Studies on multilayer film coextrusion I. The rheology of flat film coextrusion

Abstract

Abstract Multilayer flat film coextrusion was studied, both experimentally and theoretically. For the experimental study, a sheet‐forming die with a feedblock was designed, and plastic films of three and five layers were coextruded. The die was provided with three pressure transducers in the axial direction in order to determine the pressure gradient in the die, allowing the determination of the reduction in pressure drop when different combinations of two polymer melts were coextruded. Polymers used for coextrusion were: (1) low density polyethylene and ethylene‐vinyl acetate; (2) low density‐polyethylene and high density polyethylene; (3) low density polyethylene and polystyrene. For the theoretical study, the z ‐component of the equations of motion for steady fully‐developed flow were solved using a power law non‐Newtonian model, Comparisons were made between the experimental and the theoretically predicted volumetric flow rates. Predictions of the velocity distributions, shear rate profiles, and shear stress distributions were made as functions of the processing conditions and the rheological properties of the individual polymers concerned.