Abstract: A method and apparatus capable of continuously discharging contaminated gas out of metal molds, the gas being generated prior to the casting step and including moisture. A gas vent valve is provided at a gas vent passage in communication with a mold cavity. A suction unit and a vacuum pump are selectively connected to the gas vent valve. After closure of the metal molds, the suction unit is connected to the gas vent valve to perform sucking interior of the mold cavity. By this suction, an external air is introduced into the metal molds through a pouring port of a shot sleeve. Thus, contaminated gas in the cavity is replaced with a clean air. During injection molding, the gas vent valve is connected to the vacuum pump for decompressing the mold cavity. Vapor explosion is avoidable since moisture content is removed from the mold cavity even if the molten metal is introduced into the cavity.

Abstract: A gas venting system is connected to a downstream side of a gas vent valve through a gas exhaust line. An electromagnetic valve and a suction unit are connected to the gas exhaust line. A pressure switch is directly connected to the gas exhaust line to detect the pressure variation within the line. By the suction through the suction unit and the opening of the electromagnetic valve, the gas exhaust line is decompressed. If the line has a perforation, the pressure does not reach a predetermined pressure. If clogging occurs, the pressure reaches a predetermined pressure while the metal molds are open and the gas vent valve is open. Thus, accurate detection to the defects can be achieved regardless of the state of the metal molds and the internal volume of a vacuum tank.