Process for the production of highly expanded polyolefin insulated wires and cables

In a process for producing an expanded polyolefin insulated cable of excellent high frequency characteristics comprising the steps of feeding a polyolefin, a nucleating agent and a blowing agent to an extruder wherein the blowing agent is fed into the polyolefin as in a conventional swelling process or a conventional injection process and then heat extruding the resulting mixture through the extruder onto an inner conductor passing through the extruder to cover the outer periphery of the inner conductor, thereby forming an insulation layer of an expanded polyolefin foam, the improvement wherein said expanded polyolefin foam is highly expanded and which includes the steps of preheating the inner conductor and uniformly applying a polyolefin adhesive layer having a film thickness of less than 0.3 mm directly onto the inner conductor which is a metallic conductor by passing said inner conductor through a metal squeeze die which is separate from the extruder and heated to a temperature greater than the heat flow temperature of said polyolefin adhesive, and which contains therein the polyolefin adhesive in a heat flowable condition, to thereby increase the adhesive strength between the highly expanded polyolefin insulation layer and the inner conductor prior to forming the highly expanded polyolefin insulation layer having a foaming ratio in excess of 2.5 times (60% expansion degree) on the adhesive layer on the inner conductor and thereafter heat extruding the foaming polyolefin mixture which contains a volatile liquid as a blowing agent on the adhesive layer on the inner conductor, the feeding of the volatile liquid being prior to extrusion wherein the polyolefin of the expanded insulation layer consists essentially of a material which is selected from the group consisting of low density polyethylene, medium density polyethylene, high density polyethylene, polypropylene, and mixtures thereof and wherein said squeeze die has a fixed diameter or has a smooth inner surface and comprises an inlet section, for receiving said inner conductor, of a first diameter and an outlet section for exit of said inner conductor, of a second diameter, said first diameter being greater than said second diameter and said squeeze die being shaped like a truncated cone.