They are available as: 1. Occurs usually near the sprue-casting junction. A binocular microscope is extremely helpful in detecting and removing them. So the proper length of sprue is important to keep the pattern away from heat centre close to the end of the casting ring. The hottest portion of the casting ring is located near the centre of the ring heat centre. The produced Vapor is used to force the molten alloy into the mold. Progress in Investment Castings.