The dimensional accuracy of improved dental stone, silverplated, and epoxy resin die materials.

1 mproved dental stone is the predominant material in dentistry for making dies used in the lost-wax process. Although improved dental stones have served the profession well, the main disadvantages of their use as dies are (1) susceptibility to dimensional change due to abrasion, (2) limited reproduction of fine detail,’ and (3) lack of strength. The lack of strength can result in breakage of dies, especially those duplicating long, narrow preparations. In the fabrication of all-porcelain labial margins or some of the new generation cast porcelain restorations, dies that exhibit greater strength and abrasion resistance than gypsum dies are desirable. Alternate die materials that may yield dimensionally accurate dies, are more resistant to abrasion, and are stronger than the improved dental stone dies presently in use are available. The literature concerning dimensional accuracy of various die materials is somewhat equivocal. Gypsum die materials generally exhibit an average setting expansion of O.l%.* Such minimal expansion is thought to be beneficial in terms of aiding compensation for metal shrinkage, wax pattern dimensional change, and other unavoida.ble inaccuracies in the casting process. Electroplated dies have been found to be less accurate dimensionally than improved stone dies when formed with polysulfide rubber or condensation-reaction silicone impression m.aterials.3 One study demonstrated that silver-plated dies tend to be undersized by as much as 0.2%; whereas another found that they tend to be oversized.5 It has been shown that vinyl polysiloxane impression ma.terials are the most accurate for making silver-plated dies.+ Castings made on silver-plated dies have been shown to have smaller marginal openings than those made on improved stone dies.6 Epoxy resin dies have been found to be generally undersized, pri.marily because of polymerization shrinkage.’ The manufacturer of the new generation epoxy resin die systern claims to have controlled this shrinkage and that dimensional accuracy can be precisely con-