Abstract
In dental casting technique, it has been said that the metal shrinkage should be fully compensated with the same amount of expansion obtained by setting and heating the investment bulk. However, some reports have recently pointed out the anisotropy and heterogeneity of the latter. So, the author measured the displacement of 'crystobalite' investment material in the casting ring and the dimensional change of 'K-metal' casting under the conditions very close to the clinical practice to know the actual circumstances of shrinkage compensation. The displacement of two inner portions of 'crystobalite' investment in the casting ring was measured along its axis by using differential transformer, of which the core was connected to a plate embeded in the investment bulk balancing by a bob so that no gravitative effect might work. Net setting expansion of the investment bulk under the plate was generally greater at higher W/P ratio and more than 3% when the ring was lined with an appropriate number of wet asbestos layers. It increased as the number of asbestos layers increased up to a certain critical value, which was lesser at higher W/P ratio. Dimensional changes of several important portions as well as volume change of crown type and MOD type castings were also evaluated with reference to those of the wax pattern. Even in case that setting expansion by itself of the investment exceeded 3%, the casting could not always become larger than the wax pattern in every portion. Although asbestos lining had remarkable enlarging effects on the dimensions along ring radius and anyhow supported the shrinkage compensation, thickness and volume of casting could almost never become larger than those of the wax pattern even with 4 layers of asbestos. Nevertheless, at least the inside dimensions of crown and MOD castings could be sufficiently larger than those of the wax pattern under certain proper conditions. Such castings could well be fit to the model. Therefore, it may be concluded that the well-fitting casting can be produced due to a favorable deformation even with considerable shrinkage as a total.
