Measurement and modelling of high density polyethylene melt extrudate swell

This paper describes experimental and computational modelling of the extrudate swell behaviour in two grades of high density polyethylene, one with long chain branching and one linear, and for comparison, a low-density polyethylene melt. Results from the experiments were compared with models using the Pom-Pom and Roliepoly constitutive equations implemented in a Lagrangrian finite element solver. To obtain good agreement it was found necessary to include the temperature changes in the extrudate (convection and skin forming), and surface tension, as well as the elasticity of the melt. The Roliepoly model (which is proposed as a model for linear polymer melts) performed better at modelling the linear polymer, however the Pom-Pom model (which is designed for branched polymers) also produced a good result suggesting that the decoupling of orientation and stretch degrees of freedom, which is implicit in the model, is true of all branched polymers.