具体实施方式：
[0030]Fused Filament Fabrication (FFF) is a type of additive fabrication that extrudes a build material (also called a feedstock) onto a substrate such as a build platform to form an object. Generally the build material includes some kind of thermoplastic that is pushed through a heated extruder head. In some cases, the heated extruder head moves relative to the build platform during the deposition process resulting in the successive formation of the object. In one approach to FFF-style fabrication sometimes referred to as Bound Metal Deposition (BMD), the build material may include one or more powders so that the fabricated part includes a solid in powder form in addition to other components. The powders within the build material may include, either individually or as a part of a mixture of powders, metallic powders, ceramic powders, oxide powders, and/or carbide powders. In some approaches, these other components may include one or more binders that hold the powder(s) together, and which are removed subsequent to fabrication of the part. The debound part may then be sintered in a furnace to produce a solid metal object.
[0031]The debinding stage conventionally includes several steps and utilizes several different pieces of hardware. For example, a first debinding stage may occur in a solvent bath which chemically removes at least some of the binders, followed by a second debinding stage that may occur within a furnace. This solvent debinding stage may be problematic for several reasons, however. First, this stage may take several hours or days since the chemical solvent must permeate the part to dissolve the binder, and the primary binder may take time to dissolve in the solvent. Second, the dissolving of the primary binder may create open channels in the part. Third, solvent may cause swelling of the part. Relatively thinner portions of the part may swell quicker or may swell more than relatively thicker portions of the part, due to uneven permeation time of the solvent into the part. Such swelling may result in distortion or cracking of the part, particularly at the intersection of thinner and thicker portions.
[0032]Other debinding approaches may not rely on solvent debinding. One such approach instead utilizes a catalytic debinding process in which parts are debound in nitric acid vapor at an elevated temperature. This approach can also be problematic, however, because a separate furnace must be used to perform the catalytic debinding separate from the subsequent sintering process. In addition, the nitric acid vapor must be managed to protect users from its dangerous health effects and components of the oven must be designed to handle the acidic and aggressive nature of the nitric acid vapor at temperatures between 100° C. and 140° C.
[0033]The inventors have recognized and appreciated techniques for debinding additively fabricated parts that do not require solvent debinding or catalytic debinding, but may be performed by only thermal debinding in a furnace. As a result, in at least some cases debinding and sintering may take place sequentially within a single furnace. In some embodiments, the techniques may utilize particular materials as binders that allow for a thermal debinding process that does not negatively affect the parts.
[0034]Conventionally, one challenge with thermal-only debinding is that if binder material is incompletely removed during debinding, the remaining material may begin to flow while the parts are heated, thereby deforming or otherwise changing the shape of the part. If the primary binder is incompletely removed, for instance, it will typically have a plasticizing and lubricating effect upon the remaining components of the build material and will encourage the flow of the build material as the temperature is raised in the furnace. Such flow may be similar to the manner in which the material flowed when initially forming the parts using FFF, which may cause the object to flow, warp, tilt, slump, collapse, form blisters, and/or crack.
[0035]According to some embodiments, a build material (also referred to herein as a feedstock) for additive fabrication may include a primary binder component that has particular material properties that allow the component to be fully (or nearly fully) removed from parts during thermal processing without causing the aforementioned undesirable flow of the remaining material within the parts. The inventors have recognized and appreciated that the melting point and/or vapor pressure of the primary binder component are important properties to consider when selecting a suitable primary binder component (also referred to herein as a “primary binder”), so that the component can be completely removed through heating (thereby causing evaporation and/or sublimation of the primary binder component) without causing deformation of the parts.
[0036]The melting point of the primary binder component, for instance, may dictate the temperatures at which the primary binder component can be removed through thermal treatment and may also influence at least some of the mechanical properties of a part containing the binder at room temperature. If the melting point is too high, it may not be possible to remove the primary binder component without causing deleterious effects on the part during thermal treatment; conversely, if the melting point is too low, the parts may be too soft at room temperature. As a contrasting example, it has been observed by the inventors that use of paraffin wax as a conventional primary binder component within a feedstock can cause deformation of fabricated parts if only thermal debinding is performed. The inventors have recognized and appreciated that this occurs because the temperature at which the paraffin wax evaporates is sufficiently high to cause the remaining components of the feedstock to soften and flow, thereby damaging the part, before the paraffin wax can be safely removed. Thus, conventionally, parts formed from a feedstock that includes paraffin wax are typically chemically debound prior to thermal treatment.
