Control of the physical blowing agent (PBA) concentration in polymers is critical in achieving stable production of polymeric foams. However, there are very few in-line chemical monitoring techniques for foam injection molding (FIM) processes. The difficulty in sensor application with regard to FIM comes from nonsteady-state operations, where the temperature and pressure temporarily change dynamically. We invented an in-house fiber optic near-infrared (NIR) probe with high-pressure and high-temperature resistances and applied it to the transmission NIR spectroscopy of FIM processes to monitor the PBA, i.e., CO 2 , dissolved in the molten polymer. The CO 2 concentration was successfully monitored from the CO 2 absorption spectra at 4952 cm −1 and the polymer's absorption spectra even at a 120 MPa injection pressure and 200 °C processing temperature. Furthermore, the phase separation of CO 2 from the polymer before injection was detected from the decrease in transmittance of the NIR spectra.
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