Dimensional changes in dental plaster during the setting process.
The authors, using an NIN apparatus, examined the dimensional changes occurring in dental plaster during the setting process from its slurry state immediately after being mixed to its solid state, and the following results were obtained : 1) Linear shrinkage that can be seen in the initial stage of the setting process shows a tendency to become larger when the water/powder ratio is increased, and the length of the specimen is shorter. 2) Expansion initiation time shows a tendency to be quickened when the water/powder ratio is reduced, but little difference due to the length of the specimen is recognizable. 3) The apparent linear expansion shows a tendency to become larger when the water/powder ratio is smaller, and when the length of the specimen is shorter. 4) Velocity (time constant) from the beginning of expansion to the maximum expansion is almost constant, being little influenced by the water/powder ratio and the length of the specimen.
Development of measurement apparatus for expansion and shrinkage during setting, using a non-contact micro displacement meter.
Dental plaster and investment are used after being mixed with water or another liquid. Therefore, when they start setting, their condition changes from fluid to non-fluid. In other words, for some time after they are mixed with water or liquid, they look slurry, but as soon as setting begins they cease to be slurry and become solid, while at the same time they start expanding. In order to measure the dimensional change of this kind of material at the time of setting, it is preferable to measure it automatically under a loadless, non-contact condition immediately after it is mixed with water until the time when it has become solid. The conventional apparatus used for this purpose includes the following: comparator [1-4], micrometer microscope [5-8], dialgauge micrometer [1,2,7-20], straingauge extensometer [21,22], cathetometer & photographing [23], electromicrometer [24-28], micro-displacement meter [29-31], optomicrometer [32], etc. However, none of these seem to satisfy, in the strictest sense, the exact measuring condition stated above. Therefore we, the authors of this report, thought of pursuing and examining the actual dimensional change occurring in the mixture of plaster and investment from its slurry to solid condition at the time of setting. First of all, we tried to develop a kind of measuring apparatus which would fit the purpose of our research. We test-developed and examined the automatic measuring apparatus with a built-in, non-contact micro-displacement meter, which we named "NIN Setting Expansion and Shrinkage Apparatus" (which hereinafter we shall call "NIN Apparatus" for short). We actually used this test-developed NIN Apparatus by using for material the
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