Patent Grant
11766718
Various aspects of the present disclosure relate generally to furnaces, and particularly to furnaces configured for debinding and/or sintering operations.
Metal injection molding (MIM) is a metalworking process useful in creating a variety of metal objects. A mixture of powdered metal and one or more binders (e.g., a polymer such as polypropylene or wax) may form a “feedstock” capable of being molded, when heated, into the shape of a desired object. The initial molded part, also referred to as a “green part,” may then undergo a preliminary debinding process (e.g., chemical debinding or thermal debinding) to remove primary binder while leaving secondary binder intact, followed by a sintering process. During sintering, the part may be heated to vaporize and remove the secondary binder (thermal debinding) and brought to a temperature near the melting point of the powdered metal, which may cause the metal powder to densify into a solid mass, thereby producing the desired metal object. Applicants recognize that extreme temperatures above 1000 C tend to be destructive to many familiar engineering materials even including many metals and ceramics. This is especially the case as temperatures exceed 1250 C. For example even the nickel alloys such as Kanthal® and other high temperature nickel based alloys can be utilized below 1250 C but these materials can rapidly become impractical as temperatures climb above 1250 C. One of the challenges of designing low cost sintering furnaces is that there are very few reasonably priced engineering materials that can survive repeated cycles at sintering temperatures. When faced with narrowing material choices and options, furnace engineers traditionally accept designs that are compromised in various ways including using extreme amounts of power to minimize amount of contaminable insulation while nevertheless suffering significant issues with contamination.
Additive manufacturing, such as three-dimensional (3D) printing, includes a variety of techniques for manufacturing a three-dimensional object via a process of forming successive layers of the object. Three-dimensional printers may in some embodiments 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 debinding and sintering processes to produce the object.
In addition to MIM based additive manufacturing, there are systems using powder beds and loose powder, optical resin curing, and others. These methods, and others, may involve the use of a furnace to produce the final part or to enhance the properties of the part.
In order to reduce contamination and improve the quality of the part, a vacuum furnace may be used for thermal debinding and/or sintering. Thermal treatment with a vacuum furnace may be useful, for example, to reduce the occurrence of oxidation. While vacuum furnaces may assist in reducing oxidation, these furnaces may be prone to contamination that reduces the quality of the part.
In order to produce higher quality parts, it is beneficial to reduce the amount of contamination present within the furnace especially during sintering. One common source of contamination is from insulation included in the furnace. For example, insulation may retain contaminants, such as moisture, binder released from parts during debinding, and various compounds that offgas from the parts and the structures of the furnace itself during thermal processing. Generally, increased insulation is associated with increased contamination, as these contaminants are often retained by the insulation and released during subsequent thermal processing. Therefore, while thick insulation may reduce the amount of power necessary to maintain desired temperatures within the chamber of the furnace, thick insulation may increase the quantity of contaminants present within the chamber. Some furnaces, such as graphite insulated and molybdenum insulated furnaces, may employ minimal insulation with the aim of reducing contamination. However, the use of minimal insulation may greatly increase the power required (e.g., high power requirements of approximately 20 kW to 100 kW). Moreover, the use of minimal insulation may require the use of water cooling at least to protect the metal chamber surrounding the insulation, which may involve the use of two nested and hermetically-sealed chambers (e.g., steel chambers) with a structure between the two chambers to facilitate the flow of water for heat exchange, adding complexity and cost. Moreover, even when the quantity of insulation is reduced in this or another manner, contamination due to water vapor from the atmosphere (introduced during loading and unloading of parts) and/or condensed binder products, which may become re-volatized during sintering, may still adversely affect part quality.
In some aspects, the presence of contamination within a furnace may lead to contamination of parts or reduced quality in the parts. For example, moisture present within the furnace may increase oxidation of metal powder, or may influence complex chemical reactions throughout the furnace that change and/or influence the carbon content of an alloy in complex and often unpredictable ways during sintering. Some types of insulation, such as ceramic insulation, may be particularly susceptible to contamination, including moisture contamination. However, contamination may occur in various types of insulation, as well as in other components of the furnace.
The apparatus and systems of the current disclosure may address one or more of the problems described above, or address other aspects of the prior art.
Disclosed is a furnace for high-temperature sintering of metal parts with significantly less contamination—especially during sintering—than previously existing designs available at comparable overall cost. This is accomplished by sealing an inner insulation layer from an outer insulation layer and heating the inner insulation layer sufficiently during debinding of the parts that the temperature of the inner insulation layer causes it to not readily allow condensation of contaminants outgassing from parts during debinding. The contaminants can instead be successfully outgassed from the furnace, rather than building up in the insulation. Heating in the furnace is preferably accomplished using a combination of an inner set of heaters and an outer set of heaters. The outer heaters heat the inner insulation from the outside to ensure the outer portion of the inner insulation is heated sufficiently. The inner heaters provide the majority of the heating necessary to heat a work zone to a sintering temperature to sinter the parts.
Also disclosed is a high temperature seal for the door required to load and unload parts in the above furnace that is capable of both withstanding the very high temperatures required for sintering parts formed from MIM build material and also prevent contaminates from entering the furnace from the ambient environment. Preferably this is accomplished by having an inner seal and an outer seal. The inner seal is capable of withstanding the high temperatures it is exposed to during the sintering process. The outer seal prevents contaminates from reaching the inner seal where they may otherwise enter the furnace at undesirable rates. Preferably inert gas is flowed between the outer seal and the inner seal, where it exits the outer seal, preventing backstream flow of contaminants. The low level of contamination achieved by the above described furnace can therefore be maintained.
In one embodiment, a compound sintering furnace with managed contamination for debinding and sintering parts includes an outer insulation layer and an inner insulation layer disposed within the outer insulation layer and having an internal hot face surrounding a work zone. A sealed housing surrounds the inner insulation layer and is composed of a refractory material capable of withstanding a service temperature, the service temperature being greater than a debinding temperature that is less than 1000 C, and less than a sintering temperature that is greater than 1250 C. An outer heater system is configured to heat at least a portion of the sealed housing and externally heat the inner insulation layer to, in conjunction with an inner heater system, heat the work zone to the debinding temperature, and inhibit condensation of a binder within and upon the inner insulation layer during a debinding process. The inner heater system is configured to internally heat the inner insulation and contribute a majority of heating necessary to heat the work zone to the sintering temperature during a sintering process.
