James Nixon

A novel methodology to characterize the constitutive behaviour of polyethylene terephthalate for the stretch blow moulding process

Abstract The stretch blow moulding (SBM) process is the main method for the mass production of PET containers. And understanding the constitutive behaviour of PET during this process is critical for designing the optimum product and process. However due to its nonlinear viscoelastic behaviour, the behaviour of PET is highly sensitive to its thermomechanical history making the task of modelling its constitutive behaviour complex. This means that the constitutive model will be useful only if it is known to be valid under the actual conditions of interest to the SBM process. The aim of this work was to develop a new material characterization method providing new data for the deformation behaviour of PET relevant to the SBM process. In order to achieve this goal, a reliable and robust characterization method was developed based on an instrumented stretch rod and a digital image correlation system to determine the stress-strain relationship of material in deforming preforms during free stretch-blow tests. The effect of preform temperature and air mass flow rate on the deformation behaviour of PET was also investigated.

Analysis and Simulation of the Free-Stretch-Blow Process of PET

This paper is concerned with understanding the behaviour of Polyethylene Terephthalate (PET) in the injection stretch blow moulding (ISBM) process where it is typically bi-axially stretched to form bottles for the packaging industry. Preforms which have been pre sprayed with a pattern and heated in an oil bath have been stretched and blown in free air using a lab scale ISBM machine whilst being monitored via high speed video. The images have subsequently been analysed using a digital image correlation system (VIC 3D). The results have been used to validate appropriate simulations of the free-blow process using ABAQUS®/Explicit FEA with a suitable viscoelastic material model, along with experimental process data obtained using an instrumented stretch rod.

Processing - property relations from biaxial deformation of PET (polyethylene terephthalate)

A numerical simulation with correlation between the stretch blow moulding process parameters and the eventual room temperature mechanical properties of the formed PET (Polyethylene Terephthalate) bottle enables the bottle manufacturers to optimize the design of the preform and the mechanical performance of the bottle. In this study PET samples were processed at conditions relevant to stretch blow moulding process and subsequently Young’s moduli at room temperature were determined from the uniaxial tensile tests. A correlation was established between the network stretch of the Buckley material model and the Young’s modulus.

Determining Volumetric Strain in Biaxial Deformation of PET at Temperatures and Strain Rates for Stretch Blow Moulding

For materials formed in the rubbery region eg. PET in the stretch blow moulding process, the normal assumption is that the material is incompressible. In this paper the validity of this assumption is challenged by conducting a series of experiments that measure the volumetric strain under different strain and temperature histories. Experiments have been conducted on a biaxial testing machine instrumented with lasers for measuring the strain through the thickness in combination with and digital image correlation for measuring the in plane strain for PET stretched in uniaxial and biaxial deformation between 90°C and 100°C. Results will be presented that show that the Poisson’s ratio for PET can vary between 0.4 and 0.55 depending on the test conditions. It is concluded that the values measured greater than 0.5 are due to the strain induced crystallinity that occurs with PET during the biaxial deformation process.______________________________________________________________________