Blow Moulding Research Articles

Dies with adjustable flow channel geometries open up new processing possibilities in extrusion

A new technique to produce dies for extrusion was developed which enables an on the fly reduction of the attainable thickness tolerances in extruded products. It realises to combine an interesting improvement of the quality of extruded products with a reduction of material consumption. The new technology is suitable for a fine tuning of the flow channel geometry in dies in order to minimise remaining thickness tolerances in extruded products. While using the new generation of sensitively adjustable dies, it is even possible to vary the thickness of an extracted parison over its circumference during the blow moulding process. The long term goal of the development is to provide processors with dies for all extrusion processes that allow the thickness to be controlled during production.

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.

Melt Strength and Thermal Properties of Organic Peroxide Modified Virgin and Recycled HDPE

Abstract Commercial blow moulding grade recycled high density polyethylene (r-HDPE) and blow moulding grade virgin HDPE were reactively extruded with various compositions (0.00 to 0.15 %) of different peroxides in a twin screw extruder. The aim was to improve the melt strength properties of a blow moulding grade HDPE homopolymer – a polymer resin comprising a significant part of the post consumer recycled plastic stream in Australia. Melting behaviour and crystallinity were investigated by modulated differential scanning calorimetry (MDSC). Uniaxial melt extensional (UME) measurement of the modified materials was carried out using Göttfert Rheotens melt strength tester to measure melt quality of the material. Melt strength and zero shear viscosity were also measured to assess and correlate changes in melt properties due to peroxide modification. Melt strength and zero shear viscosity of both recycled and virgin material were enhanced with the increase in composition of peroxide. However, addition of peroxide also brought with it flow instabilities like draw resonance and necking, both of which lead to limitations in polymer processing.

On-line optical measurement of parisons in blow moulding

Blow moulding is a process whereby a cylindrical parison is extruded first, then pinched by the two halves of a closing mould and finally blown into the product. The actual size and shape of the parison is the result of complex interactions between the die and mandrel geometries, the viscoelastic properties of the polymer and the processing parameters. In practice, obtaining a satisfactory parison is a time-consuming trial and error process. A novel technique using laser lighting of the parison at different angles, during extrusion, gives access to the size and thickness along the entire length of the parison and to the shape of the cross-section at a given location. Results are given for a polypropylene an a polyethylene materials, together with extrusion simulations of a parison with an elliptic cross-section. Increasing the flow rate is increasing the swelling of the parison and its thickness. This effect is more marked with Polypropylene than with Polyethylene.

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.

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.

Advances in infrared technology for the online monitoring of injection moulding: application to the understanding of the nature of contact at the polymer—mould interface

We describe a novel online infrared method for remote sensing of the surface and the bulk temperatures of polymers during injection moulding. The method may also be applied to other polymer forming processes such as extrusion and blow moulding. The key feature of the new method is the use of a hollow optical fibre that is incorporated into the injection mould to transmit the thermal radiation from the target to the sensor. The main characteristic of the hollow optical fibre is that it exhibits low transmission loss of the thermal energy in the mid-and far-infrared, and no end reflection. This allows measurement of quite low temperatures, as low as near room temperature. Conventional optical fibre thermometers can neither measure such low temperature ranges nor measure the polymer surface temperature. In this article, we present the first online results of critical tests of the new device. A Husky injection moulding press was used for the experiments. Good correlation was found between the radiometric results and those obtained with a thermal probe inserted near the polymer—mould interface, and with infrared imaging after the polymer part was ejected from the injection mould. In the second part of the paper, we show how the new infrared device can be used to give a better insight on the time evolution of the thermal contact between polymer and mould through the different phases of a typical injection moulding cycle. The experimental results show that thermal contact between polymer and mould is not negligible and not constant with time.

Rheological characterisation of recycled poly(ethylene terephthalate) modified by reactive extrusion

Recycled poly(ethylene terephthalate) (R-PET) has poor melt strength and viscosity. Hence, the use of R-PET in blow moulding applications, where high melt strength is required, is limited. In this study reactive extrusion of post-consumer R-PET with a low molecular weight modifier: pyromellitic dianhydradide (PMDA) has been used as a way of improving the rheological properties of R-PET. Rheological characterisation of the reactive extruded R-PET (RER-PET) was performed by means of a parallel plate rheometer. The unmodified R-PET and virgin PET were also tested for comparison purposes. The RER-PET samples exhibited higher complex viscosity and higher storage modulus compared to unmodified R-PET sample. Increase in complex viscosity and storage modulus became more pronounced as the percentage of PMDA in R-PET was increased. At high PMDA concentrations, the modified Cole–Cole plots demonstrated a shift towards higher storage modulus values at a given loss modulus value. Viscoelastic properties observed were related to the molecular structure of modified samples. Rheology studies confirmed the increase in molecular weight of post consumer R-PET with an increase of PMDA concentration, and the formation of branching at concentration above 0.25 wt.% PMDA.

