Studies on New Dental Investment containing AlPO₄　Part 3 Adaptability of Castings and Properties of Investment

Abstract

As reported previously, the thermal expansion of a new dental investment, which was composed of AlPO₄(berlinite)refractory and phosphate binder, was no more than 7%. However, it is necessary to control the amount of thermal expansion for practical use.New dental investments for dental casting were prepared by mixing SiO₂ refractorys with AlPO₄(berlinite) refractory(5〜30wt%) and phosphate binder(14wt%) to control overall thermal expansion.Adaptability of castings made of various types of dental alloys was examined with MOD metal mold by only thermal expansion techniques.The relation between the distortion of castings and the setting expansion were examined.The properties of the new dental investments were compared with those of commercially available phosphate bonded investment.The rapid increase in the linear expansion of the new dental investment occurred on heating both at the temperatures of α-β inversion of quartz, cristobalite and berlinite and at the temperature of transion of β-berlinite from β-cristalphite.The more SiO₂ blended with AlPO₄, the less was the resulting thermal expansion.In this manner, the thermal expansion was varied within a range of approximately 1.5 to 3.0 at 700 or 800℃.The casting shrinkage of dental alloy could be compensated for because the overall thermal expansion could be controlled by changing the ratio of components.It was found that the less the setting expansion, the less was the distortion of castings from the point of defined distortion amount.Furthermore, the coefficient of correlation between the setting expansion and the distortion of castings was 0.86.Such physical properties of the new dental investments as compressive strength, setting time and setting temperature were no less or better than those of the available phosphate bonded investment.