In another embodiment, a low-power high-purity furnace system for high-temperature thermal processing of parts includes an inner furnace including an inner furnace insulation arrangement having an inner hot face surrounding a work zone heated by at least one inner furnace heater. An outer oven that contains the inner furnace and includes a sealable oven housing having an oven door, where the oven door in an open position is operable for loading and unloading parts into and out of the work zone, and where in a closed position the oven door is gaseously sealed to the sealable oven housing. The outer oven includes an outer oven insulation arrangement that supports at least one oven heater therein, the at least one over heater being arranged outside the inner furnace and configured to heat an outside of the inner furnace. The at least one inner furnace heater is configured to controllably heat the work zone, and at least one part therein, to within a range of processing temperatures below a maximum inner furnace temperature. The at least one outer oven heater is configured to controllably heat the outside of the inner furnace to a range of oven temperatures less than the maximum inner furnace temperature.
Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the features, as claimed. As used herein, the terms “comprises,” “comprising,” “including,” “having,” or other variations thereof, are intended to cover a non-exclusive inclusion such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements, but may include other elements not expressly listed or inherent to such a process, method, article, or apparatus. Additionally, the term “exemplary” is used herein in the sense of “example,” rather than “ideal.” It should be noted that all numeric values disclosed or claimed herein (including all disclosed values, limits, and ranges) may have a variation of +/−10% (unless a different variation is specified) from the disclosed numeric value. In this disclosure, unless stated otherwise, relative terms, such as, for example, “about,” “substantially,” and “approximately” are used to indicate a possible variation of ±10% in the stated value. Moreover, in the claims, values, limits, and/or ranges of various claimed elements and/or features means the stated value, limit, and/or range +/−10%. The terms “object,” “part,” and “component,” as used herein, are intended to encompass any object fabricated using the additive manufacturing techniques described herein.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate various exemplary embodiments and together with the description, serve to explain the principles of the disclosed embodiments. There are many aspects and embodiments described herein. Those of ordinary skill in the art will readily recognize that the features of a particular aspect or embodiment may be used in conjunction with the features of any or all of the other aspects or embodiments described in this disclosure.
Embodiments of the present disclosure include systems and methods to facilitate and improve the efficacy and/or efficiency of sintering printed objects. Reference now will be made in detail to examples of the present disclosure described above and illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. While exemplary embodiments of a furnace system or furnace may be discussed with different reference numbers, it is understood that the features of any the furnaces described herein may be combined or replaced as desired.
As described above, furnaces for additive manufacturing may experience contamination due to the introduction of moisture and/or binder into a chamber containing one or more parts. This contamination may result from substances that are absorbed and released from one or more layers of insulation. However, contamination may result from other mechanisms, including release of contaminants from other structures of the furnace system or from within the furnace, leaky seals at inlets, outlets, and/or a door of the furnace, etc. Thus, it may be possible to improve the functioning of a furnace system by addressing one or more of these sources of contamination, such as with the exemplary solutions disclosed herein, which may be individually incorporated in to a furnace system or may be used in combination with one another.
Furnaces, and in particular sintering furnaces, often have large power requirements. For example, a relatively small sintering furnace with a work zone of about 1 cubic foot may require approximately 20 kW to 40 kW of power. Larger furnaces (e.g., furnaces with work zones of about 4 cubic feet to about 8 cubic feet) may require even more power, on the order of one-hundred to several-hundred kW of power. Sintering may require the application of this power for a prolonged period of time (e.g., multiple hours). The application of high levels of power for such extended periods of time represents significant costs and energy usage. For example, sintering cycles in relatively larger furnaces may be associated with energy costs in the hundreds of dollars, or more, per cycle. Additionally, commercial furnaces may operate frequently (e.g., on a nearly-continuous basis), in order to maximize productivity. Each of these furnaces may operate for about 1,000 hours per year, corresponding to several megawatt hours of power on an annual basis.
Smaller furnaces (e.g., smaller tube furnaces typically used in laboratory research) may tend to require less energy by including additional insulation. However, these furnaces may not be suited for use in manufacturing. Furthermore, these furnaces may not be usable under a vacuum, which applies a controlled, negative-pressure environment to the interior of the furnace, reducing the amount of contamination in the atmosphere, and the amount of undesired products, such as binder, that is emitted into the atmosphere.
For at least the above-described reasons, commercial and laboratory furnaces may be inefficient and may increase pollution, both directly (from emissions) and indirectly (due to large power requirements).
The solutions described below may provide one or more environmental benefits, including reduced pollution, costs, and energy usage. For example, one or more of the furnaces described herein may have power requirements (e.g., about 2 kW to about 4 kW) that are approximately 90% less as compared to the requirements of some commercial furnaces. One-thousand large, metal-sintering furnaces operating at 10 killowatts power for 1000 hours per year may be expected to consume 10 million watt hours per year. As a comparison, a typical residential American home may be expected to consume 10 million watt hours per year. Accordingly, assuming there are 1,000 or more large metal-sintering furnaces currently in operation, energy and associated reductions in pollution may be expected to a 1000 homes. Moreover, if comparable energy-saving techniques were employed in the ceramic-sintering and firing industries (which are larger than the metal-sintering industry), these energy savings may be doubled, tripled, or even greater.
However, chamber 20 may be sealed to a lesser degree, for example sealing that may inhibit or limit leakage of gas. In some case, for operation at vacuum, the airtight outer chamber may be sealed as well as sturdy enough to withstand 15 psi. I other cases where the outer airtight chamber will only need to support atmospheric pressure on both sides and does not need to be very sturdy.
Furnace 100 may include a housing wall 32 that defines chamber 20 within which insulation 22 and heaters 112 may be located. Heat generated by heaters 112 may debind and/or sinter parts 90 placed within chamber 20. In one aspect, the central cavity or chamber 20 within furnace 100 may define a parts cavity 116, which is inside of a retort 114 in which parts 90 may be supported (e.g., via one or more parts holders or shelves 34 and support brackets 35 for supporting each shelf 34, although any suitable shelving configuration is contemplated). Retort 114 may be located within a hot zone 28 defined by one or more layers of insulation 22, including inner or exposed insulation 24. An inward-facing surface of an inner-most layer of insulation 24 may form a hot face 44 that surrounds hot zone 28. Hot zone 28 may represent an area surrounded by hot face 44 of exposed insulation 24.
