the present invention comprises a method for delivering a medium such as a gas , vapor or liquid into a melted plastic in a mold via an elongated screw shaft rotatably supported in a housing , the method including the steps of providing the screw shaft with a bore longitudinally therethrough from a proximal end to a distal tip end thereof , rotating the screw shaft in the housing to move plastic therethrough , and into the mold , introducing a gas , vapor or liquid into the bore , to permit the gas , vapor or liquid to be delivered through the bore , and into the molten plastic in the mold through the distal tip end of the screw shaft .