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Experimental characterisation and modelling of polyethylene terephthalate preform for injection stretch blow moulding

Polyethylene terephthalate (PET) is a widely used polymer in the production of bottles by injection stretch blow moulding (ISBM). In this work, we present a characterisation method to identify material properties directly from a preform, considering temperature and stress-relaxation effects related to its viscoelastic response. A customised oven and gripping system were designed to perform uniaxial tests in a proper temperature range on tubular specimens obtained from preforms. A visco-hyperelastic model is then proposed: a Marlow-type strain energy function coupled with a Prony series and William-Landel-Ferry equation to include time and temperature dependency. Finally, a case study of ISBM process is implemented in a finite element code considering this constitutive model. Strain maps and predicted thickness of the bottle wall were evaluated as process quality indicators. Simulation results showed good agreement with measurements on the real processed bottle, confirming the usefulness of the approach for product or process parameters optimisation.