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 some Casting Investments
CREDIT is generally given to Taggart1 for introducing the cast gold inlay to the dental profession. Although he realized that gold shrinks when cooled from molten state to solid state of room temperature, the investment available to him did not supply sufficient compensation. In 1910, Van Horn2 published a paper dealing with a method by which he obtained the compensation by means of wax expansion. The next major research in connection with this problem was the work of Coleman3 at the National Bureau of Standards. He determined the gold shrinkage to be 1.25 per cent and presented a casting technic with compensation values of quantitative nature. The total compensation for the gold shrinkage was obtained by the combination of the setting and thermal expansion of the mold. This procedure is the general practice followed at the present time for compensation. There are some disadvantages to this method. Skinner has indicated the difficulty of obtaining smooth castings and the improper compensation for the casting shrinkage.4 It has been found by Hollenback5 and Scheu6a that castings made at a high temperature are rougher than those made at a low temperature (450° C.), depending upon the great difference in thermal expansion between plaster of Paris and silica, which causes a poor union of the fine particles. This may also result in friability and there may be some warpage due to the differential expansion. A method which provides sufficient compensation at a low mold temperature is the hygroscopic casting technic.b Wilson7 showed that certain plasters expand as much as 0.0044 inch to the inch by taking up moisture from the air. With this information at hand, Scheula, b tested some commercial investment to determine what results the addition of moisture would have. He found6b: (1) All of the casting investments that were tested expanded. (2) The thicker the mix, the greater the expansion. (3) A mix with hot water expanded more than one with cold water. (4) The time at which the mix is placed in the water is of great importance. It was found that if the mix is placed in the water early, it would not expand so much as when placed in a little later. It was also found that if the mix was placed in the water after a certain time, it could not expand to the maximal amount. (5) The temperature of the water in which the mix is placed is important, for two reasons; (a) the temperature of the water controls slightly the expansion of the investment, and (b) water at a temperature of 1000 F. slightly softens the wax pattern so that the fine margins are not marred while expansion is taking place. The mix should remain in the water at least 20 minutes to obtain the full expansion, after which time no further expansion is obtained.