Mold dilation is rarely mentioned in casting technology textbooks but it can occur in jewelry investment casting. Dilation is a slight volume ballooning of the mold cavity, and lack of mold rigidity at the instant of casting is the most important factor. There is often a connection between metal/mold reaction and dilation, and the likely mechanism is explored. Dilation below about 5% is difficult to detect without very careful measurement; above 5%, it is likely to increase the metal demand beyond the capability of the feeding system and cause interdendritic shrinkage. The paper presents evidence of dilation in platinum and 18K white gold casting with different investments and melting/casting units. In view of the small dimensions of jewelry castings, the dimensional changes have to be examined very carefully, preferably by statistical trends.

Author: Dr. John C. Wright