抄録
α-calcium sulfate hemihydrate is used as a model material or binder of investment for fabricating intraoral appliances. When mixed with water, the hemihydrate (powder) changes into the dihydrate in the setting process, which allows the mixed to seemingly expand. This expansion, although sensitive to constraints, greatly affects the accuracy of fabricated appliances. Therefore, it is necessary to evaluate the expansion pressure. In this study, the pressure under a rigid constraint condition was directly measured by using a universal testing machine. As a rule, the pressure did not appear soon after the mixing. Then, the pressure curve fell to a peak in the contraction stage and turned over toward a maximum. The maximum pressure decreased with increasing water-powder ratio (W/P). This might be because higher W/P led to less number of acicular dihydrate crystals per unit volume. On the other hand, the pressure was independent of mixing time and room temperature.
