Abstract: The formation of the rat tail is because that the partial dry molding sand layer on the internal surface of mould cavity is heated to high temperature and expanses. When the expansion force is bigger than the total slipping resistance, the hot molding sand layer slips and bulges. The majority of resistance comes from the neighboring hot molding sand layer. The resistance decreases intensely when the slenderness ratio of the hot molding sand layer increases. The increase of molding sand’s hot strength and the decrease of the slenderness ratio of the hot molding sand layer can increase the critical stress effectively and prevent rat tail effectively. When the slenderness ratio is less than 1/2 or the hot compression stress is not bigger than the wet compression strength of the molding sand, the rat tail should never come into being.

Key words: Critical stress, Hot strength, Rat tail, Slenderness ratio