Gary Menary

Biaxial Characterisation of Materials for Thermoforming and Blow Moulding.

Abstract During free surface moulding processes such as thermoforming and blow moulding, heated polymer materials are subjected to rapid biaxial deformation as they are drawn into the shape of a mould. In the development of process simulations, it is therefore essential to be able to accurately measure and model this behaviour. Conventional uniaxial test methods are generally inadequate for this purpose and this has led to the development of specialised biaxial test rigs. In the present study, the results of several programmes of biaxial tests conducted at Queens University are presented and discussed. These have included tests on high impact polystyrene (HIPS), polypropylene (PP) and aPET, and the work has involved a wide variety of experimental conditions. In all cases, the results clearly demonstrate the unique characteristics of materials when subjected to biaxial deformation. PP draws the highest stresses and it is the most temperature-sensitive of the materials. aPET is initially easier to form but exhibits strain hardening at higher strains. This behaviour is increased with increasing strain rate but at very high strain rates, these effects are increasingly mollified by adiabatic heating. Both aPET and PP (to a lesser degree) draw much higher stresses in sequential stretching showing that this behaviour must be considered in process simulations. HIPS showed none of these effects and it is the easiest material to deform.

Modelling of poly(ethylene terephthalate) in injection stretch–blow moulding

Abstract Simulations of the injection stretch–blow moulding process have been developed for the manufacture of poly(ethylene terephthalate) bottles using the commercial finite element package ABAQUS/standard. Initially a simulation of the manufacture of a 330 mL bottle was developed with three different material models (hyperelastic, creep, and a non-linear viscoelastic model (Buckley model)) to ascertain their suitability for modelling poly(ethylene terephthalate). The Buckley model was found to give results for the sidewall thickness that matched best with those measured from bottles off the production line. Following the investigation of the material models, the Buckley model was chosen to conduct a three-dimensional simulation of the manufacture of a 2 L bottle. It was found that the model was also capable of predicting the wall thickness distribution accurately for this bottle. In the development of the three-dimensional simulation a novel approach, which uses an axisymmetric model until the material reaches the petaloid base, was developed. This resulted in substantial savings in computing time.

Finite element modelling of stretch-blow moulding of PET bottles using Buckley model: Plant tests and effects of process conditions and material parameters:

Abstract Plant tests and finite element (FE) analyses of the injection stretch-blow moulding (ISBM) process of polyethylene terephthalate (PET) bottles have been carried out in this study with a view to optimizing preform designs and process conditions. Plant tests were carefully conducted at first to make bottles in a 330 ml mould from four preform designs under different process conditions. Both a digital handheld thermometer and a FLIR ThermoCAM Imager system were used to measure the initial preform temperature distributions (IPTDs). Comprehensive FE analyses using ABAQUS were then carried out to model the ISBM of these bottles, using a physically based model (Buckley model) to model the complex constitutive behaviour of PET. It was found that the numerical simulations often resulted in free blowing or over-thinning of the bottle bottoms when the measured IPTDs and process conditions were modelled. Parametric studies of the IPTDs, the pre-blowing pressure and the material parameters of the Buckley model were carried out. It was demonstrated that all of them had considerable effects on the effectiveness of FE modelling. In particular, the stress-strain relations modelled by the Buckley model were very sensitive to two parameters used to model the strain-stiffening behaviour. By carefully adjusting the material parameters and process conditions, successful simulations with excellent bottle thickness predictions were then achieved. It is concluded that the model parameters must be obtained by accurately testing the bottle-grade PET with similar process conditions to those in industrial ISBM so that the Buckley model can be confidently used to model the ISBM process. It is also found that good predictions of bottle wall thickness alone do not necessarily justify the numerical modelling. Validation of the deformation process may be equally important.

Performance enhancement of polymer nanocomposites via multiscale modelling of processing and properties

Abstract This paper provides an overview of research on modelling of the structure–property interactions of polymer nanocomposites in manufacturing processes (stretch blow moulding and thermoforming) involving large-strain biaxial stretching of relatively thin sheets, aimed at developing computer modelling tools to help producers of materials, product designers and manufacturers exploit these materials to the full, much more quickly than could be done by experimental methods alone. The exemplar systems studied are polypropylene and polyester terephalate, with nanoclays. These were compounded and extruded into 2mm thick sheet which was then biaxially stretched at 155°C for the PP and 90 to 100°C for the PET. Mechanical properties were determined for the unstretched and stretched materials, together with TEM and XRD studies of structure. Multi-scale modelling, using representative volume elements is used to model the properties of these products.

