Wire coating and melt elasticity

Abstract

In modern wire coating, the polymer is dragged through a round cylindrical die. Onto this drag flow, we superpose pressure-driven extrusion. We devote this paper to analyzing this extrusion in eccentric cylindrical coordinates. We find that, when the molten polymer is an elastic liquid, a recentring force, Fx, is exerted on the wire. This is how the wire is then coated concentrically. The lateral force acting on the wire thus matters. This also explains why the wire cannot be coated with Newtonian or nearly Newtonian polymer. The axial force on the wire, Fz, is always positive, and we find that the die eccentricity decreases Fz. This determines the required pulling force. Thus, the axial force acting on the wire also matters. We follow the method of Jones (1964) called polymer process partitioning, to obtain the coating velocity profile, v⌣z(ξ,θ), from which we get the coating thickness profile. We integrate this profile to get the flow rate, and thus, the average thickness. We also obtain the stresses in the extrudate. We include one detailed dimensional worked example to help engineers design coating dies.