[0037]According to some embodiments, a build material for additive fabrication may comprise metal and/or ceramic powder in addition to a binder. The binder may include an organic primary binder component that comprises between 30% and 60% by volume of the binder. The organic primary binder component may have a melting point above 40° C. and below 140° C., and may have a vapor pressure above 0.05 Torr at all temperatures between 50° C. and 160° C. These properties may result in the aforementioned behavior during thermal treatment that allows removal of the binder component from the build material without damage to the parts. In some embodiments, the organic binder component may comprise both a hydrophobic moiety and a hydrophilic moiety. For instance, the organic binder component may be an alcohol, a carboxylic acid, or an amine. In some embodiments, the organic binder component may comprise a hydrophobic moeity that is a substituted or unsubstituted aliphatic chain of 10 to 20 carbon atoms. Such a moeity may result in the above-mentioned desirable melting point and vapor pressure properties.
[0038]Build materials as described above, coupled with a suitable thermal process as described below, may address the above-described deficiencies of solvent debinding and catalytic debinding. The techniques described herein may provide a faster debinding process that is also less likely to lead to defects in a final part in addition to removing the need to move debound parts (which may be delicate) between or within devices, such as is performed in solvent debinding and catalytic debinding.
[0039]The feedstock for forming a green part, and thus the final part, may comprise a powder (e.g., metal powder particles and/or ceramic powder), a primary binder and a secondary binder. The feedstock may further include wetting agents, slip agents/lubricants, and die release agents. The feedstock may also comprise a continuous phase and a discrete phase, in which components of the feedstock (e.g., binders, metal particles, lubricants, wetting agents, tackifiers, etc.) may be assigned to each phase with a predetermined function.
[0040]In some embodiments, the build material from which green parts are formed may include a primary binder (e.g., a sublimable binder, or an evaporable binder) and a secondary binder. Green parts may be placed in a furnace and kept under appropriate vacuum (or low or controlled partial pressure of a specific individual gas species or mixture of gas species) and temperature conditions for a predetermined period of time so as to cause sublimation of the primary binder. Maintaining a vacuum or low partial pressure may ensure that the component in gas form may not deposit back onto the part or within the furnace. Alternately or in addition, ensuring that the binder in gas form does not deposit back into a part and/or within a furnace may be accomplished with a sweep gas at any pressure, where the rate of gas flow (or chamber turnovers) may be increased with the pressure. In such embodiments, the primary binder may covert to a gas while the secondary binder stays intact, as described further below.
[0041]Some embodiments may include heating a furnace to just below the melting point of a primary binder component to maximize the rate of the component's conversion to gas (e.g., through sublimation), and removing the gas (from the part or vicinity of the part) by either a flowing gas sweep or by vacuum. Flowing gas may also be used in the furnace no matter what the vacuum level/pressure may be, and this can increase the rate of sublimation at any temperature/vacuum combination. This may be an advantage, since the temperature condition(s) for sublimation may be adjusted by adjusting the pressure. This may be because the melting points of most candidate sublimable materials (e.g., durene) may change with pressure. The ability to select the sublimation temperature (within a range) may advantageously provide more options on which secondary binders may be paired with the primary binders.
[0042]For purposes of illustration, FIGS. 1A-1D depict a conventional process of forming an additive fabricated part using Fused Filament Fabrication (FFF) to produce a green part, following by solvent debinding to produce a brown part, followed by sintering in a furnace to produce a final part.
[0043]FIG. 1A is a block diagram of an additive manufacturing system according to some embodiments of the disclosure. System 100 includes an additive fabrication device (also sometimes called a three-dimensional (3D) printer) such as a fused filament fabrication (FFF) subsystem 102, a debinding subsystem 104 and a furnace subsystem 106 for treating the green part after fabrication. Fused filament fabrication subsystem 102 may be used to form an object from a build material, for example, by depositing successive layers of the build material onto a build plate. The build material may include metal powder and at least one binder material. In some embodiments, the build material may include a primary binder and a secondary binder (e.g., a polymer such as polypropylene).
