Elastic effects and extrudate distortions in capillary flow of molten polyethylene resins

During the flow of polymer melts, distortions of the emerging stream begin at well defined critical shear conditions and increase in severity at higher shear. End‐corrected critical shear stresses in the capillary extrusion of molten polyethylene are independent of the length/radius ratio of the capillary but increase somewhat with rising temperature. The recoverable shear strain reaches its maximum value at the critical conditions, where it was calculated as the elastic component of the end effects and by applying Hooke's law in shear. Critical recoverable shear strain as well as the derived quantities shear modulus and normal force increase with increasing temperature. Linear polyethylene has higher values for these three parameters than two branched resins. Relaxation times calculated as the ratio viscosity/shear modulus agree rather well with values determined by other means.