Plastic and nonmetallic article shaping or treating: processes – Forming articles by uniting randomly associated particles – With subsequent cutting – grooving – breaking – or comminuting
Reexamination Certificate
1999-08-17
2001-05-08
Derrington, James (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Forming articles by uniting randomly associated particles
With subsequent cutting, grooving, breaking, or comminuting
C264S109000, C264S119000, C264S655000, C264S657000, C264S669000, C264S670000, C264S678000, C419S036000, C419S037000, C419S065000, C252S391000, C252S391000, C524S916000
Reexamination Certificate
active
06228299
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to gelcasting compositions, and more particularly to gelcasting compositions with improved drying and storage characteristics, and improved machinability in the dried state.
BACKGROUND OF THE INVENTION
Gelcast ceramic bodies have been demonstrated to be machinable in the “green” state, after drying and before firing. See S. D. Nunn, O. O. Omatete, C. A. Walls, and D. L. Barker, “Tensile Strength of Dried Gelcast Green Bodies,”
Ceram. Eng. Sci. Proc
., 15 [4] 493-498 (1994), and S. D. Nunn and G. H. Kirby, “Green Machining of Gelcast Ceramic Materials,”
Ceram. Eng. Sci. Proc
., 17 [3-4] (1996).
Polymers have been demonstrated to have utility in methods of forming complex or intricately shaped parts from ceramic powders. The forming of ceramics is important because machining ceramics into complex shapes is time consuming and expensive, and in many cases impractical. Strivens, U.S. Pat. No. 2,939,199, discloses a method of forming articles from ceramic powders wherein the ceramic powders are mixed with a vehicle comprising a thermosetting material and a plasticizer, and the resultant mixture is injection molded into a mold of a desired shape and heated to cure the thermosetting component. The vehicle is then removed from the molded part by low pressure distillation or by solvent extraction. Kingery et al., U.S. Pat. No. 3,351,688, discloses a method wherein the ceramic powder is mixed with a binder such as paraffin at a temperature where the binder is liquid, and the resulting mixture is cast into a mold of the desired shape. The binder is permitted to solidify so that a green piece is formed having a uniform density. Curry, U.S. Pat. No. 4,011,291, and Ohnsorg, U.S. Pat. No. 4,144,297, disclose the use of a paraffin wax binder for molding ceramic powders into desired shapes. Rivers, U.S. Pat. No. 4,113,480, and Wiech, Jr., U.S. Pat. No. 4,197,118, disclose methods for molding parts from metal powders by mixing the powders with binder materials and injection molding the resultant mixtures. Additional methods which employ binder materials are disclosed by Hurther et al., U.S. Pat. No. 4,478,790, and Kato, U.S. Pat. No. 4,460,527.
It is known that gelcasting can also be a useful way of forming ceramic materials. Gelcasting is a method of molding ceramic powders into green products wherein a monomer solution is used as a binder vehicle and the controlled polymerization of the monomer in solution serves as a setting mechanism. The resulting green product is of exceptionally high strength and may be dried to remove water. After drying, the product may be further heated to remove the polymer and may also subsequently be fired to sinter the product to a high density. Gelcasting methods are disclosed in Janney, U.S. Pat. No. 4,894,194, Janney et al, U.S. Pat. No. 5,028,362, and Janney et al., U.S. Pat. No. 5,145,908. Gelcasting of ceramics such as alumina is described by A. C. Young, O. O. Omatete, M. A. Janney, and P. A. Menchhofer, “Gelcasting of Alumina,”
J. Am. Ceram. Soc
., 74 [3] 612-18 (1991). Mark A. Janney, Weiju Ren, Glen H. Kirby, Stephen D. Nunn, and Srinath Viswanathan, “Gelcast Tooling: Net Shape Casting and Green Machining,”
Materials and Manufacturing Processes
, 1997 describe the use of a water-based gelcasting system to form parts using H13 tool steel powder. R. Raman, M. A. Janney, and S. Sastri, “An Innovative Processing Approach to Fabricating Fully Dense, Near-Net-Shape Advanced Material Parts,” Advances in Powder Metallurgy and Particulate Materials, 1996, Metals Powder Industries Federation, Princeton, N.J., 1996 describe the use of a water-based gelcasting system to form parts using an 83/17 aluminum/silicon alloy powder. S. D. Nunn, J. O. Kiggans, Jr., R. E. Simpson, II, and J-P Maria, “Gelcasting of Silicon Compositions for SRBSN,”
Ceram. trans
., 62, 255-62 (1996) describe the use of an alcohol-based gelcasting system and a water-based gelcasting system to form parts using silicon powder. M. A. Janney, “Gelcasting Superalloy Powders,” in P/M in Aerospace, Defense and Demanding Applications—1995, Metals Powder Industries Federation, Princeton, N.J., 1995, describes the use of a water-based gelcasting system to form parts. The disclosures of these references are incorporated fully by reference.
