Queen's Homepage


Materials Research Group



Thermoforming

Introduction

Research into the thermoforming process has been carried out at Queen's University for more than 10 years. Much of the work has focussed on optimising the production of thin-walled food packaging products from a range of polymer materials, but a wide range of associated research topics has been investigated.


Thermoformed Food Packaging


The thermoforming research group is very well equipped with 2 purpose-built thermoforming machines. The larger is a single cavity roll-fed continuous cycle machine, which is designed to replicate exactly the operation of equivalent modern industrial machines. The smaller is a specially designed laboratory scale version of the larger machine, where small sheet samples may be heated in an adjacent oven. This offers considerable flexibility for research, as it is much easier to incorporate instrumentation and to adjust the process cycle.

The Roll-fed and Laboratory Thermoforming Machines


Both machines are equipped with plug-assist for deeper draw products and may employ forming pressures up to 10 bar.

Research

Research into thermoforming has examined the entire process cycle from the production of the sheet feedstock by extrusion, through the various stages of thermoforming, to the performance of the final products. Much of the work is ongoing.

Process Modelling
A substantial part of the research effort has been directed towards the development of robust finite element simulations of thermoforming processes. In this area thermoforming lags well behind competing processes such as injection moulding. However, poor understanding of key aspects of the process operation, such as material deformation behaviour and tool/sheet tribology, has hampered further development.

Finite Element Process Models


Tool / Sheet Contact
The tribological aspects of various thermoforming tool and polymer sheet materials have been extensively investigated. Of most interest are the thermal and frictional properties of plug and mould materials. In these studies a number of experimental approaches have been employed, including thermal imaging and high speed video.



Thermal Properties

Process Optimisation
Thermoforming tests have been used to study in detail the relationships between the main thermoforming process variables and the properties of the final product. Experiments have included separate evaluation of the plugging and blowing stages in plug-assisted thermoforming. The work has also included analysis of the important role that sheet extrusion variables play in the subsequent behaviour of the material during the thermoforming process.

Investigation of Plugging and Blowing Process Steps


Process Control
Methods have been investigated to provide on-line control of the thermoforming process. Normally industrial machines utilise a minimum amount of instrumentation and automatic control, and instead rely heavily on trial and error methods. The most obvious process variable to measure and control is the sheet temperature, immediately prior to forming. However, work on the plug-assisted process has provided an alternative approach that measures the force drawn by the plug during the process stages. A prototype system has been developed, which may be used either for on-line process control or for as an aid in understanding the main process variables.



Sheet Temperature & Plug Force Monitoring

Morphological Changes
When thermoforming semi-crystalline materials, such as PP and PET, the morphological changes that are induced during the extrusion of the sheet have profound effects on the subsequent thermoforming behaviour. Studies carried out on PP materials have shown dramatic changes in the form and degree of cystallinity induced by extrusion parameters, and ageing effects have been shown to alter material properties many days after extrusion.



Morhological Changes in Polypropylene

Extrusion & Thermoforming of Cellular Polymers
Foams or cellular materials are employed in a range of applications. In this work cellular structures were produced by the addition of chemical blowing agents during the sheet extrusion process. A large number of commercial blowing agents were investigated, producing foamed sheet with widely varying cell densities. The composition of the agents was shown to have the greatest effect on the foam structure. Subsequent work is now examining the effect of thermoforming processes on the cellular structure in thermoformed parts.



Extruded and Thermoformed Cell Structures


Material Modelling
The deformation behaviour of polymer materials under realistic processing conditions is an essential element of the understanding of thermoforming processes. However, the high temperatures, high strain rates processing conditions very difficult. At Queen's University these difficulties have been overcome through the design and development of a biaxial testing machine. This can replicate thermoforming temperatures (up to 250C), deformation speeds (up to 1.5 m/s) and biaxial deformation modes from planar through to equi-biaxial.


Biaxial Testing Machine


For further information, please contact Dr. Peter Martin or Professor Eileen Harkin-Jones.






© QUB - Materials Research Group 2003
Contact Webmaster