Abstract
The qualities of dental investment are specified and assured at the minimum by Japanese Industrial Standard, and manufactureres are obliged to describe the adequate and accurate instructions for proportioning and manipulating on each unit container : the water-powder ratio, the setting time, the setting expansion, the thermal expansion curve, etc. With reference to these instructions a proper mold for each casting alloy may be successfully prepared, if the very details of the instruction were kept. From the practical point of view, however, it is quite troublesome and nonproductive to keep to such instructions each time carefully. Of course, for the professional it is most desirable for these mixing conditions and other factors to have no effects on the properties of the set investment. But it is not the case. Thus the authors checked the factors, on some dental investment, and decided the ranking of the practical importance of them which should be carefully controled in mixing investments. The results obtained are as follows : 1) The first and the most important factor to be controled is the water-powder ratio, which has a marked effect on the setting time, the setting expansion and the thermal expansion for all the investments. Thus, the powder and the water to be mixed should be properly balanced in accordance with the maker's indication and not by the empirical intuition. 2) The second important factor is the room temperature, which has a great effect especially on the setting behavior. This factor might be difficult to be controled and so the temperature dependence of each investment should be well acquainted prior to the mixing. 3) The third factor is the spatulation time. This factor may be controled to the allowable extent empirically without time measurment. 4) Other factors such as the water temperature, the interaction of water-powder ratio with water temperature or with spatulation time have less but appreciable effects. 5) Each of the foregoing factors showed respectively nearly the same effect on all the investments used in this experiment.
