The apparatus has a valve arrangement which opens rapidly due to priming or prestressing of the valve plate by the pressure in the pressure chamber. When the valve is closed, pressure, built up in the pressure chamber space closed by the valve, applies a force to the valve member in the direction of its open position. The valve member is held closed by a holder arrangement. Following the development of a predetermined pressure in the pressure chamber, the holder arrangement releases the valve member such that the pressure in the pressure chamber forces the valve member to its open position opening the pressure chamber passage and permitting the pressure medium to surge onto the surface of the molding material. This apparatus is suitable for all high-stressed gases which must be released suddenly.