Another common variation is cross-head extrusion. In this process the flow of plastic is typically altered for permitting solid material, like fiberglass strands or copper wire to feed into the melt flow, and thus become part of the extrusion. Cross head extrusion is usually used when reinforcements are not able to pass through the machine's barrel and screw.


Crossshead extrusion allows to coat plastic over steel cable, jacket multiple tubes to even wires. The basic equipment required includes an extruder fitted with suitably designed cross-head, a pay-off (or unwinding) system, a brake or tension control device, a water bath for cooling, a capstan or variable speed haul-off and an automatic wind-up or coiling system. There are generally two types of crosshead extrusion process. They are covered in the next section. Types of Crosshead Extrusion
The cross-head extrusion along with the die arrangement can be any of the two types:

• Cross-head with pressure (packing) die
  In case of Pressure Die extrusion, the extruded material comes in contact with the core for example hose, wire etc., within the die itself. The melt when applied pressure tries to penetrate the openings in the core material. This technique is ideal in cases where:
  • There is requirement of good adhesion.
  • Requirement of a smooth, regular, outer cover diameter over an rough or irregular inner core material.
  
  The following image is of typical pressure die arrangement:
  
  
  
• Cross-head with tubing (sleeving) die
  In this technique the material is extruded in the form of tube. It is “drawn down” for meeting the surface of the core material. Assistance of a vacuum is imperative in this process. Vacuum is applied inside of a torpedo and it functions through the bore of the mandrel (pin). A typical Tubing Die (sleeving) extrusion is shown in the following image:
  The advantages of tubing die extrusion are:
  • Improved control of cover wall thickness.
  • Simple to strip cover from core (for example wire coating application).