A numerical and experimental approach to optimise sheet stamping technologies: polymers thermoforming

The technology of polymers thermoforming is taken into consideration, with the aim of developing an optimisation technique for the preliminary set-up of process parameters and for the achievement of a better control of thickness distribution, material saving and cycle-time reduction. For this purpose, a FEM code is used, specifically developed to simulate sheet forming. A viscoelastic constitutive law is adopted, in order to improve the description of material performances. The troublesome characterisation of material. which is in a plastic, almost fluid state, deserves the use of non-standard experimental test methodologies (elongational and shear tests, dynamo-mechanical tests performed above glass transition, T-g), as well as analytical developments (computation of elongational. and shear viscosity, evaluation of rheological parameters as a function of temperature and shear rate). Such a behavioural model is firstly implemented in the simulation code, which is then tuned and validated through the simulation of the experimental characterisation tests. Finally, the air slip forming of a low-temperature refrigerator cell is numerically simulated and experimentally manufactured with the main purpose to predict thickness distribution and point out regions of minimum value. Notwithstanding the deep shape, which leads to remarkable thickness variations, the comparison between predicted and actual values at different locations shows a satisfactory agreement: as a matter of fact the error along the cell mid-section did not exceed 0.2 mm on average, corresponding to 5% initial thickness. (C) 2002 Elsevier Science Ltd. All rights reserved.