Polymer Coextrusion Flow Modelling

Coextrusion processes involve the simultaneous flow of several polymers with different characteristics, within the same tool, to obtain a multilayer laminate combining the qualities of each of the constituent polymers (impact resistance, scratch resistance, aesthetic appearance, barrier properties), while achieving good adhesion between the different layers. The first requirement is to obtain a multilayer product at the end of the die, with each layer of uniform thickness. This will depend on the die geometry, the rheology of the different polymers and their processing temperature. The second requirement is to ensure good adhesion between the polymers, while avoiding the appearance of instabilities between the different layers. Modeling these coextrusion flows is an effective tool for predicting thickness distributions at the die exit, as well as the velocity and stress fields inside the die that are at the origin of interfacial instabilities. Following a description of the different types of coextruded polymer products and the dies used to produce them, we propose models of increasing complexity, first applied to multi-channel dies (fed by different single-material extrusion dies) for which the actual coextrusion flow only occurs over a short distance in a simple geometry. The challenge is then to predict the shape and establishment time of velocity and temperature fields for Newtonian and pseudoplastic rheological behaviors, in isothermal and non-isothermal situations. We then turn our attention to the case of "coextrusion box" dies, in which the various polymers are assembled upstream of a die with a complex geometry. The challenge is to predict not only the velocity, stress and temperature fields, but also the distribution of the various polymers in the die and at its outlet. These co-extrusion situations are illustrated by concrete applications.