LOW PRESSURE INJECTION MOLDING OF FLAT TABLEWARE FROM METAL FEEDSTOCKS

Abstract: A process for molding flat tableware from powders and molding compositions therefor are disclosed. Parts produced by this process are formed near net shape without the need for machining or other finishing operations. The process comprises forming a mixture containing a metal powder, a gel-forming material and an aqueous gel-forming material solvent, and molding the mixture in an injection molding machine under conditions of temperature and pressure to produce a self-supporting flat tableware article.

Description: LOW PRESSURE INJECTION MOLDING OF FLAT TABLEWARE FROM METAL FEEDSTOCKS FIELD OF THE INVENTION This invention relates to a process for shaping metal parts from powders and molding compositions therefor. More particularly, the invention is directed to molding processes and molding compositions for forming long, thin sections and complex-shaped cross sections that can be readily fired to produce near net- shape articles without the need for machining or other shaping and finishing operations. BACKGROUND OF THE INVENTION There are many different techniques used in the art of fabricating flat stainless steel tableware. One process is known as forging, in which the flat tableware is formed by a series of high pressure and force impacts until the desired shape is obtained. This process entails numerous steps from start to the finished product, taking a relatively long time and requiring the use of large, expensive machinery. The first step is known as the upset in which the rod material is heated and a ball is formed on the end to create enough material in the bolster area during forging. The second step is known as the breakdown forge in which the rod is heated in a furnace to approximately 1300°F and forged into a generally flat article. The third step is called the drop forge in which the preformed article is forged again to its rough finished shape. The fourth step is known as the trim step in which the forging is trimmed to the actual outline of the flatware. The fifth step is known as the "kp" tumble in which the flatware is tumbled with stone media to remove burrs. The sixth step is known as the notch step in which the circular part of the flatware outline is machined. The seventh step is known as the mill step in which the flat surfaces of the flatware are milled. The eighth step is the heat treatment in which the flatware is furnace hardened. The ninth step is known as the line finish in which the parts are buffed with a very aggressive coated roll to remove forging scale. The tenth step entails vibratory tumbling of the flatware to prepare them for additional buffing. The eleventh step is the finish buffing step in which the flatware is buffed to a mirror finish. The final step is the inspection step. In order to overcome some of the shortcomings of the prior art method of manufacturing flat tableware, injection molding techniques have been found to be ideally suited for high volume manufacturing of near net-shape flatware. This process produces flatware that has the desired physical properties and visual appearance without the need to perform costly finishing operations. The process is relatively inexpensive and offers considerable advantages over multiple step processes that require additional machining and finishing operations to produce acceptable finished product. Low pressures and temperatures are employed to shape the finished flatware using aqueous feedstocks made from metal powders. BRIEF SUMMARY OF THE INVENTION The invention is directed to a process for forming a long, thin cross section article having a complex shape comprising the steps of forming a mixture comprising a metal powder, a gel-forming material and an aqueous gel-forming material solvent; and molding the mixture in a mold under conditions of temperature and pressure sufficient to produce a self-supporting article. The invention also provides an injection molding process for forming a flat tableware article comprising the steps of forming a mixture comprising a metal powder, a gel-forming material selected from the group of polysaccharides consisting of agaroids and an aqueous gel-forming material solvent; supplying the mixture to an injection molding machine having a mold containing a cavity for shaping the article therein, the mixture being maintained during the supply step at a first temperature above the gel point of the gel-forming material; and cooling the mixture in the mold to a second temperature below the gel point of the gel-forming material to form the tableware article. The tableware is molded at low temperatures of approximately 85°C and low pressures of approximately 400 to 1000 psi (hydraulic). The tableware is cooled in the mold to approximately 37°C. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more fully understood and further advantages will become apparent when reference is made to the following detailed description and the accompanying drawings in which: Fig. 1 is a schematic flow diagram of one embodiment of a method for the manufacture of flat metal tableware according to the present invention. Fig. 2 is a photograph of the soft tool used to form the flat tableware by low pressure injection molding using aqueous stainless steel feedstock. Fig. 3 is a photograph of flat tableware articles manufactured according to the present invention. DETAILED DESCRIPTION OF THE INVENTION Flat tableware is formed according to the present invention from metal powders, preferably in injection molding machines at low pressures and temperatures. As used herein, the term metal powders includes powders of pure metals, alloys, inteπnetallic compounds and mixtures thereof. According to the process of this invention, the metal powder is initially mixed with a gel-forming material and a solvent for the gel-forming material. This mixture is then mixed with a proportionate amount of a carrier to make it fluid enough to enable it to