[Studies on the distortion of the cast crown by measuring volume change (author's transl)].

Abstract

We made experimental cast crowns under various casting conditions, and measured their dimensional change of inner cervical margin and volume change compared with wax patterns. The relation between percentage of linear expansion (alpha) and percentage of cubic expansion (beta) of matter is beta not equal to 3alpha. When investment mold and cast metal equally expand or contract in all direction, the relation between the dimensional change (=alpha) and the volume change (=beta) of the cast crown must obey this expression beta not equal to 3alpha. When the relation between alpha and beta of the cast crown is not beta not equal to 3alpha, we consider the cast has distortion. Casting conditions were fifteen kinds by changing investment, casting method, linning material and mold temperature. The following results were obtained. 1) Placing wet asbestos liner, dimension was larger than its pattern. But, contraction of volume was great. 2) Placing kaowool liner and asbestos liner coated with vaseline contraction of volume was improved. 3) In case of ringless casting method, the cast crown gave best fit to metal die, and distortion was very small. 4) Judging from these results, hygroscopic expansion of investment by wet asbestos liner made volume contracts, and made the cast crown distort. 5) In case of casting at low temperature ceravest mold (150 degree C), dimension was as large as casting at high temperature mold (700 degree C). But, the lower the mold temperature, the larger the contraction of volume. Therefore, the cast crown fitted metal die, but had large distortion.