Effect of Bentonite on Strength Inhibition of Shell Mold using Recycled Calcination Sand from Green Sand Molds

Abstract

  We investigated the reasons for why shell mold strength decreases when green sand is reclaimed by calcination at lower temperatures. When calcination is performed at temperatures above 740℃, the shell mold strength increases. The temperature of 740℃ is close to the OH group dehydration temperature for bentonite, which is present below 740℃, and is thought to reduce shell mold strength. To investigate this relationship, we added bentonite to shell mold sand, and confirmed that it reduces the shell mold strength. Using scanning electron microscopy, we found that bentonite prevents resin adhesion to sand particles and causes embrittlement of the resin coating. Moreover, the results of gel-permeation chromatography and powder X-ray diffraction analysis indicated that intercalation of bentonite in the phenolic resin reduces shell mold strength.