[Dilatometry of gypsum bonded investment (author's transl)].

Although many investigations on the thermal expansion of dental investments have already been done, the conditions upon which thermal expansion was measured were not always definitely shown and the phenomena on thermal behavior of dental investments were not reasonably interpreted. In this investigation the effects of W/P ratio of samples, heating rate and furnace atmosphere were studied by "Push Rod" Dilatometer in order to attempt to standardise dilatometry of dental investments. All experiments were designed with two-way layout techniques. The main results were as follows, 1) W/P ratio, heating rate and furnace atmosphere have significant effects on the thermal expansion curves of the gypsum bonded investments. 2) One steep expansion and three stages of shrinkage in heating process of these investments from room temperature to about 900 degrees C were observed. 3) The quantity of the steep expansion, first and second stages of shrinkage and the expansion at 650 degrees C were affected by W/P ratio. 4) The temperature at which the steep expansion or shrinkage occurred were affected by heating rate. 5) Furnace atmosphere affected on both quantity of the steep expansion or shrinkage and their temperature. It also affected on the expansion at 650 degrees C. Then, these factors should be considered in dilatometry of gypsum bonded investments and in dental practice.