Patent Application
20230044534
The subject matter of the present disclosure generally relates to retorts for sintering objects, and more particularly relates to retorts for improved thermal processing of sinterable objects.
Various powder metallurgy processes may be used to form metal objects such as parts. Such processes typically involve thermal processing (which may include debinding or sintering) a metal powder into a solid form.
Thermal processing hardware includes several elements to successfully process parts produced via powder metallurgy processes. Powder metallurgy processes may include legacy techniques such as press and sinter and metal injection molding, or more contemporary additive manufacturing techniques such as bound metal deposition, binder jetting, or any other technique where a part is shaped from a build material which includes a powdered or otherwise finely divided material (e.g., a metal like 17-4 stainless steel, 316 stainless steel, H13 tool steel, A2 tool steel, D2 tool steel, Ti6-4 titanium, commercially pure titanium, aluminum, 4140 carbon steel, ceramics, and the like) that is later exposed to a sufficiently high temperature and appropriate atmosphere to accomplish the consolidation and densification of the finely divided material to a solid form with a desired density, typically above 90%, but perhaps above 96% or 98% depending upon the material. As one skilled in the art will appreciate, descriptions are available in the literature prescribing the combination of temperature and chemical atmosphere that is required to consolidate a wide range of materials of commercial interest (see, for example “Sintering: Theory and Practice” by R. M. German).
During thermal processing of sinterable objects, objects to be processed are exposed to a wide range of chemical and thermal conditions. While some of these conditions may be desirable (e.g., inert gas and increased temperature), others may not (e.g., outgassed and leaked gaseous species such as water, oxygen, and hydrocarbons) and may lead to undesirable changes in the chemistry of the object or wholesale failure of the process. One skilled in the art will appreciate that species which may be beneficial in a first sintering process may be deleterious to a second sintering process; crucially, however, both cases demand that the operators and designers of thermal processes utilize thermal processing hardware and devices which are robust to the entry of unknown gaseous species into regions where parts are thermally processed.
It is common in such thermal processing processes to employ a retort which rests on the furnace floor and holds or contains the objects being processed. A main purpose of the retort is to provide a controlled chemical (e.g., gaseous) atmosphere during sintering. Existing such retorts are insufficient to protect objects undergoing thermal processes from the undesirable conditions identified above.
Described are approaches to design a retort, to be placed within a furnace, such that the objects to be thermally processed within the retort and furnace hotzone will be thermally processed in an atmosphere with improved control and consistency.
The present disclosure may be employed in a wide range of sintering environments and furnaces, including without limitation the manufacture of objects via traditional powder metallurgy techniques in addition to additive manufacturing technologies such as fused filament fabrication by extrusion or powder bed binder jet printing.
Described is a new and improved method and approach to retort design coupled with process parameters to improve conditions imposed upon objects during thermal processing. The concepts and techniques disclosed may be applied in many different configurations, and may improve the processing performance of many different styles and configurations of furnaces.
An example thermal processing profile is shown schematically in
The thermal processing hardware is essential to the performance of the object during and after thermal processing. Typically, the thermal processing hardware is identified as a furnace, but other terms such as “vacuum furnace”, “tube furnace”, “Metal injection molding (MIM) furnace” should be understood as thermal processing hardware; without loss of applicability, this disclosure will refer to the collective group of thermal processing hardware as “furnaces” collectively, and as a “furnace” in the singular.
Furnaces typically include the following elements: (a) a furnace housing, typically, but not always, constructed from a high temperature material such as a refractory ceramic or temperature resistant metal alloy, (b) high temperature insulation, (c) a means of heating objects to be thermally processed, (d) a volume, centrally located within the furnace housing, high temperature insulation, and in thermal communication with the means of heating, in which the objects to be thermally processed are housed during thermal processing. In certain furnaces, the objects accomplishing the means of heating may define a hotzone, such that the hotzone is everywhere between the heating elements. A further object, such as a retort, is typically also employed to house the objects during thermal processing and may provide improved control over the chemical environment to which objects are exposed, and may also improve the uniformity of heating and reduce thermal gradients in the object to be processed.
