Field of Invention

[0001] The material and methods disclosed herein relate generally to powder metal processing, and in particular to a method for forming articles from metal or ceramic powders by continuous extrusion of shaped sections.

Background of Invention

[0002] Metal injection molding (MIM) has been practiced for years to produce articles for industries such as agricultural, automotive, business machine, food & beverage, hardware, medical, small appliance, and sporting goods. Metal injection molding is capable of producing articles that have material properties comparable to those of articles produced by forging, stamping, and machining processes, while at the same time providing the ability to produce high volumes of the articles in a cost effective manner. Additional advantages of metal injection molding include the ability to produce articles with intricate shapes, the ability to produce net and near net articles with close tolerances, and the ability to produce articles with multiple components in one mold, thereby reducing assembly costs.

[0003] A disadvantage to metal injection molding is the discrete nature of the process, which results in the production of only a limited number of article units per press cycle. The press cycle rate is controlled by the time required to fill the mold with a liquid charge and cool the resultant part sufficiently that it can be removed from the mold. Another disadvantage to metal injection molding is the difficulty in producing articles of substantial length. Yet another disadvantage to metal injection molding is the high cost of the molds which can produce only one particular unique part.

[0004] Accordingly, it would be advantageous to have a continuous powder extrusion method capable of producing non-molded articles of virtually unlimited length which may then be cut to size. It would further be advantageous to produce the elongate powder extruded non-molded articles at a rate much faster than the discrete articles could be produced by MIM. It would still further be advantageous to have a continuous powder extrusion method capable of producing non-molded articles having any desired cross- sectional profile, including solid, hollow, and multi-lumen shapes using relatively low cost, flexible tooling. It would still further be advantageous to have a continuous powder extrusion method capable of producing non-molded articles of net or near net shape. Additional advantages of the continuous powder extrusion method of the present invention will be apparent in view of the following description.

Summary of Invention

[0005] The present invention provides a method for making a non-molded article using powder extrusion. The method includes providing one or more bulk material powders, providing one or more binders, mixing the one or more bulk material powders and the one or more binders to make a mixture, and providing a die having a shape. The method further includes extruding the mixture through the die to produce a green form, debinding the green form to produce a brown form, sintering the brown form to produce a densified form, and processing the densified form to produce an article.

[0006] A powder extrusion method can be performed continuously, and can be used to make long lengths of net or near net forms with a custom-shaped cross section, or to make long lengths of preforms that can be thermally, mechanically or thermomechanically processed to produce non-molded articles. Continuous powder extrusion is a cost effective method of producing a non-molded article having precise features that may otherwise need to be machined from comparable cast and wrought material.

[0007] The method of continuous powder extrusion transforms one or more bulk material powders into a non-molded article. The method includes providing one or more bulk material powders and one or more binders. The bulk material powders and binders are combined into a substantially uniform mixture. The mixture is extruded through a die to produce a green form. The die has a shape corresponding to the desired external shape of the finished article but at a somewhat larger size. The die optionally includes a mandrel having one or more shapes corresponding to one or more desired internal hollow shapes in the finished article. The green form is produced by extruding the mixture into an elongate section having a profile corresponding to the die and the optional mandrel. The green form is debound to produce a brown form. Debinding comprises removing some portion of the binder material by thermal, solvent, supercritical fluid, or aqueous extraction, or by a reactive chemical process, or by some combination thereof. The brown form is sintered at one or more elevated temperatures to produce a densified form of the desired size. The densified form is cut to net length to produce a powder extruded non-molded article having a desired cross-sectional shape.

[0008] The continuous powder extrusion method can be used to produce shaped sections that are difficult or costly to fabricate by other methods, such as a section having an internal hexagonal or octagonal shape. The continuous powder extrusion method can further be used to produce shaped bars or tubes of custom materials, including reactive, brittle, or hard materials that may be difficult to melt and cast or reduce by traditional metal working processes including extrusion, forging, rolling, drawing or stretching. Additionally, the continuous powder extrusion method can be used to make sintered powder metal preformed shapes for subsequent hot forging or hot rolling operations to heal small internal voids and promote homogeneity, whereby the output of the combined processes is a low cost finished wrought product having a high density and similar mechanical properties to cast and wrought metal products. Additionally, the continuous powder extrusion method can be used to make custom materials that include more than one composition of metallic powders or both metallic and ceramic powders.

