DISTORTION OF CASTINGS DUE TO REHEATING IN INVESTMENT

Abstract

Cast restoration s are some times reheated in investment for soldering or repairing. In this study, distortion of castings made of gold alloys of types B and C and a dental copper alloy due to reheating in investment was investigated by reading the degrees of their cervical misfit on the original dies before and after reheating. The effect of reheating without investing was also compared as control. Findings were as follows: 1. Reheating could distort the castings remarkably in investment but little without investment. 2. Slender U-form castings distorted more than heavier M-O-D castings did. 3. Distortion was greater in a quartz investment than in a cristobalite investment. 4. The reheating· temperature to produce clinically significant distortion was higher than 600℃ for the copper alloy and higher than 800℃ for the gold alloys. Dental castings are sometimes reheated in investment. For instance, a defect in a cast restoration may be repaired by adding another cast or solder with reheating the casting. If the units for a bridge or splint are connected by soldering, they are also reheated in investment. The change in relative position of those units during soldering has so far been discussed as the main cause distorting a bridge or splint1-5). The distortion of the units themselves due to reheating in investment must also be considered. Such a distortion was investigated in this study, varying form of casting, types of investment and alloy, and heating temperature.