The Use of Evanescent Wave Fluorescence Spectroscopy for Control of the Liquid Molding Process

Fluorescence has been demonstrated to be an accurate measurement of resin cure and is measured using an evanescent wave fiber-optic sensor. An economical optical fiber sensor has been developed with a refractive index in excess of 1.6, permitting evanescent wave monitoring of epoxy resins. In this work, the fluorescence wavelength-shift, which has been correlated with monomer conversion, is monitored during the liquid molding process. A hierarchical control strategy is being developed for liquid molding processes that will use the cure measurement provided by the evanescent wave fluorescence sensor. Single input/single output control loops are implemented at the base level to regulate the mold temperature, the vacuum pressure in the mold, either the inlet flow or pressure during injection, and the final mold pressure. A model-based feedback controller is implemented to control the chemical cure and manipulates the setpoint of the temperature controller to achieve cure control. A general high-level controller is under development to optimize the process under a wide range of operating conditions.