Molding finely divided sinterable material

Details Molding finely divided sinterable material Molding finely divided sinterable material

1994-01-07

1995-04-25

Fiorilla, Christopher A.

Plastic and nonmetallic article shaping or treating: processes

Including step of generating heat by friction

264109, 264234, 2643281, C04B 35634

Patent

active

054096502

DESCRIPTION:

BRIEF SUMMARY
This invention relates to the molding of finely divided sinterable material, such as a powder of metal or high-melting-point non-metal (e.g. silicon) or ceramic substance such as silicon carbide, silicon nitride or alumina.
The invention provides a method by which a dispersion of sinterable material can be injection-molded under moderate pressure to a product whose shape is of considerable complexity, the product being not only of uniform density but also such that the binder employed can be readily removed from it, so as to obtain a porous structure with density still uniform (though, of course, lower) and with shape essentially unaffected. This in turn enables subsequent sintering to be carried out with no substantial change in shape from that of the direct product of injection molding.
The method of the invention depends on the use of a novel binder of our discovery, which has the necessary property of being solid at room temperature and fluid at the elevated temperature of injection molding. The binder comprises (i) a solid polystyrene, which forms the predominant weight ingredient of the binder, and (ii) in lower weight proportion, an organic compound which is soluble in the molten polystyrene and is more soluble than polystyrene in a lower aliphatic alcohol or ketone, examples of which are methanol, ethanol, propanol, isopropanol, acetone and methyl ethyl ketone.
Preferably the organic compound has a melting point of at least 50.degree. C. Preferred organic compounds are those whose molecular structure includes an aromatic ring, for example diphenyl carbonate (melting point about 80.degree. C.).
According to the invention there is provided a method which comprises injection-molding at elevated temperature a dispersion of finely divided sinterable material in a binder, to obtain a molded product of uniform density, both the dispersion and the binder being solid at room temperature and fluid at the temperature of molding, said binder comprising (i) polystyrene, which forms the predominant weight proportion of the binder, and (ii) in lower weight proportion, an organic compound which is solid at room temperature, is soluble in molten polystyrene and is preferentially soluble in a lower aliphatic alcohol or ketone; and removing said binder in non-solid form from the molded product, to leave a porous sinterable structure of uniform density.
The solid dispersion may additionally include a wax and/or a dispersant. It may also include an ethylene/vinyl acetate copolymer suitably in an amount 0.5-2.5.sup.w /o of the dispersion to improve the rigidity of the molded product while the binder is being removed from it in non-solid form. In the latter expression we include removal by sublimation, by dissolution in a liquid solvent such as one of those (methanol etc.) previously referred to, and by controlled oxidation to form volatile products, e.g. benzoic acid, carbon dioxide and water vapor.
According to a preferred feature, the binder is removed from the molded product in two stages, in the first of which the major part of the organic compound is removed as vapour at elevated temperature; and subsequently the polystyrene, with any residue of the organic compound, is removed at further-elevated temperature.
According to an alternative preferred feature, the binder is removed from the molded product in two stages, in the first of which the major part of the organic compound is removed by dissolution in a solvent liquid; and subsequently the polystyrene, with any residue of the organic compound, is removed at elevated temperature.
The solid dispersion may conveniently by prepared by mixing the finely divided sinterable material with a solution of the polystyrene and the organic compound in a relatively volatile organic solvent (preferably of boiling point below 100.degree. C.), and evaporating said solvent from the mixture thus formed.
Preferred proportions of the various ingredients of the dispersion are as follows:


______________________________________ % Ingredient Main Function By Weight ________________

REFERENCES:
patent: 4403056 (1983-09-01), Giolito et al.
patent: 4680154 (1987-07-01), Matsubara et al.
patent: 4784812 (1988-11-01), Saitoh et al.

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