In an exemplary configuration, one or more heaters 112 (e.g., a plurality of separate heaters with helical heater elements, separate or continuous serpentine heater elements, or any other suitable heater or group of heaters) may include electrically-resistive heater elements that extend between retort 114 and inner insulation 24 in hot zone 28. These heaters may generate heat 14 within hot zone 28 to provide uniform heat to parts 90. One or more layers of isolated or outer insulation 26 may surround the one or more layers of inner insulation 24. An outer periphery or cold face 46 of an outermost layer of outer insulation 26 may define a cold zone 30. An outer cooling jacket 38 may be provided, if desired, to prevent excess heat accumulation within cold zone 30. The inclusion of cooling jacket 38 may decrease the amount of dissipated heat 92 that exits furnace 100.
In order to generate heat in a controlled manner, furnace power system 82 may supply electrical energy that is converted into heat by heaters 112. Each heater 112 may be connected to a suitable power supply 84 which may receive power from main power source 86. In some aspects, main power source 86 may correspond to a commercial or residential standard power source (e.g., a 240V power source). Electrical connections between power supply 84 and one or more heaters 112 may be provided via one or more power feedthroughs 88 that extend through housing wall 32. Power supply 84 may provide AC or DC power to heaters 112 based on commands from furnace controller 76.
Environment control system 58 may include systems for applying a vacuum to furnace 100, as well as systems for injecting an inert gas into furnace 100. For example, a gas manifold 74 may include one or more process gas feedlines 78 and effluent gas or vacuum lines 73 outside of furnace 100. Gas feedthroughs 66 may connect each external line 73, 78 with an interior of furnace 100. As shown in
Environment control system 58 may facilitate application of a vacuum to furnace 100. This vacuum may facilitate removal an effluent 75, which may contain a mixture of process gas, vaporized binder, and other offgas from parts 90 and/or components of furnace 100. Vacuum-applying components of environment control system 58 may include, for example, one or more vacuum pumps 60 connected via gas feedthroughs 66 (e.g., through an exhaust and or vacuum manifold) to cold zone 30 and hot zone 28 via vacuum valves 62. Therefore, vacuum may be applied to an interior of retort 114 and/or chamber 20 defined by vacuum housing wall 32. When vacuum is applied to chamber 20, vacuum housing wall may be formed as an air-tight sealed housing that is configured to withstand vacuum pressure. In the exemplary configuration illustrated in
One or more temperature sensors 80 may be provided within hot zone 28 to monitor and provide feedback information indicative of a current temperature within hot zone 28. Additional temperature sensors 80 may be positioned in other locations of hot zone 28, such as on hot face 44, for example. Additional temperature sensors 80 may be positioned on outer insulation 26, on or in vacuum housing wall 32, or any other desired location. One or more vacuum pressure sensors 68 may be provided to measure a strength of the vacuum applied at one or more locations of furnace 100. For example, a pressure sensor 68 may be applied to measure a pressure of chamber 20, as shown in
Furnace system 10 may be configured to generate an atmosphere-controlled furnace environment via environment control system 58. While a vacuum furnace may include an atmosphere control system, such as system 58, not all atmosphere-controlled furnaces may be vacuum furnaces. A vacuum furnace may form an atmosphere-controlled furnace, when appropriately sealed to provide and withstand vacuum pressure. In at least some embodiments, vacuum furnaces, such as furnace 100 (and the other exemplary furnaces described herein), may be configured to operate at a range of pressures, including deep vacuum, such as about 10−6 millitorr, to about one atmosphere (760 Torr). If desired, furnace 100 may be suitable for use at modest positive pressures. Typical operation involves operating at debinding pressures that are between 1 torr and 500 torr and sintering pressures from 10 millitorr to 400 torr, or in certain applications atmospheric pressure. One of the advantages of the present application is that purity levels can be achieved similar to those typically found in ultra-high vacuums but without the necessity to operate at extremely low pressures.
Furnace 100, which may be formed as a vacuum sintering furnace, may utilize a controlled flow of processing or process gas 71 while simultaneously pumping with vacuum pump(s) 60 in a manner balanced by controller 76. The introduction of process gas 71 and application of vacuum (which removes effluent 75) from chamber 20 and/or parts cavity 116 may control the atmosphere to which parts 90 are exposed. Vacuum pressure may be measured with vacuum pressure sensor 68 in order to facilitate control over the vacuum by controller 76. For example, controller 76 may be configured to balance an inlet flow rate (e.g., of process gas 71) in opposition to outlet gas flow (e.g., of effluent 75). A manual pressure gauge may be provided instead of, or in addition to, sensor 68 to facilitate manual monitoring and/or control over flow balancing. The flow of outlet gas or effluent 75 may be controlled manually or with controller 76, by adjusting the position of one or more electronically-controlled or adjustable valves 62 and/or by changing the pumping rate of pump(s) 60. In a similar manner, flow of process gas 71 may be controllably varied by mass flow controller 70, which may be controlled by controller 76. While mass flow controller 70 and furnace system controller 76 are illustrated as separate devices, as understood, mass flow controller 70 and system controller 76 may be combined in a single system controller. One or both of controllers 70 and 76 may be implemented by any suitable combination of programmable logic controllers (PLCs), computers, etc. Controllers may include open-loop feedback devices, closed-loop feedback devices, and/or state machines. If desired, suitable controllers may include custom microchip-controlled embedded controllers. In some embodiments, furnace controller 76, mass flow controller 70, or both, may be connected to one or more computers through serial or parallel buses, Ethernet, WIFI, Bluetooth, intranet, cellular, LAN, WAN, internet or any other suitable wired connection, wireless connection, or combination thereof. During thermal processing, parts 90 may outgas, especially during debinding processes, as can the housing of furnace system 10 itself, including insulation 22 and retort 114. In some aspects, these outgassing rates may affect the pressure and the control over pressure achieved by controllers 70 and/or 76. The above-described balancing may be influenced by, and/or may be performed in response to, outgassing from parts 90 and/or components of system 10.
In order to reach and maintain temperatures suitable for thermal treatments, such as debinding or sintering, an arrangement of high-temperature (high-temperature resistant) thermal insulation 22 may be located in an interior of furnace 100 with respect to wall 32. Insulation 22 may allow furnace 100 to operate at power requirements that are within desired or practical limits. In one aspect, insulation 22 may be sufficient to allow heaters 112 to reach sintering temperatures when furnace system 10 is connected to a standard power source (main power source 86). Insulation 22 may further avoid excessive heating of components located outside of insulation 22, such as system components and/or components within the room in which furnace 100 is present. Insulation 22 may also limit temperatures that furnace parts themselves, such as wall 32, feedthroughs 66, 88, etc., are exposed to.
