Stretch Blow Moulding Research Articles

A novel approach of measuring the dielectric properties of PET preforms for stretch blow moulding

AbstractPolyethylene terephthalate (PET) is the most common plastic material used in injection stretch blow molding (ISBM) process for the manufacturing of bottles. The injection‐molded PET preform has to be preheated uniformly to a pliable state before the blowing stage of the process. Microwave technology offers a potentially more economical means for preheating the preform in place of the conventional infrared oven heating. For such applications, determination of dielectric properties of PET preform of a given geometry is essential. This article describes a novel approach of measuring the dielectric properties of PET preforms at microwave frequencies instead of the common experimental procedures used for such measurements. The dielectric properties are determined by using both CST Microwave Studio electromagnetic simulation software and the automatic network analyzer together. No complex mathematical solutions are required. Validation of this new approach has been made by comparing the measured dielectric properties of some polymers with those available in the published literature. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci 2008

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

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.

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.

Optimization by the C-NEM Method of the Stretch-Blow Molding Process of a PET Bottle Near T g

The stretch-blow molding process of poly(ethylene terephthalate) bottles generates some important modifications in the mechanical properties of the material. Considering, the range of temperature (T > T g ) that is usually used, the material has a very high viscosity and shows a strain hardening effect linked to the microstructure evolution. A simple visco-plastic model had been identified from experimental results of uniaxial and biaxial tensile tests [1]. In the present work, we use this model and a numerical technique to simulate the inflation of a preform under an internal pressure and also submitted to the elongation of a stretch rod. The finite elements method has a poor efficiency in the stretch-blow molding process because the final material strain is up to 300%. This strain level generates strong element distortions and necessitates to often re-mesh. In order to carry out simulations with strain higher than 300%, the constrained natural elements method (C-NEM [2]) is used. In fact, the C-NEM method allows simulation with high strain level without re-meshing. The original “mesh” can be use from the beginning to the end of the simulation. So the properties of the material can be recorded in each node, without loss of information. In a first approach C-NEM are applied to simulate and optimize stretch-blowing of a PET preform by using axis symmetric assumption, the mold is not taking into account.

Optimisation of Preform Temperature Distribution For the Stretch-Blow Moulding of PET Bottles

This study presents an optimization strategy developed for the stretch-blow moulding process. The method is based on a coupling between the Nelder-Mead optimization algorithm, and Finite Element (FE) simulations of the forming process developed using ABAQUS®. FE simulations were validated using in situ tests and measurements performed on 18.5 g – 50 cl PET bottles. To achieve that, the boundary conditions were carefully measured for both the infrared heating and the blowing stages. The temperature distribution of the perform was predicted using a 3D finite-volume software, and then applied as an initial condition into FE simulations. Additionally, a thermodynamic model was used to predict the air pressure applied inside the preform, taking into account the relationship between the internal air pressure and the enclosed volume of the preform, i.e. the fluid-structure interaction. It was shown that the model adequately predicts both the blowing kinematics and the thickness distributions of the bottle. In a second step, this model was combined to an optimization loop to automatically compute the best perform temperature distribution, providing a uniform thickness for the bottle. Only the last part will be fully detailed in this paper.

Influence of chemical architecture of PET on ability to stretch blow moulding

PET specific properties make this material a good candidate for stretch blow moulding. However chemical architecture of the resin can be varied by the supplier. These changes induce different draw abilities or different behaviours whilst blowing. In this study some well controlled PET resins are characterized both from the point of view of their intrinsic properties and from the point of view of their process abilities. Results enlighten clearly the tight correlation between the chemical structure and the behaviour of the polymer. Coupling effects between the structure, the laboratory properties and the process ability exist making this process a highly not intuitive processing. This study allows, in parallel, proposing a global approach to estimate blowing ability from laboratory analysis.

Visualization study and analysis on preform growth in polyethylene terephthalate stretch blow molding

AbstractIn stretch blow molding (SBM) process, the preform growth during the stretching and blowing is critical to the thickness distribution and properties of the final bottle. Whereas the thickness distribution is one of the most important criteria in the production of bottles. So this work focused on the polyethylene terephthalate (PET) preform growth using a transparent mold, through which the instantaneous images of the preform in the stretching and blowing stage were captured. By changing the delay time of the preblow, the three preform growth types, referred to as dolphin, sandpile, and two‐bubble, were observed. The longitudinal and hoop stresses acting on the preform segment during the stretching and blowing were analyzed. Two parameters, on which the longitudinal and hoop stresses depend, respectively, were defined. Then combining the geometry and sizes of the preform, the stresses and temperature distribution on it, and the stress–strain curves of the PET material used, the cause for different preform growth types was systematically analyzed. On the basis of preform growth types, the thickness distributions of the bottles obtained under different delay times of the preblow were explained. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 564–573, 2007

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

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.

