, the post-processing station 306 may serve multiple printers so that a number of objects can be debound and sintered concurrently, and the conveyor 304 may interconnect the printers and post processing station so that multiple print jobs can be coordinated and automatically completed in parallel. Alternatively, the object 303 may be manually transported between the two corresponding stations.

[0052] The post-processing station 306 may be any system or combination of systems useful for converting a green part formed into a desired net shape from a metal injection molding build material by the printer 302 into a final object. The post-processing station 306 may, for example, include a chemical debinding station and a thermal sintering station that can be used in sequence to produce a final object. Some contemporary injection molding materials are engineered for thermal debinding, which makes it possible to perform a combination of debinding and sintering steps with a single oven or similar device. While the thermal specifications of a sintering furnace may depend upon the powder to be sintered, the binder system, the loading, and other properties of the green object and the materials used to manufacture same, commercial sintering furnaces for thermally debound and sintered MIM parts may typically operate with an accuracy of +/- 5 degrees Celsius or better, and temperatures of at least 600 degrees C, or from about 200 degrees C to about 1900 degrees C for extended times. Any such furnace or similar heating device may be usefully employed as the post-processing station 306 as described herein. Vacuum or pressure treatment may also or instead be used. Identical or similar material beads with a non-binding coating may be used for a furnace support— e.g., packing in a bed of this material that shrinks similar to the part, except that it will not bond to the part.

[0053] Embodiments may be implemented with a wide range of other debinding and sintering processes. For example, the binder may be removed in a chemical debind, thermal debind, or some combination of these. Other debinding processes are also known in the art (such as supercritical or catalytic debinding), any of which may also or instead be employed by the post-processing station 306 as described herein. For example, in a common process, a green part is first debound using a chemical debind, which is following by a thermal debind at a moderately high temperature (in this context, around 700-800 C) to remove organic binder and create enough necks among a powdered material to permit handling. From this stage, the object may be moved to a sintering furnace to remove any remaining components of a binder system densify the object. Alternatively, a pure thermal debind may be used to remove the organic binder. More general, any technique or combination of techniques may be usefully employed to debind an object as described herein.

[0054] Similarly, a wide range of sintering techniques may be usefully employed by the post-processing station. For example, an object may be consolidated in a furnace to a high theoretical density using vacuum sintering. Altematviely, the furnace may use a combination of flowing gas (e.g., at below atmosphere, slightly above atmosphere, or some other suitable pressure) and vacuum sintering. More generally, any sintering or other process suitable for improving object density may be used, preferably where the process yields a near-theoretical density part with little or no porosity. Hot-isostatic pressing (“HIP”) may also (e.g., as a postsinter finishing step) or instead be employed, e.g., by applying elevated temperatures and pressures of 10-50 ksi, or between about 15 and 30 ksi. Alternatively, the object may be processed using any of the foregoing, followed by a moderate overpressure (greater than the sintering pressure, but lower than HIP pressures). In this latter process, gas may be pressurized at 100-1500 psi and maintained at elevated temperatures within the furnace or some other supplemental chamber. Alternatively, the object may be separately heated in one furnace, and then immersed in a hot granular media inside a die, with pressure applied to the media so that it can be transmitted to the object to drive more rapid consolidation to near full density. More generally, any technique or combination of techniques suitable for removing binder systems and driving a powdered material toward consolidation and densification may be used by the post-processing station 306 to process a fabricated green part as described herein.

[0055] The post-processing station 306 may be incorporated into the printer 302, thus removing a need for a conveyor 304 to physically transport the object 303. The build volume of the printer 302 and components therein may be fabricated to withstand the elevated debinding/sintering temperatures. Alternatively, the printer 302 may provide movable walls, barriers, or other enclosure(s) within the build volume so that the debind/sinter can be performed while the object 303 is on a build platform within the printer 302, but thermally isolated from any thermally sensitive components or materials. [0056] The post-processing station 306 may be optimized in a variety of ways for use in an office environment. The post-processing station 306 may include an inert gas source 308. The inert gas source 308 may, for example, include argon or other inert gas (or other gas that is inert to the sintered material), and may be housed in a removable and replaceable cartridge that can be coupled to the post-processing station 306 for discharge into the interior of the post-processing station 306, and then removed and replaced when the contents are exhausted. The post-processing station 306 may also or instead include a filter 310 such as a charcoal filter or the like for exhausting gasses that can be outgassed into an office environment in an unfiltered form. For other gasses, an exterior exhaust, or a gas container or the like may be provided to permit use in unventilated areas. For reclaimable materials, a closed system may also or instead be used, particularly where the environmental materials are expensive or dangerous.

