Strain induced crystallization of PET under biaxial conditions. From laboratory tests to injection stretch-blow molding

Abstract

Conventional biaxial testing and free stretch blow molding (ISBM) of preforms were conducted in parallel to analyze the biaxial behavior of PET and the resulting strain-induced crystallization (SIC) under bi-axial conditions. Stretch blowing has promoted complex loading paths combining most of the simple paths used at the laboratory scale, which are analyzed locally using marked preforms. The time-temperature superposition principle is shown to apply under biaxial conditions up to high strain, which has facilitated the definition of experimental sets and mechanical comparisons. X-ray diffraction was used to analyze the crystal microstructure and texture. In ISBM, the texture is not a simple image of the final deformation state in terms of orientation. The organized phase that is induced is probably an imperfect crystal capable of evolving in time and the strain hardening under biaxial conditions could be a gradual phenomenon as in uniaxial stress. A scenario is proposed.