Hong, Ai LinAi LinHongChiu, Chun YangChun YangChiuKao, Cheng YiCheng YiKaoFang, Chen-YuChen-YuFangGebremedhin, Kiday FisehaKiday FisehaGebremedhinYeh, Shu KaiShu KaiYehSEN-YEU YANGPark, Chul B.Chul B.Park2026-03-162026-03-162026-0608968446https://www.scopus.com/record/display.uri?eid=2-s2.0-105028965573&origin=resultslisthttps://scholars.lib.ntu.edu.tw/handle/123456789/736348Foam injection molding (FIM) has remained largely empirical due to the opacity of molds preventing direct observation of foaming dynamics. This study establishes a visualization platform for systematic investigation of FIM processes, enabling future research across diverse polymer systems. Building upon established methodologies, this work successfully reproduces and validates previous findings while extending investigations to unexplored parameter regimes. A custom-designed mold with integrated optics and pressure sensors enabled simultaneous visual and pressure monitoring of polystyrene foaming with CO2. Systematic experiments investigated both low-pressure (LPFIM) and high-pressure (HPFIM) FIM with core-back technology, demonstrating the platform's capability to capture fundamental process-structure relationships across different processing modes. In LPFIM, an insufficient mold venting design can affect mold filling, with better mold filling achieved at lower injection speeds (50 cm3/s) compared to higher speeds (80 cm3/s). This finding contrasts with previous studies using optimized venting systems, underscoring the crucial role of mold design in achieving LPFIM success. In HPFIM with core-back, a critical transition was observed at 0.8 mm core-back distance, triggering cell density increases from < 60 cells/cm3 to > 4500 cells/cm3. An optimal core-back velocity of 33 mm/s was identified, with higher velocities causing pressure rebound and rapid gas consumption that suppress further nucleation. These fundamental insights provide qualitative guidelines for process optimization while establishing a robust research platform for future investigations across different polymer-blowing agent systems.falseCell nucleationCoreback processExtensional stressFoam injection moldingReal-time visualizationSupercritical carbon dioxidejournal article10.1016/j.supflu.2026.1068972-s2.0-105028965573