Rauri McCool

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.

Process modelling for control of product wall thickness in thermoforming

Abstract The present paper describes the results of an investigation into the modelling of plug assisted thermoforming. The objective of this work was to improve the finite element modelling of thermoforming through an enhanced understanding of the physical elements underlying the process. Experiments were carried out to measure the effects on output of changes in major parameters and simultaneously simple finite element models were constructed. The experimental results show that the process creates conflicting and interrelated contact friction and heat transfer effects that largely dictate the final wall thickness distribution. From the simulation work it was demonstrated that a high coefficient of friction and no heat transfer can give a good approximation of the actual wall thickness distribution. However, when conduction was added to the model the results for lower friction values were greatly improved. It was concluded that further work is necessary to provide realistic measurements and models for contact effects in thermoforming.

Thermoforming carbon fibre-reinforced thermoplastic composites

The use of carbon fibre composites is growing in many sectors but their use remains stronger in very high value industries such as aerospace where the demands of the application more easily justify the high energy input needed and the corresponding costs incurred. This energy and cost input is returned through gains over the whole life of the product, with for example, longer maintenance intervals for an aircraft and lower fuel burn. Thermoplastic composites however have a different energy and cost profile compared to traditional thermosets with notable differences in recyclability, but this profile is not well quantified or documented. This study considers the key process control parameters and identifies an optimal window for processing, along with the effect this has on the final characteristics of the manufactured parts. Interactions between parameters and corresponding sensitivities are extracted from the results.

Thermoforming process simulation for the manufacture of deep-draw plastic food packaging

The development of an effective and representative finite element-based simulation of the deep-draw plug-assisted thermoforming process is presented. It has been clearly demonstrated that the frictional and thermal effects of plug contact are of critical importance and appropriate relationships for these have been developed and included in the simulation. A constitutive material model based on the van der Waals strain energy function has been used to represent the deformation response of the high-impact polystyrene polymer sheet and the model parameters have been closely fitted to biaxial test data. The prototype simulation is stable and its output is generally in good agreement with experimental measurements. It is recognized that model accuracy could be improved further through additional experimental investigation of the process. The working simulation provides a powerful platform for in-depth analysis of the thermoforming process.

Integrating Digital Manufacturing, Processing, and Design of Composite Structures

The need for sustainable transport in the future, emphasised recently with the dramatic increases in energy and fuel costs, is driving the development and use of light weight, composite materials. The predictive methods currently used for material specification, component design and the development of manufacturing processes, need to evolve beyond the current ‘metal centric’ state of the art, if composites are to realise their potential in delivering sustainable transport solutions. There are however, significant technical challenges associated with this process. The SME supply chain faces a changing market place where their expertise in traditional manufacturing methods and small component design has less relevance. They need to apply new technologies and develop new capabilities to ensure commercial sustainability in the face of twenty first century economic and climatic conditions as well as transport market demands. There is a major technology gap between design, analysis and manufacturing in both the OEMs, and the smaller companies that make up the SME based supply chain. As regulatory requirements align with environmental needs, manufacturers are increasingly responsible for the broader lifecycle aspects of vehicle performance. These include not only manufacture and supply but disposal and re-use or re-cycling. This paper presents a project structure which has been designed to address these issues using at its core, a digital framework for the creation and management of performance parameters related to the lifecycle performance of thermoplastic composite structures.

The prediction of process-induced deformation in a thermoplastic composite in support of manufacturing simulation:

Digital manufacturing techniques can simulate complex assembly sequences using computer-aided design–based, ‘as-designed’ part forms, and their utility has been proven across several manufacturing sectors including the ship building, automotive and aerospace industries. However, the reality of working with actual parts and composite components, in particular, is that geometric variability arising from part forming or processing conditions can cause problems during assembly as the ‘as-manufactured’ form differs from the geometry used for any simulated build validation. In this work, a simulation strategy is presented for the study of the process-induced deformation behaviour of a 90°, V-shaped angle. Test samples were thermoformed using pre-consolidated carbon fibre–reinforced polyphenylene sulphide, and the processing conditions were re-created in a virtual environment using the finite element method to determine finished component angles. A procedure was then developed for transferring predicted part forms from the finite element outputs to a digital manufacturing platform for the purpose of virtual assembly validation using more realistic part geometry. Ultimately, the outcomes from this work can be used to inform process condition choices, material configuration and tool design, so that the dimensional gap between ‘as-designed’ and ‘as-manufactured’ part forms can be reduced in the virtual environment.

