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
Hygroscopic expansion of dental investments.
Hygroscopic expansion is manifested by the increase in volume of a gypsum prod­ uct when it is allowed to set under water or in contact with water other than that incorporated in the mixture at the time of setting. The expansion appears to differ chiefly in magnitude from the usual setting expansion which occurs when a gypsum product sets or hardens in air, out of contact with water other than the gauging water. Hygroscopic expansion was first dis­ covered by Scheu,1 and was used by him for the compensation of casting shrink­ age.2’ 3 Hollenback later perfected the technic for producing precision castings by shrinkage compensation with hygro­ scopic expansion, and the well-known “Hollenback casting technic” 4 resulted. There are a number of factors which are known to affect the magnitude of the hygroscopic expansion of a casting in­ vestment, such as the water: powder ratio and the length of time that the invest­ ment is subjected to the water bath after the initial set.1,2 The lower the water:powder ratio, the greater is the hygroscopic expansion, and the sooner the investment is immersed in water at or after the initial setting time, the greater is the expansion. Scheu determined the effect of many other variables on the hygroscopic ex­ pansion. Many of his conclusions were fundamental and his papers are basic to all research in this field. The following experiments are in part repetition of some of Scheu’s work under controlled laboratory tests with more modern equip­ ment and materials. In brief, .the technic advocated by Hollenback5 is to invest the pattern under vacuum in an investment with a pre­ scribed water:powder ratio. The casting ring containing the invested pattern is immediately immersed in a water bath at a temperature of 37°C. (99°F.) for 30 minutes. The wax pattern is then elimi­ nated by suction, and the mold is heated to 800° to 900°F. (430° to 480°C .), at which temperature the castings are made. The castings so obtained are justly famous for their precision. Although accurate castings can be ob­ tained with the Hollenback technic, little