CUSTOM IMPRESSION TRAYS .3. A STRESS-DISTRIBUTION MODEL

All resins used to make custom impression trays exhibit plastic deform ation at some force value; therefore it is important to compare the ph ysical property values of such materials with the stresses to which im pression trays are subjected during dental procedures. A simple mathem atical model of a custom tray was developed to predict stress distribu tions in this final part of a three-part investigation. Experimental s tress analysis of such a tray confirmed the accuracy of the model, whi ch was then used to predict the maximum stress experienced by the tray during removal of a completed impression from the oral cavity. The re sults of this analysis indicated that these stresses would be signific antly lower than the yield stress for a commonly used polymethyl metha crylate resin or a light-polymerized resin. The stresses were also suf ficiently low for us to conclude that thermoplastic resins would not p ermanently deform; however, the stresses encountered in the experiment al confirmation procedure were close to the yield stress values for th ese materials.