New insights on the layer uniformity of multilayered films produced by coextrusion with interfacial surface generators

Polymer coextrusion is a manufacturing process whereby at least two polymer melts feed a die to form an extrudate made up of different layers, typically up to 12, to achieve a better product performance. Extruded films with hundreds or thousands of layers can be produced by using sequences of layer multipliers, also known as interfacial surface generators (ISG), at the die inlet. Each ISG splits, stacks, and merges the flow by means of various geometrical channel transformations (GT), in such a way that the number of layers at the outlet is doubled. However, in practice, the thicknesses of the individual layers generated after each transformation are often non-uniform. This work deals with the numerical simulation of multilayer co-extrusion of Newtonian polymer melts, aiming at providing new insights on the influence of the ISG geometry on the layer uniformity. It was found that depending on the combination of GT taking place, uneven layers may form due to the development of unsymmetrical velocity profiles along the direction perpendicular to the fluid interface. From this information, improved ISG dies, capable of producing coextruded films with uniform layer distribution, are proposed and assessed numerically.