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Variation

Ceramic Shell Casting

A variation of the investment-casting process is ceramic-shell casting. It uses the same type of plastic pattern, which is dipped first in a slurry with colloidal silica or ethyl silicate binder, then into a fluidized bed of fin-grained fused silica or zircon flour. The pattern is then dipped into coarse-grain silica to build up additional coatings and thickness to withstand the thermal shock of pouring. This process is economical and is used extensively for precision casting of steels, aluminum, and high- temperature alloys.

If ceramic cores are used in the casting, they are removed by leaching with caustic solutions under high pressure and temperature. The molten metal may be poured in a vacuum to extract evolved gases and reduce oxidation, thus improving the quality of the casting. To further reduce microporisity, the castings made by this and other processes are subjected to hot isostatic pressing. Aluminum castings, for example, are subject to a gas pressure of up to about 100 MPa at 500 degrees Celsius.

Source: Kalpakjian, Manufacturing Processes for Engineering Materials.