MOLD DISTORTION BY FOUR DIFFERENT INVESTING TECHNIQUES

Abstract

The dimensional changes and distortions of the castings made by the following four techniques were compared: (1) the hygroscopic expansion technique by water addition, (2) the conventional thermal expansion technique with wet asbestos lining, (3) the improved thermal expansion technique with dry asbestos lining using a commercial investment and (4) the same technique using a modified experimental investment having a small setting expansion. The U-form wax patterns were invested in rings varying position and direction and the changes of the transverse dimension of the castings at the levels of the occlusal floor and the cervical ends were determined by placing them on the original mold. The changes at the level of the cervical ends were used for comparing the mold expansions and the differences between the changes at the two levels were used for comparing distortions opening the cervical ends. Findings were as follows: 1) The hygroscopic expansion technique showed the greatest variation in mold expansion depending on the position and direction of the wax patterns in rings and also the greatest distortions. 2) The conventional thermal expansion technique showed the next greatest variation and distortion. 3) The improved thermal expansion technique with dry asbestos lining remarkably decreased the variation and distortion using the same commercial investment as used in the above technique, though it did not completely eliminate them. 4) The improved thermal expansion technique using a modified experimental investment having the ideal 0.05 per cent setting expansion could completely eliminate the variation and distortion. Its total expansion was, however, yet insufficient for clinical use.