Post-and-core assemblies made with an accelerated pattern elimination technique.

Abstract

PURPOSE: Accelerated pattern elimination has the potential for increasing productivity. This study evaluated the accelerated pattern elimination technique using three commonly used phosphate-bonded investments. MATERIALS AND METHODS: Part one of this study determined the mean time interval from start of mixing to the maximum exothermic setting reaction temperature for each investment. A chromel/alumel thermocouple was placed at the heat center of a methylcellulose lined casting ring, using wet or dry ring liner. Investments were vacuum mixed at the recommended ratio for the accelerated technique. Colloidal silica solution and ddH2O were combined at a 50:50 ratio to meet the manufacturer's recommended liquid volume. Part two determined the dimensional reproduction of a standardized pattern and its casting using both casting techniques. Mixing ratios were the same as in part one for the accelerated technique and 75% colloidal silica to 25% double-distilled water (ddH2O) for the conventional technique. The accelerated technique used the mean setting time established in part one followed by a 15-minute furnace holding time at 725 degrees C (1350 degrees F). The conventional technique used a 1-hour bench setting time, followed by placing the mold into a cold furnace. A controlled rate of climb to a maximum temperature of 725 degrees C (1350 degrees F) was used with a 1-hour soak time. Each pattern and its casting were measured at four sites: (1) Length of the post-and-core assembly, (2) maximum core diameter, (3) post diameter at the core base, and (4) post diameter at its apex. RESULTS: A significant difference was found between the time interval to maximum exothermic setting reaction temperature for all the investments (P < .01). The accelerated technique produced castings with a relative dimensional increase of 0.11% to 4.80%. The conventional technique ranged from a 0.04% decrease in size to an increase of 3.65%. Castings made with the accelerated technique were significantly different than those made with the conventional technique (P < .01). CONCLUSIONS: Differences in the time interval to maximum exothermic setting reaction temperature indicate that each phosphate investment should have a recommended setting time before introduction into the furnace. The carbon-containing investment showed the least relative change of the three investments evaluated for both casting techniques.