The Role of Tool/Sheet Contact in Plug-Assisted Thermoforming.

Plug-assisted thermoforming produces a wide range of polymer products through a combination of deformation by air pressure and contact with tool surfaces. In this paper the role of tool/sheet contact in determining the process output is investigated. A combination of thermoforming, friction and heat transfer tests were carried out on common tool and sheet materials. The results show that the typical friction coefficients for the material combinations are within the range 0.1 to 0.3, but the values rise sharply on approaching thermoforming temperatures. Thermal imaging tests demonstrate that all of the plug materials significantly cool the heated sheet on contact, even over very short periods of time. The temperature of the plug is very important. At low plug temperatures heat transfer effects predominate, whereas at high plug temperatures friction effects predominate. A plug temperature of approximately 100oC balances these effects and creates the most effective material distribution.

Biaxial Characterisation of Materials for Thermoforming and Blow Moulding

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 this paper the results of several programmes of biaxial tests conducted at Queen’s 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.

Polystyrene and asphaltene micelles within blends with a bitumen of an SBS block copolymer and styrene and butadiene homopolymers

Using fluorescence microscopy, DSC and DMTA we have explored blends of a bitumen with a styrene-butadiene-styrene (SBS) block copolymer, and with blends of the bitumen with SBS and one or two homopolymers - a polystyrene and a poly(cis-butadiene). The SBS polymer was progressively replaced with quantities of the homopolymers both together in the proportions found in the block copolymer and then by each homopolymer separately. At low temperatures the blends are all softer than the bitumen itself, so the polymers plasticise the bitumen-rich phase, and above 50°C the blends' stiffness (E') falls below a plateau only when a critical proportion of the block copolymer has been replaced with the two homopolymers: this supports the idea of an extensive network created by the polystyrene-rich spherical microphases that is effective even when the polystyrene microphases have melted. In one polymer blend the stiffness rose as the temperature was raised above 100°C, suggesting the development of a mesophase based upon polybutadiene plus asphaltenes, in another E' was enhanced and E" remained constant as the temperature rose above 70°C, perhaps for a similar reason; in some loss process appeared and the stiffness fell as temperature rose; but in others a good part of the SBS was replaced by either polystyrene or polybutadiene without changing the appearance of a rubbery plateau, that is, without a diminution of the mechanical properties of the soft matter.