Optimization by the C-NEM Method of the Stretch-Blow Molding Process of a PET Bottle Near T g

Abstract

The stretch-blow molding process of poly(ethylene terephthalate) bottles generates some important modifications in the mechanical properties of the material. Considering, the range of temperature (T > T g ) that is usually used, the material has a very high viscosity and shows a strain hardening effect linked to the microstructure evolution. A simple visco-plastic model had been identified from experimental results of uniaxial and biaxial tensile tests [1]. In the present work, we use this model and a numerical technique to simulate the inflation of a preform under an internal pressure and also submitted to the elongation of a stretch rod. The finite elements method has a poor efficiency in the stretch-blow molding process because the final material strain is up to 300%. This strain level generates strong element distortions and necessitates to often re-mesh. In order to carry out simulations with strain higher than 300%, the constrained natural elements method (C-NEM [2]) is used. In fact, the C-NEM method allows simulation with high strain level without re-meshing. The original “mesh” can be use from the beginning to the end of the simulation. So the properties of the material can be recorded in each node, without loss of information. In a first approach C-NEM are applied to simulate and optimize stretch-blowing of a PET preform by using axis symmetric assumption, the mold is not taking into account.