be readily supplied to a mold by any of a variety of techniques. A preferred technique is injection molding. Generally, the amount of powder in the mixture is between about 35 to 65% by volume of the mixture. Preferably, the powder constitutes between about 40 to 62% by volume of the mixture, and most preferably constitutes between about 45 to 60% by volume of the mixture. The aforementioned amounts are especially well suited for production of near net shape injection molded flat tableware. The gel-forming material employed in the mixture is an agaroid, which has been defined as a gum resembling agar but not meeting all of the characteristics thereof (See H.H. Selby et al., "Agar," Industrial Gums, Academic Press, New York, NY, 2 nd ed., 1973, Chapter 3, p. 29). As used herein, however, agaroid not only refers to any gums resembling agar, but also to agar and derivatives thereof such as agarose. An agaroid is employed because it exhibits rapid gelation within a narrow temperature range, a factor that can dramatically increase the rate of production of articles being manufactured. The preferred gel- forming materials are those that are water-soluble and include agar, agarose and carrageenan. The most preferred gel-forming materials include agar, agarose and mixtures thereof. The gel-forming material is provided in an amount preferably between about 0.5 to 6 wt% based upon the amount of solids in the mixture. It should be understood that more than about 6 wt% of the gel-forming material may be employed in the mixture. Such higher amounts are not believed to have any adverse impact on the process, although these higher amounts may begin to reduce some of the advantages produced by the novel compositions of the present invention, especially with respect to the production of near net shape bodies. Most preferably, the gel-forming material comprises between about 1 to 3% by weight of solids in the mixture. The mixture further includes a gel-forming solvent in an amount sufficient to dissolve the gel-forming material. While any of a variety of solvents may be employed depending upon the composition of the gel-forming material, especially useful solvents for agaroid-containing gel-forming materials are polyhedric liquids, particularly polar solvents such as water or alcohols. It is, however, most preferable to employ a solvent which can also perform the dual function of being a carrier of the mixture, thus enabling the mixture to be easily supplied to a mold. Water has been found to be particularly well suited to perform this dual function. A liquid carrier is normally added to the mixture to produce a homogeneous mixture having a viscosity that allows it to be readily molded by the desired molding process. Ordinarily, the liquid carrier is added in an amount necessary to ensure the proper fluidity of the mixture. Generally, the amount of a liquid carrier is between about 40 to 60% by volume of the mixture depending upon its desired viscosity. In the case of water, which performs the dual function of a solvent and a carrier for agaroid-containing mixtures, the amount is generally between about 35 to 60% by volume of the mixture, with amounts between about 40 to 55% by volume being preferred. In addition, because of its low boiling point, water is easily removed from the body prior to and/or during firing of the molded article. The mixture may also contain a variety of additives that can serve any number of useful purposes. For example, dispersants may be employed to ensure a more homogeneous mixture. Biocides may be used to inhibit bacterial growth in the molding compositions, especially if they are to be stored for a long period of time. A gel strength enhancing additive may be employed to further improve the processability and yield of the molded flatware. The preferred gel strength enhancing agents are chosen from the class of borate compounds including, but not limited to, calcium, magnesium, zinc and ammonium borate. The most preferred compound has been found to be calcium borate. The gel strength enhancing compound is preferably used in an amount of approximately ca. 0.2 to 1 wt% based on the liquid carrier. The components of the molding formulation are compounded in a heated blender that provides shearing action thereto, creating a homogeneous mixture of high viscosity. The shearing action is instrumental in producing compositions of high solids loading in a dispersed and uniform state, highly suitable for subse- quent injection molding. Ability to form uniform compositions of high solids loading is desirable in the production of injection molded parts. Use of compositions with high solids concentration results in lower shrinkages when the molded parts are dried and fired, facilitating close dimensional control and mitigating the tendency for cracks to form during the densification process. The benefits afforded by this process include higher yields of acceptable product and lower scrap rates. This can have a significant effect on the cost of the overall process and may determine whether injection molding is lower in cost relative to other fabrication processes for a particular component. The mold for fabricating the flat tableware may be made by any number of methods well known to those skilled in the art. For example, a metal mold for forming the desired tableware article shape may be made by machining a cavity in the shape of the desired article into a metal block. Soft tooling in the form of resins or particulate reinforced resins can be made using casting techniques. In the latter case a cavity in the shape of the desired article may be formed by casting around a master. The master can be made by any number of suitable methods well known to those skilled in the art, such as by machining or grown SLA masters. Resin, most preferably urethane or epoxy, is pre-mixed with the reinforcement filler and cast around the master. After the resin cures to a solid, the master is removed and secondary operations