China Foundry (Aug 2010)
Research on cracking mechanism of the thin shell mould in expendable pattern shell casting during pattern removal process
Abstract
Aiming at the cracking phenomenon of the thin shell mould in the expendable pattern shell casting during the pattern removing process, some systemic researches are presented. The influence of the pattern removing method and temperature on the pattern removing were investigated. The shell mould cracking mechanism was analyzed by using thermo-gravimetric analysis (TGA), and combining the temperature field and the volume change of the expanded polystyrene (EPS) foam pattern being tested. The results indicated that the shell mould was not easily cracked when the pattern removing process was carried out with the furnace being heated little by little because of the shell slowly shrinking with dehydration and shell strength gradually increasing. The shell mould was soon destroyed when it was set directly into the furnace at above 400 ℃ because of the thin shell mould rapidly shrinking and the foam pattern hindering. However, the shell mould had no cracking when it had been preheated for a long time even if the furnace temperature was above 400 篊 and the shell was put into the furnace directly. Moreover, when the shell mould was directly set into the furnace at lower temperatures, 250 to 300 ℃, the shell would shrink slowly and the foam pattern would stay at the maximum expansion stage temperature of 100 to 110 ℃ for a long time; and the shell mould would experience an expansion force from the foam pattern for a long time. The expansion force is related to the pattern removing temperature, holding time, foam pattern thickness and density. Therefore, the foam pattern with higher density could make the shell crack.
