Biaxial deformation of PET at conditions applicable to stretch blow moulding and the subsequent effect on mechanical properties

• Main Author: Tan, Cheong Wang
• Published: Queen's University Belfast 2008
• Subjects: 668.9
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.486212

The injection stretch blow moulding (ISBM) process is the main method used to mass-produce polyethylene terephthalate (PET) bottles for the carbonated soft drink and the mineral water industries. At present researchers have been developing numerical simulations of the process with the aim of gaining a better understanding of the process and ultimately optimize the design of new containers more efficiently. However, one of the drawbacks for these simulations is the lack of knowledge on the biaxial stress-strain behaviour of PET at processing conditions relevant to ISBM and the effect of biaxial stretching on the final properties. The main objective ofthis study was to understand and.characterize the biaxial behaviour of PET and quantify how biaxial stretching under different processing conditions influenced the final mechanical and structural properties. The results were. implemented within a numerical simulation with the aim of accurately predicting both the thickness and modulus distribution in the final container. Initially, a comprehensive stu:dy of the biaxial defonnation behaviour of three different grades of PET was carried out at conditions applicable to the ISBM process using the Queen's Biaxial Stretcher. The stress strain data generated from these biaxial tests was used to generate material constants for the Buckley-Jones-Adams material model. A new procedure for generating the constants was developed using the mathematical package MATLAB®. This semi-automated procedure was found more efficient than the original manual procedure by Buckley et al. Post-stretching analysis was carried out by using tensile and DSC methods. The modulus development of PET was found to be strongly dependent on the stretching temperature, strain rate, stretch ratio, and defonnation mode. Among those parameters, stretch ratio and defonnation mode play the most important role. A two dimensional ISBM simulation of a 1 litre PET bottle was developed for the prediction of thickness and modulus distributions. The comparison of the numerical results with the experimental measurement showed a satisfactory prediction ofbottle thickness and modulus in the side wall region.