Modeling Polymer Flow Instabilities

Abstract

Since about 40 years it is known that polymer melts passing through a die may exhibit flow instabilities. This is a serious problem in chemical industry, because it distorts the extrudate and therefore limits production rates. The instabilities are most conveniently described in terms of the simple experimental set-up sketched in Fig.1. The polymer melt in the barrel is kept moving by means of a plunger with constant speed making the inlet flow rate Q _in constant. The pressure P(t) is measured in the barrel. The outlet flux Q _out is observed, but its exact value is hard to measure. For low plunger speeds, and thus small Q _in values, P(t) and Q _out (t) appear to be constant in time, and Q _in = Q _out . If the plunger speed is gradually increased, the so-called shark-skin phenomenon becomes more and more appreciable. This is a more or less regular pattern of ridges at the surface of the extrudate, perpendicular to the flow direction. Typical values for the period and the amplitude observed in the pressure signal are 0.1s and 10⁵ Pa, respectively. When Q _in reaches a higher, critical level, both P(t) and Q _out (t) start to exhibit large amplitude oscillations. This ‘volume’ flow instability is referred to as the spurt effect giving rise to an alternating smooth and distorted extrudate. Typical values for the period and the amplitude in the pressure signal are now 10s and 10⁷ Pa, respectively. The corresponding measured P(t) signals show a typical saw-tooth behaviour (Fig.2).

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