The orientation of each of these elements may vary slightly from furnace to furnace, but the general architecture is as follows: The objects to be processed are placed within the retort, the retort is disposed among heating elements, insulation is located outside of the heating elements opposite the retort, and a housing surrounds the insulation, in a manner to also at least surround the retort. The retort is typically designed, configured, and placed such that heating of the retort is intended to be uniform, in some cases, the retort may experience a warmer or colder temperature, with the expectation that the objects to be thermally processed will still experience a uniform thermal processing schedule (in terms of temperature and gas flow, for example).
The construction of most furnaces encourages the transport of contaminants to the objects undergoing thermal processing. The transport mechanism is typically a combination of gaseous diffusion and convection to the surface and then interior of the objects.
The sources of contamination may include:
The furnace insulation may contribute to the generation and transport of contamination. There are several mechanisms whereby the furnace insulation may adsorb contaminants that may later be released during the thermal processing of an object. In a first example, the furnace insulation may provide a venue for gaseous species to deposit when the furnace is open, vented to an uncontrolled or laboratory atmosphere, cold, or otherwise in an unheated state exposed to contaminants such as water vapor or oxygen. In a further example, the furnace insulation may provide a venue for gaseous debind species, such as hydrocarbons, soot, or other chemical derived from a binder chemistry of the object during thermal processing, to become absorbed and/or adsorbed. As one skilled in the art may appreciate, species which may become absorbed or adsorbed at a first temperature and pressure or first range of temperatures and pressures may later become evolved at a second temperature and pressure or second range of temperatures and pressures. In certain instances, and to aid in the removal of doubt, the second range of temperatures may be greater than the first range of temperatures, for example. Thus, the furnace insulation may act to capture and store contaminants and later foul the furnace upon release of the originally captured contaminants or other cognate contaminants derived from the originally captured contaminants. By way of non-limiting example, the furnace insulation may adsorb water vapor at a temperature between 20 and 40 degrees centigrade, as may occur when the furnace is cold and open for the loading of parts for thermal processing. As the thermal processing cycle is executed, heating elements will first heat the surface of the insulation nearest to the heating elements, and the temperature of the insulation will gradually rise from regions of the insulation proximal to the heating elements toward distal regions of the insulation. The propagation of temperature (caused by heat flow) through the insulation may evolve water vapor continuously as the furnace executes a thermal processing cycle. The presence of water vapor at elevated temperatures may lead to chemical and/or physical damage and/or degradation for objects undergoing thermal processing—for example, including the oxidation of metals. Further, as may be appreciated by one skilled in the art, furnace insulation typically exhibits a high surface area per volume of area occupied by the insulation, and may therefore act as a large reservoir or high-capacity storage venue for a wide variety of contaminants.
The furnace plumbing may also contribute to the generation and transport of contamination. Similar to the manner in which furnace insulation may absorb and adsorb contaminants, the furnace plumbing may adsorb some amount of contamination on the wetted surfaces of the plumbing. As one skilled in the art will appreciate, wetted surfaces both upstream and downstream of the objects undergoing thermal processing (where upstream and downstream are taken using reference from the direction of gas flow identified from the gas inlet and outlets) may constitute regions where contamination may deposit onto and evolve from.
The objects to be sintered may also serve as sources of contamination. The objects to be sintered may consist, at certain periods of time during thermal processing, a mixture of substances such as polymers, metal powders, metal oxides of the metal powders, metal oxides dissimilar from the metal powders, ceramic powders, and any other materials necessary to accomplish the thermal processing of the object, as will be appreciated by one skilled in the art. Depending upon the temperature and stage of thermal processing, a first material in the object to be processed, such as a metal powder, may become contaminated from a second material in the object to be processed, such as a polymeric binder.
Other surfaces in the furnace may serve as sources of contamination. These surfaces may include any boundaries of the furnace, portions of the furnace frame, measurement equipment (such as pressure, temperature, or flow sensors), electrical feedthroughs, electrical insulation, electrical wiring, mechanical fasteners, or any other apparatus that is in gaseous communication (via convection or diffusion, for example) with the heated region in the furnace where the objects to be thermally processed will be housed.