[0009] The present invention further provides a non-molded article having a profile. The article is produced by the method comprising providing one or more bulk material powders, providing one or more binders, mixing the one or more bulk material powders and the one or more binders to make a mixture, providing a die having a shape, extruding the mixture through the die to produce a green form, debinding the green form to produce a brown form, sintering the brown form to produce a densified form, and processing the densified form to produce an article. The profile has an outer shape corresponding to the shape of the die.

[0010] The present invention still further provides a method for making a non-molded article using powder extrusion. The method includes providing one or more bulk material powders, providing one or more binders, mixing the one or more bulk material powders and the one or more binders to make a mixture, and providing a die having a shape. The method further includes heating the mixture, extruding the mixture through the combination of the die and the mandrel to produce a green form having an outer profile corresponding to the shape of the die, and receiving the green form onto a setter tray. The method also includes debinding the green form to produce a brown form;, sintering the brown form to produce a densified form, and processing the densified form to produce an article. The setter tray enables the brown form to shrink without fracturing.

[0011] Other objects, advantages, and features of the present invention will become apparent to those skilled in the art upon reading the following detailed description, when considered in conjunction with the appended claims and the accompanying drawings briefly described below.

Brief Description of Drawings

[0012] The accompanying drawings, which are incorporated herein and constitute a part of this specification, illustrate preferred embodiments of the invention, and together with the general description given above and the detailed description given below, serve to explain features of the invention. However, it should be understood that this invention is not limited to the precise arrangements and instrumentalities shown in the drawings.

[0013] FIG. IA is a schematic overview of an embodiment of the continuous powder extrusion method of the present invention for producing a non-molded article.

[0014] FIG. IB is a schematic overview of an embodiment of the continuous powder extrusion method of the present invention for producing a non-molded article.

[0015] FIG. 1 C is a schematic overview of an embodiment of the continuous powder extrusion method of the present invention for producing a non-molded article.

[0016] FIG. ID is a schematic overview of an embodiment of the continuous powder extrusion method of the present invention for producing a non-molded article.

[0017] FIG. 2A shows an exemplary powder extruded non-molded article having a custom cross-sectional shape formed using the method of the present invention. [0018] FIG. 2B shows an exemplary powder extruded non-molded article having a round outside shape and a round inside shape formed using the method of the present invention.

[0019] FIG. 2C shows an exemplary powder extruded non-molded article having a plurality of internal lumens formed using the method of the present invention.

[0020] FIG. 2D shows an exemplary powder extruded non-molded article having a round outside shape and a hexagonal inside shape formed using the method of the present invention.

[0021 ] FIG. 2E shows an exemplary powder extruded non-molded article having a round outside shape with external threads and a hexagonal inside shape formed using the method of the present invention. The hexagonal inside shape is formed by the extrusion mandrel, while the threads are formed by a finishing operation such as machining or thread rolling.

[0022] FIG. 2F shows an exemplary powder extruded non-molded article having an oval outside shape and a round inside shape formed using the method of the present invention.

[0023] FIG. 2G shows an exemplary powder extruded non-molded article having a rectangular outside shape and a rectangular inside shape formed using the method of the present invention.

[0024] FIG. 2H shows an exemplary powder extruded non-molded article having a non-symmetrical outside shape and a non-symmetrical inside shape formed using the method of the present invention.

[0025] FIG. 21 shows partially cut-away view of an exemplary powder extruded non- molded article having an external diameter and an internal diameter with a single bump formed using the method of the present invention. Detailed Description of the Illustrated Embodiments

[0026] The present invention provides a method of a making a powder extruded non- molded article having a custom shaped cross-section using a continuous extrusion method. The method uses a mixture of metallic and/or ceramic powders in a polymer and/or inorganic binder that are shaped by being forced under pressure through a die, causing the mixture to be extruded into an elongate form. The resultant elongate form comprises a desired cross-sectional shape and can extend indefinitely from the outlet of the die as a continuous elongate form. The elongate form can be subsequently subjected to mechanical then thermal, and/or thermomechanical processing to achieve a wrought densified form having density and mechanical properties equal to cast and wrought metal. Cutting the form to a net length produces a completed net size article. Net length and net size are the finished length and finished size, respectively, of an article. Further finishing by thermal and various other processes can be applied to refine the properties and appearance of the article and for producing a finished article.

[0027] An elongate form produced by a continuous powder extrusion method of the present invention may have a solid or hollow cross-sectional profile. The profile of the elongate form corresponds to the die, in combination with the optional mandrel, through which the mixture of bulk material and binder is extruded. Accordingly, the profile of the elongate form may include external or internal shapes that are limited only by the intricacy of the die and mandrel combinations that may be created. A small sampling of exemplary cross-sections, illustrated in Figures 2A through 21, are discussed in greater detail below. Custom shaped cross-sections may include, for example, the combination of a round external shape with a hexagonal internal shape, or a round multi-lumen hypotube or surgical tube. Custom shaped cross-sections may further include external shapes of round, oval, rectangular, and any combination of flat and arc surfaces such as symmetrical and nonsymmetrical polygons, as well as T-sections, U-sections, L-sections, I-sections, or virtually any regular or irregular permutation thereof.

