Casting shrinkage of thin-walled castings

The casting shrinkage of thin-walled castings was measured using a tungsten die method. The thickness and the surface area of the casting were simulated to those of either partial veneer crowns or complete denture frameworks. Gold alloys were cast at two different pressures (0.35 and 0.49 MPa) and the shrinkage measurement was made at different locations on the castings to investigate the effect of these factors.

Both overall and local shrinkages were significantly influenced by size and casting pressure. These shrinkages were significantly lower for the denture-sized (DS) specimens than for the crown-sized (CS) specimens, and significantly decreased with the increase of the casting pressure. The effect of location on the local shrinkage significantly varied with the change of the other two factors, size and pressure. At the higher casting pressure, the DS specimens sustained a significantly smaller shrinkage at the margin than at the centre, whereas the CS specimens had a significantly smaller shrinkage at the centre than at the margin at the lower pressure.

The results suggest that the casting shrinkage of a thin-walled casting is influenced by the temperature gradient in the alloy and the resultant time difference in alloy solidification as well as the volume supply of the molten alloy in the mould.