a combination plastic and gas injection nozzle assembly and method of operation in which the nozzle assembly includes a plastic resin passage , a closure member for the plastic resin passage , a gas flow passage , and a closure member for the gas flow passage all coaxial with respect to one another . actuating cylinders are provided for moving both closure members between open and closed positions . the actuating cylinders being positioned coaxial with the other elements to eliminate off - set loading of the nozzle components . a sequential molding operation is disclosed in which a plurality of mold cavities defined by a single mold assembly are injected by one or more nozzles with plastic and a gas to form hollow spaces in the molded article . the cavities are injected with plastic one at a time to reduce the required mold clamping forces and enable independent control of plastic injection into each cavity . after partially filling each mold cavity , the nozzles are used to inject pressurized gas into the molten resin forcing the resin against the surface of the mold cavities and forming hollow spaces within the molded articles .