[0028] Referring to Figure IA, there is shown a schematic overview of a first exemplary method of making a powder extruded non-molded article. The method includes a step 10 of providing bulk material powders, a step 20 of providing binders, a step 30 of mixing the bulk material powders with the binders to form a mixture, a step 40 of providing a die (optionally including a mandrel for creating one or more lumens), a step 50 of extruding the mixture through the die (optionally including the mandrel) to produce a green form, a step 60 of debinding the green form to produce a brown form, a step 70 of sintering the brown form to produce a densified form, and a step 75 of processing the densifϊed form to produce an article. The processing step 75 includes a substep 90 of cutting the densifϊed form to a length.

[0029] Referring to Figure IB, there is shown a schematic overview of a second exemplary method of making a powder extruded non-molded article. The method includes a step 10 of providing bulk material powders, a step 20 of providing binders, a step 30 of mixing the bulk material powders with the binders to form a mixture, a step 40 of providing a die (optionally including a mandrel for creating one or more lumens), a step 50 of extruding the mixture through the die (optionally including the mandrel) to produce a green form, a step 60 of debinding the green form to produce a brown form, a step 70 of sintering the brown form to produce a densifϊed form, and a step 75 of processing the densified form to produce an article. The processing step 75 includes a substep 80 of subjecting the densified form to mechanical and then thermal processing, and/or to thermomechanical processing, using metal processing methods, to produce a wrought form. The processing step 75 further includes a substep 90 of cutting the wrought form to a length.

[0030] Referring to Figure 1 C, there is shown a schematic overview of a third exemplary method of making a powder extruded non-molded article. The method includes a step 10 of providing bulk material powders, a step 20 of providing binders, a step 30 of mixing the bulk material powders with the binders to form a mixture, a step 40 of providing a die (optionally including a mandrel for creating one or more lumens), a step 50 of extruding the mixture through the die (optionally including the mandrel) to produce a green form, a step 60 of debinding the green form to produce a brown form, a step 70 of sintering the brown form to produce a densified form, and a step 75 of processing the densified form to produce an article. The processing step 75 includes a substep 90 of cutting the densified form to a length and a substep 100 of finishing the article. [0031 ] Referring to Figure ID, there is shown a schematic overview of a fourth exemplary method of making a powder extruded non-molded article. The method includes a step 10 of providing bulk material powders, a step 20 of providing binders, a step 30 of mixing the bulk material powders with the binders to form a mixture, a step 40 of providing a die (optionally including a mandrel for creating one or more lumens), a step 50 of extruding the mixture through the die (optionally including the mandrel) to produce a green form, a step 60 of debinding the green form to produce a brown form, a step 70 of sintering the brown form to produce a densified form, and a step 75 of processing the densified form to produce an article. The processing step 75 includes a substep 80 of subjecting the densified form to mechanical and then thermal processing, and/or to thermomechanical processing, using metal processing methods, to produce a wrought form. The processing step 75 further includes a substep 90 of cutting the wrought form to a length to produce an article and a substep 100 of finishing the article to produce a finished article.

[0032] In the step 10 of providing bulk material powders, the bulk material powders can include one or more powders of any metallic or ceramic material suitable for powder extrusion. The bulk material powders may encompass any metal, ceramic, or other material that can be sintered. The type and size of powders selected will depend on the alloy, the type of form to be made, and whether the form will be processed to a wrought article.

[0033] Forms can include net forms, near net forms, and preforms. If the form produced by extrusion, debinding, and sintering has an outer shape that is at the size of the article to be produced, the form is termed a net form. If the form produced by extrusion, debinding, and sintering has an outer shape that is near, but usually slightly larger than, the size of the article to be produced, the form is termed a near net form. A near net form can also be slightly smaller than net size, where, for example, it is expected that the article will be coated or otherwise processed to enlarge the outer shape to net size. If the form produced by extrusion, debinding, and sintering has an outer shape that is larger or smaller than net or near net size, the form is generally termed a preform. A preform can be smaller than net size, where, for example the preform will be upset (deformed in such a way that the length is reduced along the original extrusion axis). A net form, a near net form, and a preform can have the same physical or mechanical properties, the term "preform" merely being used to designate a form that is being larger or smaller than a net form or near net form. A green form is subjected to the same debinding step to produce a brown form, and a brown form is subjected to the same sintering step to produce a densified form, regardless whether the form is a net form, a near net form, or a preform. However, when a preform has been produced by the extruding, debinding, and sintering steps, the densified form is generally mechanically then thermally, and/or thermomechanically processed before the cutting step to produce a wrought form that is at net or near net size.