In some aspects, insulation 22 may include high-performance insulation. Insulation 22 may completely surround heaters 112 and may have a low number of cracks, holes, and other paths through which parasitic heat leakage may occur for a given amount of heating power. In some aspects, the maximum achievable temperature (e.g., a temperature within hot zone 28) for a set of heaters 112 may be associated with a combination of factors including one or more of: (i) a surface area of hot face 44 (increased surface area requires more power), (ii) a type and quality of insulation 22, (iii) a thickness of insulation 22, (iv) the overall condition of insulation 22 with respect to aging, wear, and damage, and (v) the quantity of vacuum pressure applied by pump(s) 60, for example. Regarding the surface area of hot face 44, larger furnaces (having larger hot zones) may require more power for a given insulation type and thickness. Power requirements may be generally proportional to this surface area.
In an exemplary type of insulation 22, heat shielding insulation 22 may include multiple thin layers of refractory metal, such as molybdenum and/or tungsten. This insulation may be particularly useful with refractory heaters 112 that include refractory metal material inside sealed vacuum housing 32. Each layer of material of insulation 22 may act as a radiation shield with a plurality of layers acting together in a layered or stacked arrangement to maintain hot zone 28 at high temperatures (such as sintering temperatures), while maintaining an exterior (e.g., wall 32) at significantly lower temperatures, and in some embodiments, nearly at room temperature. System 10 may optionally include a cooling jacket 38, such as a water cooling path that surrounds a portion or entirety of an exterior of chamber 20.
One or more layers of insulation 22 may include metal materials, such as refractory metal materials. Suitable refractory metal materials may include or may be based on molybdenum and/or tungsten. Refractory metal insulation material may be advantageous for use in layered insulation for establishing and maintaining a high purity atmosphere within furnace 100. For example, molybdenum and tungsten have sufficient resistance to degradation from heat, vacuum, and exposure to process gas. These materials may also experience limited water and/or binder uptake or absorption. However, molybdenum and tungsten may provide a lower resistance to heat transmission in comparison to other insulation materials, and may tend to increase power requirements and cost.
Instead of, or in addition to refractory metal, insulation 22 may include high-temperature fiber insulation that operates, in principle, in a manner similar to fiberglass fiber insulation used in traditional home construction. High-temperature fiber insulation suitable for insulation 22 may include lightweight graphite fiber material. As used herein, the phrase “graphite insulation” may include graphite fiber insulation. Graphite insulation included for use in insulation 22 may be produced in rigid form as rigid fiber board with a volumetric fill factor (e.g., the ratio of fiber volume divided by the total spatial volume occupied by the rigid board) of less than 100% such that the board has low density, and is thus lighter in weight, as compared to solid graphite. Individual graphite fibers may tend to be most thermally conductive in the direction of the fibers. Thus, highly-oriented sheets or boards of graphite fiber insulation may exhibit anisotropic performance. Insulation 22 may include graphite fiber insulation fabricated in flat or curved planar layers with fibers generally extending parallel to the layer and perpendicular to the direction of heat flow (e.g., perpendicular to heat 14 on a given side of insulation 22). In square or rectangular furnaces, insulation 22 may include flat boards with fibers oriented along lateral extents of the boards such that highest resistance to heat conduction occurs in a direction perpendicular to the board. Insulation 22 may also include graphite fiber formed as a semi-rigid or non-rigid graphite felt with fibers oriented along the lateral extents of the felt. Cylindrical furnaces with cylindrical hot zones may be constructed by wrapping layers of such felt to form a layered cylinder of insulation 22. When insulation 22 includes graphite fiber, suitable heaters 112 may include electrically-resistive graphite heaters 112.
In at least some aspects, insulation 22 may include a relatively lightweight ceramic fiber insulation material. Similar to graphite insulation 22, ceramic fiber insulation 22 may be in the form of one or more rigid fiber boards with a volumetric fill factor of less than 100% such that the board has lower density and lighter weight as compared to solid ceramic. While improved thermal performance may be achieved by arranging ceramic fibers generally perpendicular to the direction of heat flow, ceramic insulation may be relatively thermally isotropic regardless of the arrangement of the fibers. Thus, ceramic fibers may generally be arranged parallel to the direction of heat flow if desired. In rectangular furnaces, ceramic insulation 22 may include flat boards including ceramic fibers oriented at least partially in parallel with the lateral (long) direction of the board. Similar to graphite fiber insulation 22, ceramic fiber insulation 22 may include non-rigid ceramic felt. Cylindrical furnaces 100 with cylindrical hot zones 28 may be constructed by wrapping layers of ceramic felt to form a layered cylinder of insulation 22. As used herein “ceramic insulation” may include ceramic fiber insulation. Exemplary ceramic insulation 22 materials may include alumina and mullite mixtures, or other ceramic materials, in any suitable grade or density. Any suitable heater 112 may be used in conjunction with ceramic insulation, such as SiC heaters, molybdenum disilicate heaters, or refractory metal heaters.
Each of the above-described materials for inclusion in insulation 22 (refractory metal insulation, graphite insulation, and ceramic insulation) may be selected based at least in part on the desired design and application of furnace 100. For example, graphite insulation 22 may remain mechanically robust at temperatures up to or greater than 2,000 degrees C. for hundreds or thousands of cycles, while ceramic fiber insulation 22 may be useful at somewhat lower temperatures, such as 1,600 degrees C. Commercially-available graphite insulation products include rigid, semi-rigid and flexible felt configurations, which may be suitable for inclusion in insulation 22. The maximum operating temperature of ceramic insulation 22 may be influenced by the purity of the ceramic material and density. In order to maximize the temperature resistance of ceramic insulation 22, it may be desirable to employ high purity alumina and/or high-density boards. Various forms of ceramic insulation 22 may provide a higher degree of thermal insulation as compared to graphite, even for forms of ceramic that have a lower maximum operating temperature.
