Abstract: Methods are provided for processing expandable thermoplastic materials with a typical compression relief design extruder screw having a helical flight for advancing and working a thermoplastic material. The screw is modified to shorten the feed and compression sections and to include a plurality of force-producing components in at least the compression section of the screw such that at least some portion of the components in any one turn of the flight lie in one plane essentially normal to the axis of rotation of the screw and such that the walls of the flight are uninterrupted. This arrangement controls the location of the rupturing or fragmenting of what is referred to as the solid bed and controls the manner in which it is fragmented to provide an extrudate at a die end of the extruder which has a constant temperature and which is uniform throughout each successive section of the thermoplastic material.

Abstract: A typical compression relief design extruder screw having a helical flight for advancing and working a thermoplastic material is modified to shorten the feed and compression sections and to include a plurality of force-producing components in at least the compression section of the screw such that at least some portion of the components in any one turn of the flight lie in one plane essentially normal to the axis of rotation of the screw and such that the walls of the flight are uninterrupted. This arrangement controls the location of the rupturing or fragmenting of what is referred to as the solid bed and controls the manner in which it is fragmented to provide an extrudate at a die end of the extruder which has a constant temperature and and which is uniform throughout each successive section of the thermoplastic material.

Abstract: The capacitance and diameter-over-dielectric of successive sections of a conductor having a layer of cellular plastic insulation extruded thereon are monitored continuously. The monitoring provides a continuous indication of the capacitance and the diameter with respect to the percent expansion of the cellular plastic insulation and the weight of the insulation per unit length of the conductor. This facilitates the regulation of process variables to maintain the capacitance and diameter-over-dielectric within acceptable ranges of values.

Abstract: The capacitance and diameter-over-dielectric of successive sections of a cellular plastic material are monitored continuously. The monitoring provides a continuous indication of the capacitance and the diameter with respect to the percent expansion of the cellular plastic material and the weight of the material per unit length of the conductor. This facilitates the regulation of process variables to maintain the capacitance and diameter-over-dielectric within acceptable ranges of values.

Abstract: 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.