The process gas may be a source of contamination. Depending upon the material comprising the object to undergo thermal processing, impurities within the process gas may damage the chemical and/or physical performance of the object after thermal processing. For example, carbon steels and tool steels (such as the midcarbon steels 4140 and 4340, and the tool steels H13, A2, D2, and S7, and the reactive metals such as aluminum, titanium, and alloys thereof, for example) are sensitive to the amount of water vapor at several temperatures throughout thermal processing. At relatively low temperatures (below approximately 600 degrees centigrade), the water may interact with the various metals to produce a metal oxide, as the temperature increases in the furnace, the oxide may devolve into the metal and a gaseous species containing oxygen. In some cases, carbon containing gases such as carbon monoxide or carbon dioxide may be evolved (or may otherwise form as a result of a reaction, as is the case of water and/or oxygen with carbon, such as graphite, for example), where the source of carbon may be from the chemistry of the metal alloy which may serve to deplete the chemical concentration of carbon below the acceptable level of carbon required to satisfy the chemistry of a given alloy; thus, water contamination in the process gas may lead to decarburization of the metal from which the objects in the furnace are comprised. Other gaseous compounds such as carbon monoxide, carbon dioxide, oxygen, nitrogen, methane, or other species may serve as contaminants depending upon the alloy to be processed. By way of non-limiting example, the presence of methane when processing a steel may result in carbon uptake (carburization) affecting the chemistry of the steel to the point where the content of carbon is no longer acceptable, either throughout the entirety of the object or in specific regions of the object (as would be performed in a case hardening operation).
It is thus strongly desired to control and mitigate any sources of contamination from the sources identified ((1) to (6)). Control and mediation is a crucial aspect of furnace design.
Now with reference to
For example, during the debind segment, the process gas may contain a relatively high concentration of hydrogen, to hydrogenate the binder chemistry to be degraded, or, in an alternative embodiment, may include dry air to oxidize and degrade the binder chemistry. Furthermore, the temperature in the debind segment may increase at a first rate to a first hold temperature for a first hold time, and then increase at a second rate to a second hold time, or exhibit any number of changes in temperature (up or down) to any number of hold temperatures for any given duration of time, the rates of temperature change, hold temperatures, and hold durations required to remove the binder or otherwise cleanse the objects and prepare them for further processing steps.
During sintering, the process gas may contain a high concentration of hydrogen, or may contain a high concentration of inert gas (such as argon), or may contain a high concentration of nitrogen, or any other gas required to thermally process the objects to be processed. As will be understood by one skilled in the art, the determination of a ‘high’ or ‘low’ concentration is made in reference to the sum total of gaseous species within the furnace, and, thus, discussions and specifications of relative concentration should not be confused with measurements of the total gas content, as may be indicated, at least in part, by the pressure in the furnace. In some embodiments, mixtures of gases may be required during the sinter segment. The pressure of the furnace during sintering may be larger than atmospheric (e.g., >760 Torr), atmospheric (e.g., approximately 760 Torr), moderately lower than atmospheric (e.g., in the range from and including 300 to slightly less than 760 Torr), or substantially lower than atmospheric (e.g., any pressure lower than 300 Torr including pressures less than 0.01 Torr). In some embodiments, pressures may be considered as low vacuum (25 Torr to atmospheric pressure), medium vacuum (25 Torr to 1e-3 Torr), high vacuum (1e-3 to 1e-9 Torr), ultrahigh vacuum (1e-9 to 1e-12 Torr), or other terms denoting the vacancy of matter in a volume. Applicant appreciates that several sets of terminology may exist to describe the range of pressure or ranges of pressures utilized during thermal processing, but note that the specific terms utilized are significant only to the extent they indicate the dominant physical processes which prevail in affecting the thermal processing of sinterable objects. The flowrate may be any flow rate of gaseous species used as a process gas.
During cooling, the temperature may be controlled to decrease at a set rate, and in certain embodiments may be configured to hold at a specific temperature for a specific period of time for reasons including heat treatment of the object to be processed. In some embodiments, the gaseous species to which the objects to be thermally processed may be reducing, oxidizing, carburizing, decarburizing, or inert. The flow rate of process gas into the furnace may be commanded to any required flow rate. In certain embodiments, the gas flow may be larger than or equivalent to the largest flow rate of process gas utilized during the sinter segment. In certain embodiments, the gas flow may be much smaller than the flow rate of process gas used during the sinter segment. In certain embodiments, there may be no gas flow during the cooling segment. The pressure may be controlled to a specific value of pressure, either above or below atmospheric pressure. In certain embodiments, the flow of process gas may act in concert with command of a pumping system to achieve and control the desired furnace pressure.