It has recently been observed that the machinability of gelcast blanks is sensitive to the ambient humidity. Parts that are stored in high humidity conditions machine well, while those that are stored in low humidity conditions machine poorly. Poor machining characteristics are evidenced by increased chipping and tool chatter, with a poor surface finish for equivalent machining conditions. It has further been observed that green parts that have been stored for extended periods of time at ambient conditions develop cracks that make them unusable.
The gelcasting of parts having a large cross sectional thickness presents particular problems. These parts can be cast and gelled, however, they are difficult to dry without introducing cracks that prevent further processing, such as firing and densification. It accordingly would be desirable to provide gelcasting compositions which would not be as sensitive to humidity conditions, would machine well, with relatively less chipping and tool chatter, and would result in an improved surface finish for equivalent machining conditions. It would further be desirable that the parts be capable of storage for extended periods of time in ambient conditions without the development of cracks. It further would be desirable to provide gelcasting compositions which would permit the manufacture of gelcast parts having large cross-sectional thicknesses which can be dried without introducing cracks.
SUMMARY OF THE INVENTION
It is an object of the invention to provide gelcasting compositions which have improved drying characteristics and machinability.
It is another object of the invention to provide gelcasting compositions which result in gelcast blanks which are less sensitive to changes in the ambient humidity.
It is yet another object of the invention to provide gelcasting compositions which result in gelcast blanks which, after storage in low humidity conditions, machine relatively well, with relatively less chipping and tool chatter, and after machining result in products having an improved surface finish. It is still another object of the invention to provide gelcasting compositions which can be stored for extended periods of time in ambient conditions without the development of cracks. It is another object of the invention to provide gelcasting compositions which can produce gelcast parts having large cross-sectional thicknesses, and which can be dried without introducing cracks that will prevent further processing such as firing and densification.
These and other objects are provided by a gelcasting formulation comprising an inorganic powder, solvent, a monomer system soluble in said solvent, an initiator system for polymerizing said monomer system, and a plasticizer soluble in the solvent. Dispersants and other processing aides to control flow properties during casting can also be included. Additives known in the art to modify gelcast properties can also be included. The plasticizer improves the mechanical properties of the gelcast part after polymerization. Parts cast with gelcasting formulations according to the invention demonstrate improved drying behavior, machinability, and shelf-life in the dried and unfired state.
The functional group of the monomers is preferably selected from vinyl and allyl groups. The monomer system preferably comprises at least one multifunctional monomer (di, tri, etc.). In a most preferred embodiment, the monomer system comprises at least one monofunctional monomer and at least one difunctional monomer. Aqueous solutions of hydroxymethacrylamide (HMAM) with optional comonomers, can also be utilized for the monomer system.
A method for forming inorganic powders according to the invention includes the steps of making a
Janney Mark A.
Walls Claudia A. H.
Akerman Senterfitt & Eidson, P.A.
Derrington James
UT-Battelle LLC
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