IN-LINE OPTICAL MONITORING OF POLYMER INJECTION-MOLDING

An optical sensor, consisting of optical fibers to transmit light to a nd from the mold cavity, was constructed for the purpose of measuring the onset of polymer solidification during injection molding. The sens or was used to detect characteristic fluorescence radiation from a dye which had been doped into the resin at very low concentration. By mea suring changes in fluorescence intensity it was possible to detect whe ther the state of the resin was liquid or solid. We observed that, as the resin cooled in the mold. the onset of solidification was indicate d by highly characteristic and distinct changes in the fluorescence in tensity/time profile. Application of the method involved the use of a calibration relationship between the fluorescence intensity and temper ature of the doped polymer in order to deter-mine the distinct feature s which characterize the onset of solidification. Injection molding of a glass forming polymer (polystyrene) and a crystallizable polymer (p olyethylene) was monitored by this technique.