Flow shear stress in the co-extrusion of polymer multilayer film: variations and effects on interfacial instability

Multilayer co-extrusion combines two or more layers of materials in a single extrusion process. To improve the quality of multilayered film products fabricated by co-extruding two polymers with different rheological properties, suppressing interfacial instability is vital. This study designed and fabricated a mould with layer-multiplying sets to increase the number of layers in co-extruded polymer multilayer films. Flow pressure drops were measured by two pressure sensors set sequentially on the mould wall along the flow direction. Pressure drops in the flow were measured to calculate flow shear stress. Variations of shear stress are then used to analyse interface slipping. Experiments were performed using polypropylene (PP) and polycaprolactam (PA ₆ ). Extrusion rate was experimentally increased to raise the pressure within the flow. The experiments showed that, when shear stress reaches a certain threshold, a large number of interfaces causes interfacial slip, which leads to a decline in shear stress. Increasing the number of interfaces increases interfacial slip. A large interfacial slip results in much lower shear stress on the mould wall.