Polymer Processing Extrusion Instabilities and Methods for their Elimination or Minimisation

Abstract

This paper presents and reviews findings in relation to three key areas where polymer processing instabilities occur. The paper also describes methods that can be utilised to reduce, or eliminate, the particular instability. Using previously published results in each of the three areas and work presented in the paper, physical insight into the three mechanisms is reviewed and compared. Extrusion instabilities develop with increasing extrusion rate and the onset of extrusion instability is often a key limitation to the maximum output of an extrusion line. The sharkskin instability is an exit effect instability that can be modified by changing exit geometries and eliminated using certain additives. The stick-spurt instability is intimately related to wall boundary conditions which can be influenced by certain wall and polymer formulations. Finally volume instabilities occur in the entry region of a die and result in ahighly distorted product. The instabilities are related to viscoelastic effects within the die and can be minimised by appropriate die and polymer modification. The paper provides sufficient experimental background to identify the key physical aspects associated with each of the instabilities and this in turn provides insight into the different way each instability occurs and how they can be minimised.

Experimental evidence of a helical, supercritical instability in pipe flow of shear thinning fluids

“…Among these, an ubiquitous one, usually referred to as "melt fracture", results in a distortion of the extrudate into a more or less pronounced and regular helical shape. Visualization of the velocity and stress fields in the reservoir [3][4][5][6] has lent strong support to the idea that melt fracture was ascribable to the well documented destabilization of the elongational flow upstream of the sudden contraction at the entry of the die [4,7]. Due to viscous damping, such velocity oscillations are expected to decay over a finite length as the fluid is advected through the pipe.…”

“…Due to viscous damping, such velocity oscillations are expected to decay over a finite length as the fluid is advected through the pipe. Hence, it is not surprising that helical instabilities were reported by many authors as being more severe when using short dies, rather than pipes with larger length/radius (L/R) aspect ratios [3]. However, as pointed out by Larson [1], one cannot rule out that the flow in a long pipe may itself develop a bulk helical instability, in which case the amplitude of the extrudate distorsion should not decrease upon increasing the die aspect ratio.…”

“…Competition between pipe and contraction flow instabilities -In the field of extrusion instabilities, a touchstone is the broad phenomenology pertaining to the so-called "melt fracture" of polymer melts [3,5,25]. In their early review, Petrie and Denn [26] pointed out the apparent contradiction between two groups of experimental results dealing with either branched or linear polyethylene.…”

“…Then (b), upon increasing V , although the extrudate remains straight, a characteristic fine pattern appears on its surface. The pattern contrast gradually increases with V , indicating the growth of the surface distorsion until it shows up as a sawtooth deformation of the profile edges (c), unmistakably attributable to the "sharkskin" instability [1][2][3].…”

“…Extrusion, which consists in forcing viscoelastic polymer melts or solutions out of a reservoir through an axisymmetric pipe, or "die", is an important shaping process, unfortunately prone to a variety of instabilities which spoil the surface or the bulk of the extrudate [1][2][3]. Among these, an ubiquitous one, usually referred to as "melt fracture", results in a distortion of the extrudate into a more or less pronounced and regular helical shape.…”