[0034] Net or near net forms generally do not require further thermal, mechanical, and/or thermomechanical processing after sintering. Preforms, being larger or smaller than net or near net size, will frequently be processed as a metal subsequent to extrusion, debinding, and sintering, for example by forging, hot working, or cold working then annealing.

[0035] For producing forms having a net or near net size, particularly parts having a small cross-section and smooth surface requirements, the bulk materials preferably include fine metal, ceramic, or metal and ceramic powders. More preferably, the bulk materials include fine metal powders having a diameter of less than 50 microns. Most preferably, the bulk materials include fine metal powders having a diameter of less than 25 microns. Small net or near net parts may include, but are not limited to, surgical biopsy needles, surgical screws, and multilumen hypodermic needles.

[0036] For producing an elongate preform that will be subjected to further thermal, mechanical, and/or thermomechanical processing, particularly a preform having a larger or smaller cross-section than the net article, the bulk materials preferably include coarser metal or metal and ceramic powders. Although larger sections can readily be formed using fine metal powders, less costly coarse powders may be used for these preforms. Because preforms will be mechanically then thermally, and/or thermomechanically processed in order to produce a wrought article, the preform at the conclusion of the sintering step may have lower density and a rougher surface finish than would be permissible in a net or near net form. The greater porosity and rougher surface consistent with the use of coarser powders will be corrected during the further thermal, mechanical, and/or thermomechanical processing steps to which these preforms may be subjected. Accordingly, bulk material powders having a diameter of 40 microns or finer may preferably be used to make preforms of larger cross sections, and particle sizes of the bulk material powders having a diameter of 80 microns or finer may be used without detrimentally affecting the properties of the finished wrought article.

[0037] For producing articles by a method of continuous powder extrusion, the composition of the bulk material powders may be elemental, master alloy, or prealloyed. The bulk materials preferably include powders capable of forming various metals and alloys. The bulk materials may include, but are not limited to, powders capable of forming any metals and alloys suitable for the medical industry, in particular metals and alloys suitable for implantation. The bulk materials may also include reactive powder metal alloys.

[0038] For example, the bulk materials can include powders capable of forming the following medical metals and alloys: austenitic, ferritic, martensitic and precipitation hardenable stainless steels per ASTM F899, 316L stainless steel per ASTM Fl 38, commercially pure Titanium per ASTM F67, Ti 6Al 4V per ASTM F 1472, Ti 6Al 4V ELI per ASTM F136, Ti 6Al 7Nb per ASTM F1295, Nitinol per ASTM F2063, Cobalt Chromium Molybdenum (CoCrMo) per ASTM F75 or ASTM Fl 537, Cobalt Chromium Tungsten per ASTM F90, Cobalt Nickel Chromium per ASTM F562, and combinations thereof, and other novel alloys designed to have specific properties conducive to medical device function such as radiopacity or MRI compatibility.

[0039] The bulk materials can include austenitic, martensitic, ferritic, and precipitation hardenable stainless steels per ASTM F899, which may be particularly useful in forming instrument and tooling parts.

[0040] The bulk materials can include powders capable of forming an implantable austenitic stainless steel alloy (e.g., 316L stainless steel per ASTM F138), which may be particularly useful in forming parts for medical and surgical applications. An austenitic stainless steel generally has sufficient corrosion resistance for biocompatibility, excellent elongation, is relatively low cost and is readily fabricated.

[0041] The bulk materials can include powders capable of forming a precipitation hardened stainless steel alloy (e.g., 17-4PH stainless steel per ASTM F899), particularly for producing non implanted parts that exhibit a good balance between corrosion resistance and strength. The strength and hardness can be modified by adjusting the temperature at which the part is heat-treated as part of the finishing step after sintering. The precipitation-hardened alloy generally provides for better corrosion resistance than 400 series stainless steels and better strength than 300 series stainless steels.

[0042] The bulk materials can include powders capable of forming a martensitic stainless steel (e.g., 420 or 440 per ASTM F899). These non implanted material exhibits the highest hardness and strength among the stainless steels with some tradeoff in corrosion resistance.