Due to the presence of gaps, cracks, or other imperfections in insulation 22, including door insulation 595, one or more paths of heat leakage 592 may occur when heaters 112 are activated. As shown in
Furnace 590 may include a sealed housing formed, at least in part, by housing wall 32. In at least some configurations, housing wall 32 may provide a vacuum-resistant housing that can withstand vacuum pressure up to at least about 13.5 psi (about 700 Torr) or at least 15 psi (about 775 Torr). Furnace 590 may also be operable at room temperature environments. The housing of furnace 590 may include door 602, as well as door seal 594 to facilitate loading and unloading of a mass of one or more parts 90. The furnace housing may enclose an arrangement of insulation 22, which defines inner hot face 44 that faces heaters 112 and work zone 228, and an outer cold face 46 that faces housing wall 32. Insulation 22 may include one or more layers of inner insulation 24, and one or more layers of outer insulation 26. In one aspect, these layers of insulation may have the same or similar grade (density and/or maximum use temperature). However, if desired, inner insulation 24 may be relatively high grade insulation (e.g., high density and/or higher maximum use temperature) that extends closer with respect to the inward-facing hot face 44, while outer insulation 26 may include relatively lower grade insulation (e.g., lower density and/or lower maximum use temperature) that extends closer with respect to the outward-facing cold face 46.
Furnace heaters 112 may include any of the materials described herein, such as silicon carbide or graphite. Heaters 112 may receive electrical current through one or more heater power feeds 630 which could include a conductive electrode such as a wire that is electrically insulated relative to the housing and sealed with respect to the housing by an electrical feedthrough such as a commercially available ceramic to metal sealed feedthrough, and the electrical current may be converted into heat by resistive heater elements of heaters 112 to heat work zone 228. In an exemplary configuration, work zone 228 may provide approximately one cubic foot of space. Work zone 228 may be configured to reach temperatures suitable for sintering, such as approximately 1,250 to 1,500 degrees C. To facilitate the maintenance of such temperatures, insulation 22 may include approximately 4-inch to approximately 5-inch thick graphite and/or ceramic fiber insulation. One or more differing grades and/or densities of insulation may be included, as described above, or each layer of insulation may have approximately the same grade and/or density.
The part or parts 90 loaded into work zone 228 may have a total mass of approximately 5 kg or less, although larger furnaces 590 may have larger capacities. In some aspects, furnace 590 may be capable of operation at approximately 1,250 to 1,500 degrees C. with maximum ramp rate (temperature rate of rise) of between approximately 5 degrees C. per minute and approximately 10 degrees C. per minute. Furnace 590 may be configured to maintain a steady-state temperature of 1,250 to 1,500 degrees C., or higher, and may have a holding power within a range of approximately 1.5 kW to approximately 4 kW. Different steady-state temperatures and holding powers may be achieved by modifying insulation 22. One or more temperature sensors 626, such as a thermocouple temperature probe or thermistor, may be operably coupled to work zone 228 or another portion of furnace 590 and may transmit temperature information to control system 650.
In some aspects, furnace 590, as shown in
Furnace 590 may include insulation 22 having a relatively large thickness. In some aspects, the thickness of insulation 22 may be a larger than insulation provided in a high-power water-cooled industrial furnace having a similar work zone. However, as discussed above, insulation may be porous and/or prone to absorption and re-emission of contaminants. The amount of contamination retained and subsequently emitted by insulation 22 may be further exacerbated by eliminating water cooling and thus requiring thicker insulation. By replacing water cooling with air cooling, the outer surface of the shell (e.g., outer surface of wall 32) may experience temperatures of approximately 100 degrees C. during sintering operations. In such a configuration, cold face 46 of insulation 22 may experience temperatures of between approximately 200 degrees C. and approximately 300 degrees C., or lower, by providing relatively thick insulation. Furnaces having a configuration corresponding to furnace 590 of
Furnace 600 may include a sealed chamber 628 as a space having a shape and volume that is defined by an interior volume of temperature-resistant sealed housing 635. Housing 635 may be formed, for example, by a tube (or other suitably shaped structure) including a high temperature resistant refractory metal, ceramic or otherwise temperature-resistant material. While housing 635 and chamber space 628 may be described as having cylindrical shapes, as understood, housing 635 and chamber space 628 may have square, rectangular, or other suitable shapes. Housing 635 may be sealed at a closed or sealed end 622 and may include a main opening or open end 624 (which may be closed by door 602). Door 602 may mate with, e.g., be received through, open end 624 of housing 635 such that an end of door 602 (e.g., an end that receives inlet 618) may extend to ambient air outside of furnace 600. Main door seal 610 may, when door 602 is closed, be positioned adjacent to ambient air outside of furnace 600. Therefore, main door seal 610 may experience sufficiently cool temperatures (e.g., less than or equal to approximately 300 degrees C.) to allow and/or facilitate the inclusion of an elastomeric seal for seal 610. In one aspect, an elastomeric seal 594 may include a gasket or o-ring including a silicone-based material or other appropriate polymer.
A temperature-resistant a housing 635 including one or more of: (i) a refractory non-metallic material (e.g., mullite, alumina, or SiC); (ii) a refractory metal material; or (iii) a temperature resistant metal (e.g., nickel, nickel alloys, or other metals that have maximum use temperatures exceeding approximately 800 degrees C. and/or melting temperatures exceeding approximately 1,350 degrees C.).
A relatively thin layer of exposed inner insulation 24 (e.g., high-temperature-resistant exposed insulation) may be secured within chamber 628 inward of the housing walls. Insulation 24 may define a hot face 44 due to heat from heaters 112. The area within hot face 44 may define work zone 228, such that one or more parts 90 may be supported within work zone 228. When housing 635 includes metal material, the metal may include nickel, nickel alloys, 310S stainless steel, Kanthal metal (FeCrAl), or a refractory metal that can withstand operating temperatures of approximately 800 degrees C. or greater, or in particular, approximately 1,000 degrees C. or greater. While it may be desirable to form housing 635, which may be in the shape of a tube or rectangle, for example, of a metallic material, there is no requirement that housing 635 include metal. For example, housing 635 may include a ceramic, various forms of SiC (e.g., nitride bonded SiC, reaction bonded SiC, sintered SiC, etc.) and/or other refractory materials. It is noted that the materials forming housing 635 may be materials that are compatible with ambient oxygen at high temperatures.
As can be seen in
Referring to the embodiment of
As illustrated in
In some aspects, housing 635 may include 310S stainless steel alloy having a melting temperature of approximately 1,400 degrees C. and a service temperature (considered as a maximum use temperature) of approximately 900 degrees C. to approximately 1,200 degrees C. When 310S stainless steel or a similar material is used in housing 635, the thicknesses of inner 24 and outer 26 insulation may be established to cooperatively balance one another such that, for steady-state sintering temperatures of approximately 1,350 degrees C. or approximately 1,400 degrees C. within work zone 228, the alloy tube or housing 635 may reach a temperature of approximately 1,100 C, or less. Thus, the thicknesses of insulation 24, 26 may be balanced such that, for a particular amount of power applied to heaters 604, 606, housing 635 reaches a particular temperature.