[0044]Debinding subsystem 104 may be configured to treat the green part produced by fused filament fabrication subsystem 102 by performing a first debinding process in which a primary binder material may be removed from the green part. As discussed above, the first debinding process may traditionally be a solvent debinding process, as will be described in further detail with reference to FIG. 1C. In such cases, the primary binder material may dissolve in a debinding fluid while the secondary binder material remains, holding the metal particles in place in the brown part.
[0045]Furnace subsystem 106 may be configured to treat the brown part by performing a secondary debinding process in which the secondary binder and/or any remaining primary binder may be vaporized and removed from the printed part. In some embodiments, the secondary debinding process may comprise a thermal debinding process in which the furnace subsystem 106 may be operated to heat the part to vaporize the secondary binder (or otherwise convert the secondary binder to a gas).
[0046]As shown in FIG. 1A, system 100 may also include a user interface 110, which may be operatively coupled to one or more components, for example, to fused filament fabrication subsystem 102, debinding subsystem 104, and furnace subsystem 106, etc. In some embodiments, user interface 110 may be a remote device (e.g., a computer, a tablet, a smartphone, a laptop, etc.) or an interface incorporated into system 100, e.g., on one or more of the components. User interface 110 may be wired or wirelessly connected to one or more of fused filament fabrication subsystem 102, debinding subsystem 104, and/or furnace subsystem 106. System 100 may also include a control subsystem 116, which may be included in user interface 110, or may be a separate element.
[0047]Fused filament fabrication subsystem 102, debinding subsystem 104, furnace subsystem 106, user interface 110, and/or control subsystem 116 may each be connected to the other components of system 100 directly or via a network 112. Network 112 may include the Internet and may provide communication through one or more computers, servers, and/or handheld mobile devices, including the various components of system 100. For example, network 112 may provide a data transfer connection between the various components, permitting transfer of data including, e.g., part geometries, printing material, one or more support and/or support interface details, printing instructions, binders, heating and/or sintering times and temperatures, etc., for one or more parts or one or more parts to be printed.
[0048]Moreover, network 112 may be connected to a cloud-based application 114, which may also provide a data transfer connection between the various components and cloud-based application 114 in order to provide a data transfer connection, as discussed above. Cloud-based application 114 may be accessed by a user in a web browser, and may include various instructions, applications, algorithms, methods of operation, preferences, historical data, etc., for forming the part or object to be printed based on the various user-input details. Alternatively or additionally, the various instructions, applications, algorithms, methods of operation, preferences, historical data, etc., may be stored locally on a local server (not shown) or in a storage and/or processing device within or operably coupled to one or more of fused filament fabrication subsystem 102, debinding subsystem 104, sintering furnace subsystem 106, user interface 110, and/or control subsystem 116. In this aspect, fused filament fabrication subsystem 102, debinding subsystem 104, furnace subsystem 106, user interface 110, and/or control subsystem 116 may be disconnected from the Internet and/or other networks, which may increase security protections for the components of system 100. In either aspect, an additional controller (not shown) may be associated with one or more of fused filament fabrication subsystem 102, debinding subsystem 104, and furnace subsystem 106, etc., and may be configured to receive instructions to form the printed object and to instruct one or more components of system 100 to form the printed object.
[0049]FIG. 1B is a block diagram of an illustrative Fused Filament Fabrication (FFF) subsystem, according to some embodiments. In the example of FIG. 1B, the fused filament fabrication subsystem 102 may extrude a build material 124 (which may also be referred to as a feedstock 124) to form a three-dimensional part. As described above, the build material may include a mixture of metal powder and a binder containing one or more components. In some embodiments, the build material may include a mixture of a ceramic powder and a binder containing one or more components. In general, the build material may include any combination of metal powder, plastics, wax, ceramics, polymers, among others. In some embodiments, the build material 124 may come in the form of a rod or filament comprising a composition of metal powder and one or more binder components (e.g., a primary and a secondary binder).
[0050]Fused filament fabrication subsystem 102 may include an extrusion assembly 126 comprising an extrusion head 132. Fused filament fabrication subsystem 102 may include an actuation assembly 128 configured to move the build material 124 into the extrusion head 132. For example, the actuation assembly 128 may be configured to move a rod of build material 124 into the extrusion head 132. In some embodiments, the build material 124 may be continuously provided, e.g., as a spool of feedstock filament or fiber from the feeder assembly 122 to the actuation assembly 128, which in turn may move the build material 124 into the extrusion head 132. In some embodiments, the actuation assembly 128 may employ a linear actuator to continuously grip or push the build material 124 from the feeder assembly 122 towards and into the extrusion head 132.
