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
Setting Changes of Hydrocal in Free State and in Various Hydrocolloid Impressions
A vertical measuring apparatus was devised by Fusayama and used for measuring the free setting expansion of hydrocal. This apparatus is very easy to use and is accurate enough.Perfect curves of dimension, temperature and hardness of hydrocal during setting were made and compared in fig. 2. Plastone and Duroc were used as hydrocal material.Their setting expansions occured at the time of the initial setting. The maximum expansion of Plastone was +0.54% 1 day after pouring, and that of Duroc was +0.05% 6 hours after pouring.The maximum temperature rise from room temperature in the center of a 23×23×50 mm3 block of Plastone was 7.5°C 30 minutes after pouring. That of Duroc was 16°C 13 minutes after pouring. The times requested to reach to these peaks substantially corresponded with those of the highest piches of their setting expansions.At first the hardness increased generally parallel to the curves of their setting expan-sions, and then continued to increase during their drying shrinkage also. They reached B. H. N 7 after 50 minutes in the case of Plastone and B. H. N 8.5 after 30 minutes in the case of Duroc. These results were about 60% of their final hardness measured after 1 week.Elastic hydrocolloid impressions restricted the setting expansion of hydrocal poured in them to some extent. Harder impression colloid supported closer by tray walls restricted it more.The Plastone models poured in various hydrocolloid impressions expanded after one day. This is illustrated in the following data :Technicol-Plastone model +0.26 (±0.06) %Algix-Plastone model +0.51 (±0.10) %Deelastic-Plastone model +0.27 (±0.07) %D. P. Im. Col.-Plastone model +0.35 (±0.09) %