Extrusion die for externally ribbed plastic tubing

Claim 1

An extrusion die for an apparatus producing seamless plastic tubing having a smooth inner surface and an annularly ribbed outer surface defined by alternating ribs and grooves, including: an extrusion head having a central bore with a lateral opening for receiving an extrudate of a thermoplastic material under pressure therefrom, an elongate nozzle attached to said head and providing a supply passage extending in the longitudinal direction thereof for the delivery of said extrudate to the delivery end thereof, a hollow mandrel placed in said bore in substantially coaxial relation with said nozzle and spaced from the latter to provide a plastic conducting passage extending in the longitudinal direction thereof for the delivery of said extrudate to the delivery end thereof, the nozzle and mandrel defining respectively the annular extrusion orifice of diminishing interior dimensions, through which the extrudate can flow uninterruptedly and form a parison within the nozzle and around the mandrel, said delivery end of the mandrel having a coaxial external conical portion; a threaded adjusting member for adjusting longitudinally said mandrel, effective to vary the size of the extrusion orifice when the mandrel is axially adjusted; a coupling means interconnecting said mandrel and the head and allowing only the axial, without rotation, movement of the mandrel.