In this study, different mechanisms contributing to the structural development of amorphous and semi-crystalline thermoplastics in foam injection molding (FIM) process have been discussed. A visualization technique was employed which enabled us to monitor various steps of the process, in-situ, and led us to gain new insights and achieve clear understanding of cellular structure development in FIM process. Polystyrene (PS), amorphous, and polypropylene (PP), semi-crystalline, were physically foamed using carbon dioxide (CO2) as the blowing agent. The FIM process was performed with and without mold opening. We learned that the cell nucleation due to the cavity’s pressure drop is the major cell nucleation mechanism in FIM of pure amorphous polymers. Furthermore, a proper melt packing pressure can be used to re-dissolve pre-mature cells, which nucleated during the mold-filling stage, to (i) enhance the structural uniformity and (ii) to increase the obtainable pressure-drop rate either at melt-shrinkage stage or at mold-opening stage. Furthermore, it became evident from the in-situ visualization results that the cell nucleation in FIM process of semi-crystalline polymers differs from that of amorphous polymers by the distinct contribution of the formed crystals as suitable nucleating site for cells. Thus, in addition to controlling the pressure drop and its rate inside the mold cavity, the effects of processing conditions on the crystallization behaviour and kinetics must also be considered to govern the cell structure development in FIM process of semi-crystalline polymers.