Thermal expansion of dental casting alloys and phosphate-bonded investments.
For a dental investment to render accurate castings, it is essential 1) that the setting expansion is low enough not to cause any distortion of the wax pattern, and 2) that the thermal expansion is high enough to compensate fully for the solid thermal contraction of the casting alloy used1-8). However, all phosphate-bonded investments known today when used according to the manufacturers' instructions, are characterized by a relatively high setting expansion, and by a thermal expansion insufficient to compensate adequately for the solid thermal contraction of conventional casting alloys. Recent studies9) have shown that the setting expansion of representative European brands of phosphate-bonded investments may be reduced to an acceptable low value by modifying the mixing liquid. It was the purpose of the present work to study the thermal expansion of thus modified products, and for comparison also the thermal expansion of several dental casting alloys.
Dimensional change in mould space on setting of phosphate bonded investment.
— The effect of setting expansion and of temperature rise during setting of phosphate bonded investment on selected dimensions of an MOD casting mould space was studied. Excessive mould expansion and distortion was demonstrated. It was concluded that investments having zero setting expansion and sufficient thermal expansion to compensate for casting shrinkage should be used with the die-investing technique. However, such investments are not at present available for all alloys.
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.