Manufacturing in blow molding: Time reduction and part quality improvement

In this work, a concurrent engineering approach was applied to the development of a plastic dumbbell, a part used in the recreational sector. The design goal for the part was to reduce the overall weight and to improve the thickness distribution. Simulation of the extrusion blow moulding process and preliminary experimental trials were performed concurrently to assist in the development of the part. Once the numerical modelling of the part was done, improvement of the production process was performed based upon the desired objective function, i.e., a uniform part thickness distribution and/or minimal part weight. The optimisation was performed in two sequential steps, weight optimisation then thickness optimisation, by the systematic manipulation of the operating conditions, such as the parison dimensions. A process modelling methodology was employed to demonstrate the reduction in the part development time using the new model-based approach. Results showed that span time for the part development cycle could be reduced by approximately 60%, while the actual man-hours could be reduced by approximately 30% using a concurrent engineering approach.

New technology for thermal processing of poly(vinyl alcohol)

Thermal processing of poly(vinyl alcohol) (PVA) has been a big challenge worldwide because its melting point is very close to its decomposition temperature. Based on intermolecular complexation and plasticisation, an amido containing compound (Ac), which has a complementary structure to PVA, together with water, were adopted to improve thermal processing of PVA. The results show that water and Ac can control the supramolecular structure of PVA, confine its crystallisation, decrease its melting point, thus obtain the thermal processing window for PVA. In this way, melt spinning of PVA fibres, extrusion blowing of PVA films and extrusion blowing moulding of PVA containers are realised. Then PVA high performance fibres with tensile strength over 1·9 GPa, PVA blown films with good mechanical properties and transparency, and PVA containers with good gasoline barrier properties and thermal resistance are obtained. Compared with the traditional wet processing of PVA solution, the thermal processing technology adopted for modified PVA is much more simple, effective and cost saving. More encouragingly, the PVA container with excellent chemical resistance and physical properties will open a new application field for PVA.

Effect of pinch-off welding on quality of food packaging bottle

The objective of this study was to investigate the influence of processing parameters on the quality of the pinch-off welding of an ethylene-vinyl alcohol/polypropylene (EVOH/PP) six-layer bottle for tomato ketchup, evaluated in terms of its structure and properties imparted to the corresponding bottles. The pinch-off welding process was studied on coextruded blow moulded bottles by varying the mould temperature, the mould closing speed and the hold time according to a L9-Taguchi experiment design. It was observed that the mould closing speed is the more influential parameter, determining the layered structure at the pinch-off and the mechanical performance of the bottle. Better properties were obtained when the mould closing speed was set to the highest value allowed by the blow moulding equipment.

Determination of the elongational viscosity of polymer melts by melt spinning experiments. A comparison with different experimental techniques

In this work, melt spinning experiments were tentatively used for the determination of the elongational viscosity of polymer melts at different levels of tensile strain and strain rate. The materials examined were two high-density polyethylene grades for blow moulding with similar number-average molecular mass but different polydispersity index. The data from melt spinning tests were compared with transient extensional viscosity data obtained by uniform isothermal tensile tests, performed by means of an extensional rheometer, as well as with those produced by converging flow tests (Cogswell model). The results showed that for high strain and strain rate levels, the melt spinning experiments provide elongational viscosity data quite close to the transient extensional viscosity values obtained from the tensile tests.

Microwaves thermal conditioning of preforms

Free blowing of PET preform has been performed successfully and quickly in a rotating system which has been designed for dielectric loss heating. Investigations of the thermomechanical behaviour of PET and biaxial-stretch blow moulding have shown that an inverted temperature profile is needed prior to its stretching. A temperature profile modelling was carried out using the permittivity at different temperatures of the amorphous PET. It was necessary to solve the Maxwell's and heat equations to determine the best profile and power tuning. Transient external surface wall predicted temperature was found to agree well with IR result. This work needs further investigation for a complete heating of the preform and better energy efficiency. Industrial application involves the integration of the moulding step and the design of the overall process. So far it has been found that the microwave assisted process is at least five times faster than IR heating.