can be performed to create a finished, multiple-use tool for production of parts from powder feedstocks. The tool may incorporate other desirable features, such as cooling lines, removable sprue and ejector systems. The mixture is supplied to the mold by any of a variety of well-known techniques including gravity feed systems and pneumatic or mechanical injection systems. Injection molding is the most preferred technique because of the fluidity and low processing temperatures and pressures of the mixtures that are required. These features, low processing temperatures and pressures, are especially attractive in reducing abrasive and erosive wear of the injection molding equipment. The mixture is transported to the mold at a temperature above the gel point (temperature) of the gel-forming material. Ordinarily, the gel point of the gel-forming material is between about 10 to 60°C, and most prefer- ably is between about 30 to 45°C. A wide range of molding pressures may be employed. Generally, the molding pressure (hydraulic) is between about 100 to 1500 psi, although higher or lower pressures may be employed depending upon the molding technique used. Preferably, the molding pressure is in the range of about 150 to 1000 psi, and most preferably, between about 250 to 800 psi. The mold temperature must, of course, be at or below the gel point of the gel-forming material in order to produce a self-supporting body. The appropriate mold temperature can be achieved before, during or after the mixture is supplied to the mold. Ordinarily, the mold temperature is maintained at less than about 40°C, and preferably is between about 15 to 25°C. Thus, for example, optimum production rates would be expected to be achieved with an injection molding process wherein the preferred gel-forming materials (which exhibit gel points between about 30 to 45 °C) are employed to form a mixture, and wherein the mixture is injected at less than about 100°C into a mold maintained at about 55°F. After the tableware article is molded and cooled to a temperature below the gel point of the gel-forming material, the "green" part is removed from the mold, dried to remove the water and then fired at elevated temperatures to remove the binder and densify the part. Drying may be accomplished at ambient and/or above ambient temperatures. The firing times and temperatures (firing schedules) are regulated according to the powder material employed to form the tableware article. Firing schedules are well known in the art for a multitude of materials and need not be described herein. The process for manufacturing flat tableware by injection molding according to the present invention is illustrated in Fig, 1. The tool for producing the tableware article is placed in an injection molding machine. The article is molded as described hereinabove and cooled below the gel point of the material. The tool then opens and the "green" part is removed from the tool and allowed to dry at ambient or above ambient temperatures to remove the water. The dried part is then sintered at elevated temperatures according to well-known firing schedules for the material being used in order to obtain the desired properties. The sintered part needs little or no further finishing operations and possesses the desired visual appearance without the need for further buffing or polishing. Among the most important advantages achieved by the present invention is the production of flat tableware by the aforementioned process in which many of the previous steps required have been eliminated and the cost of manufacture has been greatly reduced. Another advantage is the relative speed with which the flatware can be manufactured compared to conventional manufacturing techniques. Yet another advantage is the desirable visual appearance of the finished flatware produced by this process. Having thus described the invention in full, clear and concise terminology, the following example is provided to illustrate an embodiment of the invention. The example, however, is not intended to limit the scope of the invention to anything less than is set forth in the appended claims. Example Flat tableware in the form of a fork was made from a machined steel master using the following procedure as shown schematically in Fig. 1. Poly- urethane was premixed with aluminum filings constituting approximately 30 volume% of the casting medium and was cast around the master/pattern, which was supported in a wooden form. The urethane was allowed to cure undisturbed for approximately 24 hours, setting to a rigid solid. The master was then removed, leaving a cavity in the urethane mold in the shape of a fork as shown in Fig. 2, and the mold was removed from the supporting wooden form. Secondary machining operations were performed as necessary on the urethane tool. The tool shown in Fig. 2 was installed on a Cincinnati Milicron 55 ton reciprocating screw injection molding machine, and the near net shape forks were molded from 316L and 17-4PH stainless steel feedstock material using hydraulic molding pressures in the range of approximately 400 to 700 psi and a barrel temperature of about 185°F. The mold temperature was controlled at about 55°F by means of circulating fluid from a chiller. The finished flatware shown in Fig. 3 was dried for approximately 24 hours and then sintered in a hydrogen atmosphere using standard sintering schedules for 316L and 17-4PH stainless steels. The flatware manufactured according to the above-described process possessed the required properties with regard to strength, surface finish and density. The present invention satisfies a longstanding industry need to manufacture high quality flat tableware in an economical and expedient manner. The tooling cost is relatively low and the manufacturing time is relatively short compared to conventional processes for making metal tooling. The finished parts require little or no additional processing, resulting in a very economical manufacturing process.