[0051]In some embodiments, the fused filament fabrication subsystem 102 includes a heater 134 configured to generate heat 136 such that the build material 124 moved into the extrusion head 132 may be heated to a workable state. In some embodiments, the heater 134 may be integral to the extrusion head 132. As used herein, a “workable state” of a build material may refer to a build material that is able to be actuated through the extrusion head. In some cases, a “workable state” may also refer to a build material able to adhere to the build plate and/or cohere to successively deposited volumes of build material.
[0052]The build material 124 may be heated to a temperature at or below the temperature of the heater, depending on how long the build material is in proximity to the heater and/or how close the build material is to the heater. For example the heater may, when operated, heat to a temperature that is between 160° C. and 200° C., and when the build material is extruded through the extrusion apparatus (to which the heater may be integral as noted above), the build material may be heated to within several degrees of the operating temperature of the heater. By way of example, for a heater temperature of 165° C., the temperature of the build material may be heated up to, at most of 165° C. The temperature of the build material during extrusion may be referred to herein as a “deposition temperature.” In the above example, for instance, the deposition temperature of the build material is 165° C.
[0053]According to some embodiments, a deposition temperature during additive fabrication may be equal to or greater than 100° C., 120° C., 140° C., 160° C., 170° C., 180° C., 190° C. or 200° C. According to some embodiments, a deposition temperature during additive fabrication may be less than or equal to 230° C., 220° C., 210° C., 200° C., 190° C., 180° C., 170° C., 160° C., 140° C., 120° C., or 100° C. Any suitable combinations of the above-referenced ranges are also possible (e.g., a deposition temperature during additive fabrication of greater or equal to 140° C. and less than or equal to 170° C.).
[0054]In some embodiments, the heated build material 124 may be extruded through a nozzle 133 to extrude workable build material 142 onto a build plate 140. It is understood that the heater 134 is an exemplary device for generating heat 136, and that heat 136 may be generated in any suitable way, e.g., via friction of the build material 124 interacting with the extrusion assembly 126, in alternative embodiments. While there is one nozzle 133 shown in FIG. 1B, it is understood that the extrusion assembly 126 may comprise more than one nozzle in other embodiments. In some embodiments, the fused filament fabrication subsystem 102 may include another extrusion assembly (not shown in FIG. 1B) configured to extrude a non-sintering ceramic material onto the build plate 140.
[0055]In some embodiments, the fused filament fabrication subsystem 102 comprises a controller 138. The controller 138 may be configured to position the nozzle 133 along an extrusion path (also referred to as a toolpath) relative to the build plate 140 such that the workable build material is deposited on the build plate 140 to fabricate a three-dimensional green part 130. The controller 138 may be configured to manage operation of the fused filament fabrication subsystem 102 to fabricate the green part 130 according to a three-dimensional model. In some embodiments, the controller 138 may be remote or local to the metal printing subsystem 102. The controller 138 may be a centralized or distributed system. In some embodiments, the controller 138 may be configured to control a feeder assembly 122 to dispense the build material 124. In some embodiments, the controller 138 may be configured to control the extrusion assembly 126, e.g., the actuation assembly 128, the heater 134, the extrusion head 132, or the nozzle 133. In some embodiments, the controller 138 may be included in the control subsystem 116.
[0056]Fused filament fabrication subsystem 102 may separate the ambient environment inside and outside of the printer (e.g., the laboratory or office space) to provide for a controlled ambient environment within the printer. The subsystem may also include a filtration mechanism to capture any material that leaves the binder (from sublimation, evaporation, or reaction to a gaseous compound) from the printing process. Alternately or in addition, fused filament fabrication 102 may include or be in connection with a system that may provide a controlled fabrication temperature and atmosphere, and a filtration mechanism.
