A first step towards the modelling of the thermoforming of plastic sheets

A computer code is developed to improve the understanding of the process of the thermoforming of plastic sheets. The physical basis of the model deals with the constitutive laws of several polymeric materials (PP, PS, PMMA/ABS) under large strains and strain rates, assuming viscoelastic behaviour and temperature-dependent mechanical properties. Tensile tests are carried out on a servohydraulic Instron device, the deformation of the sample being measured on line by means of a CCD camera. Hyperelastic and memory-integral constitutive laws are investigated. Parameter identification is carried out using theoretical incompressible tensile behaviour for several models. The stretching of the plastic sheet under isothermal conditions is formulated using simple finite elements. The non-linear dynamics of the thermoforming process, involving large strains, finite rotations, large thickness variations and non-conservative loads, are accounted for by using a fully-explicit algorithm and co-rotational stresses and strains. A membrane assumption is made, whereby no gradients are considered through the thickness.