Influence of the viscosity of polymer melts on the coextrusion process based on wall slip conditions

Abstract Three-dimensional viscoelastic flow numerical models for the polymer coextrusion process were established on the basis of two different wall conditions and solved via finite element methods. The influence of polymer melt viscosity on the coextrusion process was investigated by analyzing die swell, melt interlayer interface stability and flow field distribution under two different wall slip conditions, namely, sticky adhesive condition and fully slip non adhesive condition. The results indicate that in the traditional coextrusion process with sticky adhesive condition, the die swell, interface stability and flow field distribution change with the melt viscosity, but the effects of the shell melt viscosity differ from those of the core melt viscosity. The die swell, interface offset degree and radial flow strength (orthogonal to the flow direction) increase with increasing shell melt viscosity and decrease with increasing core melt viscosity. The die pressure drop, shear rate and first normal stress difference increase with increasing shell or core melt viscosity, but the influence of the shell melt viscosity is much greater than that of the core melt viscosity. In the gas-assisted coextrusion process with fully slip non adhesive condition, the die swell, interface stability and flow field distribution are not affected by the viscosity of both polymer melts which means that the flow behavior in the coextrusion channels is independent of the polymer viscosity. There is no die swell or interface deviation phenomenon, nor any melt radial flow. The die pressure drop, shear rate and first normal stress difference are close to zero, except in the close vicinity of the die inlet. This approach not only ensures product quality but also solves the problem of material selection for polymer coextruded products. In addition, the goal of energy conservation and consumption reduction has been achieved, and the energy savings and consumption reduction effects are particularly significant for high-viscosity melts.