Plastic and nonmetallic article shaping or treating: processes – Including step of generating heat by friction
Patent
1988-02-24
1990-05-08
Derrington, James
Plastic and nonmetallic article shaping or treating: processes
Including step of generating heat by friction
264 63, 264344, C04B 3334
Patent
active
049236520
DESCRIPTION:
BRIEF SUMMARY
FIELD OF ART
The present invention relates to a method of removing a binder from powder molded products and more particularly to a method of removing a binder used as a molding assistant from powder molded products produced according to injection molding or slip casting to obtain sintered ceramic products, thus relating to a so-called powder molded products dewaxing method.
BACKGROUND ART
Sintered ceramic products as mass-produced products complicated in shape are industrially produced by molding such powders as alumina, zirconia, silicon carbide and silicon nitride as raw materials into desired shapes according to injection molding or slip casting, followed by dewaxing, and then igniting the thus-obtained powder molded products at a temperature required for sintering.
The injection molding referred to above is a molding method in which a binder exhibiting plasticity as a whole and making molding easier such as, for example, polystyrene, polyethylene, diethylene phthalate, paraffin, fatty acid ester, or polyvinyl alcohol, is added to and kneaded with, for example, alumina powder as mentioned above in an amount of 20 to 35 parts by weight based on 100 parts by weight of the powder, and the kneaded mixture is charged under pressure into a desired shape of a mold and molded. The resulting powder-molded product is taken out of the mold and the binder is vaporized and removed by heating, followed by igniting to obtain a sintered ceramic product of a desired shape.
The slip casting referred to above is a casting method in which 20 to 40 parts by weight of a binder which is water or a mixture of water with an alcohol, as well as a small amount of a peptizer such as HCl, AlCl.sub.3, NaOH or water glass, are added to and thoroughly mixed with 100 parts by weight of, for example, alumina powder as mentioned above to obtain a stable slip having fluidity and difficult for the powder to precipitate, then this slip is poured into a mold of a porous material such as gypsum, allowing at least the binder contained in the slip to be absorbed into the mold until the slip has no longer fluidity, and then the resulting powder-molded product is taken out from the mold. The powder-molded product thus obtained still contains the binder, e.g. water, usually in an amount of 10% to 15% by weight. Therefore, as in the case of injection molding, such remaining binder is vaporized and removed by heating, followed by igniting to a temperature of, say, 1,300.degree.-2,300.degree. C., whereby a sintered ceramic product can be obtained.
A thermoplasticizer, a plasticizer, a dispersant and a solvent added to a powder in the injection molding and slip casting in the present invention will hereinafter be named generically as "binder". And the operation for vaporizing and removing, by heating or any other suitable means, the binder remaining in a molded product obtained according to the foregoing injection molding or slip casting, will hereinafter be referred to as "dewaxing" which term is commonly used by those skilled in art.
However, sintered ceramic products resulting from dewaxing and subsequent igniting of powder molded products obtained by the foregoing method, i.e., injection molding or slip casting, involve the problem that they are often defective (incapable of being used as products) due to cracking or delamination.
Further, once such defects occur in the interior of the sintered ceramic products, it is difficult to find out the defects at the stage of commercialization, so parts of such defective products are commercialized as they are, thus causing breakage in use. This is a serious problem.
In this connection, it is to be specially noted that such defects as cracking and delamination occur in the dewaxing step in most cases.
More particularly, if a binder remains in a powder-molded product, the remaining binder will vaporize rapidly when igniting the powder-molded product into a sintered ceramic product, thus causing fracture or cracking in the same product. To prevent this, that is, to remove the binder, the powder-molded pr
REFERENCES:
patent: 4248813 (1981-02-01), Hottori et al.
patent: 4612146 (1986-09-01), Huther
patent: 4647414 (1987-03-01), Mizuno
patent: 4696777 (1987-09-01), Ito
Partial Translation of Japanese Patent 59198105.
Aoki Shin-ichi
Isomura Hisashi
Miura Kazukiyo
Murakawa Norihiro
Derrington James
Mitsui Toatsu Chemicals Inc.
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