The steady state balance between insulation thickness and housing 635 temperature may be understood with reference to the DC electrical analogy illustrated in
This balancing approach may be used to achieve a furnace 600 that may require less power to achieve the same or similar work zone temperatures, as compared to furnace 590 of
Furnace 600 may be thermally tuned to maintain one or more desired temperature balances such that desired sintering temperature is achieved in the work zone without exceeding maximum service temperature of the sealed chamber. In general, in some designs, it may be challenging to balance two insulations 24, 26 perfectly and to accommodate various temperature drifts and/or imperfections that may be expected, especially if there is a narrow design margin with regard to the maximum temperature housing 635 may experience without becoming damaged (maximum acceptable temperature). As used herein “design margin” refers to a difference between intended operating temperature of temperature-resistant housing 635 and the maximum acceptable temperature of the constituent material of housing 635. In general, it may be possible to build and tune individual furnaces 600 to achieve a narrow margin (e.g., an approximately 200 degrees C. margin). For example, when furnace 600 achieves a margin of 200 degrees C., insulation 24 and heaters 604 may be formed of a material that is configured to operate at temperatures at least 200 degrees C. higher as compared to the maximum acceptable temperature of housing 635. With a sufficiently high margin (e.g., approximately 400 degrees C.) the system may be manufactured in a way that overcomes the above-mentioned challenges by using thicker exposed inner insulation 22 to ensure that the temperature of housing 635 is significantly lower than the maximum acceptable temperature of housing 635. However, as the margin is increased, system cost may increase, and the performance benefit may diminish as thicker exposed insulation 24 may be required. For example, systems may use a nickel tube or 310S stainless steel as temperature-resistant housing 635, which may be operable at sintering temperatures of approximately 600 degrees C. In order to sufficiently insulate housing 635 from work zone 228 having significantly higher temperatures, relatively thick insulation 24 may be employed. However, such thick inner insulation 24 may be susceptible to contamination and/or may be expensive. Additionally, it may be beneficial to ensure that the expected temperature of chamber 635 during sintering is sufficiently cooler than the maximum acceptable temperature, while taking into account the thickness of insulation 24.
Stated differently, the furnace of
As illustrated in
Door 602 may be moveable to an open or loading position, as illustrated in
Feeds 618 and 620 may have cross-sectional areas that are approximately 20%, or smaller, as compared to a cross-sectional area of chamber 628. The configuration shown in
Feeds 618 and 620 may be connected to a work zone controller 650 that may include a suitable control device (e.g., computing system) and a power supply. Details of these feeds will be discussed shortly. Work zone controller 650 may be operably connected to inner heaters 604 by electrodes 654. One or more of the electrodes 654 may extend within a housing of one or more inlets 618, as described in greater detail below. Electrodes 654 may be electrically isolated with respect to inlet 618 and outlet 620, by forming a tubing of inlets and outlet 618, 620 from an electrically-insulating material and/or providing an insulating layer of material surrounding electrodes 654 but preferably by maintaining the electrodes in the center of the tube spaced apart from the tube such that there is no physical contact therebetween. Electrode 654 (e.g., within outlet 620) may be connected to temperature sensor 626 and may generate a temperature feedback signal to work zone controller 650. Inner heaters 604 may be electrically connected to work zone controller 650 via electrodes 654. Thus, power may be controllably supplied to inner heaters 604 from controller 650 (or a separate power source) via electrodes 654.
Outer heaters 606 may be connected to one or more outer heater controllers 652 via electrodes 654, as schematically shown in
In at least some aspects, outer heaters 606 may be activated simultaneously with the inner heaters by outer heater controller 652 during initial thermal processing, such as debinding, to assist in warming the outer peripheral surface of inner (exposed) insulation 24 to reduce or prevent condensation of volatized binders on or within inner exposed insulation 24. If desired, outer heater controller 652 may operate furnace 600 by applying an increased amount of power to outer heaters 606. The power applied to heaters 606 may gradually decrease or ramp down toward zero while power is increasingly applied to inner heaters 604 to increase or ramp up the temperature in work zone 228 to a sintering temperature. In some cases, it may be desirable to balance the power applied to inner and outer heaters during debinding such that a spatially uniform temperature (e.g., a suitable temperature for a debinding cycle) is maintained throughout, e.g., throughout the entire thickness, of the inner insulation 24. Work zone controller and power supply 650 may monitor temperature of the work zone 228 with at least one temperature sensor 626, while outer heater controller and power supply 652 may monitor the temperature of secondary or outer chamber 636 by at least one outer chamber temperature sensors 702.
It is noted that no insulation is “ideal” and that there may be thermal leaks in the outer insulation such as assembly joints and cooling paths and other imperfections. Thus, the theoretical optimum thickness and geometry should account for these thermal leaks. In one approach to mitigating leaks a designer could choose to provide design margin by erring on the side of adding thickness to the outer insulation. While this will ensure that the system remains well within a given power budget, it may cause the system to be more prone to overheating the housing especially if there is any failure or instability in the control system. Note that if the insulation is made thicker than optimum then it could be necessary to controllably add very low flow air cooling (for example as described with reference to
Referring to
As shown in
Process gas 71 introduced via inlets 618 may impede diffusion and condensation of contaminants that may otherwise tend to diffuse toward the cold end of chamber 635. A flow of gas through outlet tube 620 (e.g., via a vacuum pump connected to tube 620) may impede back diffusion of contaminants that may tend to condense in relatively colder regions as they flow from the hot end or work zone 228.
With ongoing reference to
Peclet sealing may be conceptualized as resulting from the flow of a relatively clean, pure sweep gas 422 that is provided with a sufficiently uniform and rapid flow velocity within Peclet channel 697 so as to overwhelm diffusion of contaminants (such as oxygen, moisture and/or hydrocarbons) upstream against the sweep gas flow.
