Process Monitoring of Hot-Press Molding of FRP Laminates by Fiber-Optic Index Sensor

Abstract

We have developed a measurement method of refractive index for monitoring degree of cure of resin. The present paper describes the application of fiber optic refractive-index sensor on hot-press molding of GFRP laminates. From the experimental results, it appeared that the transverse molding pressure to optical fibers did not affect the optical loss. The increase of optical power resulted from reflection light between air and resin flow closing to the sensor. It was found that the cure process of FRP laminates during hot-press molding could be monitored by this method after the sensor edge was filled with resin. During gelation of resin, it could be seen that large fluctuation existed in the sensor signal due to scattering from micro air-voids in resin. The fluctuation disappeared within 20 minutes because the voids were vacuumed completely. The trend of index curve turned from decrease to increase because of proceeding of cure reaction and then became constant when cure reaction finished. From the experimental results, it was found that the embedded refractive index sensor could detect resin flow and void vacuuming as well as degree of cure.