The theoretical prediction of thermoformed carbon fibre reinforced thermoplastic materials in support of optimal process design

This paper develops a theoretical model capable of predicting the deformation behaviour of a thermoplastic composite, 90° angled part during manufacture using a thermoforming process. The model is intended to support a virtual approach to understanding process capability. This in turn will increase the utility of digital manufacturing methods by enabling the use of more realistic part forms in product and process design. A typical V-shape carbon fibre reinforced polyphenylene sulfide (PPS) composite part was selected as the demonstrator. A custom built thermoforming cell was used to manufacture a series of samples to investigate the process-induced shape variation of the part based on a range of tooling temperatures during cooling. It was found that the influence of mould temperature on the deformation is more dependent on the composite's thermal properties. After de-moulding, the part deforms because of the thermal and crystallization shrinkage during the cooling from mould temperature to room temperature. For the same ply orientation, the final bend angles decrease with increasing mould temperature. The processing conditions were then used as the basis of a theoretical model designed to predict the final part angle. The theoretical model was developed according to basic equilibrium, compatibility, and constitutive equations. The displacement model was supplemented with boundary and continuity conditions to solve final laminate deformations. In the constitutive model, thermal and crystallization shrinkage strains are considered because of semi-crystalline and very low moisture absorption properties of PPS. The stiffness matrix is considered as temperature dependent as the behaviour of PPS is highly temperature dependant. The largest difference between experimental and predicted results was 16.26% for the sample formed at the 170°C mould temperature whereas the calculation outcome for the sample formed with 110°C mould was within 1.44% of the experimental result. The calculations show that composite part deformation is dominated by temperature change induced anisotropic thermal strains.