[0057]FIG. 1C depicts a block diagram of a traditional chemical debinder subsystem 104 for debinding a green part 130. The chemical debinder subsystem 104 may include a process chamber 150, into which the green part 130 may be inserted for a first debinding process. The first debinding process may be a solvent debinding process that may be performed in a storage chamber containing debinding fluid, e.g., a solvent. The storage chamber 156 may comprise a port which may be used to fill, refill, and/or drain the storage chamber 156 with the debinding fluid. The storage chamber 156 may be removably attached to the debinder subsystem 104. For example, the storage chamber 156 may be removed and replaced with a replacement storage chamber (not shown in FIG. 1C) to replenish the debinding fluid in the debinding subsystem 104. The storage chamber 156 may be removed, refilled with debinding fluid, and reattached to the debinding subsystem 104. As discussed above, embodiments of the present disclosure may remove the need for a chemical debinder, and the primary binder may be removed via sublimation and/or evaporation, as will be described further below. In such embodiments, all thermal processing, including removing of the primary binder through sublimation, removal of the secondary binder through vaporization, and sintering of the metal part, may occur in a furnace.
[0058]FIG. 1D is a block diagram of the furnace subsystem 106 according to exemplary embodiments. The furnace subsystem 106 may include one or more of a furnace chamber 162, an isolation system 164, an air injector 169 (also referred to as an oxygen injector, which may introduce air or oxygen gas into the system), and a catalytic converter system 170.
[0059]The furnace chamber 162 may be a sealable and insulated chamber designed to enclose a controlled atmosphere. In some cases, the atmosphere in the chamber may be controlled to be substantially free of oxygen. In some embodiments, the atmosphere may be configured to be substantially free of oxygen to prevent combustion. In the context of the current disclosure, a controlled atmosphere refers to an atmosphere being controlled for at least composition and pressure. The atmosphere may be controlled to be substantially free of oxygen, in some embodiments the atmosphere is configured to be substantially free of oxygen to prevent combustion.
[0060]The furnace chamber 162 may include one or more heating elements 182 for heating chamber contents enclosed within the furnace chamber 162. As shown in FIG. 1D, the brown part 131 may be placed in the furnace chamber 162 for thermal processing. e.g., a thermal debinding process or a densifying process. In some embodiments, the furnace chamber 162 may be heated to a suitable temperature as part of the thermal debinding process in order to remove any binder components included in the brown part 131 and then may be heated to a sintering temperature to densify the part. The furnace chamber 162 may include a retort 184 with walls partially or fully enclosing the region where the brown part 131 is located. In some embodiments, the furnace chamber 162, specifically the retort 184, may include one or more shelves on which the green part 130 may be placed within the furnace chamber 162.
[0061]Gas may be introduced to the furnace chamber to affect the atmosphere surrounding the printed object during thermal processing as the brown part 131 is heated during a thermal processing, e.g., during the thermal debinding process. In some embodiments, some amount per time of the furnace atmosphere may be pumped out of the furnace chamber 162, flowed through the isolation system 164, and directed towards the catalyst converter system 170. The isolation system 164 may be configured to prevent downstream species, gas, or gas components (e.g., gas, particularly oxygen gas from air injector 169) from transporting back towards the furnace chamber 162. The isolation system 164 or catalytic converter system 170 may be configured to remove at least a portion of the toxic fumes, e.g., at least a portion of the volatilized binder components.
[0062]As noted above, the system of FIGS. 1A-1D represents a conventional system for additive fabrication that includes a solvent debinding step. The techniques described herein improve over such a system by eliminating the solvent debinding step and requiring only a thermal debinding step. The resulting system may be represented by FIG. 2A, which is a block diagram of an additive manufacturing system suitable for practicing embodiments of the present disclosure, according to some embodiments.
[0063]As may be seen in FIG. 2A, additive manufacturing system 200 includes components 102, 110, 112, 114 and 116 as shown in FIG. 1A and described above, but notably does not require debinding subsystem 104, rather only requiring a furnace subsystem 206 to be described below. In the example of FIG. 2A, the furnace subsystem 206 may be operated to debind the green part produced by fused filament fabrication subsystem 102 and to sinter the resulting brown part.
[0064]FIG. 2B depicts a cross-sectional view of the furnace subsystem 206 of FIG. 2A, according to some embodiments. As will be discussed further below, according to the techniques for thermal debinding described herein, proper control of pressure and gas flow through the furnace is important to properly remove the primary binder components. Furnace subsystem 206 is configured to heat green parts 210 while controlling the pressure within the furnace.