[0043] The bulk materials can include powders capable of forming novel metallic alloys or mixtures of ceramics and metals designed to have specific properties conducive to medical device function such as radiopacity or MRI compatibility.

[0044] The bulk materials can include a combination of low alloy steel, carbon, nickel, and molybdenum. This combination provides for a multi-purpose, economical material for non-medical applications that provides flexibility in various properties such as strength, hardness, and wear resistance.

[0045] The metal or metallic powders of the bulk materials may be produced by gas atomization or by other methods. Particularly with regard to bulk materials used to extrude preforms that will be further processed thermally, mechanically, and/or thermomechanically, the bulk materials may include low cost titanium powders having a very coarse structure that may be made by the direct reduction of titanium chloride.

[0046] The bulk materials can further include any one or more radiopaque materials that preclude penetration of x-rays or other types of radiation commonly used in diagnostic imaging, which may be particularly advantageous in the medical device field. [0047] In the step 20 of providing binders, the binders can include one or more materials commonly used as binders in metal injection molding. The binder materials must be compatible with the metallic or ceramic bulk materials. The binder materials must further be compatible with the demands of the extruding step 50, the debinding step 60, and the sintering step 70. In particular, the binders can include any melt extrudable organic or inorganic compound that can be removed via a thermal, solvent, supercritical fluid, or aqueous extraction, or via reactive chemical process using a chemical debinding agent. Preferably, the binders include a polymer binder. The binders may include a combination of two or more polymeric materials. More preferably, the binders include a polymer that provides sufficient melt strength to maintain a cross-sectional shape during the extrusion method. The binders may include various plastics, thermoplastics, waxes, and acrylic thermosetting resins. The binders may include inorganic materials, particular for use in combination with bulk material powders comprising ceramics.

[0048] In the step 30 of mixing the bulk material powders with the binders to make a mixture, the bulk material powders and the binders, and any other components such as radiopaque materials, are mixed together to make a mixture having a substantially uniform composition. In the mixture, the metal powders, ceramic powders, or combined metallic and ceramic powders are wet thoroughly by the polymer and/or inorganic binders. The proportion of binders to bulk material powders is preferably in the range of about 40% to about 65% by volume. The components may be mixed using a conventional mixing apparatus. The temperature at which the materials are mixed can vary. Preferably, the mixture is flowable at temperatures and pressures close to ambient. A flowable mixture allows the mixture to fill crevices and small dimensional features of a die, optionally including a mandrel, through which the mixture will be extruded.

[0049] In the step 40, an extrusion die is provided, the die optionally including a mandrel. The extrusion die may be selected to form a basic geometric outer dimensional shape such as a circle, oval, rectangle, or polygon, or a complex geometric outer dimensional shape such as that of a boat or car window molding, an irregular polygon, or an asymmetric shape. An exemplary extruded form having a complex irregular asymmetric shape is illustrated in Figure 2A. The form may have a complex irregular shape and may include complex profiles such as that of a boat or car window seal. In other examples, forms may be coextruded into multiple layers, stripes, or wires, or over- extruded into wire coverings or into one or more layers of coatings.

[0050] A mandrel having one or more shapes may be used in conjunction with the die to produce a form having a cross-section with one or more internal hollows corresponding to the one or more shapes of the mandrel. Single or multi-lumen structures can be produced having a variety of cross sectional profiles. Figures 2B through 21 illustrate some possible examples of extruded forms having single-lumen or multi-lumen cross sectional profiles.

[0051] As shown generally in Figures 2B through 21, a hollow tubing preform may be extruded and subsequently sintered and drawn to produce lower cost seamless metallic tubing when compared with existing methods, wherein the finished tubing may have any desired external and internal profile. Such tubing may be produced from existing alloys, and in particular may be advantageously produced from alloys that are difficult to melt or hot process. In particular, the extruded form may have cross-sectional shapes including a single lumen, as exemplified in Figures 2B, IE, 2F, 2G, and 21. Alternatively, the extruded form may have cross-sectional shapes including multiple lumens, as exemplified in Figures 2C and 2H.

[0052] As shown in Figure 2B, a tubular part can be produced having a generally circular external shape and a generally circular internal lumen. As shown in Figure 2C, a tubular part can be produced having a generally circular external shape and a plurality of internal lumens.