[0057] The post-processing station 306 may be coupled to other system components. For example, the post-processing station 306 may include information from the printer 302, or from a controller for the printer, about the geometry, size, mass and other physical characteristics of the object 303 in order to generate a suitable debinding and sintering profile. Optionally, the profile may be created independently by the controller or other resource and transmitted to the post-processing station 306 when the object 303 is conveyed. Further, the post-processing station 306 may monitor the debinding and sintering process and provide feedback, e.g., to a smart phone or other remote device 312, about a status of the object, a time to completion, and other processing metrics and information. The post processing station 306 may include a camera 314 or other monitoring device to provide feedback to the remote device 312, and may provide time lapse animation or the like to graphically show sintering on a compressed time scale. Post-processing may also or instead include finishing with heat, a hot knife, tools, or similar, and may include applying a finish coat.

[0058] Fig. 4 illustrates a typical object 400 printed by a printer of an additive manufacturing system such as the printer 100 described above. The object 400 is a cube, shown in a cross-section view, and includes a solid external layer including a top layer, bottom layer, and side walls. The external layer encompasses an infill occupying the internal volume of the solid. The object 400 is a“green” part that must be debinded and then sintered, as described above with reference to Figs. 2 and 3.

[0059] For green parts having thicker or more voluminous portions, during debinding, the debinding solvent may require more time to fully penetrate the part and dissolve the binder component of the green part. During such a debinding, solid portions of the green part can inhibit the flow of the solvent, allowing passage only after the solvent has created pores in the portion of the solid as a result of dissolving the binder within that portion. Even after the pores are formed, the small scale of those pores continue to limit the flow of solvent (also referred to as“debind fluid”) through the part. As a result, the part may require an excessive amount of time to complete the debinding process.

[0060] In order to shorten the debind process, the object 400 includes an infill that defines several vertical channels extending in parallel to one another. This configuration can shorten debind time by directing the flow of the solvent through the channels, as well as decreasing the thickness of any one portion of the object 400. However, the object 400 may also introduce a number of disadvantages. For example, the infill may not possess structural integrity that is sufficient for a intended application of the object, causing the object to fail during use. During debinding, the debinder solvent must first penetrate (and at least partially debind) the external layer of the object before it reaches the infill, and the resulting solution (i.e., binder dissolved in the solvent) must also flow through the external layer. Thus, the external layer hinders debinding of the infill. Further, once the debinder solvent flows into the infill, it is generally confined to a single vertical channel, thereby limiting the efficacy of the solvent and hindering the evacuation of the solution from the object 400.

[0061] Figs. 5A-B illustrate a printed object 500 in an example embodiment. A shell defines an exterior of the object 500 and includes one or more apertures that enable flow of a debinder solvent therethrough. An infill structure occupies a volume encompassed by the shell, and defines a network of interconnected channels. Fig. 5A illustrates a first lateral cross-section of the object 500, and Fig. 5B illustrates a second lateral cross-section, where the first and second cross-sections are parallel and separated by a distance effective to illustrate the shape of the infill structure. In particular, Fig. 5A is a cross-section depicting channels that enable flow of the solvent in a direction through the cross-section plane. Fig. 5B, in contrast, is a cross-section of the same infill structure, and depicts a portion of the structure enabling flow of the solvent through any portion within the plane of the cross- section. As shown, the object 500 has a square cross-section, but may also include portions occupying any geometry, and the composition of the cross-section (i.e., shell and infill structure) may be adapted to any such geometry. The composition of the object 500 may also be applied to printed objects of any geometry in further embodiments.

[0062] By forming a network of interconnected channels, the infill structure generally enables percolation of a debinder solvent through any portion of the infill structure. For example, if the infill structure were sampled as a cubic volume (e.g., a cubic subset of the structure), the structure would enable flow of the debinder solvent from any one face of the cubic volume to any other face of the cubic volume via the network of interconnected channels. The infill structure (or a subset of the infill structure) may have a geometry that is isotropic, possessing uniformity in all directions. The channels of the network may be scaled in size to balance solvent flow and structural integrity of the object. For example, channels of approximately 2mm in diameter may enable acceptable solvent flow while maintaining acceptable strength of the structure.

[0063] Although the object 500 is shown to include a single aperture at the top surface of the shell and a single aperture at the bottom surface of the shell, the object 500 may include a greater or fewer number of apertures to facilitate the flow of the solvent. The apertures may be sized to accommodate adequate flow of solvent (e.g., 2mm). During a debinding of the object 500, the object 500 may be submerged in the debinder solvent, enabling the debinder solvent (and resulting solution) to flow into and out of the infill structure via the apertures.

The solvent may also debind the shell and enable passage of the solvent and/or solution through the pores of the debinded shell. The apertures may also be omitted if it is determined that the object 500 can be debinded within acceptable parameters (e.g., debind time) without the apertures.