Several state-of-the-art retorts are shown in
With reference to
Now described are concepts for improved retorts, including:
An embodiment improved retort is detailed in
As used within the present disclosure, Peclet sealing refers to tubing long in length relative to a lengthscale of the tube cross-section (such as, in some embodiments, a tube diameter, a tube radius, a square root of a cross-sectional area, or a value in proportion to a square root of a cross-sectional area, for example) combined with an outflow through the tubing of a process gas that prevents backflow or diffusion of contaminants and ambient air through the tubing. The performance of a Peclet sealing element may be evaluated in terms of the unitless Peclet number, which corresponds to the product of gas velocity and channel length divided by a diffusivity of a gaseous species transiting the tube and the flow of which is intended to be affected by the Peclet sealing element. The Peclet number may correspond to: Pe=u×L/D, where Pe represents Peclet number, u represents average velocity of sweep gas within a tube or channel, L represents the length of the tube, and D represents the diffusivity of the species in question (e.g., a source of contamination). Increased Peclet number may be associated with increased sealing. Thus, as velocity or tube length increases, gas flow may more effectively impede diffusion of outside air, which may include contamination. Accordingly, for a given gas flow velocity, the Peclet number can be increased by using a thinner or longer channel.
In certain embodiments, it is desirable to maintain a flow of gas in the Peclet tube such that the flow maintained prevents or substantially reduces a counterflow of contaminant gas species. In certain embodiments, a downstream portion of a tube may be exposed to a contaminant such as a hydrocarbon, oxygen, or water vapor and the upstream portion of the tube may connect, terminate, or otherwise be connected to a volume for thermally processing and object such as a sinterable material. Since, in certain embodiments, it is desirable to maintain a contaminant free atmosphere (or at least substantially contaminant reduced atmosphere as compared to the atmosphere in the downstream region of the tube), a flow may be maintained in the Peclet tube directed from the region where the contaminants are desired to be free toward the region where the contaminants are present. The degree to which transport of contaminants is sealed or arrested in the Peclet tube is determined by the Peclet number.
Applicant estimates that for an upstream contaminant level of c0 and a downstream contaminant level c1 (where c1>c0 and the contaminant levels may be understood as an intensive measure of concentration such as parts per million, mass per volume, moles per volume, or similar), and where the upstream contaminant level is determined entirely by back transport of the downstream contaminant, that the upstream contamination may be estimated using the expression c0=c1*exp(−Pe), where exp( ) is the exponential function. Thus, specification of a Peclet number in the tube will determine the ratio of c0/c1: for Pe>12 parts per million contaminant reduction may be achieved, for Pe>20 parts per billion contaminant reduction may be achieved. Applicant typically operates with a Peclet number of (or in excess of) 25. In certain embodiments, the Peclet number may range from 10 to 25, or from 25 to 100, or even from 1 to 10, depending upon the level of sealing required in the Peclet tube. Once a level of sealing and therefore Peclet number have been determined, and in the case where the diameter has been specified, an estimation or determination of the diffusivity for the contaminant species may be found or computed. Common resources for determination of the diffusivity of molecules in gaseous transport may be made using the text “The Properties of Gases and Liquids” by Poling, Prausnitz, and O'Connell, using tabulated data, theoretical predictions, or any other suitable means. The Peclet number, diffusivity, and tube cross-sectional lengthscale determined, the velocity may then be computed, and a flow rate of a process gas determined by the product of the gas velocity and the tube cross-sectional area. As one skilled in the art will appreciate, the diffusivity of a species in a gas may be strongly temperature dependent, and thus the flow rate required for sufficient sealing may depend upon the temperature in the furnace during thermal processing. For a tube of 10 mm inner diameter, 65 mm length, a Peclet number of 25, and a contaminant molecule modeled as oxygen (or other small gas), inventors use flows to accomplish sealing above 1 standard liters per minute at 1325 degrees C., above 0.8 standard liters per minute at 600 degrees C., and above 0.6 standard liters per minute at 200 degrees C., for example. As one skilled in the art will appreciate, this procedure may be modified to determine a geometry of the Peclet seal for a fixed flowrate, known diffusivity, and defined value of Pe to achieve a specified reduction in contamination on the upstream end of the Peclet tube. Inventors recognize that furnaces of different construction (either in overall architecture or choice of materials), or objects to be thermally processed containing different materials may require atmospheres with varying amounts of cleanliness or contaminant reduction. In certain embodiments, a larger or smaller Peclet number may be required to achieve target process performance, which may be accomplished easily through variation of the rate of flow of process gas.