Principles of polymer processing

1. History, Structural Formulation of the Field Through Elementary Steps, and Future Perspectives. 1.1 Historical Notes. 1.2 Current Polymer Processing Practice. 1.3 Analysis of Polymer Processing in Terms of Elementary Steps and Shaping Methods. 1.4 Future Perspectives: From Polymer Processing to Macromolecular Engineering. 2. The Balance Equations and Newtonian Fluid Dynamics. 2.1 Introduction. 2.2 The Balance Equations. 2.3 Reynolds Transport Theorem. 2.4 The Macroscopic Mass Balance and the Equation of Continuity. 2.5 The Macroscopic Linear Momentum Balance and the Equation of Motion. 2.6 The Stress Tensor. 2.7 The Rate of Strain Tensor. 2.8 Newtonian Fluids. 2.9 The Macroscopic Energy Balance and the Bernoulli and Thermal Energy Equations. 2.10 Mass Transport in Binary Mixtures and the Diffusion Equation. 2.11 Mathematical Modeling, Common Boundary Conditions, Common Simplifying Assumptions, and the Lubrication Approximation. 3. Polymer Rheology and Non-Newtonian Fluid Mechanics. 3.1 Rheological Behavior, Rheometry, and Rheological Material Functions of Polymer Melts. 3.2 Experimental Determination of the Viscosity and Normal Stress Difference Coefficients. 3.3 Polymer Melt Constitutive Equations Based on Continuum Mechanics. 3.4 Polymer Melt Constitutive Equations Based on Molecular Theories. 4. The Handling and Transporting of Polymer Particulate Solids. 4.1 Some Unique Properties of Particulate Solids. 4.2 Agglomeration. 4.3 Pressure Distribution in Bins and Hoppers. 4.4 Flow and Flow Instabilities in Hoppers. 4.5 Compaction. 4.6 Flow in Closed Conduits. 4.7 Mechanical Displacement Flow. 4.8 Steady Mechanical Displacement Flow Aided by Drag. 4.9 Steady Drag-induced Flow in Straight Channels. 4.10 The Discrete Element Method. 5. Melting. 5.1 Classification and Discussion of Melting Mechanisms. 5.2 Geometry, Boundary Conditions, and Physical Properties in Melting. 5.3 Conduction Melting without Melt Removal. 5.4 Moving Heat Sources. 5.5 Sintering. 5.6 Conduction Melting with Forced Melt Removal. 5.7 Drag-induced Melt Removal. 5.8 Pressure-induced Melt Removal. 5.9 Deformation Melting. 6. Pressurization and Pumping. 6.1 Classification of Pressurization Methods. 6.2 Synthesis of Pumping Machines from Basic Principles. 6.3 The Single Screw Extruder Pump. 6.4 Knife and Roll Coating, Calenders, and Roll Mills. 6.5 The Normal Stress Pump. 6.6 The Co-rotating Disk Pump. 6.7 Positive Displacement Pumps. 6.8 Twin Screw Extruder Pumps. 7. Mixing. 7.1 Basic Concepts and Mixing Mechanisms. 7.2 Mixing Equipment and Operations of Multicomponent and Multiphase Systems. 7.3 Distribution Functions. 7.4 Characterization of Mixtures. 7.5 Computational Analysis. 8. Devolatilization. 8.1 Introduction. 8.2 Devolatilization Equipment. 8.3 Devolatilization Mechanisms. 8.4 Thermodynamic Considerations of Devolatilization. 8.5 Diffusivity of Low Molecular Weight Components in Molten Polymers. 8.6 Boiling Phenomena: Nucleation. 8.7 Boiling-Foaming Mechanisms of Polymeric Melts. 8.8 Ultrasound-enhanced Devolatilization. 8.9 Bubble Growth. 8.10 Bubble Dynamics and Mass Transfer in Shear Flow. 8.11 Scanning Electron Microscopy Studies of Polymer Melt Devolatilization. 9. Single Rotor Machines. 9.1 Modeling of Processing Machines Using Elementary Steps. 9.2 The Single Screw Melt Extrusion Process. 9.3 The Single Screw Plasticating Extrusion Process. 9.4 The Co-rotating Disk Plasticating Processor. 10. Twin Screw and Twin Rotor Processing Equipment. 10.1 Types of Twin Screw and Twin Rotor-based Machines. 10.2 Counterrotating Twin Screw and Twin Rotor Machines. 10.3 Co-rotating, Fully Intermeshing Twin Screw Extruders. 11. Reactive Polymer Processing and Compounding. 11.1 Classes of Polymer Chain Modification Reactions, Carried out in Reactive Polymer Processing Equipment. 11.2 Reactor Classification. 11.3 Mixing Considerations in Multicomponent Miscible Reactive Polymer Processing Systems. 11.4 Reactive Processing of Multicomponent Immiscible and Compatibilized Immiscible Polymer Systems. 11.5 Polymer Compounding. 12. Die Forming. 12.1 Capillary Flow. 12.2 Elastic Effects in Capillary Flows. 12.3 Sheet Forming and Film Casting. 12.4 Tube, Blown Film, and Parison Forming. 12.5 Wire Coating. 12.6 Profile Extrusion. 13. Molding. 13.1 Injection Molding. 13.2 Reactive Injection Molding. 13.3 Compression Molding. 14. Stretch Shaping. 14.1 Fiber Spinning. 14.2 Film Blowing. 14.3 Blow Molding. 15. Calendering. 15.1 The Calendering Process. 15.2 Mathematical Modeling of Calendering. 15.3 Analysis of Calendering Using FEM. Appendix A: Rheological and Thermophysical Properties of Polymers. Appendix B: Conversion Tables to the International System of Units (SI). Appendix C: Notation. Author Index. Subject Index.

