The Effect of Melt Elasticity on Extrusion and Other Melt Processing Operations

Abstract

It has long been known that grossly irregular extrudate is obtained at excessively high extrusion pressures, especially at low melt temperatures when the viscosity is very high. The gross irregularities suggest that an elastic effect is superimposed on the viscous effect. The stress history of the emerging extrudate must have involved a major flow disturbance— departure from laminar flow, turbulence, ‘melt fracture’—and the die channel was too short to give the melt sufficient dwell time to enable it to relax and recover and to re-establish fully laminar flow. Tordella¹ has pointed out that the site of major disturbance must be the die inlet region, where a fat and sluggish flow is converted to a thin and fast flow. Kendall² studied the effect of the die inlet taper angle on the degree of distortion and found that the elastic stresses in the melt and the severity of the extrudate defects were much reduced when the taper was gentle. It was also possible to estimate the relaxation time of the melt, taking it as the minimum dwell time in the die which is necessary to eliminate the elastic memory and obtain good extrudate.