In certain embodiments there may be portions of the retort that reside outside of a hot zone of the furnace without departing from the present disclosure. It is desirable, however, for the Peclet seal to be in the hotzone or otherwise heated to the temperatures of the objects being processed. In certain embodiments, there may be portions of the retort that reside outside of a hot zone of a furnace, and the retort may be further configured such that objects to be thermally processed are situated in the retort and within the hotzone, such that the objects to be thermally processed are within the retort and hotzone.
With reference to
In certain embodiments the retort material may be formed from a ceramic such as alumina, zirconia (with or without a stabilizer such as yttria or magnesia), magnesia, yttria, or other ceramic material. In certain embodiments, the retort material may be formed by a silicon carbide, reaction bonded silicon carbide, silicon nitride, reaction bonded silicon nitride, or other silicon containing refractory material. In certain embodiments the retort material may be formed from a glass or glass-like material such as silica, fused silica, fused quartz, borosilicate glass (such as Borofloat by Schott A G of Mainz, Germany or Pyrex by Corning of Corning, N.Y.). In certain embodiments, the retort may be formed by a silicate-containing material such as porcelain, cordierite, steatite, forsterite, or mullite. In certain embodiments, the retort may be formed by silicon.
In certain embodiments, the retort may be formed from a metallic alloy such as stainless steel (such as 310S, 316, 316L, and 304, for example), a nickel-based superalloy such as Inconel (such as type 625, or 718, for example), high nickel content alloys (e.g., those above about 20% by weight such as HK40, HP, H101, 30Cr Micro, 45Pa, 45HT, and 60HT, for example). In certain embodiments, inventors recognize that the use of a particular metallic alloy may be preferred based upon the ease of fabrication, ductility, and/or resistance to thermal shock. In certain embodiments, the metallic alloy may be formed, constructed, or shaped into a tubular or cylindrical structure. In certain embodiments, the metallic alloy may be formed, constructed, or shaped into a cuboid.
In certain embodiments, the retort may be formed from a refractory metal such as molybdenum, niobium, tantalum, tungsten, or rhenium. In certain embodiments, a variety, or combination, of retort materials may be utilized.
With reference to
In certain embodiments, the base plate including a fore volume may be positioned on the top of the retort assembly rather than the bottom with the exclusion of a lid and a flow path to the base plate. In certain embodiments, the fore volume may be placed to the side of the sintering volume (i.e., the area where parts are placed and sintered), assuming an upright orientation of the retort, while remaining in uninterrupted fluid communication with the sintering volume, within the retort. In certain embodiments, a plate including a fore volume may be positioned at arbitrary positions within the retort assembly, rather than an uppermost or lowermost position relative to adjacent retort components.
In certain embodiments, a fore volume housing, with an interior cavity defining a fore volume, may house a cleansing object. The interior cavity may be fluidly connected to a process gas inlet via a first gaseous connection apparatus (such as a tube or conduit), and may be connected to a retort inlet via a second gaseous connection apparatus (such as a tube or conduit). In some embodiments, the length of the second gaseous connection apparatus may be shorter than the length of the first gaseous connection apparatus, the shorter length may be desired to reduce the possible ingress of contaminants into the gaseous connection apparatus between the fore volume and retort, said ingress of contaminants due to any permeability through the material of which the connection apparatus is comprised. In certain embodiments, the retort, second gaseous connection apparatus, and fore volume housing may be disposed entirely within and interior to the heating elements within the furnace. In certain embodiments, the first gaseous connection apparatus may mate, interface, or otherwise connect with a process gas inlet outside of the interior region of the furnace defined by the heating elements.