Thermoforming of Continuous Fibre Reinforced Thermoplastic Composites

The introduction of new materials, particularly for aerospace products, is not a simple, quick or cheap task. New materials require extensive and expensive qualification and must meet challenging strength, stiffness, durability, manufacturing, inspection and maintenance requirements. Growth in industry acceptance for fibre reinforced thermoplastic composite systems requires the determination of whole life attributes including both part processing and processed part performance data. For thermoplastic composite materials the interactions between the processing parameters, in‐service structural performance and end of life recyclability are potentially interrelated. Given the large number and range of parameters and the complexity of the potential relationships, understanding for whole life design must be developed in a systematic building block approach. To assess and demonstrate such an approach this article documents initial coupon level thermoforming trials for a commercially available fibre reinforced t...

Integrating Materials, Manufacturing, Design and Validation for Sustainability in Future Transport Systems

The predictive methods currently used for material specification, component design and the development of manufacturing processes, need to evolve beyond the current ‘metal centric’ state of the art, if advanced composites are to realise their potential in delivering sustainable transport solutions. There are however, significant technical challenges associated with this process. Deteriorating environmental, political, economic and social conditions across the globe have resulted in unprecedented pressures to improve the operational efficiency of the manufacturing sector generally and to change perceptions regarding the environmental credentials of transport systems in particular. There is a need to apply new technologies and develop new capabilities to ensure commercial sustainability in the face of twenty first century economic and climatic conditions as well as transport market demands. A major technology gap exists between design, analysis and manufacturing processes in both the OEMs, and the smaller c...

Preparation and Properties of Polyphenylene Sulfide/Multi‐walled Carbon Nanotube Composites

Polyphenylene sulphide (PPS)/multi‐walled carbon nanotube (MWCNT) composites were prepared using a melt‐blending procedure combining centrifugal pre‐mixing and twin‐screw extrusion. A homogeneous dispersion of MWCNTs throughout the matrix was revealed by scanning electron microscopy for the nanocomposites with MWCNT contents ranging from 0.5 to 8.0 wt%. The presence of the MWCNTs showed both promotion and retardation effects on the crystallization of PPS. The competition between these two effects results in an unusual change of the degree of crystallinity with increasing MWCNT content. The mechanical properties of PPS were markedly enhanced by the incorporation of MWCNTs.

A Virtual Test System Integrating Materials and Manufacturing to Aid Design Choices

The rapid increase in usage of fiber reinforced, structural composites in commercial aviation and beyond is providing many challenges to designers in understanding how they can be used more effectively to exploit their advantages. One such challenge is in the appropriate selection of a composite lay-up or type which can be most efficient in a given application scenario. The difficulty lies in the variability that is possible in composite part production. Each new layup or configuration is effectively a new material and requires an extensive test programme to validate the performance, from coupons which give basic material characteristics, up through the test pyramid to large sub-components which contain basic assemblies. This variety of testing gives confidence in understanding the material and how it performs after processing in structural assemblies. Naturally the manufacturing process is also important here with different processes sometimes needed for different materials or thicknesses. However, inevitably this is a time consuming and expensive process requiring many thousands of small tests leading up to a few major tests which are complex to set up and perform. This work is attempting to address this by developing a virtual test system which will sit hand-in-hand with a physical test system. The concept is based on the performance of as many validated simulations as possible. These can subsequently be used for variants or derivatives to inform design choices and establish new validation programmes where appropriate.