Ef. Morey et R. Earnshaw, THE EFFECT OF POTENTIAL INVESTMENT EXPANSION AND HOT STRENGTH ON THE FIT OF FULL-CROWN CASTINGS MADE WITH A GYPSUM-BONDED INVESTMENT, Dental materials, 11(5-6), 1995, pp. 311-316
Objectives. It has been claimed that the strength of the investment mo
ld at the casting temperature affects the dimensional accuracy of cast
ings (e.g., Markley, 1953; Earnshaw, 1957; Asgar, 1972), but the relat
ionship has not been studied quantitatively. In this investigation, th
e effects of both mold expansion and hot strength on the relative inac
curacy of full-crown castings have been measured and analyzed. The lik
elihood that a strong investment could cause distortion of the casting
by non-uniform restriction of casting shrinkage (Earnshaw, 1969b) was
also considered. Methods. Castings were made with a commercial gypsum
-bonded inlay investment, used both as supplied and with modifications
that increased its expansion and reduced its hot strength. In both se
ries of tests, the investments were used over a wide range of liquid/p
owder (UP) ratios in casting rings fitted with dry ceramic liners, and
set under dry conditions. Results. Casting inaccuracy showed a signif
icant linear correlation with total expansion and a highly significant
linear correlation with the combination of total expansion and hot st
rength. The modified investment, with its low hot strength, gave less
distortion of casting shape than did the much stronger unmodified mate
rial. However, it was found that to ensure sound castings, the hot com
pressive strength should not be less than 1.8 MPa. Significance. This
investigation showed that while investment expansion is the major vari
able affecting casting inaccuracy, hot strength is an important modify
ing factor which also has to be considered when predicting casting ina
ccuracy from measured properties of the investment.