Heating Rate of a Gypsum Investment Related to Crack Formation

¹ Tokyo Medical and Dental University, Department of Operative Dentistry, Tokyo, Japan

The relationship between the heating conditions of investment molds and crack formation was investigated using a cristobalite investment. The following findings were obtained by comparing the results from heating cylindrical specimens, casting in molds with rings, and measuring the inner temperatures of those molds.

In the experiment with cylindrical specimens, cracks were produced when the specimens were quickly heated (40° C/min) only at the stage from 250° to 350° C.; but no cracks were produced when slowly heated (10° C/min) at this stage, even if they were quickly heated before and after it. Consequently, quick heating at the stage of cristobalite transformation between 250° and 350° C. seems the main cause of crack formation. This is considered due to rapid increase in the difference between the inner- and the outer-layer expansions of the investment molds when heated quickly at this stage.

Fins due to mold cracks were produced on the castings also when quick heating was started from 350° C. (furnace temperature) with casting rings of regular size (30 mm. in inner diameter and 38 mm. in height). This is probably because the mold investment was quickly heated when the transformation was not yet finished in the inner layer, where the temperature rose much later than the atmospheric temperature. No fins were, however, produced on the castings when heated quickly after the furnace temperature reached 400° C. Quick heating immediately after set always produced fins.