This description of Peclet sealing may be understood and quantified approximately according to a unitless Peclet number 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 channel, and D represents the diffusivity of the contamination species (for example Oxygen, moisture and/or hydrocarbons) in the sweep gas. The degree of isolation R is a ratio of concentration contaminant on the upstream end (the clean end) of the Peclet channel divided by the concentration of contaminant on the downstream (contaminated side) and R can be estimated as R=Exp(−Pe). Accordingly, for a given gas flow velocity, the Peclet number and hence the degree of isolation can be increased by any combination of (i) using a thinner and/or longer channel (ie smaller G and/or longer L) (ii) increasing gas flow. We have validated the reliability of this approximation for atmospheric pressure conditions as long as L>>G and we have routinely demonstrated seals as shown in
The exemplary configuration of furnace 600 illustrated in
Each outer heater control module 706 may receive temperature information for a portion of chamber 636 corresponding to one or more coils 704. A system-level controller 708 may receive information and output control signals to each of the outer heater control modules 706. Additionally, system-level controller 708 may receive feedback information from, and provide commands to, work zone power supply 651, and/or work zone controller 650 so that the plurality of control modules 706 may be cooperatively controlled by a central system level control device such as controller 708.
The above-described configurations of furnace 600 depicted in
With ongoing reference to
Furnace 600 may, with respect to a particular holding power (for example 4 kW), tend to experience overheating of housing 635 when blowers 716 are deactivated. In order to avoid such overheating of housing 635 at steady-state sintering power, a closed-loop air cooling system may include smaller blowers 716, cooling channels 714 for providing cooling air from blowers 716, a control module for each blower 716 (or subset of blowers) that may be integrated into blowers 716 or provided separately, and chamber temperature sensors 702. Blowers 716 may controllably circulate cooling air to maintain the temperature of secondary chamber 636 and/or housing 635 approximately constant or within a pre-determined temperature range. The design of furnace 600 may take fail-safe considerations into account to ensure that, in the event of a power failure, or other failure of the cooling system, the thermal mass in work zone 228 may not cause thermal damage, especially to the temperature resistant housing 635. Options for air cooling include, but are not limited to, relatively small, low-cost displacement pumps, such as piston pumps or diaphragm pumps, small squirrel cage fans, or one or more other suitable mechanisms. In exemplary configurations, cooling requirements may reach approximately several hundred watts or approximately 1,000 watts.
While a plurality of blowers 716, 718 are illustrated in
The values shown in Table 1 may correspond to a thermal conductance of 0.5 W/mK (e.g., corresponding to graphite insulation) at 1,400 degrees C., 3 kW of holding pattern at steady-state, and may assume that insulation 24 is formed as a uniform slab of material with no variation in thermal conductivity. Exemplary equations for determining thickness of insulation 24 include: Thickness=ΔT×Conductance×Area/Power; or Thickness=[1,400−Housing temp]×Conductance×Area/Power. In general, thermal conductivity may tend to drop rapidly with a reduction in temperature dropping from approximately 0.5 W/mK at 1,400 degrees C. to approximately 0.05 W/mK at 200 degrees C. If desired, thinner layers of insulation may be used.
It is evident from the above table that For a particular sintering temperature, such as 1,400 degrees C., higher permissible temperatures of housing 635 may allow for the use of progressively thinner insulation 24 and, thus, may provide increasingly cleaner atmospheres during sintering. It is recognized that reducing the margin or difference between the maximum use temperature of housing 635 and the actual maximum steady state operating temperature of the housing 635 may increase cleanliness and atmospheric purity by (i) facilitating the use of less (or thinner) insulation; and (ii) increasing the temperature of insulation 22 during thermal processing, in particular during the beginning stages of a sintering process during which the temperature within work zone 228 may be increasing prior to reaching steady state holding temperature and/or maximum peak temperature. Outer heaters 606 may be utilized early in the sintering cycle to facilitate the increase in temperature. In exemplary processing, excellent atmospheric cleanliness may be achieved by one or more of: (1) using outer heaters 606 in addition to the inner heaters during debinding to achieve debinding temperature(s) in work zone 228 such that heat from the outer heater keeps the outside at the same or even higher temperature as the inside hot face of the inner insulation. For example, as the inner heaters bring the inner hot face to debinding temperature (for example 500 C) outer heaters 606 may heat the outside of the inner insulation 24 and to approximately the same or possibly even higher temperature. This processing with outer heaters 606 will generally tend to prevent or at least inhibit condensation within exposed insulation 24 and on housing 635 and/or chamber 636 by elevating the entire insulation 22 and housing 635 to approximately the same temperature as the temperature of work zone 228 during debinding; (2) increasing the power applied to outer heaters 606 to raise the temperature of housing 635 to approximately the maximum use temperature (e.g., about 1,100 degrees C. for a 310S stainless steel housing 635, which may initiate and progress the ramp up to sintering temperature until the inner heaters 604 are powered to maintain the ramp rate); (3) activating inner heaters 604 when the temperature of housing 635 approaches a maximum acceptable temperature to drive the temperature of work zone 228 towards a predetermined sintering temperature (simultaneously, a secondary feedback control may be activated and operated as described above so that outer heaters 604 do not overheat the housing 635 beyond its maximum acceptable temperature); (4) optionally, employing cooling channels and a cooling system as part of the feedback control to avoid overheating of housing 635; (5) de-activating heaters 604 and 606 at the conclusion of sintering, and activating rapid cooling systems to rapidly cool work zone 228. This process is illustrated in
At least some embodiments may include refractory metal materials in insulation 24 as a layered stack of thermal reflectors arranged such that emission from each plate is, to some extent, reflected by the next outer layer, with the power between each layer being a function of emissivity and temperature difference. It is noted that the manufacturing and assembly of such insulation 24 may employ support mechanisms that may bring a degree of thermal conductivity. Nevertheless, it may be possible to take the thermal conductivity of such support mechanisms into account for furnace 600. Exemplary design guidelines and assumptions useful for inclusion of refractory metal materials may include: an effective emissivity of approximately 0.3 for the refractory layers. In particular, emissivity of clean molybdenum may be below 0.25 at a temperature below 1,500 degrees C. (taking into account that emissivity generally rises going from relatively low (100 degrees C.) to higher (1,400 degrees C.) temperatures. Emissivity of steels may be relatively high, e.g., at 1,000 degrees C., the emissivity may be about 0.8. It may be possible to compensate for such emissivity by adding one more refractory layer(s).
Heat flow from one layer to the other may be related to, e.g., proportional to, emissivity. Heat flow through different insulation materials may be approximately the same (e.g., emissivity through metal shields and through nickel isolated insulation may be approximately the same). The design considerations described above may also be applied to graphite materials for insulation 24, (taking into account that it may be difficult to assign a conductivity value). In some embodiments that include graphite materials for insulation 22, it may be beneficial to perform an iterative calculation to determine the number of required layers associated with: (i) a given sintering temperature, (ii) an assumed steady-state power, and (iii) an assumed maximum chamber temperature.
