Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ – Of inorganic materials
Reexamination Certificate
1999-08-06
2001-04-24
Derrington, James (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
Of inorganic materials
C264S645000, C264S638000, C264S639000, C264S669000, C264S670000, C264S328200
Reexamination Certificate
active
06221289
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to sintered ceramic elements or parts for use in molding a moldable material such as metal. More particularly, the invention relates to processes and binder compositions for forming unfired or green ceramic elements that may be fired to form sintered ceramic elements such as inserts, cores or mold wall components to be used in molds to shape the mold cavity and the moldable material. Following solidification of the material being molded, if the ceramic element comprises a core or insert, it is removed as by leaching, and the molded part is complete.
The inventive processes and binder compositions may be used in various types of shaping processes to form the ceramic element. For example, the invention may be used in extrusion or injection molding processes, but it is particularly useful in connection with injection molding processes.
In the case of extruded ceramic elements, the prior art extrusions contain relatively large amounts of water, e.g. 19% based on the total weight. (Hereinafter, weight percentages are based on the total weight of the green ceramic component materials to be introduced into the shaping apparatus unless otherwise noted or indicated by the context). Any frictional or flow resistance is overcome by the extrusion force which may be several thousands of pounds, e.g. 3000 pounds.
The prior art extrusion materials contain ceramic, water and colloidal silica. A cereal flour is also used to contribute to the binder function. Immediately following extrusion, the extruded parts are dried at 70° F. for several days depending upon the part size. This drying step removes liquids at a slow rate in order to avoid detrimental high temperature removal in the firing cycle.
In the case of inserts that are to be injection molded, the ceramic insert material is initially bound together using a binder formed of about 22 to about 25% wax based on the total weight of the blend or mixture of the materials used to form the green ceramic element. The wax used typically comprises paraffin.
It is believed that the relatively high prior art level of the wax component, e.g. 22 to 25%, is necessary in order to achieve a homogeneous dispersion. The use of reduced amounts of wax is believed to result in non-uniform ceramic distribution. The wax is initially melted and the ceramic powder is mixed with the melted wax in a blender with the addition of ceramic powder occurring over a prolonged period of time, e.g. 4 hours, in a mixing cycle. This melt-blend process is very slow since it is necessary to form a homogeneous dispersion of the ceramic powder in the wax. The powder and wax mixture is then formed into a thin sheet and crushed into small pieces suitable for addition to the injection molding apparatus. This process may take eight to 12 hours.
The crushed wax and ceramic mixture is heated in the injection molding process, and the wax is melted or made tacky to cause it to act as a binder for the ceramic powder. The injection molded insert or piece is then slowly dried to remove organic components such as water. The typical prior art drying cycles are performed at about 70° F. or room temperature, and may last for about five days or more. One prior art technique uses a 10 day drying cycle before firing.
A major disadvantage of the prior art relatively high wax levels is that the green ceramic element must be held in the desired shape during firing. If the green part is not held in shape, puddling may occur with the green ceramic being melted to a puddled mass or some other defect in the part shape may result. If a setter or a sager is not used, the relatively large amount of wax used in the prior art will melt and cause the ceramic to puddle or otherwise be misshapen as opposed to maintaining its desired configuration.
In view of the large amount of wax used in prior art melt-blending techniques, setters must be used to support the part as it is fired. The setters retain the shape of the part during firing since the removal of such a large amount of wax would otherwise result in the part being misshapen. An alternative technique is to place the part in a sager and surround it with alumina type powder in order to hold the shape of the part. (A sager is a ceramic box used to hold a part during firing.) This alternative technique is called wicking since the wax wicks from the part and into the powder as the firing cycle progresses. When the wax reaches the temperature at which it will ignite, it is burned from the powder as well as the green part.
About five years ago, the industry attempted to achieve a 24-hour cycle. That is, the time from injection molding to the completion of the firing is to be as close as possible to a 24 hour time period. The more practical goal is a 72 hour cycle. It is not believed that these goals were previously met.
SUMMARY OF THE INVENTION
It has now been found that ceramic powder and binder compositions suitable for shaping and firing may be prepared using dry blending techniques. That is, the wax component is not melted in order to combine it with the ceramic powder, but rather, the wax component is combined in particulate form with the ceramic powder. The particulate wax is mixed in conventional blender apparatus in a relatively short time period.
Such dry-blending enables the relative amount of wax to be reduced as compared with prior art techniques. For example, the particulate wax component may comprise less than about 20%, and more preferably, less than about 15%, and most preferably about 10 to 12% by weight based on the total weight of the mixture.
The reduction in the amount of wax has been found to enable substantial elimination of prior art drying steps. The green ceramic element may be fired immediately after it is formed without an intermediate drying cycle. The only delay between the completion of the forming or injection molding of the green ceramic and the beginning of its firing is the handling time necessary to position the green ceramic element in the firing furnace.
As noted above, the reduced wax component yields improved green structures that are self-supporting during firing. That is, they do not require the use of sagers or setters and may be fired without external shape supports.
The green ceramic element may include about 8 to about 12% by weight of the wax component, and more preferably, from about 10 to 20% wax. The wax should be of suitable particle size to enable homogenous mixing with the ceramic powder and water. Preferred particle sizes range from about 6 to 22 microns, and more preferably about 100 mesh.
A portion of the wax may comprise a specially formulated lubricity enhancer or slip agent. The slip-agent improves the lubricity of the ceramic material as it is injected into the mold. In the absence of such a lubricity agent, the ceramic material will scrape or scratch the mold. In preferred arrangements, known dispersion or colloidal binders may be included also. For example, about ½ to 40% of colloidal silica may be used. Colloidal silica is a dispersion of fine silica particles in water.
The inventive binder includes wax particles of a size enabling a homogeneous dry-blend with ceramic powder and a melting point that assures binding with pressure and/or heating in the shaping apparatus. Typical particle sizes may range from 6 to 22 microns. The melting or softening point may be in the range of about 140° F. or less to about 200° F. Generally, lower melting point materials are preferred.
A suitable wax may be determined in accordance with the particular shaping apparatus and operating conditions, and the wax should melt or soften and become tacky during shaping to a sufficient degree to lubricate the flow along metallic surfaces and, in cooperation with other binding materials present, bind the ceramic powder together during firing without external surface support. The required wax may be readily identified by trial and error.
In particularly preferred arrangements, the wax component comprises a low molecular weight alkane of C
24
to C
40
, e
Corbett James E.
Toth John G.
Wegenek Arvid
Core-Tech, Inc.
Derrington James
Pearne & Gordon LLP
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