Injection Stretch Blow Moulding Process Research Articles

Experimental characterisation and modelling of polyethylene terephthalate preform for injection stretch blow moulding

Polyethylene terephthalate (PET) is a widely used polymer in the production of bottles by injection stretch blow moulding (ISBM). In this work, we present a characterisation method to identify material properties directly from a preform, considering temperature and stress-relaxation effects related to its viscoelastic response. A customised oven and gripping system were designed to perform uniaxial tests in a proper temperature range on tubular specimens obtained from preforms. A visco-hyperelastic model is then proposed: a Marlow-type strain energy function coupled with a Prony series and William-Landel-Ferry equation to include time and temperature dependency. Finally, a case study of ISBM process is implemented in a finite element code considering this constitutive model. Strain maps and predicted thickness of the bottle wall were evaluated as process quality indicators. Simulation results showed good agreement with measurements on the real processed bottle, confirming the usefulness of the approach for product or process parameters optimisation.

Measuring and modelling air mass flow rate in the injection stretch blow moulding process

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 ABAQUS. 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 ABAQUS/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.

Numerical simulation of the thermodependant viscohyperelastic behavior of polyethylene terephthalate near the glass transition temperature: Prediction of the self‐heating during biaxial tension test

The poly ethylene terephthalate near the glass transition temperature highlights a strongly non linear elastic and viscous behaviour when biaxially stretched at high strain rates representative of the injection stretch blow moulding process. A non linear visco‐hyperelastic model, where characteristics are coupled to the temperature, has already been identified from equi‐biaxial tension experimental results. The weak form of the mechanical part of the model is presented and implemented into a finite element code developed in the Matlab environment and validated by comparing numerical simulation of equi‐biaxial testing with the analytical solution in the isothermal case. Considering the thermal aspects, an experimental study, where PET sheets are heated using infrared (IR for short) lamps is also presented. The modeling of the IR radiation of the sheet helps to identify the thermal properties of the PET. The thermal model is then implemented in the finite element code, coupled to the 2D viscoelastic model. A discussion is made to justify the accuracy of the assumption made on homogeneity of the temperature field through the thickness. The simulation of the 2D plane stress equibiaxial test shows the important influence of the thermal aspects and the coupled thermo‐mechanical software is used to quantify the self‐heating phenomenon in the case of the biaxial elongations of PET sheets at high strain rates. POLYM. ENG. SCI., 53:2683–2695, 2013. © 2013 Society of Plastics Engineers

Numerical Simulation of the Viscohyperelastic Behaviour of PET near the Glass Transition Temperature

The presentation deals with the non linear strongly elastic and viscous behaviour of poly ethylene terephthalate near the glass transition temperature and biaxially stretched at high strain rates representative of the injection stretch blow moulding process. A non linear visco-hyperelastic model inspired from [1] and identified from the experimental results of the equi-biaxial tension test [2], have been developed and presented in [3] is implemented into a finite element code developed with Matlab. The thermal behaviour modelling, identification and simulation has also been managed. First, a numerical simulation of 2D plane stress case has been performed involving 2 fields (global velocity V and elastic Cauchy Green tensor Be). Rectangular finite elements with quadratic and linear interpolations have been employed for velocity and the elastic left Cauchy Green tensor. Second, an axi symmetric formulation involving 4 fields (global velocity V, lagrange multiplier p associated with the global incompressibility condition, and multiplier q associated with the incompressibility of the elastic part) has been performed using rectangular elements. Degree of interpolation have been tested for all possible combinations to test the LBB like condition. Both simulations are compared with equi biaxial or sequential biaxial testing in order to reproduce the strain hardening effect and the self-heating observed. The final goal of this work is to perform the free blowing simulation to compare with experimental data. Therefore, we should solve an iterative procedure for a thermo-mechanical equation. At each time step, a four-field approach is adopted for the mechanical part, and a classical heat transfer equation is discretised for the thermal part.

A study of blow moulding simulation and structural analysis for PET bottles

Simulation of the injection stretch blow moulding (ISBM) process for the PET (polyethylene terephthalate) bottle manufacturing process is important in improving quality of PET bottles and reducing production time. This paper presents an investigation of finite element simulation of the ISBM process utilising B-SIM simulation software. For a given perform shape, results are obtained on the effect of various process parameters on wall thickness distribution of PET bottles. On completion of the simulation, the simulated bottle file is exported from B-SIM to ANSYS finite element analysis software for structural analysis. Top load analysis and internal pressure analysis are conducted in verifying the mechanical strength of PET bottles. Results indicate that by proper selection of process parameters and by B-SIM simulation, PET bottles of desired strength can be produced by the injection blow moulding process with more uniform thickness distribution.

Modelling of the Injection Stretch Blow Moulding of PET Containers via a Pressure-Volume-time (PV-t) Thermodynamic Relationship

A 2-D isothermal finite element simulation of the injection stretch-blow moulding (ISBM) process for polyethylene terephthalate (PET) containers has been developed via the commercial finite element package ABAQUS/standard. In this work, the blowing air to inflate the PET preform was modelled via two different approaches; a direct pressure input (as measured in the blowing machine) and a constant mass flow rate input (based on a Pressure-Volume-time relationship). These two methods were tested with a simplified stretch blow moulding process where a preform was blown with and without a stretch rod in free air (no mould). The results clearly show that simulation with a constant mass flow rate as input gave an excellent prediction of volume vs. time curve and preform shape evolution when compared with the direct pressure approach. In addition to this, rapid inflation of the PET preform (∼0.03s) was found to occur in the direct pressure approach which was not observed in reality. This result reveals that the constant mass flow rate approach is more appropriate in modelling the blowing stage in ISBM process.

Injection stretch blow moulding process of reactive extruded recycled PET and virgin PET blends

Reactive extruded recycled PET (RER-PET) was blended with virgin PET in order to find optimum composition for producing bottles using injection stretch blow moulding (ISBM) process. Two variables were investigated namely, RER-PET concentration in the blend with virgin PET and RER-PET intrinsic viscosity, [ η]. The variability of [ η] was facilitated by changing PMDA chain extender concentration in the production of RER-PET. Different molecular, mechanical, barrier and optical tests such as infrared, burst pressure, top load strength, drop impact, environmental stress cracking, liquid permeation, gas permeation, and clarity were conducted. The aim was to evaluate the properties of the bottles in response to different blends compositions. Bottles made from RER-PET and virgin PET blends showed similar and better mechanical and barrier properties at certain compositions to those made from Virgin PET. The effects of RER-PET molecular properties such as intrinsic viscosity, chain orientation and conformation on bottles mechanical, barrier and optical properties are detailed.

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

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.

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

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.