Void formation by axial expansion of a gypsum-bonded investment

The lengths of castings have been found to be dependent, in part, on the length of the ring lining material relative to the length of the casting ring. Five MOD inlay wax patterns were invested individually in open metal, in closed metal, and in plastic casting rings 35 mm or 42 mm long and 28 mm in diameter. The gingival walls of the wax patterns were placed either 8 mm or 15 mm from the uppermost ends of the rings. Specimens were broken in a mesiodistal direction, and the voids at the gingival walls were measured. The largest voids (139 microns) occurred in 42-mm-long plastic rings with the gingival walls 8 mm from the uppermost ends of the rings. The smallest voids (20 microns) occurred in the 35-mm-long closed metal rings. The boundaries between the investment and the voids were curved rather than accurately reflecting the straight gingival walls of the wax patterns.