Online characterization of viscoelastic and stress-relaxation behavior in thermoforming

The thermoforming process is subject to a lot of variability with regard to the viscoelastic properties of the material, and the level of residual stresses caused by the manufacturing (extrusion and calandering) process of a plastic sheet. A robust control system must be able to identify and adjust to these perturbations in real time during the thermoforming process.The research project proposes two process sub-models, which predict the behavior of the elastic modulus and the level of frozen-in stresses. These two sub-models will be used in a model based control system, consisting of a number of process submodels encompassing different material properties and machine characteristics.A series of experiments was conducted using an industrial thermoforming machine to measure the elasticity and the level of frozen-in stresses in the material. A bubble inflation technique was used to determine the behavior of the elastic modulus in the transition state between solid and liquid. Additionally, a simulation of the sag that occurs during plastic sheet heating was developed to identify the level of frozen-in stresses