[0053] As shown in Figure 2D, a medical tool can be produced having a continuous internal hexagonal lumen for applying a driving torque to a device with a corresponding male hexagonal form. Such a medical tool can be used both to drive a suture anchor and to concomitantly contain the attached suture material within the lumen. Similarly, a medical tool can be produced having a continuous octagonal lumen for driving a device with a corresponding male octagonal form. [0054] As shown in Figure 2E, a generally tubular medical implant can be produced from an implantable alloy having a continuous hexagonal internal shape for receiving a hexagonal driver tool and a threaded external surface. Such an implant can be used as a trauma fixation screw without drilling a hole for a guide wire or broaching the hexagonal internal shape. Similarly, a generally tubular medical implant can be produced from an implantable alloy having a continuous octagonal internal shape for receiving an octagonal driver tool and a threaded external surface.

[0055] As shown in Figure 2F, a generally tubular part can be produced having a generally ovalized external shape and a generally circular internal lumen. As shown in Figure 2G, a generally tubular part can be produced having a generally square or rectangular external shape and a generally square or rectangular internal lumen.

[0056] As exemplified in Figure 2H, an extruded generally tubular part may be produced having a complex polygonal external shape and a plurality of internal lumens having various shapes.

[0057] As exemplified in Figure 21, an extruded generally tubular part may be produced having one or more bumps, or variations in external and internal diameter.

[0058] In the step 50 the mixture is extruded through the die to produce a green form having a desired cross-sectional shape which may be solid or hollow. The mixture of bulk material (metal, ceramic, or combined metal and ceramic) powders with polymer and/or inorganic binders is fed into an extruder. The mixture is heated to form a flowable mixture. Then, the flowable mixture is continuously extruded through the die or through the annulus or annuli between the die and the optional mandrel having one or more shapes. The extruded mixture emerges from the die, or die and mandrel combination, as a green form, or green-state extrusion. The green form has a desired profile corresponding to the shape of the die or the die and mandrel combination.

[0059] The extruder comprises an extrusion press. The extrusion press is preferably a screw-driven extrusion press. The extrusion press forces the heated mixture through the die to produce an elongate form having the desired profile. The extrusion press applies a force to the mixture forcing the mixture through the die or die and mandrel combination. [0060] During extrusion, frictional heat is generated. Accordingly, the die is typically constructed with internal cooling passages. Cooling of the die serves to solidify the flowable mixture as it emerges from the die as a green form. The overall dimensions of the die are larger than that of the desired completed continuous powder extruded non- molded article to account for the shrinkage that will occur during binder removal and sintering.

[0061] During extrusion of the mixture, temperature, pressure, shear, and velocity within the extruder are controlled to enable production of a sound, pore-free product. Temperature is preferably controlled by using external heaters. In addition, the generation of shear heating may be controlled by regulating the extrusion rate. The extrusion temperature is dependent on process factors including the composition and size bulk material powders, the composition of the binders, and the ratio of bulk material powders to binders in the mixture, as well as on the rate of extrusion. The temperature is preferably less than about 950°F and is more preferably in the range of about 200°F to about 65O ⁰ F, but may range higher than 950°F if the binder materials used to carry the bulk material powders have an especially high melting temperature.

[0062] Pressure is preferably controlled by regulating the feed rates of the screw- driven extruder, the feed rate being a combination of the speed and capacity of the screw. In addition pressure may be controlled by regulating the temperature of the melted mixture. The extrusion pressure is dependent on process factors including the composition and size of the bulk material powders, the composition of the binders, and the ratio of bulk material powders to binders in the mixture, as well as on the rate of extrusion. The pressure is preferably in the range of 50 PSI to 7500 PSI, but may range higher or lower depending upon variations in the process factors.

[0063] Shear and velocity are incidentally controlled and are generally a result of a combination of several inputs, including but not limited to extrusion rates, the composition and size of the bulk material powders, the composition of the binders, the ratio of bulk material powders to binders in the mixture, temperature, pressure, draw down, and quenching. Velocity gradients (shear) can range from a near perfect 0.998 to a turbulent 0.005. Shear is dependent on factors including tooling design (i.e., the shape of the die and the optional a mandrel), feed rate, and internal and external friction characteristics of the materials in the mixture being extruded. Shear may cause or induce stress concentrations and other undesirable properties in the green form. Ultimately, the undesirable properties imparted to the green form by shear during extrusion can be removed in the subsequent sintering step or by other thermal, mechanical, and/or thermomechanical processing steps.

[0064] An advantage of a continuous extrusion method is product length. Continuous powder extrusion of the heated mixture of bulk material powders and binders produces a continuous elongate form of a desired profile whose length is theoretically limited only by the processing equipment or factory in which it is produced, and the need for a steady flow of input mixture. If a continuous sintering oven is set up in sequence with the extruder, theoretically, an infinite length of extrusion can be continuously debound and sintered into a densified form. Typically, an extruded article will be produced in sections of between 1 foot and 20 feet, although longer and shorter sections can also be readily made.

