Extrusion Coating Process for Wire and Cable Jacketing

The principle is to fully surround a continuous substrate, like a wire or cable, with a cover of polymeric material. Overjacketing extrusion is a coating process in which individual bare wires or bundles of precoated wires are coated with a layer of insulating polymer. A wide variety of plastics may be used and that can be tailor-made for the cable. For many applications, such as insulated cables, the plastic should be a good insulator, flexible, and impact resistant. In this process, as shown in Fig. 13.13, a wire (or bundle of wires) is preheated to above the glass transition or melting temperature of the polymer coating that is to be applied. This is to ensure good adhesion of the new coating.

Next, this preheated bare wire is pulled very rapidly through the middle of the die and places a thin coating of polymer around the wire. Due to the geometry of the dies used, relatively high extrusion rates are possible while still avoiding melt fracture. The newly coated wire is then drawn through an air or gas flame to smooth the surface of the coating, and finally a water bath to fully cool the coated wire. Coated wires are now spooled to prepare for further processing if desired. The equipment includes substrate payoff (input) system, substrate preheater, extruder, cooling device, and take-up system. The tooling is a wire coating with extrusion covering die mounted with extrusion axis perpendicular to the extruder axis. The materials include PVC and polyethylene which are most widely used. Nylon, fluoropolymers, and cellulosics are used for wire insulation or where transparency is desirable. The products include wire and cable coating, sheathing, and jacketing, single conductor for wire or multiple conductors for cable covering, flat ribbon multiconductor cables for appliance cords, TV antenna cables, and decorative trim elements with polished brass, Al, or chrome-plated substrates in a protective, transparent cover for plastic bag ties.

Fig. 13.13