[0065]The furnace 206 comprises a vacuum (or other isolating) chamber 202 separating the surrounding atmosphere from regions interior to the chamber, a thickness of high temperature insulation 204, and a series of heating elements (not shown for clarity), that may exist on several sides of the retort 208 which sits interior to the aforementioned elements. As shown, the retort may contain a series of shelves to support the parts 210 and the retort may be further configured with an integrated (or separate) gas flow manifold to distribute sweep gas across the objects to be sintered. After the gas has interacted with the objects, a downstream manifold may be configured to collect and remove the gas through an outlet toward a pump or other similar mechanism. Various apparatus upstream or downstream of the inlet and outlet manifolds may be employed to control the flow and pressure of gaseous species into and out of the furnace chamber. For instance, one or more pumps (e.g., pressure pumps, vacuum pumps) may be operated coupled to the inlet and/or outlet to control the pressure within the chamber of the furnace 206.
[0066]Furnace 206 may be configured for use with a primary binder component that may be converted to a gas through evaporation, sublimation, or other means, to be described further below. For example, the furnace may be capable of achieving the temperature and pressure conditions to sublimate and/or vaporize primary and secondary binders within a part. The furnace may comprise a flow line or in-line heating element configured at a temperature above which the sublimed component may be converted, combusted, reacted, or pyrolized to a material which can be exhausted through the pump or reacted by the catalytic converter and exhausted. For example, the furnace may, at least, be capable of achieving and sustaining vacuum conditions (e.g., 10−4 to 10 Torr, as low as 0.2 Torr, or a range including at least about 5 to 10 Torr, but perhaps as high as 300 Torr or 2200 Torr) and in some embodiments, temperature ranges of approximately 25° C.-1400° C. (as may be used in the sintering of ferrous metals, for example), and a range of approximately 25° C.-2800° C. (as may be used in the sintering of ceramics and carbides, for example) in other embodiments. The temperature ranges may also include ranges suitable for sintering the metal ceramic powder. For example a peak temperature of 800-1200° C. may be used for Cu, Ag, Au, or other metals, alloys, and ceramics with a melting temperature in this range. A peak temperature of 1200-1400° C. may be used for ferrous, Ni-based, or Ti-based metals and alloys. A peak temperature of 1400-1600° C. may be used for some carbides or ceramics.
[0067]The furnace 206 may further be configured to manage gaseous effluent from the binder(s) or other byproducts of the thermal debinding process to avoid contamination of the parts. This may be achieved, e.g., by allowing for the use of sweep gas, including one or more getters (e.g., activated charcoal, or metal powders, flakes, turnings, chips, or granules of a material with a high affinity for oxygen), including a catalytic converter, or any suitable structure or combination of structures available to manage effluent. The effluent may be captured as positions distal to the retort and furnace chamber. In another embodiment, the sublimed and/or evaporated material may be converted, pyrolized, or otherwise reacted by passing the effluent through an assembly of high temperature elements in such a manner that the effluent is brought to intimate and direct contact with the high temperature elements; in some embodiments, the elements may be held at a temperature between 200° C. and 500° C. In other embodiments, the elements may be held at a temperature between 400 and 800° C. Once the effluent has passed through the assembly of high temperature elements, the sublimed material is substantially converted to gaseous constituents or inert solids which can be managed by a catalytic converter, pumping system, or the like. Further, the furnace may include pressure or temperature gauges to monitor the status of thermal processing, including thermal debinding and/or sintering. The furnace subsystem 206 described in connection to FIG. 2 may include one or more of these described features.
[0068]To further describe how the furnace 206 may be operated to thermally debind and sinter a green part, FIGS. 3A-3B depict charts showing the furnace temperature over time, according to some embodiments. As discussed above, techniques described herein allow for thermal debinding without causing the build material within a part to flow. This result may be produced by either of the processes depicted in FIGS. 3A and 3B coupled with a suitable binder within the build material. Both the process and materials are described in further detail below, although it is assumed within FIGS. 3A and 3B that the parts contain a primary binder component, a secondary binder component, and at least some other material such as metal or ceramic.
[0069]In the example of each of FIGS. 3A-3B, the furnace initially is heated to a first temperature T1. In the subsequent discussion, when the temperature of the furnace is referenced, this refers to the temperature at which