Due to the simulation, a better understanding has been gained of the deformation of the membrane during the forming step, the evolution of the temperature in the membrane and the pneumatic aspects linked to the discharge of the air through the vents. It is now possible to simulate the plastic thermoforming process. The code, that takes full account of inertia effects, solves the equations of motion using a second order, explicit, time integration algorithm. The thickness distribution of the finished parts agrees well with the predictions. A new method of strain measurement has been developed based on an optical method without contact has been introduced. Also, the non linear viscoelastic characterization of polystiren, polypropilen and a bilayer material polymethyl methacrylate/acrylonitrile-butadiene-styrene (PMMA/ABS) has been completed. An innovative method for measuring the membrane displacements during the process has been set up and the critical parameters of the process have been highlighted, (ie the importance of the temperature control during the heating step and the importance of the air vents location and pressure control during the forming step). The simulation has clearly demonstrated that significant progress can be made to remove the art of this process but the control of temperatures and pressures by the end users must be improved for the simulation to be really predictive.