In-line Spectroscopic Monitoring of Foaming Gas Concentration during Foam Injection Molding with Chemical Blowing Agents

Polymeric foams are increasingly used across industries, contributing to material and energy efficiency. In particular, chemical blowing agents (CBAs) are widely used in foaming processes due to their ease of use without the need for high-pressure equipment. However, controlling the amount of foaming gas released during processing remains a challenge due to the variability in decomposition depending on the temperature, residence time, and material history. This study presents a method to enable in-line monitoring of the gas concentration during foam injection molding using near-infrared (NIR) spectroscopy, addressing the need for precise control of the concentration. A transmission-type NIR spectroscopic system was installed in the nozzle of an injection molding machine to monitor the concentration of CO2 gas produced from sodium bicarbonate. Due to the opaque and multicomponent nature of the polymer melt, CO2, H2O, and decomposition residues, absorption peaks were significantly overlapped and distorted by scattering. To resolve this, various pellet samples (Sample 1: untreated CBA-MB; Sample 2: predecomposed CBA-MB; and Sample 3: partially decomposed CBA-MB) were prepared to isolate and quantify the absorption peaks associated with foaming gases. Difference spectra confirmed that absorption peaks at 1930 and 2020 nm corresponded to H2O and CO2, respectively. A robust Partial Least Squares (PLS) regression model, trained on a combination of Sample 1 and Sample 2 data, successfully predicted CO2 concentrations even under low-gas and compositionally varied conditions (Sample 3), outperforming models trained on limited data. Finally, the model enabled real-time tracking of the gas concentration during a material changeover process, demonstrating the feasibility of this system for practical, in-line process monitoring. These results establish a foundational technology for real-time control of CBA reaction progress in foam injection molding, which is potentially extendable to other chemical foaming systems.