Three factors affecting investment setting expansion and casting size

T he production of accurate dental castings by the lost wax process involves casting molten alloy into a refractory mold which is sufficiently and precisely oversized to accommodate the shrinkage of the alloy during cooling. The total expansion of the investment mold is determined by the combined effects of setting behavior and thermal expansion during burnout. Several investigators have studied the multitude of variables which affect setting expansion.1-1o The major variables include (1) number of liners in the ring, (2) position of liners, (3) position of wax pattern in the ring, and (4) water/powder ratios of investments. The combined roles of each of these variables in regulating expansion has been well documented and generally accepted by the profession. Clinical experience has suggested, however, that other factors may also be significant in determining the setting expansion of investment and in the subsequent regulation of casting size. The purpose of this study was to investigate the related effects of (1) mixing rate, (2) ring liner position, and (3) storage conditions on the setting expansion of both gypsum-bonded and phosphatebonded investment molds; and subsequently to correlate casting size with measured expansion data. Although there are many studies of investment expansion behavior in the literature, few have actually correlated these data with actual casting size.