Heuristically optimized RBF neural model for the control of section weights in stretch blow moulding

The injection stretch-blow Moulding (ISBM) process is typically used to manufacture PET containers for the beverage and consumer goods industry. The process is somehow complex and users often have to heavily rely on trial and error methods to setup and control it. In this paper, a novel identification method based on a radial basis function (RBF) network model and heuristic optimization methods, such as particle swarm optimization (PSO), deferential evolution (DE), and extreme learning machine (ELM) is proposed for the modelling and control of bottle section weights. The main advantage of the proposed method is that the non-linear parameters are optimized in a continuous space while the hidden nodes are selected one by one in a discrete space using a two-stage selection algorithm. The computational complexity is significantly reduced due to a recursive updating mechanism. Experimental results on simulation data from ABAQUS are presented to confirm the superiority of the proposed method.

The Effect of Temperature, Strain Rate and Strain on the Induced Mechanical Properties of Biaxially Stretched PET

The study is focused on the effect of strain rate, temperature and stretch ratio on the room temperature mechanical properties of PET (Polyethylene terephthalate) following biaxial deformation. Specimens were biaxially stretched within a temperature range 80-110°C, a strain rate in the range 1-16/s and stretch ratio in the range 1-2.8. The tensile moduli of the stretched specimens were obtained using tensile testing. Results show that post-stretching room temperature modulus increases with decreasing temperature, increasing strain rate, and stretch ratio.

Modeling the Constitutive Behaviour of PET for Stretch Blow Moulding

There are a substantial amount of constitutive models that have been developed to capture the finite deformation behavior of polymers for forming simulations. Most of these models have been used to capture the behavior in uniaxial and simultaneous biaxial modes of deformation. However, very few have attempted to model the sequential biaxial deformation behavior which is more appropriate for the stretch blow moulding process. The aim of this work is to develop a model for PET to successfully capture the sequential stress‐strain behavior as a function of temperature and strain rate, thus making it suitable for use in simulations of stretch blow moulding. Biaxial test data has been generated at temperatures and strain rates appropriate for stretch blow moulding and a model developed by Buckley et al. has been implemented within the commercial finite element package Abaqus/Explicit. In parallel, an efficient automatic curve fitting procedure has been developed to enable the material parameters to be easily fo...

Numerical Simulation of Injection Stretch Blow Moulding: Comparison with Experimental Free Blow Trials

The aim of this study is to develop validated simulations of the stretch blow moulding process. This paper describes experimental studies where a preform is inflated into free air (free blow trials) and the subsequent development of simulations based on the experimental data. The free blow trials were monitored via video and instrumentation was use to measure the pressure vs. time in the process. One of the key conclusions from these trials was that the pressure inside the preform is not an input variable but is in fact an output variable highly dependent on the preform temperature, timing of the application of pressure and the nominal line pressure. This effect was also shown to be hugely significant when considering the development of simulations of the manufacturing process. Two free blow simulations were developed 1: where the pressure was applied directly as measured and 2: where the application of pressure was calculated by the simulation based on a specified flow rate of air. The results clearly sh...

Experimental study and numerical modelling of injection stretch blow moulding of angioplasty balloons

Abstract Angioplasty balloons are a class of medical device used to clear clogged arteries. They are manufactured via a process similar to that of injection stretch blow moulding. At present the forming of angioplasty balloons is something of a black art. When a new balloon is being developed, the process parameters and tube dimensions are determined through a mixture of trial and error and experience. The present paper describes the first phase in the development of a finite element simulation of the process which will ultimately be used to optimise the design and manufacture of these devices. A data acquisition system has been developed that measures the temperature and the tension/force in the tube during forming as well as the displacement applied to the tubing to initiate the forming process. Additionally a high speed video camera has been used to visualise the process. The balloon can be seen to form within 0·03 s with an average strain rate of 2000 s−1. Based on the data supplied from the data acquisition system a process simulation has been developed which replicates the formation of the balloon as seen on the high speed video camera. A series of biaxial tests of Nylon 12 in the temperature range between 60 and 120°C and strain rates between 1 and 32 s−1 show that the behaviour of the material is sensitive to temperature but is not dependent on strain rate. Further work is required in terms of the development of a suitable material model capable of capturing this behaviour.

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.