Developments in the analysis of steady screw extrusion of polymers

Surging in screw extruders

A comprehensive study of polyethylene extrusion was carried out with an instrumented extruder on several screws differing in geometry and paralleled with capillary rheometry of the extrudate leading to melt viscosity and elasticity functions. Assuming Oldroyd's constitutive equation for molten polyethylene and the extrudate melt temperature oscillation (ΔTm) as a measure of the process stability and an indication of melt mixing quality, it was found that the product of the dimensionless numbers (Deborah, De and Brinkman, Br) correlates well (r = 0.93) with ΔTm. Since the Br and De groups follow from the rheological characteristics of the melt and the extrusion process parameters, a relationship of type: ΔTm = a + b (De) (Br) appears useful in designing an extrusion process (for a polymer of known rheology) with respect to its stability, and indirectly, its mixing efficiency.

Evaluating screw performance: Extrusion process stability and the degree of mixing from melt rheology data and from measurement on an instrumented extruder

A simplified model is used for calculating the time-dependent velocity of polymeric fluid in an extruder. The flow properties of the fluid are characterized by a simple constitutive equation based on two parameters: a constant viscosity μ and a constant elasticity modulus G. It was found that the transient velocity fluctuates periodically, and the time tt needed to restore the steady-state velocity from a disturbance varies with the ratio G/μ and the dimensionless group ρH2G/μ2, where ρ is the density of the fluid and H is the screw depth of the extruder.

R. D. Biggs

Papers

Transient behavior of viscoelastic fluid in an extruder