Online real-time acquisition for transient temperature in blow molding

In blow molding, the temperature is an important parameter which influences the process, cycle times, and part properties, etc. In the current work, a PC-based temperature acquisition system, including hardware and software, was developed to measure the transient temperatures of an extrusion blow-molded part during its cooling and solidification, and of a reheated preform in stretch blow molding (SBM). Two issues were addressed. Firstly, the thermocouples in the acquisition system were specially designed by the authors to allow for their penetration into the wall of the cooling part or reheated preform. Secondly, in order to measure the temperature of reheated preform as it rotates, a special brush mechanism was designed. The acquisition system was tested through a series of experiments on industrial blow-molding machines under different conditions, such as die temperature, blowing pressure, and blow mold material in extrusion blow molding, and the kind of heating lamps and their voltage settings in SBM. The ability of fast and repeatable online measurement of transient temperature profiles using the acquisition system should prove to be helpful to better understand the molding phenomena, to validate the numerical simulation results and to derive optimum processing parameters for blow molding.

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.

Statistical models for optimisation of properties of bottles produced using blends of reactive extruded recycled PET and virgin PET

Blends of chain-extended reactive extruded recycled PET (RER-PET) and virgin PET were fed in injection stretch blow moulding (ISBM) process. The RER-PET blends have different RER-PET concentrations and RER-PET have different pyromellitic dianhydride (PMDA) chain extender concentrations. RER-PET composition and percentage in the blend were used as process variables. The resulted ISBM bottles were tested for burst pressure, top load, liquid permeation and carbonation loss. The experimental results were used in an optimisation process aiming to identify the best blend composition that produces best bottles properties. Models were fitted and statistically analysed to represent the bottles properties in the optimisation calculations. Two optimum points were found in which highest chain extender concentration in RER-PET were favoured. The optimum points showed different optimum RER-PET concentrations in the blend. The first optimum concentration is 20% RER-PET which resulted in best burst pressure and top load resistance. The second optimum point with RER-PET concentration of 29% produced minimum liquid permeation and carbonation loss. The results of the optimisation procedure are beneficial in identifying the possibility of the use of RER-PET for bottling application.

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.

Hot‐fillable containers containing PET/PEN copolymers and blends

AbstractBottles and containers made of PET are not suitable for hot filling since the limiting upper‐use temperature of this polymer is about 85°C. In the present study the properties and performance of bottles made from copolymers and blends of PET containing NDC groups and manufactured by the Injection Stretch Blow Molding (ISBM) process were investigated. These compositions possess advanced properties and can be used up to 95°C. The properties of these bottles were found to depend on their composition and microstructure. The glass transition temperature, the degree of crystallinity and the induced strains that were measured by differential scanning calorimetry and differential mechanical thermal analysis are reported. It was concluded that NDC‐containing PET based copolymers and blends could be processed by a one‐stage ISBM process into containers of improved properties and hot‐filling capabilities. Polym. Eng. Sci. 44:1670–1675, 2004. © 2004 Society of Plastics Engineers.

Coupled temperature–displacement modelling of injection stretch-blow moulding of PET bottles using Buckley model

This study presents a fully coupled temperature–displacement finite element modelling of the injection stretch-blow moulding (ISBM) process of polyethylene terephthalate (PET) bottles using ABAQUS with a view to optimising the process conditions. A physically-based material model (Buckley model) was used to predict the mechanical behaviour of PET at temperatures slightly above its glass transition temperature. A model incorporating heat transfer between the stretch rod, the preform and the mould was built using axisymmetric solid elements. Extensive finite element analyses were carried out to predict the deformation, the distribution and history of strain and temperature during ISBM of a 20g–330ml bottle, which was made in an in situ test on a Sidel SB06 machine. Comparisons of numerical results with the measurements demonstrate that the model can satisfactorily model the sidewall thickness and material distributions. It is also shown that significant non-linear differentials exist in temperature and strain in both bottle thickness and length directions during the process. This justifies the employment of a volume approach to accurately predict the final mechanical properties of the bottles governed by the orientation and crystallinity which are highly temperature and strain dependent.

