As described in Doles U.S. Pat. No. 6,540,013 B1, and Doles et al. U.S. Pat. No. 6,770,699 B2, investment casting is used to produce high quality metal articles that meet relatively close dimensional tolerances. The process has also been called lost wax, lost pattern and precision casting. Typically, an investment casting is made by first constructing a thin-walled ceramic mold, known as an investment casting shell, into which molten metal can be introduced.
Shells are usually constructed by first making a facsimile or pattern, from a meltable substrate, of a metal object to be made by investment casting. Suitable meltable substrates may include, for example, wax, polystyrene, or other plastics.
Next, a ceramic shell is formed around the pattern. This may be accomplished by dipping the pattern into a slurry containing a mixture of liquid refractory binders such as colloidal silica, ethyl silicate, and/or various organic polymers, plus a refractory powder such as quartz, fused silica, zircon, alumina, or aluminosilicate (aluminum silicate), and then sieving dry, refractory grains onto the freshly dipped pattern (this step is usually referred to as stuccoing the mold). Typically, each coat of slurry and refractory grains is air-dried before subsequent coats are applied.
The shells are built up to a thickness in the range of about ⅛ to about ½ inch (i.e. about 0.3 cm. to about 1.3 cm). After final dipping and sieving (stuccoing), the shell is thoroughly air-dried. The shells made by this procedure have been called “stuccoed” shells, because of the texture of the shell's surface. The shell is then heated to at least the melting point of the meltable substrate. In this step, the substrate comprising the pattern is melted away, leaving only the shell and any residual, meltable substrate. The shell is then heated to a temperature high enough to vaporize or otherwise remove any residual, meltable substrate from the shell. Usually, before the shell is cooled from this high temperature heating, the shell is filled with molten metal in a conventional manner, such as a gravity, pressure, vacuum or centrifugal method. When the molten metal in the shell, called a casting mold, has solidified and cooled sufficiently, the shell is broken away to separate it from the casting.
As described in U.S. Pat. Nos. 6,450,243; 6,755,237; and 6,769,475 of Shaw and Duffey, slurries for the preparation of investment casting shells are disclosed, in which the slurries carry a small amount of short plastic fibers, which fibers improve the performance and characteristics of the material. However, such fiber-containing slurries also have certain technical difficulties: for example, the fibers are difficult to keep dispersed in the slurry. They also make the slurry viscosity more difficult to measure, and they can reduce accurate contact of the slurry to the pattern, so that they do not always reproduce small holes, slots, grooves, and other fine detail to the degree desired, although the material works well in many commercial uses.
In accordance with this invention, filler components for the preparation of investment casting slurries are disclosed in which various improvements are provided over the fiber containing slurries of the prior art. Specifically, improvements can be achieved in the uniformity of shell thickness, while providing rapid buildup of shell thickness and improved coverage of holes, corners, and details. Also, the drainage time of the slurry, compared with the fiber-containing slurries of the prior art, can be shortened.