Interfacial instabilities in slit coextrusion dies: Experimental and simulation research
Phil Coates, Michael Martyn, Martin Zatloukal, Peter Saha
University of Bradford
UK

Keywords: coextrusion, geometry, instability


Experimental studies and simulation results are presented for two LDPE melts using a manifold type slit coextrusion flow visualization cell with 30° and 90° stream angles. Digital image processing has shown wave type extrudate instabilities have the same periodicity as perturbation in the flow field in the die land. Experimental analysis suggests that pronounced wave instabilities are caused by the minor layer break at the confluent region of the melt streams where extensional viscosity is a driving parameter. Simulation is performed by FEM analysis with full u-v-p-t numerical scheme employing viscoelastic constitutive equations. The predicted pressure, stresses, velocities and interface location have been found to be in a good agreement with the measurements. A recently proposed “TNSD sign criterion” has been successfully tested for the prediction of the onset of wave interfacial instabilities.