Dimensional changes of MOD mold cavity corresponding to the degree of setting expansion

The correlation between the degree of linear setting expansion and the dimensional changes in a mold space was examined. The dimensional changes in the mold were determined by investing the MOD-type wax pattern using a die-investing material of which the degree of the setting expansion was controlled by diluting a concentration of colloidal silica solution. The wax was eliminated in an electric furnace at 130 degrees C and a fusible alloy, m.p. 48 degrees C, was cast into the mold space at room temperature. The dimensions of the wax pattern and the casting were compared. The degree of linear setting expansion was measured in a paper ring by means of a dial gauge. A high correlation was found between the dimensional changes of the mold space and the degree of the linear setting expansion. The expansion was highest at the axiogingival portion while those at both the axial dimension and the occlusal dimension were smallest. Linear regression lines (Y = AX + B) revealed the differences in the expansion mode at both the external and internal portions to show a discrepancy in value B.