THE EFFECT OF POTENTIAL INVESTMENT EXPANSION AND HOT STRENGTH ON THE FIT OF FULL CROWN CASTINGS MADE WITH PHOSPHATE-BONDED INVESTMENT

In an earlier investigation, it was shown that when full crowns are ca st in gypsum-bonded investments, their relative inaccuracy is affected by both the investment's potential expansion and its hot strength. Th is study repeated the earlier one, but used a high-melting gold alloy and two phosphate-bonded investments. The investments were used under conditions which gave a range of potential expansions and hot strength s. Casting inaccuracies were determined both diametrally and axially. All castings showed distortion, which varied under the different condi tions. All were oversized axially, by amounts varying from +0.8% to +2 .3%. Diametral inaccuracies ranged from -0.2% to +0.7%. Investment exp ansion had a strong effect on axial inaccuracy, but a negligible effec t on diametral inaccuracy. Conversely, hot strength had a strong effec t on diametral inaccuracy, but only a very weak effect on axial inaccu racy, With phosphate-bonded investments, both potential expansion and hot strength are important parameters of relative casting inaccuracy. In combination, these properties showed very strong correlations with both diametral and axial inaccuracies. The observed distortions were t he result of anisotropic mould expansion and anisotropic alloy shrinka ge. The best fit, and least distortion, occurred with an investment se tting under dry conditions.