“Centrally located retort” In certain embodiments, the retort may be constructed in the form of a cylinder, tube, or other extruded shape. In certain embodiments, parts may be located within a cavity interior to the retort. In certain embodiments, a fore volume may be adjacent, coupled to, or integral with the retort. In certain embodiments, a cleansing object may be placed within the fore volume. In certain embodiments, the generators of the extruded shape (the flat end-surfaces of a tube, for example) may serve as connection points for gaseous inflow and outflow (via a Peclet tube) to and from the retort. In certain embodiments, the coupling of the inlet gas may proceed through the fore volume prior to entering the retort cavity. In certain embodiments, gaseous connection points may reside entirely within the interior region of the furnace defined by the heating elements. In certain embodiments, a fore volume may be placed between the gaseous inlet tube and the retort cavity.
The purpose of the retort is to control the chemical and thermal environment such that parts undergoing sintering (or other thermal processing) experience the desired chemical and thermal environment. The control of the chemical and thermal environment is accomplished by reasons which may include (1) chemical isolation (via sealing) of the objects to be processed within the retort from regions outside of the retort, (2) control of gaseous species brought to the objects to be thermally processed, (3) control of the direction of flow for various sources and types of gaseous species into and out of the retort, (4) transferring heat from the heating elements or other objects accomplishing heating to the objects to be thermally processed. The selection of retort materials, in some embodiments, also includes concerns related to the rate of degradation or accumulated damage that a retort may encounter during its useable life (which may, in some embodiments, exceed 100, 150, or 300 thermal processing cycles). Further, in some embodiments, the material of construction may be a determining factor in the degree to which the retort may chemically and physically isolate the materials to be thermally processed from regions in the furnace with a lesser degree of atmosphere control (that is, regions of the furnace which may be considered dirty). Materials of composition may include: ceramics (alumina, mullite), quartz/glass (typically with a lower service temperature, pyrex, borosilicate glass, fused quartz, fused silica), carbides (like silicon carbide), nitrides (aluminum nitride, silicon nitride), graphite (many different grades with varying density, porosity, and permeability), stainless steel (typically with a lower surface temperature, alloys like 310s), refractory metals (molybdenum), and combinations of various materials (including those listed above).
The nature of leaks will vary depending upon the regime of gas flow at the various regions where the leaks will occur. In general, there are three regimes of flow: (1) molecular, (2) laminar (or continuum), and (3) transition. These flow regimes are described in the reference “Vacuum Technology” by Alexander Roth.
In molecular flow, the motion of gas molecules with a vanishingly small number of collisions between molecules. In the molecular flow regime, the molecules interact with fixed bodies such as the retort, furnace walls, insulation, the objects to be sintered, and any other stationary objects. The objects may include pores, holes, orifices, or tubes in any of the fixed objects, such as pores in the retort that may exist, according to some embodiments, or the interface at mating/joining surfaces which may permit a slit, channel, or other open space connecting volumes on either side of the mating/joining surfaces, according to some embodiments. Since the molecules tend not to interact with one another, and interact only with stationary objects (such as the walls of the objects just described), the molecules are free to transport from one end of a channel (e.g., pores, holes, orifices, or tubes in the fixed object) to the other end of the channel without molecule-molecule collisions. Thus, while in a state where molecular flow is dominant, the net flux of molecules is dependent upon the difference in concentration of a molecular species on either side of the channel, a factor depending upon the geometry of channel (including, for example, a characteristic diameter and length), and other physicochemical properties of the system including the diffusive properties of the molecules undergoing transport and the system temperature.
In laminar flow, the motion of gas molecules is dominated by collisions between and among the gas molecules. The molecules will tend to move from regions of high collision frequency to regions of low collision frequency, tending to push molecules from high to low pressure. In the case of the channel discussed in the molecular flow case, a channel in which laminar flow is the dominant transport mechanism will be dominated by collisions between molecules, and these collisions will bias the diffusion of gas molecules from the high pressure toward the low pressure side.
In transition flow, neither molecular nor laminar flow is the dominant transport mechanism for molecules, and both must be considered in the analysis of transport phenomena.
