Measurement of Interfacial Instabilities During Coextrusion of LDPE’s in Flat Dies, 2003 PLACE Conference & GLOBAL HOT MELT Symposium

Coextrusion has been used in the extrusion process for the last four decades. At the same time, it has been studied to understand better the material behavior [1,2,3]. Up until the experimental work of the mid 1990’s [4,5,6,7,8], it was not completely clear that there exist at least two types of instabilities in coextrusion. The thesis by Perdikoulias [7] has clearly shown that these two types of instabilities have not only different appearance but also a different origin. This work also distinguishes the so-called zig-zag instabilities that are related to the shear stress at the interface and the so-called wave instabilities related to the merge point of coextruded material. The appearance of the zig-zag instabilities has been linked to the shear stress at the interface and therefore to the shear viscosity. In the studies by Perdikoulias et al [7,8], the appearance of the wave instabilities has been linked to the material stretching. In several following publications by Zatloukal et al [9,10] this behavior has been studied more rigorously and it was shown that the effect could be linked to the extensional viscosity of the coextruded materials. The experimental work by Perdikoulias et al [7,8] was performed on a film-blowing coextrusion line. This means a multi-manifold, annular geometry. The aim of this present work is to study experimentally, coextrusion in a flat multi-manifold geometry. At first, we want to perform an experiment in a way that we coextrude two layers of the same material similarly to the previous work [7]. The temperature will be chosen to fit the cast film process conditions. The next step will be to study the role of the shear and elongational viscosity on the appearance of the coextrusion instabilities. At the end, the shear stresses at the interface will be calculated to determine the critical shear stress for the materials used in this study.