[0065] Another advantage of a continuous powder extrusion method is increased efficiency when compared with a discrete operation such as metal injection molding. In contrast to discrete metal injection molding, where parts may be produced individually or in lots of between two and eight in each press cycle, a continuous powder extrusion method may yield the equivalent length of hundreds of parts having the same profile in a single extrusion cycle.

[0066] Another advantage of a continuous powder extrusion method is the much lower cost and greater flexibility of the dies and optional mandrels used to create the extruded green form. These dies are about 10% of the cost of a multi-cavity MIM die, and can be used for other products by mixing and matching to produce a wide range of extruded green form shapes.

[0067] After extrusion of the mixture through the die, the resultant elongate green form is received onto a custom setter tray with a shaped support surface. The green form may be cut to length or may be retained as a longer piece to be later cut to length in the cutting step after debinding and sintering. The setter tray is sized to match the length of the product. The shape of the setter tray is preferably customized to the outside shape or profile of the product. For example, the setter tray may be machined with a half-round (semi-circular) groove to support a form with a round external profile, or the setter tray may be machined with a half-hexagonal groove to support a form with a hexagonal external profile. Preferably, the setter tray is made from refractory ceramics such as alumina, zirconia, or magnesia. Most preferably, the setter tray is made from alumina.

[0068] The product will preferably remain on, and be supported by, the setter tray throughout the thermal debinding, drying, and sintering operations. The total cycle time of thermal debinding and sintering may require between approximately 12 hours and approximately 30 hours, depending on factors such as whether the sintering furnace is a batch or continuous furnace, the length of the continuous furnace, and the required time during which the material must be maintained at critical process temperatures.

[0069] In the step 60, an initial debinding is performed to transform the green form (also know as a green-state extrusion) into a brown form (also known as a brown-state extrusion). Initial debinding is the first of a two-step or three-step process in which the density and mechanical properties of the part are made to approach those of cast and wrought metal. During initial debinding, some portion of the binders, including but not limited to organic and volatile components, is removed. The binders can be removed by heating the green form, generally with a low-temperature thermal treatment. The initial debinding temperature is typically less than approximately 250°F. However, this temperature can range from 150°F to 750 ⁰ F. When the green form is heated, a portion of the binders will melt, decompose, sublime, and/or evaporate. The binders can also be removed by solvent, supercritical fluid, or aqueous extraction, by a combination of heating and solvent extraction, or through reactive chemical decomposition.

[0070] In the step 70, after initial debinding, the brown form is further thermally debound and sintered to produce a densified form of the required density and material properties. During sintering, the brown form is heated to sufficient temperatures for the metal powders to adhere to each other, and for diffusion processes to occur, thereby producing a substantially solid form having nearly the same mechanical properties as the metal in its cast and wrought form. [0071] The sintering step can include heating the brown form to a temperature close to the melting point of the constituent materials of the brown form. Preferably, the sintering temperature is about 20°F to about 200°F less than the melting point of the bulk material. The sintering temperature is maintained for a set period of time that may depend on several factors, including but not limited to the composition of the bulk material and the size of the form section. At the sintering temperature, interparticulate melting and substantial diffusion can occur, thereby eliminating interstitial void spaces and causing the material densities to increase so that the form becomes a substantially solid densifϊed form.

[0072] Complete solidification of the part to 100% density is desirable but generally does not occur during sintering. Preferably, the density of the article is at least 95% of theoretical. More preferably, the density of the article is at least 97% of theoretical. These density measurement percentages are based on the theoretical density of the article, i.e., the density that would occur if there were no void spaces.

[0073] During the sintering step, the brown form typically shrinks due to the decrease in the size of the interstitial void spaces and the resultant densification of the form. The shrinkage typically causes an article to be about 10% to about 30% smaller than the green form from which it is made. More particularly, the article is about 20% smaller than the green form. The amount of shrinkage should be considered when designing and/or selecting a die (or a die and mandrel combination) for extruding a form having a particular desired cross-sectional profile.

[0074] When debinding and sintering long forms such as those that may be produced by a continuous extrusion method, shrinkage due to densification may cause significant problems with cracking and fracturing as the ends of the long form are drawn closer together causing sliding friction on the setter tray. In order to facilitate longitudinal shrinking of a long form without cracking or fracturing, the setter tray supporting the extruded form may be tilted during sintering. Tilting the tray enables gravitationally aided sliding to occur as the brown form shrinks, thereby reducing the tendency of the friction against the setter tray to cause fracture. Alternatively, the green form may be extruded to form a coil, and the coil may be received onto a dedicated setter tray, for example a conical setter tray. As the coiled form is debound and sintered, the coil contracts in both length and coil diameter on the setter tray. Accordingly, when a conical setter tray is used, as the coil diameter contracts, it slides down the conical slope, aided by gravity, allowing the form to shrink in diameter without creating longitudinal friction that would cause cracking or fracturing.