Claims: What is claimed is: 1. A process for forming a long, thin cross section article having a complex shape comprising the steps of: a) forming a mixture comprising, 1) powder containing at least one member selected from the group consisting of pure stainless steel alloys, stainless steel alloying elements, intermetallic compounds, components of metal matrix composites and mixtures thereof; 2) a gel-forming material; and 3) an aqueous gel-forming material solvent; and b) molding the mixture in a mold containing a cavity for shaping the article under conditions of temperature and pressure sufficient to produce a self- supporting article. 2 The process of claim 1, wherein the gel-forming material is selected from the group of polysaccharides consisting of agaroids. 3. The process of claim 2, wherein the agaroid is agar, agarose or a mixture thereof. 4. The process of claim 1, wherein the aqueous gel-forming material solvent is water. 5. The process of claim 1, wherein the powder comprises between about 50 to 96 wt% of the mixture. 6. The process of claim 1 , wherein the gel-forming material comprises between about 0.5 to 10 wt% of the mixture. 7. The process of claim 1, further comprising the step of maintaining the mixture at a temperature above the gel point of the gel-forming material prior to the molding step. 8. The process of claim 7, wherein the temperature of the mixture during the molding step is reduced to a value below the gel point of the gel-forming material. 9. The process of claim 1 , wherein the mixture further comprises additives including a biocide, metal borate compound, a coupling agent, a dispersant and monomeric mono and/or polyhedric alcohol. 10. The process of claim 9, wherein the borate compound is present in an amount up to about 10 wt% of the gel-forming material solvent in the mixture. 11. The process of claim 1 , wherein the hydraulic molding pressure is less than about 1500 psi and the molding temperature is less than about 212°F. 12. The process of claim 1, wherein the article is flat tableware in the form of a fork, spoon, knife or other utensil. 13. An injection molding process for forming a flat tableware article having a complex shape comprising the steps of: a) forming a mixture comprising, 1) a metal powder; 2) a gel-forming material selected from the group of polysaccharides consisting of agaroids; and 3) an aqueous gel-forming material solvent; b) supplying the mixture to an injection molding machine having a mold containing a cavity for shaping the article, the mixture being maintained during the supply step at a first temperature above the gel point of the gel-forming material; and c) cooling the mixture in the mold to a second temperature below the gel point of the gel-forming material to form the article. 14. The process of claim 13, wherein the agaroid is agar, agarose or a mixture thereof. 15. The process of claim 13, wherein the aqueous gel-forming material solvent is water. 16. The process of claim 13, wherein the powder comprises between about 50 to 96 wt% of the mixture. 17. The process of claim 13, wherein the gel-forming material comprises between about 0.5 to 10 wt% of the mixture. 18. The process of claim 13, wherein the mixture further comprises additives including a biocide, metal borate compound, a coupling agent, a dispersant and monomeric mono and/or polyhedric alcohol. 19. The process of claim 18, wherein the borate compound is present in an amount up to about 10 wt% of the gel-forming material solvent in the mixture. 20. The process of claim 13, wherein the hydraulic molding pressure is less than about 1500 psi and the molding temperature is less than about 212°F. 21. The process of claim 13, wherein the flat tableware article is in the form of a fork, spoon, knife or other utensil. 22. The process of claim 13, wherein the metal powder contains at least one member selected from the group consisting of pure stainless steel alloys, stainless steel alloying elements, intermetallic compounds, components of metal matrix composites and mixtures thereof.

Inventors: SCHOONOVER JAMES, SESNY STEVE, SCHOONOVER, JAMES, SESNY, STEVE
Applicants: ALLIED SIGNAL INC, ALLIEDSIGNAL INC.
Application year: 2000
Main classification: B22F
IPC codes: B22F1, B22F3