Extrusion of parison for suction/blowing molding system

In a molding method wherein a plastic hollow preform is drawn through a mold passage formed in a closed mold by applying suction to a downstream end of a conduit having an upstream end connected to an output end of the mold passage, the improvement comprising forming the hollow preform by the steps of:

• concentrically extruding an outer tubular strand having an inner diameter and an inner tubular strand having an outer diameter from respective annular nozzle openings defined between respective inner and outer nozzle walls, the strands being of resins that swell transversely as they issue from the nozzle openings;
• positioning the nozzle walls such that the outer diameter of the inner strand is substantially smaller than the inner diameter of the outer strand, whereby the inner strand is separated by a space from the outer strand as the strands issue from the respective nozzle openings;
• and
• engaging an outer wall of the inner strand with an inner wall of the outer strand at a spacing from the nozzle openings after substantially all swelling of the strands has taken place.