Extruding plastic pipe from eccentric dies

Abstract

To solidify plastic pipe, the pipe is transported through a long cooling chamber. Inside this chamber, inside the pipe, the plastic remains molten, and this inner surface solidifies last. The flow due to the self-weight of the molten plastic then causes the product to thicken on bottom (and to thin on top). This is why plastic pipe is normally extruded from an eccentric die, and specifically, from a die where the mandrel (also called centerpiece (Kolitawong and Giacomin, 2001) or core (Jones, 1964)) is decentered downward. This paper focuses on the consequences of this decentering in eccentric cylindrical coordinates. Specifically, when the molten polymer is viscoelastic, as is normally the case, a downward lateral force, Fx, is exerted on the mandrel. The die eccentricity also affects the positive axial force on the mandrel, Fz. These forces govern how rigidly the mandrel must be attached (normally, on a spider die, by eight bolts). We use the method of Jones (1964), called polymer process partitioning, designed for the Oldroyd 8-constant constitutive model. We produce a method for estimating Fx and Fz. We also obtain an expression for the shape of the extruded pipe, whose thickness scales with average velocity at each angular positions 〈vz⌣〉θ, by integrating the velocity profile, vz⌣(ξ,θ) through the thickness of the eccentric annulus (with respect to ξ). We further include expressions for the stresses in the extruded polymer melt. These expressions can be used to estimate the upper-bound for the stress that is frozen into the outermost layer of the plastic pipe (since this layer is quenched first, shortly after extrusion). We include detailed dimensional worked examples to help process engineers with their pipe die designs.