An extrusion system for covering a flexible conductor (14) with a crystalline thermoplastic elastomer insulating material by tubing the plastic onto the conductor includes a core tube (61) which is mounted in an extruder crosshead (41) such that its free end is positioned within the interior of the extruder die (59). The free end of the core tube is spaced from the die opening a predetermined percentage of the land length of the die. In this arrangement, which has been found to be suitable for a crystalline thermoplastic elastomer having a crystallization initiation temperature which is capable of being substantially near to its melting point temperature, the crosshead and the core tube are arranged to provide a streamlined flow path of the insulating material toward the die opening. Air at a higher pressure than has been used in the past is caused to flow through the core tube to cause the extrudate to balloon outwardly from the conductor for a length of travel to permit adequate crystallization of the extrudate so that when it engages the conductor, it will compress any upstanding metallic burrs. This arrangement substantially inhibits the adherence to and the accumulation of any plastic material on the outer surface of the core tube which would affect the concentric disposition of the insulation cover (18) about the conductor.