Experimental study and numerical simulation of preform or sheet exposed to infrared radiative heating

Thermoplastic processing like the injection stretch blow moulding and thermoforming processes provide the heating stage with infrared oven. This is a critical stage of the process, as the final part thickness is strongly dependent on the preform or sheet temperature distribution prior to forming. Optimisation of the infrared oven is therefore necessary. Experiments have been conducted in order to characterise the heat source of the infrared emitter and the interaction between the heaters and a semi-transparent PET sheet. An 880 LW AGEMA infrared camera has been used to determine the surface distribution of the transmitted heat flux by measuring the temperature distribution on the surface of the thermoplastic sheet. In addition, numerical simulations of the temperature distribution using control-volume method have been carried out and compared with experimental data.

Strain-induced crystallisation in bulk amorphous PET under uni-axial loading

Strain induced crystallisation of poly(ethylene terephthalate) (PET) designed for stretch-blow moulding is studied combining well-controlled tensile tests, different quenching protocols and X-ray diffraction technique. As well known [Polymer, 33 (1992) 3182; Polymer, 33 (1992) 3189], crystallisation begins when a minimum strain has been reached. However, crystallisation is a progressive phenomenon that could involve intermediate stages or phases as suggested by some recent papers [Macromolecules, 31 (1998) 7562; Polymer, 41 (2000) 1217]. In this study, first evidence for the crystallisation is the appearance, parallel to the tensile direction, of zones having the crystal lateral order. The spreading out of the diffraction dots suggests that these zones are small in comparison to usual lamella thickness or that the order is imperfect. Longitudinal order appears later, progressively leading to fibre-texture. At low strain, before this texture is totally developed, cooling influences the final microstructure superposing steps in which crystallisation and relaxation compete illustrating the relative importance of cooling protocol when strain induced crystallisation of PET is studied. Nevertheless, strain hardening can be observed before the crystalline microstructure is totally developed, emphasising the fact that strain hardening is mainly controlled by first stages of crystallisation and that actual crystallisation occur during a following relaxation step.

On‐going solutions to environmental issues in plastic packaging

AbstractRecently, the consumption of polyvinyl chloride (PVC) and polyvinylidene chloride (PVDC) resins as packaging materials has been declining in the face of various environmental problems they pose. To replace PVC containers, the use of PET bottles and A‐PET containers is on the increase, but PVDC resins are still in wide use as gas‐barrier materials. For gas‐barrier purposes, however, ethylene vinyl alcohol co‐polymer (EVOH) films, polyvinyl alcohol (PVA)‐coated polypropylene films, and MX nylon or silica‐ or alumina‐coated PET films are becoming mainstream. The weight reduction of plastic bottles is considered to play an important role in tackling the waste disposal problem. Recently, a new type of stretch‐blow moulding system has been developed, which is effective in reducing the weight of PET bottles. This system is called COSMOS (complete stretch‐blow moulding system) or the two‐blow system. COSMOS PET bottles are now used to hold carbonated drinks containing fruit juice or lactic acid. Copyright © 2001 John Wiley & Sons, Ltd.

Experimental and theoretical study of uniaxial deformation of amorphous poly(ethylene terephthalate) above glass transition temperature

Uniaxial deformation of amorphous thick and massive poly(ethylene terephthalate) (PET) samples above the glass transition temperature has been studied in the temperature range relevant to the stretch–blow moulding process of PET bottles. A video controlled system coupled to a novel clamping device, together with accurate control of the thermal gradient in the material, make it possible to determine true stress v. true strain data at constant true strain rate up to very high strains of ∼2.5. The mechanical response of PET in these conditions is modelled as a molecular network exhibiting a rubber-like behaviour. Sufficiently far from the alpha transition, it is shown that the Edwards and Vilgis model is able to reproduce experimental data accurately, which also offers a relevant insight into the nature of the molecular networks. Closer to the alpha transition, this purely hyperelastic model needs refining and improving to take into account an additional non-elastic contribution.

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.

Induced crystallinity during stretch–blow moulding process and its influence on mechanical strength of poly(ethylene terephthalate) bottles

Microstructural evolution in poly(ethylene terephthalate) during a stretch– blow process has an important influence on the final mechanical prop erties. To obtain information other than thickness distributions from numerical simulations of the blow moulding process, it is necessary to take into account the evolution of these characteristics (molecular orientation, crystallinity, etc). Numerical simulations of top loading tests at ambient temperature have been carried out on bottles and compared with experimental results. Good agreement was obtained if appropriate anisotropic moduli were used. In the present paper, results of tensile measurements made on stretch–blow moulded samples and experimental tensile tests on poly(ethylene terephthalate) specimens are presented. The influence of draw ratio, temperature, and elongational strain rate on the final strength of the bottle are analysed. A correlation between the mechanical characteristics and the induced crystallinity is demonstrated and used to predict mechanical characteristics at different locations on the bottle.