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 Wax Patterns During Setting of Gypsum Investments
I T HAS been reported' that a large variation exists in the size of inlay castings even when they are made by technics more standardized than those which could be expected in a dental office. It is possible that much of this variation occurs between the time the pattern is placed in the investment and when the gold enters the mold. One of the major factors involved during this time interval is the effect of the setting and hygroscopic expansion of the investment upon the size or shape of the mold. As early as 1912, Price2 recognized that there might be some distortion of the pattern due to expansion of investment. Much later, experimental evidence was obtained to indicate that restriction by the ring and by the pattern could seriously affect the setting expansion of gypsum products.3' 4 Taylor5 also indicated that distortion would occur. Smyd,6 by mathematical calculations, suggested the pattern of distortion which might take place. However, the only detailed study in this area is that of Jorgensen.7 The technic employed led to some interesting observations. However, the investigation included only one investment used in this country and although variation in mold size was noted, no detailed work was carried out to investigate the degree of distortion, its relation to the investing technic used, or to other variables encountered during the investing procedure. It was thought that an experimental method which gave, in one measurement, the actual dimension of the investment mold may be more accurate. Therefore, the purpose of this research was to evolve a better method of investigation, to determine the degree of distortion of wax patterns by the setting investment, and the effect of certain other variables on this distortion.
The Behavior and Measurement of Hygroscopic Expansion of Dental Casting Investment
SINCE the discovery of the phenomenon of hygroscopic expansion of dental casting investments when subjected to an environment of water, there has been considerable work carried out to investigate its behavior. The ultimate goal is to develop a technic based on this property capable of compensating for the casting shrinkage of dental gold alloys. The result of these efforts has been the establishment of two technics which involve the introduction of water to the investment by a surrounding wet asbestos liner or by immersion of the entire inlay ring into a water bath. Results obtained with these technics have indicated that a successful casting is dependent on an almost overwhelming number of variables. Previous investigators have used various methods of measuring hygroscopic expansion which afford different results in accordance with the restriction offered to the expansion, as well as other less significant factors. Degni,1 Docking and Chong,2 Landgren and Peyton,3 and Delgado and Peyton4 have summarized, commented on, and investigated these methods. The general conclusion is that hygroscopic expansion is affected by the method of measurement and test when initial readings are made as soon as possible after mix. When measured from the initial set time, the expansion values of the various methods are more closely in agreement with each other. It has been felt that these latter values are more indicative of the actual enlargement of the mold cavity, since they are recorded when the investment is more rigid and can hold its shape. Accordingly, the term "effective" expansion has been applied. However, for lack of further, more conclusive, information, it is difficult to accept a starting point of such arbitrary nature. In addition, the methods for measurement of hygroscopic expansion designed to determine its compensating abilities as well as its behavior as influenced by these variable factors have been based on linear changes as measured in one direction. These measurements, as well, are dependent on factors whose effects are manifest in a significant variability of test results. As a result of these difficulties, a different approach to the problem was initiated. It was postulated that the amount of water taken up by the investment might be related to the subsequent expansion, and a procedure for adding a measured or controlled quantity of water was devised. Furthermore, since the previous methods of measurement have been based on changes occurring in only one direction, consideration was given to measuring the expansion in three directions simultaneously. It was felt that this measure would establish a more