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.

Bioresorbable Vascular Scaffolds (BVS) manufactured from poly (l-lactic acid) (PLLA) offer an alternative to metal scaffolds for the treatment of coronary heart disease. One of the key steps in the manufacture of these scaffolds is the stretch blow moulding process where the PLLA is biaxially stretched above glass transition temperature (T g), inducing biaxial orientation and thus increasing ductility, strength and stiffness. To optimise the manufacture and performance of these scaffolds it is important to understand the influence of temperature and strain rate on the constitutive behaviour of PLLA in the blow moulding process. Experiments have been performed on samples of PLLA on a custom built biaxial stretch testing machine to replicate conditions typically experienced during blow moulding i.e. in a temperature range from 70°C to 100°C and at strain rates of 1 s −1 , 4 s −1 and 16 s −1 respectively. The data is subsequently used to calibrate a nonlinear viscoelastic material model to represent the deformation behaviour of PLLA in the blow moulding process. The results highlight the significance of temperature and strain rate on the yielding and strain hardening behaviour of PLLA and the ability of the selected model to capture it.

Injection stretch blow moulding is a wellestablished method of forming thin-walled containers and has been extensively researched for numerous years. This paper is concerned with validating the finite element analysis of the free-stretch-blow process in an effort to progress the development of injection stretch blow moulding of poly(ethylene terephthalate). Extensive data was obtained experimentally over a wide process window accounting for material temperature and air flow rate, while capturing cavity pressure, stretch-rod reaction force and preform surface strain. This data was then used to assess the accuracy of the correlating FE simulation constructed using ABAQUS/Explicit solver and an appropriate viscoelastic material subroutine. Results reveal that the simulation is able to give good quantitative correlation for conditions where the deformation was predominantly equal biaxial whilst qualitative correlation was achievable when the mode of deformation was predominantly sequential biaxial. Overall the simulation was able to pick up the general trends of how the pressure, reaction force, strain rate and strain vary with the variation in preform temperature and air flow rate. The knowledge gained from these analyses provides insight into the mechanisms of bottle formation, subsequently improving the blow moulding simulation and allowing for reduction in future development costs.

Processing tubes from poly (l-lactic acid) (PLLA) by stretch blow moulding (SBM) is used in the manufacture of bioresorbable vascular scaffolds (BVS) to improve their mechanical performance. To better understand this processing technique, a novel experimental setup by free stretch blow inside a water bath was developed to visualise the tube forming process and analyse the deformation behaviour. PLLA tubes were heated, stretched and blown with no mould present inside a temperature-controlled water bath whilst recording the processing parameters (axial force, inflation pressure). The onset of pressure activation relative to the axial stretch was controlled deliberately to produce a simultaneous (SIM) or sequential (SEQ) mode of deformation. Realtime images of the tube during forming were captured using high speed cameras and the surface strain of the patterned tube was extracted using digital image correlation (DIC). The deformation characteristics of PLLA tubes in SBM was quantified by analysis of shape evolution, strain history and stress-strain relationship.

The mechanical behavior of polyethylene terephthalate (PET) under the severe loading conditions of the injection stretch blow molding process is strongly dependent on strain rate, strain, and temperature. In this process, the PET near the glass‐transition temperature (Tg) highlights a strongly nonlinear elastic and viscous behavior. In the author's previous works, a nonlinear visco‐hyperelastic model has been build and identified from equi‐biaxial tensile experimental results. Despite the good agreement with biaxial test results, the model fails to reproduce the sequential biaxial test (with constant width during first step) and the shape evolution during the free blowing of preforms. In this work, an anisotropic version of this visco‐hyperelastic model is proposed and identified form both equi and constant width results. The new version of the nonlinear visco‐hyperelastic model is implemented into the Abaqus environment and used to simulate the free‐blowing process. The comparison with the experimental results managed in Queen's University of Belfast validates the approach in terms of bubble shape and thickness distribution. POLYM. ENG. SCI., 60:823–831, 2020. © 2020 Society of Plastics Engineers

The injection stretch blow moulding process involves the inflation and stretching of a hot preform into a mould to form bottles. A critical process variable and an essential input for process simulations is the rate of pressure increase within the preform during forming, which is regulated by an air flow restrictor valve. The paper describes a set of experiments for measuring the air flow rate within an industrial ISBM machine and the subsequent modelling of it with the FEA package AbaqusABAQUS. Two rigid containers were inserted into a Sidel SBO1 blow moulding machine and subjected to different supply pressures and air flow restrictor settings. The pressure and air temperature were recorded for each experiment enabling the mass flow rate of air to be determined along with an important machine characteristic known as the 'dead volume'. The experimental setup was simulated within the commercial FEA package AbaqusABAQUS/Explicit using a combination of structural, fluid and fluid link elements that idealize the air flowing through an orifice behaving as an ideal gas under isothermal conditions. Results between experiment and simulation are compared and show a good correlation.

This paper highlights for the first time a full comprehension of the deformation procedure during the injection stretch blow moulding (ISBM) process of poly(ethylene terephthalate) (PET) containers, namely thin-walled rigid bottles. The processes required to form PET bottles are complicated and extensive; any development in understanding the nature of material deformation can potentially improve the bottle optimisation process. Removing the bottle mould and performing free-stretch-blow (FSB) experiments revealed insight into the bottle forming characteristics at various preform temperatures and blowing rates. Process outputs cavity pressure and stretch-rod force were recorded using at instrumented stretch-rod and preform surface strain mapping was determined using a combination of a unique patterning procedure and high speed stereoscopic digital image correlation. The unprecedented experimental analysis reveals that the deformation behaviour varies considerably with contrasting process input parameters. Investigation into the effect on deformation mode, strain rate and final bottle shape provide a basis for full understanding of the process optimisation and therefore how the process inputs may aid development of the preferred optimised container.

Desire to accurately predict the deformation behaviour throughout industrial forming processes, such as thermoforming and stretch blow moulding, has led to the development of mathematical models of material behaviour, with the ultimate aim of embedding into forming simulations enabling process and product optimization. Through the use of modern material characterisation techniques, biaxial data obtained at conditions comparable to the thermoforming process was used to calibrate the Buckley material model to the observed non-linear viscoelastic stress/strain behaviour. The material model was modified to account for the inherent anisotropy observed between the principal directions through the inclusion of a Holazapfel–Gasser–Ogden hyperelastic element. Variations in the post-yield drop in stress values associated with deformation rate and specimen temperature below the glass transition were observable, and facilitated in the modified model through time-temperature superposition creating a linear relationship capable of accurately modelling this change in yield stress behaviour. The modelling of the region of observed flow stress noted when above the glass transition temperature was also facilitated through adoption of the same principal. Comparison of the material model prediction was in excellent agreement with experiments at strain rates and temperatures of 1–16 s−1 and 130–155 °C respectively, for equal-biaxial mode of deformation. Temperature dependency of the material model was well replicated with across the broad temperature range in principal directions, at the reference strain rate of 1 s−1. When concerning larger rates of deformation, minimum and maximum average error levels of 6.20% and 10.77% were noted. The formulation, and appropriate characterization, of the modified Buckley material model allows for a stable basis in which future implementation into representative forming simulations of poly-aryl-ether-ketones, poly(ether-ether-ketone) (PEEK) and many other post-yield anisotropic polymers.