Abstract: An extruder screw having a root, a helical primary flight on the root and a helical barrier flight on the root and forming with the primary flight a solids channel and a melt channel. The width of the solids channel gradually decreases and the width of the melt channel gradually increases along an upstream zone of the melting section. The width of the channels gradually changes in a downstream zone of the melting section but substantially less than in the upstream zone.

Abstract: A crosshead for extruding an annular layer of thermoplastic material about a central longitudinal axis. The crosshead comprises a die having an annular slotted die opening surrounding the axis, a transverse inlet passageway for receiving molten thermoplastic material, an annular cavity connected to the inlet passageway, an annular distribution passage connected to the annular cavity and an annular connecting passage extending from the annular distribution passage to the die opening. The annular cavity and annular distribution passage each extend radially with respect to the longitudinal axis. Two or more layers of thermoplastic material are extruded by the crosshead when it is equipped with two or more dies.