Modelling the constitutive behaviour of poly(ethylene terephthalate) for the stretch blow moulding process

Student thesis: Doctoral ThesisDoctor of Philosophy


The stretch blow moulding (SBM) process is the main method for the mass production of PET containers. Understanding and modelling the behaviour of amorphous PET (aPET) is critical for design the optimum product and process.

However due to its nonlinear viscoelastic behaviour, the behaviour of aPET is highly sensitive to its thermomechanical history making the task of modelling its constitutive behaviour very complex. This means that the constitutive model will be useful only if it is known to be valid under the actual conditions of the SBM process of interest. The aim of this work was to develop a material model capable of capturing the deformation behaviour of aPET subject to an arbitrary strain history, and to develop a new material characterization method providing new data for the deformation behaviour of aPET relevant to the SBM process.

In order to achieve this goal, a reliable, robust and non-touch characterization method was developed based on the data acquisition system and digital image correlation system to determine the stress-strain relationship of material in deforming preforms during free stretch-blow tests.

Considerable experimental work, including conventional biaxial stretching tests and free stretch-blow tests, was carried out to characterize the mechanical properties of the material. These tests revealed the properties of aPET under displacement-controlled deformation and load-controlled deformation.

The material model was developed based on the original Buckley model in two aspects. One was the development of the 'lock-up' process in the network system, representing the onset of strain-induced crystallization (SIC). Another was the consideration of self-heating effect of aPET during the deformation.

A finite element model which included the validated process parameters and updated material model was created to mimic the free stretch-blow process. The results from the experiments were used to provide accurate input and. validation data to it. The results show good quantitative predictions for free stretch blow simulations where the mass flow rate is high and the main mode of deformation is simultaneous biaxial and qualitative agreement with experiments when the mass flow rate is low and the main mode of deformation is sequential deformation.
Date of AwardJul 2014
Original languageEnglish
Awarding Institution
  • Queen's University Belfast
SupervisorGary Menary (Supervisor)

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