A process for injection molding of thermoplastic materials which includes a nozzle body having a bore to provide a flow path for molten thermoplastic. One end of the nozzle body is adapted for connection with a sprue of an associated mold body, and the other is adapted for connection with an end of an associated injection molding machine. The nozzle body has a shut-off valve fluid pin housing adapted for reciprocating movement in the nozzle bore to control the flow of plastic through the nozzle body. The shut-off valve fluid pin housing also includes a body having an adjustable fluid pin with a tip portion and a fluid passage formed in the body and open at the tip portion and the other end of the fluid passage communicating with a source of two pressurized fluids, one in vapor and liquid phase and another in gas. The shut-off valve is actuated to allow thermoplastic material to flow and subsequently a first pressurized fluid vapor is allowed to flow to form a hollow cavity in the thermoplastic material and then terminated and a second fluid which is a gas is allowed to flow into the cavity followed by the liquid of the first fluid. The fluid flow is controlled by the fluid pin shape and stroke adjustments. After the thermoplastic material has set, the pressurized fluid is vented through the fluid pin housing passage into a fluid condenser for recovery of the vapor and liquid and then into atmosphere.