Method of and apparatus for extruding concentric plastic sheaths

First and second concentric insulating sheaths of plastic materials are formed continuously around a filamentary core advancing longitudinally through an extrusion chamber. The core is passed axially through a tapered entrance portion and a coaxially aligned tapered throat portion formed in a separable die disposed in the chamber. Fluent plastic material for the first sheath is delivered from a main extrusion bore, impinging on the core circumferentially at the entrance portion of the die, the flow thereof being developed as it passes into the throat portion thereof. Fluent plastic material for the second sheath is delivered from an auxiliary extrusion bore flowing through a plurality of dividing channels formed in the die and radially inward at a plurality of circumferentially spaced points around the die axis into an annular manifold or reservoir where the fluid pressure of the plastic achieves a steady state condition. The plastic is forced from the manifold circumferentially, flowing in a frustoconical channel formed in the die which terminates in an annular fluid inlet formed in the throat portion of the die to impinge circumferentially around the first sheath, forming both sheaths simultaneously around the core.