The work presents both, simulation and experimental findings of the research of two LDPE melts using a flat coextrusion flow visualization cell. The simulation is performed by FEM analysis with full u-v-p-τ numerical scheme employing viscoelastic constitutive equations. The predicted stresses, velocities and interface location have been found to be in a good agreement with the measurements. The experimental analysis shows that pronounced wave instabilities are caused by the minor layer break at the merge point of the layers, and extensional viscosity is a driving parameter here. A recently proposed ‘TNSD sign criterion’ (Zatloukal et al, Int Polym Proc., 16(2) 198-207 2001), which quantifies the relative stretching of the layers in the merging area has been successfully tested for the prediction of the onset of wave interfacial instabilities in this type of the die geometry. Moreover, rheological and molecular characteristics (MWD) were experimentally determined for different batches of LDPE. The results show that extensional viscosity may significantly vary for different batches even if shear viscosities and MWDs are very similar. FEM analysis together with TNSD value calculation was consequently performed to determine the stability of the coextrusion flow in the flat die for different batches of the material and the effect was found considerable.
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