Identification of a strain induced crystallisation model for PET under uni- and bi-axial loading: Influence of temperature dispersion

Accurate modelling of PET multiaxial behaviour is fundamental for blow moulding process optimisation. In this paper, we present the identification of a strain induced crystallisation model on uniaxial and biaxial tension tests, undertaken on PET amorphous samples above T g at various draw rates and tension speeds [Marco, Y., 2003. Caractérisation multi-axiale du comportement et de la microstructure d’un semi-cristallin: application au cas du PET. Thèse de doctorat de l’ENS de Cachan]. Using an accurate equivalent deformation, a non-Newtonian viscous model coupled with induced crystallisation seems to give a good representation of strain hardening for different kinds of solicitations. Moreover, a WLF-like parameter is identified and allows to be a representative of the range of the process temperatures with a single test at 90 °C. As temperature is a crucial parameter for both process and mechanical tests, we focus on the study of the influence of the temperature regulation on the dispersion of mechanical results. We use here a probabilistic approach giving convincing results and leading to a direct industrial application: knowing the heating regulation default of the process, one can predict the variation of the mechanical behaviour of the perform during blow moulding and therefore the variability of final product properties like thickness or crystallinity ratio.

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.

Controlled temperature jaws to improve material characterization by uniaxial hot tensile test

Due to its simplicity, the uniaxial hot tensile test is widely used to characterize the mechanical properties of materials and to obtain the parameters of the mathematical models used to model processing techniques, such as thermoforming or blow moulding. However, quite a few difficulties with this type of test are frequently reported in the literature. This work describes a new clamping device developed to enhance the degree of reliability of uniaxial hot tensile tests. Thermal and stretching tests were performed in order to evaluate the effectiveness of the proposed device. The thermal behaviour tests showed that for almost all heating conditions the temperature variation in the gauge length region of specimens is smaller than the oven temperature deviations around the prescribed value (±3.7 °C). It was also shown that the influence of the material, thickness and geometry is negligible. The stretching behaviour experiments allowed confirmation that if the temperature of the jaws is set below the glass transition temperature of the material, adhesion between the specimen and the jaws, slip-stick, grip-extrusion and compression in the gripping region are minimized or eliminated. Further, as the influence of the jaws is extended to its neighbourhood, the specimen stretching is mainly confined to the gauge length region.

Thermoplastic starch as packaging material

Attempts to reduce net CO2 emission and to increase the use of sustainable materials pose great challenges at the interface of chemical engineering and agricultural engineering. One of the products involved is thermoplastic starch that can partly substitute synthetic plastics, amongst others in packaging. In a first step a semi product is produced by mixing starch and a polyol in a cooking-extrusion process. The material obtained can than be used in other polymer processes, like extrusion, film blowing and injection moulding. Films with thicknesses of 200 µm can be achieved if gelatinisation is good and moisture content constant. In injection moulding shape stability can be retained by a judicious combination of moisture content and process temperature. The final objects can reach a strength comparable to commercial packaging plastics, like polystyrene.
 A problem that still exist is the sensitivity of the material to ambient moisture, both at short terms during production as at long times during usage. These problems increase with increasing surface to volume ratio of the final product like, for instance, films. Technologies where the material is coated or where the lubricant is immobilized have to be investigatedto expand the use. From economical point of view it can be expected that the costs of the extra extrusion step during manufacturing of the semi-product can easily be covered by the price difference between starch and synthetic packaging plastics, like polyethylene, polypropylene or polystyrene.

Stability of Extrusion Blow Moulding under Wear of the Equipment

Stability of Extrusion Blow Moulding under Wear of the Equipment