Table 2 provides a summary of exemplary embodiments corresponding to: a 3 kW steady-state holding power, 6 in., 8 in., and 10 in. diameter work zones 228, 12 in. long work zones 228, and assuming atmospheric pressure. Each layer may be assumed to exhibit emissivity of approximately 0.3 for the layers and approximately 0.8 for the wall of temperature resistant housing 635. For purposes of comparison, an exemplary baseline design is included in the third column of Table 2 above. This baseline design assumes an outer housing temperature corresponding to 3 kW, which may be expected to match, within 100 degrees C., the outer temperature that may be expected for 3 kW heat dissipation. In accordance with one or more aspects of this disclosure, and similar to Table 1, the higher the temperature of chamber 628, the lesser the amount of inner insulation 24 may be required. It is noted that each row corresponds to a given work zone, and each entry includes an exemplary estimate of chamber tube diameter (diameter of housing 635).
Various considerations may be taken into account with respect to the configuration of inner exposed insulation 24 for furnace 600, as described above. Similarly, outer isolated insulation 26 may be configured based on one or more design considerations that affect operation of furnace 600. In some furnace designs, an initial design step may include selection of a type of insulation based on a particular manufacturer and/or type of insulation. Once this first design is established, the design may be improved, e.g., by reducing insulation thickness with the inclusion of one or more grades of insulation, and/or dense high-grade materials toward the hot face and microporous materials on the outer layers. For some furnaces 600 having outer insulation and outer housing temperatures of approximately 1,250 degrees C. or less, ceramic fiber materials, such as Morgan Thermal Cerablanket, which may have maximum use temperatures of approximately 1,250 degrees C. and a densities of approximately 128 kg/m3, may be considered for use as outer insulation. An appropriate outer insulation thickness may be determined based on solely hot zone 228 temperature, under the assumption of approximately 100 degrees C. outer temperature (e.g., at wall 32), and a passive convective heat loading of roughly 100 W/m2. In this approach to furnace design, the heat load for conduction through the insulation may escape at roughly 750 w/m2 (not to be conflated with heat load requirements for overcoming leakage and for achieving desired ramp rates). In at least some furnaces, in particular those requiring high mass loads and high ramp rates, the maximum heater power may be significantly higher than the steady-state heat power conducting through the insulation.
Table 3 below lists exemplary insulation thicknesses of an exemplary insulation material (Morgan Cerablanket) for various temperatures of housing 635. Other suitable products, which may provide relatively low-cost solutions (in comparison to relatively high grade graphite), may be used instead of, or in addition to, Morgan Cerablanket.
Table 4 includes exemplary amounts of heat convection from the outside of outer insulation 26, for different tube sizes (sizes of housing 635) and thicknesses of outer insulation 26. For the values of Table 4, it is assumed that the outside of the insulation pack in steady-state is approximately 100 degrees C.
Table 4 may illustrate that one or more packs of outer insulation 26 may dissipate less than the 3 kW of power, which may be desired for maximum steady-state holding power. Accordingly, for exemplary configurations of furnace 600 described herein, there may be adequate margin to facilitate the inclusion of heat leaks, such as air cooling channels 616, without exceeding power requirements for furnace 600. In cases in which channels 616 may introduce excessive heat loss, it may be possible to include channels 616 in combination with relatively thicker outer insulation 26 to compensate for the increased heat loss. Additionally, thinner insulation may be desired in some applications (e.g., applications in which it is desired that furnace 600 have a smaller footprint). This may be achieved by employing more complex designs having different types of insulation for progressively cooler outer layers. For example, suitable microporous insulation materials (e.g., microporous insulation manufactured by Promat) may be employed. Such microporous insulation materials may have a maximum use temp of approximately 1,000 degrees C. and a relatively high R value as compared to typical ceramic blanket materials.
Each configuration of furnace 600, like furnaces 100 and 400, may be operable in a vacuum (e.g., a vacuum applied via outlet 620). Vacuum operation may be facilitated by use of walls having thickness thicker than approximately 0.125 inches. High temperature alloys, such as 310S, may exhibit some degree of softening and/or creep at elevated temperatures, such as 1,100 C. Therefore, in some embodiments, as is illustrated in
At the end of ramp up 738, the temperature within hot zone 28 may reach approximately 500 degrees C. Once this temperature is reached, the temperature within hot zone 28 may be held approximately constant during a debinding dwell period 740 that occurs for a debinding dwell time DT. This dwell time DT may extend for approximately one hour, but may be shorter or longer based on one or more of the above-described factors.
At the conclusion of dwell time DT, the temperature within hot zone 28 may again begin to ramp up during a sintering ramp up 742. As can be seen in
At the conclusion of sintering dwell time 744, a cooling period 746 may occur. If desired, one or more post-sintering heat treatments may be performed on parts 90 in the same furnace, or in another furnace.
The inner heaters 604 and outer heaters 606 can be operated in conjunction. In an exemplary embodiment, during the debinding ramp 738 and debinding dwell 740 inner heaters 604 and outer heaters 606 can have the same or similar temperature. During sintering ramp up 742 the outer heaters can have the same or lower temperature as the inner heaters. Temperature line 747 represents the maximum service temperature 747 for the sealed housing 635. During the portion of the sintering ramp up 742, the sintering dwell time 744, and the sintering ramp down 748 the outer heaters 606 are not raised above the maximum service temperature 747, while the inner heaters 604 can be raised accordingly as described above to accomplish sintering in the work zone.
This application claims the benefit of priority of U.S. Provisional Application No. 62/899,358, the entirety of which is incorporated by reference into this application.
| Number | Name | Date | Kind |
|---|---|---|---|
| 3708285 | Scheyer | Jan 1973 | A |
| 4501717 | Tsukamoto | Feb 1985 | A |
| 5048801 | Johnson | Sep 1991 | A |
| 5366679 | Streicher | Nov 1994 | A |
| 6163020 | Bartusch | Dec 2000 | A |
| 10578361 | Woodard | Mar 2020 | B2 |
| Number | Date | Country |
|---|---|---|
| 1496325 | Jan 2005 | EP |
| Number | Date | Country | |
|---|---|---|---|
| 20210078075 A1 | Mar 2021 | US |
| Number | Date | Country | |
|---|---|---|---|
| 62899358 | Sep 2019 | US |