[0064] The object 500 may be printed from a feedstock including metal and/or ceramic powder(s) and one or more binders. Alternatively, the shell and/or infill structure may be printed from one or more different feedstocks comprising metal and/or ceramic powders and one or more binders. For example, a printer 100 as described above may print the shell of the object 500 from a first feedstock and the infill structure from a second feedstock having a different composition of metal or ceramic powder(s) and binders.

[0065] In example embodiments, the infill structure enables for fluid movement through the structure in the x, y, and z directions. The infill structure should also exhibit strength in the x, y, and z directions sufficient for the intended application of the object. Sufficient strength also ensures that the part maintains its shape through the debinding and sintering process. Because the infill structure is a component of the finished, post-sintering part, the part’s mechanical properties will be effected by the infill structure.

[0066] In order to ensure sufficient strength, one or more test parts may be printed of the desired infill geometry. The geometry can be isolated, or can have a shell, or may be attached to solid features that would be pulled in a tensile test (e.g., a dogbone-type structure). The test structure may be tested in compression and tension in the x, y, and z directions. From such testing, the strength, elastic modulus, ductility, and other properties can be obtained for the structure. A high strength-to-weight ratio, as well as equal performance in all directions, may be desirable for the intended application of the part.

[0067] In an example embodiment, an infill structure may include a few notable features. First, the structure may have a pattern that can be printed at each layer by print paths that are substantially continuous and uninterrupted within the layer. Small struts, in contrast, may not be easily printed, as they require the extruder of the printer to frequently start and stop extrusion. Fused filament fabrication (FDM) printers may encounter difficulty with such printing, leading to missed extrusion, or excess material deposited between the struts. Thus, it is beneficial for the infill structure to have long, continuous extrusion paths in the XY plane. Second, the infill structure may be self-supported during printing. FDM-type processes can often print overhangs of up to 50-65 degrees. If the layers are offset by too much (i.e., in high overhangs), the extruder will be printing most of the bead in mid-air, which may prevent the bead from attaching to the previous layer, thereby reducing the strength of the infill structure. The above features can be determined based on extrusion paths and overhang angles, and can be verified by generating test prints, observing those prints for defects, and modifying the structure accordingly.

[0068] Example embodiments may also provide advantages during sintering. The object may undergo a second debind (a“thermal debind”) during sintering, and the infill structure may improve the thermal debind, by improving the efficacy and speed at which thermal debind products can leave the structure. The percolating structure of the infill structure may allow remaining binder to escape from the part and prevent any build-up of pressure inside the object. The gas may also enter and exit through the apertures in the shell, rather than passing only through the smaller pores in the solid portion of the shell. Thus, the structure and apertures generally improve gas flow within the part. The time of the thermal debind can also be reduced due to the ability of binder to escape throughout the infill and out the apertures at the shell. The object may also exhibit improved resistance to cracking, warping and slumping during the sintering process. To provide such resistances, a structure that has uniform and high strength in the x, y, and z directions may be advantageous. Such a structure can contribute to maintaining even shrinkage of the part during sintering, and have good strength particularly in the x and y directions where the possibility of cracking is highest.

[0069] Figs. 6A-C illustrate an infill structure 600 in a further embodiment. Figs. 6A,

6B and 6C show isometric, top and side views, respectively, of the infill structure 600. The infill structure 600 may be incorporated into a printed object such as the object 500 described above, and may include features of the infill structure of the object 500 described above. The pattern of the infill structure 600 may be comparable to the pattern shown in Fig. 5, and, in particular, enables percolation of debinder solvent through the network of channels formed by the structure. The size of the pattern may be scaled for use in a given printed object to ensure sufficient debinding and object strength.

[0070] Figs. 7A-B illustrate an infill structure 700 in a further embodiment. Figs. 7A and 7B show isometric and side views, respectively, of the infill structure 700. The infill structure 700 may be incorporated into a printed object such as the object 500 described above, and may include features of the infill structure of the object 500 described above. The infill structure 700 exhibits an isotropic gyroid pattern that enables percolation of debinder solvent through the network of channels formed by the structure. The size of the pattern may be scaled for use in a given printed object to ensure sufficient debinding and object strength.

[0071] Figs. 8A-B illustrate an infill structure 800 in a further embodiment. Figs. 8A and 8B show isometric and side views, respectively, of the infill structure 800. The infill structure 800 may be incorporated into a printed object such as the object 500 described above, and may include features of the infill structure of the object 500 described above. The infill structure 800 exhibits an isotropic gyroid pattern that enables percolation of debinder solvent through the network of channels formed by the structure. The size of the pattern may be scaled for use in a given printed object to ensure sufficient debinding and object strength. In alternative embodiments, an object may include an infill structure exhibiting other patterns, such as a diamond matrix or a diamond lattice pattern.

[0072] While example embodiments have been particularly shown and described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the embodiments encompassed by the appended claims.