Investigation into thermoformability of hot compacted polypropylene sheet

Abstract

This paper describes an investigation into the thermoformability of a new class of oriented polymeric material recently developed, namely hot compacted polypropylene sheet. Exploitation of any new material requires an intimate understanding of a whole range of factors, amongst which thermoformability is pre-eminent. This is particularly true for oriented polymeric materials, for while the preferred molecular alignment gives enhanced properties such as stiffness, strength, and resistance to impact, the downside is that the stretched molecular chains tend to limit further flow under stress, making thermoforming difficult. The aim of the present study was to establish the critical parameters for successful thermoforming of hot compacted polypropylene sheet.

Elevated temperature tensile tests were used to investigate the stress–strain behaviour of the compacted materials. The crucial parameters were found to be the post-yield modulus, which gives a measure of the resistance of the material to large scale deformation, and the strain to failure, which gives the upper limit on deformation. The post-yield modulus was found to be significantly affected by the test temperature and the high strain hardening behaviour of the material confirmed that significant force is required to thermoform the compacted polypropylene sheets. A hemispherical mould, with built-in gripping plate, was used to carry out a study of the thermoforming behaviour of the compacted sheets, and the results were found broadly to confirm the conclusions of the tensile tests. A linear relationship was found between the tensile force and the postforming force, reinforcing the synergy between the two tests. In addition the forming tests showed that the best temperatures to use were either side of the melting point of the melted and recrystallised phase, depending on the amount of postforming deformation required. Different gripping arrangements were investigated both in which the sheet was fully gripped and in which the sheet was allowed to flow into the mould during forming. The different schemes were found to control whether a successful component could be produced under different conditions and at different ultimate strains. Finally, the tests with the hemispherical mould showed that thermoforming this shape requires significant interlaminar shear deformation, and above 15% strain this resulted in destruction of the interlayer bond. For strains greater than this, successful thermoforming could only be achieved by allowing the material to flow into the mould.