Hygroscopic technique for inlay casting using controlled water additions

Abstract

In summary, a hygroscopic technique for dental gold inlay casting has been described which is based on the results of a fundamental study 6 of the nature of hygroscopic expansion. It was found in this study that the amount of hygroscopic expansion which occurs when a specific amount of water is brought into intimate contact with a dental casting investment during setting is directly related to or defined by this amount of water. In addition, a critical point was found to exist which represents the maximum capacity of the investment for water pick-up and subsequent expansion. Water added below this point was found to define precisely the concomitant expansion. A hygroscopic technique based on the results of this study was designed. It features a flexible inlay ring in order to eliminate the asbestos liner, yet not restrict the expansion, as well as a calibrated syringe for adding controlled amounts of water to the investment during the setting process, The advantages of this technique which were established in the measurements made on investment in the previous study 6 and corroborated by the results of practical castings as reported in this study, are stated as follows: 1. 1. The effects of influencing factors such as water/powder ratio, spatulation conditions, age and batch number of investment are eliminated. 2. 2. A convenient and accurate means of changing the expansion is provided by the addition of different amounts of water, if such a prodecure is desired by the operator. 3. 3. More than adequate reproducibility of the investment expansion, and subsequent casting fit is achieved by adding a controlled amount of water.