Forming processes for hollow plastic bodies, such as blow moulding and thermoforming, involve viscoelastic deformation phenomena of polymers materials at high temperature. In order to numerically simulate such processes by the finite elements method, the knowledge of the non linear behaviour of polymers is of chief importance. In this context, we designed an experimental apparatus based on the bubble inflation technique, which permits to produce the biaxial deformation states encountered in industrial processes. A series of tests with constant blowing air flow rates was carried out on ABS (Acrylonitrile butadiene styrene). The viscoelastic behaviour of this material and the importance of the internal pressure evolution were highlighted. Moreover, the constants of some hyperclastic and non linear viscoelastic constitutive equations were identified. Secondly, a FE code for the simulation of blow moulding and thermoforming processes was developed, in which the comportment laws determined at the previous step were implemented. A dynamic formulation of the problem is adopted and 3-node triangular elements are used for the meshing of the parison. The contact with the mold is assumed to be sticky. The numerical solution uses a central finite differences scheme. As two special features aimed at addressing problem of practical interest, our program was fitted with an adaptative mesh refinement procedure which takes account of the geometric changes and the choice was given to blow the parison with an air flow rate instead of a pressure. Comparisons with semi-analytical and experimental results allowed us to validate our program.
Numerical and experimental contribution to blow moulding and thermoforming processes