Refractory die-investing technique: the tendencies of setting expansion on die-investments in the process of constructing working models.

Die-pattern technique is presently the most prevalent in use as a general method of constructing inlays, and crown and bridge work. A wax pattern is prepared on a stone die and taken out to be invested and cast. This method and process of taking wax patterns out of the die occasionally causes deformation or damage to patterns due to handling or the properties of the wax itself, and this affects the adaptation accuracy of the cast products. To find a solution to this problem, the following method has been reexamined. Wax patterns are taken on a die (model) made of investment and the die itself, with pattern on it, is invested and cast (refractory dieinvesting technique). As a result, die-investments well adaptable for this technique have been developed and the technique has become applicable to the realm of inlays, and crown and bridge work[1-8]. Unlike conventional casting investments, die-investments have certain properties, large in setting expansion and small in thermal expansion. That is, compensation for the casting shrinkage of alloys depends mainly on setting expansion and additionally on thermal expansion. Consequently, to obtain cast products preferable in adaptability, it is required that working models made of die-investments should have even expansion. The setting expansion of models in the process of construction is, however, under the influence of various factors and even expansion cannot always be expected. We therefore have to decrease the unfavorable factors causing the uneven expansion as much as possible and gather further data on all the factors having effects on the setting expansion so as to obtain preferable working models. The authors, in the process of constructing working models, examined the factors presumably having effects on the setting expansion of die-investments: that is, kinds and thicknesses of impression material, shape and taper of the abutment tooth, shape of the tray and the existence of a base block on the models[9-14], and several findings concerning the phase of setting expansion under the influence of these factors were obtained. On the basis of these findings the authors herewith will try to explain the setting expansion tendencies of die-investments which should be required for constructing working models of crown and bridge.