Apparatus for molding of plastic tubing

An apparatus for molding of plastic tubing, including an elongate nozzle adapted for the attachment to an extrusion head for receiving an extrudate of an expandible plastic material under pressure therefrom, said nozzle providing a supply passage extending in the longitudinal direction thereof for the delivery of said extrudate to the delivery end thereof, a mandrel disposed in substantially coaxial relation with said nozzle and defining therewith generally an annular extrusion orifice of diminishing interior dimensions, through which said extrudate can flow uninterruptedly and form a parison within the nozzle and around the mandrel, the latter having a pressurized air supply passage communicating with at least one vent positioned within said parison for introducing said air to the interior of the latter, said interior communicating with the atmosphere, and a tubular mold cavity formed by a series of forwardly moving molds, characterized in that said air supply passage communicates with a tangential inlet of a vortex generation tube adapted to churn and heat up its peripheral air stream and to expand and cool its central air stream, said tube having two axially disposed opposite outlets adapted to exhaust respectively said central and peripheral streams and communicating accordingly with said vent and a hot air evacuating passage disposed within said mandrel, said tube being enclosed within an elongate housing attached to said mandrel, said housing being axially disposed within said parison and defining a chamber having said vent and connected to said supply and evacuating passages of the mandrel.