Post-and-Core Assemblies Made With an Accelerated Pattern Elimination Technique

Abstract

Accelerated pattern elimination has the potential for increasing productivity. This study evaluated the accelerated pattern elimination technique using three commonly used phosphate-bonded investments. 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. 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). 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.