A NEW METHOD FOR CASTING DISCREPANCY - SOME RESULTS FOR A PHOSPHATE-BONDED INVESTMENT

Objectives: An accurate and realistic casting discrepancy method appli cable to base metal alloys has hitherto been lacking. The purpose of t his study was to develop a method for determining casting discrepancy free of interference from oxide, slag and surface defects, working und er realistic conditions. In addition, a variable was sought that could be used for calibrating the casting process to allow for local errors . Methods: A crown pattern was designed to incorporate circular V-groo ves on the margin and the inside surface of the occlusal part for dete rmination of the 'groove root diameter' (GRD) with a measuring microsc ope. Castings using a phosphate-bonded investment were made to test th e effects of 'hygroscopic' expansion, burn-out temperature, powder/liq uid ratio and groove location. Results: The tested investment variable s showed the expected effects, but the distortion between marginal and pulpal regions was clearly shown, as were interactions between some v ariables. 'Special liquid' proportion appears to be a good candidate v ariable for process calibration. Conclusions: The GRD method was shown to be sensitive and reproducible. It is also applicable to many other casting systems, and in particular to monitoring overall process disc repancy, i.e. from tooth preparation to casting, which is suggested as being the key issue. (C) 1997 Elsevier Science Ltd.