Investigation into thermoformability of hot compacted polypropylene sheet

This paper describes an investigation into the thermoformability of a new c lass of oriented polymeric material recently developed, namely hot compacte d polypropylene sheet. Exploitation of any new material requires an intimat e understanding of a whole range of factors, amongst which thermoformabilit y is pre-eminent. This is particularly true for oriented polymeric material s, for while the preferred molecular alignment gives enhanced properties su ch as stiffness, strength, and resistance to impact, the downside is that t he stretched molecular chains tend to limit further flow under stress, maki ng thermoforming difficult. The aim of the present study was to establish t he critical parameters for successful thermoforming of hot compacted polypr opylene sheet. Elevated temperature tensile tests were used to investigate the stress-stra in behaviour of the compacted materials. The crucial parameters were found to be the post-yield modulus, which gives a measure of the resistance of th e material to large scale deformation, and the strain to failure, which giv es the upper limit on deformation. The post-yield modulus was found to be s ignificantly 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 c onfirm the conclusions of the tensile tests. A linear relationship was foun d between the tensile force and the postforming force, reinforcing the syne rgy between the two tests. In addition the forming tests showed that the be st temperatures to use were either side of the melting point of the melted and recrystallised phase, depending on the amount of postforming deformatio n required. Different gripping arrangements were investigated both in which the sheet was fully gripped and in which the sheet was allowed to flow int o the mould during forming. The different schemes were found to control whe ther a successful component could be produced under different conditions an d at different ultimate strains. Finally, the tests with the hemispherical mould showed that thermoforming this shape requires significant interlamina r shear deformation, and above 15% strain this resulted in destruction of t he interlayer bond. For strains greater than this, successful thermoforming could only be achieved by allowing the material to how into the mould.