Abstract: A thermal gate for turning on or off the flow of molten molding material into a mold cavity comprises a narrow passage formed by an extremely thin-walled tube of iron-nickel alloy surrounded by a thin sleeve of insulation, this sub-assembly being inserted into a water-cooled mold. When the tube is supplied with electric current, it rapidly heats to a temperature above the melting point of the molding material, permitting such material to flow into the mold cavity. The thin insulating sleeve maintains a temperature gradient across its wall thickness to permit the tube to remain at the desired temperature without undue power input. When it is desired to stop flow of material through the passage, the current is shut off, and the low thermal inertia of the tube and sleeve permit the tube to rapidly transfer its heat to the water-cooled mold, thereby freezing the molding material in the passage.