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Gypsum Products, Dental Waxes, and Impression Materials

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3-17. DIMENSIONAL STABILITY

a. Water Content. The water content of agar-type hydrocolloid impression materials is most important for dimensional stability.

(1) Syneresis. When an impression made of this material is removed from the mouth into the air at room temperature, the surface contracts by giving off water to the air. This process is called syneresis and causes the outer layer of the impression to shrink and become distorted.

Figure 3-1. Instruments and materials for agar hydrocolloid impressions.

(2) Imbibition. If the impression is placed in water, it will expand (take up water). This process is called imbibition. Unfortunately, the expansion caused by imbibition will not restore an impression to its original dimensions.

(3) Expansion after shrinkage. The expansion does not equal the shrinkage either in volume or direction. Therefore, any attempt to restore an impression after syneresis has occurred will result in a distorted cast or die.

b. Brief and Abrupt Stress Recommended. The structure of the hydrocolloid gel is such that it can withstand an abrupt, brief strain (change of form or size) without fracture or permanent distortion more easily than it can withstand a gentle, prolonged strain. Therefore, the impression should be removed from undercut areas quickly rather than be "teased" over these areas. The impression always undergoes stress during removal. Stress is the internal resistance of a material to forces that disarrange its normal molecular structure. After the internal force is removed, the gel "relaxes," but the impression does not quite resume its original shape because the material is not perfectly elastic. The material is also under some slight pressure while the impression is being made. The stresses induced either relax or set when the pressure is removed. This set distorts the impression after a short time. Therefore, it is important that the casts made from agar-type hydrocolloid impression materials be poured immediately after the impressions are made.

David L. Heiserman, Editor

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Revised: May 22, 2017