On-line Adaptive Control for Thermoforming of Large Thermoplastic Sheets

Large sheet thermoforming is widely used for the manufacture of parts such as twin sheet formed gas tanks, body panels and windshields. These complex technical parts require a precise temperature map to be realized prior to forming. In particular, the material used for forming gas tanks incorporates an EVOH barrier layer that is susceptible to tearing when exact processing conditions are not strictly respected. The problem is compounded by the fact that the temperature is presently controlled at the heating elements, while the temperature distribution across the thickness of the sheet is the main process variable. This makes the thermoforming process very susceptible to perturbations and greatly increases the number of rejected parts. In order to increase productivity and quality, the actual sheet temperature distribution before forming must adhere to the optimized temperature map as predicted by a process simulation or the recipe as determined by previous runs. The system presented here controls the amount of energy received by every sheet zone, which is equivalent to controlling the sheet temperature. It is tuned on-line by identifying the heating zone to sheet zone gains matrix using a flux meter and by correlating the matrix to the output of an infrared scanning thermometer located at the exit of the heating oven. The control system will realize the map of the required sheet surface temperature by adjusting the heating elements temperature. The system has been implemented on a Monark twin-sheet thermoforming machine that has dual 1.8mx1.8m square ovens and 504 heating elements in re-configurable zones.