The regime of flow is determined by ratio of a system lengthscale, L, which is representative of the geometry in which the flow is proceeding (for example the diameter of a tube, the height of a slit, the height of a step in an external flow, etc. . . . ) and comparing this lengthscale L to the mean free path, z, of the molecules undergoing transport. When the mean free path is very small as compared to the system lengthscale, z/L<100, the flow is considered laminar (or continuum), and when the mean free path is large as compared to the system lengthscale (z/L>100), the flow is considered molecular. In the intermediate range between these two limits, the flow is generally considered to be in transition flow.
It is desirable to minimize leakage through joints vs. walls. For the example case of a retort, which may consist of several walls with a sum total area A_w and thickness t_w, and have joints (e.g., along a mating interface) with a total perimeter P_J, height H_J, and thickness t_J, there is a rate at which material is transported through the retort (proportional to the area of the retort, A_w, and inversely proportional to the thickness t_w raised to some power), and a rate at which material is transported through the joints (proportional to the product of the perimeter and height P_J*H_J and inversely proportional to the thickness t_J raised to some power). In certain embodiments, the retort wall and joints will each exhibit a permeability, which is the coupling constant between the flux of gaseous molecules and the difference in species pressure on either side of either the retort wall or joints. In certain embodiments, the permeability may be dominated by molecular, transition, or laminar flow. In certain embodiments, the permeability through the retort wall may be modeled, captured, or otherwise considered as an array of tubes of a given length (likely not less than the thickness of the retort wall t_w), diameter, and concentration per area. While such a computation could, in principle, yield a permeability for the retort wall, the permeability is likely more easily measured with appropriate instrumentation, as will be appreciated by one skilled in the art. However, an approximate model for the permeability, as will be appreciated by one skilled in the art, will yield an initial estimation for the geometry of the pores, holes, orifices, cavities, or other permeable regions of the retort (which, in some embodiments, may encompass all, none, or a portion of retort area) which may be used to estimate the regime of flow and dominant flow physics. Likewise, in certain embodiments, the joints of the retort may be modeled using standard relations for slits, joints, tubes, or other standard shapes in molecular, transition, or laminar flow (as may be found in “Vacuum Technology” by Roth or “Scientific Foundations of Vacuum Technique” by Dushman). In certain embodiments, the joints may consist of more regular, estimable, and/or calculable geometries (as may be found in “Vacuum Technology” by Roth or “Scientific Foundations of Vacuum Technique” by Dushman), or may be measured with an appropriate test apparatus, as appreciated by one skilled in the art. Further, the total flux of gaseous species anticipated at a given operating point of the furnace may be considered for the retort wall and retort joints, and, in certain embodiments, the comparison between the two values may inform the selection of retort material and the joint design (including the tolerances and surface finish of the shaping and machining operations required to construct the joints).
In summation, improved retorts according to the present disclosure may contain several features which improve the quality of objects to be thermally processed, including:
The concepts encompassed by the present disclosure may act in concert, or, in some cases, individually depending upon the requirements of the retort, furnace architecture, materials comprising the objects to be sintered, and other process variables (for example, the temperatures, gas flow rates of the process gas, and process gas composition defined in the thermal processing cycle).
Now disclosed are definitions useful for the present disclosure:
Parting interface—A region formed where two opposing surfaces are brought to contact or near contact.
Hermetic seal—A degree of sealing wherein a leak source has a rate at least two orders of magnitude less a rate of the sum total of all leaks.
Semi-sealing—A degree of sealing wherein a leak source has a rate within an order of magnitude of the rate of the sum total of all leaks.
Permeable—A degree of sealing wherein a leak source is the largest fraction of the sum total of all leaks.
Process gas—Gas deliberately supplied to the objects during thermal processing, typically at a defined rate of flow. Examples include: air (typically during debinding and/or at low temperatures before and after the completion of a thermal cycle), nitrogen, hydrogen, argon, methane, and mixtures thereof such as, for example, 4% hydrogen-96% argon (forming gas).
Effluent gas—Gas of any constitution, flow, or pressure which exits the retort or furnace from an exit port toward a pump, vent, or other discharge apparatus.
