Wire-Coating Extrusion

The process of coating a wire with a polymeric material is basically an extrusion operation in which either the molten polymer is extruded continuously over an axially moving wire or the wire is pulled through the extruded molten polymer. As schematically shown in Figure 5.1, the typical wire-coating unit consists of a pay-off device, a wire preheater, an extruder equipped with a cross-head die, a cooling trough, and a take-off device. Various control and measuring instruments are utilized in the commercial line (Griff 1962). The two basic wire-coating dies are pressure-type dies and tubing-type dies, as shown schematically in Figure 5.2. The tubing-type dies are annular in cross-section. The flow geometry outside the tubing die is important from the point of view of obtaining a coating with better mechanical and electrical properties and surface smoothness. However, little effort has been spent on studying this particular aspect of the process. The pressure-type wire-coating die is an annulus, the side surface of which is the wire to be coated, moving at a constant speed. The flow through this type of die is analogous to the flow through an annulus formed by coaxial cylinders with the inner cylinder moving in the axial direction. In the past, analysis of wire-coating extrusion for pressure-type die has been carried out for Newtonian and power-law fluids (Bagley and Storey 1963; Bernhardt 1962; Carley et al. 1979; Han and Rao 1978; McKelvey 1963). Like in the film coextrusion process presented in Chapter 9, in wire-coating coextrusion two different polymers may be concentrically coated on the wire in a single step (LeNir 1974). Tough abrasive-resistant nylon, for example, can be coated over a much less expensive polyethylene core, or one can have a thin coat of color compound over unpigmented insulator, thus taking advantage of the different properties of two components at a reduced cost. Considerable savings in the cost of processing can be achieved by applying two coats in a single step.