[0075] In the step 70, the densified form is processed to produce an article. Processing can include one or more of the step 80 of thermal, mechanical, and/or thermomechanical processing the densified form to produce a wrought form, the step 90 of cutting the densified or wrought form to a length, and the step 100 of finishing the densified or wrought form whether or not it has been cut to a length.

[0076] In the step 80, the densified form may be subjected to mechanical then thermal and/or thermomechanical processing as a metal to produce a wrought product having a specific size, shape, and properties. Not all densified forms will require thermal, mechanical, and/or thermomechanical processing. In particular, net or near net densified forms typically are not subjected to such processing, but densified preforms (i.e., densified forms that have been extruded to larger or smaller than net or near net dimensions) typically are.

[0077] The forms produced by extrusion, debinding, and sintering may be net or near net in section size, or they may be preforms. Net products require essentially no processing or finishing after sintering, except for cutting the densified form to a desired length. The final size and shape of a net part section is determined by the size and shape of the extrusion die and the amount of shrinkage that occurs during debinding and sintering. Near net products require minimal processing after sintering. Minimal processing may comprise finishing processes including but not limited to machining, forming, bending, straightening, grinding, abrasive blasting, surface finishing, polishing, tumbling, coating, plating, electropolishing, chemical etching, pickling, passivating, or some combination thereof.

[0078] Preforms may be produced in profiled sections similar to those produced for net or near net products, but because preforms will be subjected to more substantial processing or finishing after sintering, they may be larger or smaller than the net and near net extrusions for making similarly sized finished parts. Prior to cutting to net length and finishing, preforms are processed like cast metals to produce wrought forms. Processing of preforms may include, but is not limited to, classical methods of thermal, mechanical, and/or thermomechanical processing. For example, a preform may be subjected to cold working then annealing and/or hot working. An article having the material properties of cast and wrought metal and a precise dimensionally controlled profile may be produced by applying one or more thermal, mechanical, and / or thermomechanical processes to a densified preform.

[0079] In the step 90, the densified form, or the wrought form if mechanical, thermal, and/or thermomechanical processing has been performed in the step 80, can be cut to any length. Preferably, the form is cut to net or finished length desired for the article. As part of the post-sintering cutting step the densified or wrought form may be straightened, if necessary, before or after the form is cut to length.

[0080] In the step 100, the article may be finished to refine the properties and/or appearance of the article to produce a finished article. In some cases, after cutting the densified or wrought form to a desired length to produce an article, no additionally finishing is required, particularly if a net shape was extruded. However, for a near net part, or for a part produced by thermomechanically processing a preform, finishing may include subjecting the densified form to conventional metal processing to refine the shape and size of the part. Any completed article produced by the continuous powder extrusion method can be further processed, regardless whether it was produced from a net form, a near net form, or a thermomechanically processed preform.

[0081] Finishing can include various processes to further refine the material properties of the article. These processes can include, but are not limited to, polishing, coating, grinding, machining, coining, heat treating and passivation. Finishing may improve mechanical properties of the finished article such as ultimate tensile strength, yield strength, and elongation, or improve surface finish, corrosion resistance, or dimensional precision. An article can be finished after being cut to a length, regardless whether the extrusion, debinding, and sintering steps produce a net form, a near net form, or a preform. An article can be machined to provide a smooth surface, threads, or other similar characteristics. An article can be electropolished, passivated, plated, or coated to enhance functionality, such as by improving corrosion resistance. An article can be annealed to relieve internal stresses or to soften the metal.

[0082] An article made by continuous powder extrusion provides various benefits, including material properties the same as or similar to those of cast and wrought metals. Moreover, the method of continuous powder extrusion provides the design flexibility to produce articles with intricate external and internal shapes, the ability to produce articles with precise tolerances, and the ability to produce articles having indefinite length.

[0083] While the invention has been disclosed with reference to certain preferred embodiments, numerous modifications, alterations, and changes to the described embodiments are possible without departing from the sphere and scope of the invention, as defined in the appended claims and equivalents thereof. Accordingly, it is intended that the invention not be limited to the described embodiments, but that it have the full scope defined by the language of the following claims.