Object support—A structure which mates with a part to act as a support during the fabrication of the object. The object support may provide support during a deposition (printing) stage, a crosslinking or curing stage, a debinding stage, a sintering stage, an annealing stage, or some or all of the various stages. The same object support may be used across more than one stage, or different object supports may be used at each stage. In some embodiments, the object support may be comprised, at least in part, of material identical to the part. In some embodiments, the object support may be comprised, at least in part, or in select regions, of material which is dissimilar than the part.
Cleansing object—A substance which sequesters, cleans, scrubs, sorbs, or otherwise captures specific gaseous species when heated to and/or within particular temperature ranges; such action is often referred to as ‘gettering’. These materials are often referred to as getters or getter materials. Getter materials may be provided in several different physical forms depending upon the application pelletized, puck, pill, granule, powder, film, wire, sheet, or other forms are commonly available from various manufacturers of getter materials, or may be custom ordered. As one skilled in the art may appreciate, to maximize the ability of the getter material to interact with flowing gas, the getter material may be selected to exhibit a large surface area to volume ratio or, in some embodiments, a large surface area to mass ratio. The specific materials to be cleaned by the getters are often determined by the temperature (which affects the rate of gettering) and the composition of the getter material: in some embodiments, an aluminum getter may capture oxygen; in some embodiments, a barium getter may capture carbon dioxide, hydrogen, nitrogen, and oxygen; in some embodiments, a magnesium getter may capture oxygen; in some embodiments, a rare earth element such as cerium or lanthanum may capture carbon dioxide, hydrogen, nitrogen, and oxygen; in some embodiments, a titanium getter may capture hydrogen, nitrogen, and oxygen; in some embodiments, a zirconium getter may capture oxygen, hydrogen, carbon monoxide, carbon dioxide, water, and various hydrocarbons. In some embodiments, the getter may capture the gas species directly, or the gas species may react on, near, or within the surface of the getter material to related or constituent molecules which may then be captured by the getter. In some embodiments, the getter may be an alloy of various materials. Commercially available getter alloys include St 171, St 172, St 707, and St 101 from SAES Getters/U.S.A., Inc. of Colorado Springs, Colo. In some embodiments, other materials such as alloys of titanium with, for example, vanadium, aluminum, iron, zirconium, or alloys of zirconium with, for example, hafnium, or iron, or alloys of iron with, for example, vanadium, chrome, silicon, or manganese; or silicon; or alloys containing iron, silicon, and zirconium; or alloys containing iron, silicon, and vanadium. In some embodiments, the material may be formed into a simple object such as a puck, disc, block, or pill. In some embodiments, a single simple object may be used. For objects which are small (typically on the order of 1-10, or 10-20 millimeters), several simple objects may be utilized in combination, in some embodiments. In some embodiments, the getter material may be fabricated into a shape with characteristics including or similar to: (1) a labyrinthine pattern, (2) an open-cell structure, (3) a tubular structure, or other similar tortuous and high surface area shape. In some embodiments, the choice of getter is determined by both (1) the specific gaseous materials to be gettered from a gas stream and (2) the temperature at which a sufficient amount of gettering occurs to satisfy the chemical constrains of the process (thermal processing of sinterable objects, in this case). A mass of getter material comprising the cleansing object may be selected based upon attributes which may at least include (a) the cleanliness of the process gas utilized, (b) the hermiticity of the process gas inlet lines everywhere between the process gas source and the cleansing object(s), (c) the sensitivity of the materials from which the objects to be thermally processed are comprised, (d) the permeability, area, and thickness of the retort material, (e) the accessible surface area per mass of the getter material, (f) the pressure of the gas surrounding the cleansing object(s), (g) the temperatures to which the cleansing object(s) are exposed, (h) the temperatures at which the cleansing object(s) are to clean the gas, or (i) the composition of the cleansing object(s).
Fore volume—An enclosed cavity with port for gas entry and gas exit, configured such the gas flows into the entrance and out of the exit, and placed such that the exit of the fore volume is fluidly connected to the entrance of the retort. The fore volume is capable of housing a cleansing object.
Interior cavity—A volume formed within the retort to house parts during thermal processing.
Retort—A housing to provide specific chemical and thermal conditions for objects to be thermally processed.
| Number | Date | Country | |
|---|---|---|---|
| 63228906 | Aug 2021 | US | |
| 63329534 | Apr 2022 | US |
