62 communicates with a passage 64 in the core rod support 12 there is a circumferential groove 66 around the outside of the flange 18 within the socket 30 this circumferential groove cummunicates through a passage 68 which communicates with another passage 70 formed by a groove in the outside surface of the core rod 26 this groove passage 70 leads to another circumferential groove 72 in the outside surface of the core rod 26 there are angularly spaced openings through the sleeve 20 leading from the annular groove 72 through a part of the outside surface of the sleeve 20 over which the parison 48 extends small poppet valves 76 in the openings 74 prevent plastic material from being injected into the openings 74 when the core rod structure is extending into an injection mold when pressure is supplied from the tube 62 through the passages 64 66 and connecting passages to the radial passages 72 at the end of a blowing operation this pressure moves the poppet valves 76 outwardly far enough to permit air to enter between the blown parison and the balloon channels 78 in the outside surface 60 of the sleeve 20 can be provided leading axially from the openings 72 however these channels are not essential because the air pressure expands the mouth portion of the parison when the blow mold opens and permits free flow of air axially to the outside surface of the balloon 50 the inner end of the core rod 26 is in axial alignment with a tube 82 that forms a continuation of the center passage through the.