Influence of microstructure evolution on mechanical strength of blown poly(ethylene terephthalate)

Evolution of the microstructure in poly(ethylene terephthalate) during the blowing process has an important influence on final mechanical properties. In order to simulate the blow moulding process it is necessary to take into account this evolution (molecular orientation, crystallisation, etc.). In the present paper, experimental results on free blown bottles are presented. These results are related to geometrical characteristics, mechanical characteristics, and physical properties. Global shape and thickness were measured to estimate strain distribution during the process. Axial and circumferential specimens were cut from several points on the bottle. Tensile tests were carried out to determine the distribution of mechanical properties. The crystallisation rate was measured on a small specimen cut from the bottle in order to correlate mechanical properties with thickness. This work is the first step in a more global study of the blow moulding process of injected poly(ethylene terephthalate) preforms. A numerical simulation of free blowing is presented, using classical viscous behaviour law. This simulation clearly shows that the evolution of the microstructure is important in modelling the behaviour of poly(ethylene terephthalate). An initial model is proposed that takes into account the evolution of crystallisation rate.