THERMAL EFFECTS IN THE NUMERICAL-SIMULATION OF THE THERMOFORMING OF MULTILAYERED POLYMER SHEETS

This paper mainly treats the thermal effects during the thermoforming process while most of the previous analyses consider an isothermal def ormation. A non isothermal three dimensional finite element model of t he thermoforming process is proposed. It couples the thermal equations in the thickness and mechanical equations on the mean surface of the sheet. The mechanical resolution is done by a finite element method us ing a membrane approximation. The deformation is driven by a pressure difference through the sheet. The thermal resolution uses a one dimens ion finite element method in the thickness with convection or conducti on at the surface and dissipation of mechanical energy. The polymer co oling is very efficient during the contact with the tools. The couplin g is done by the thermal dependent rheology. The respective contributi ons of friction and thermal effects in the thickness of the part durin g the process are discussed. The model also considers a possible multi layered material, with specific rheological parameters inside each lay er. The rheology of a polystyrene was measured under elongation as a f unction of temperature, strain and strain-rate and described by a visc o-plastic law. The predictions of the model were compared with measure ments on an instrumented thermoforming machine and with the local thic kness of axisymmetrical parts and with 3-D parts thermoformed with the same polystyrene.