We studied the mechanism of cellular structure development in high-pressure foam injection molding (FIM), using amorphous and semi-crystalline polymers. We used a visualization approach to investigate the mold-filling and foaming phenomena. Polystyrene (PS) and polypropylene (PP) were blown using carbon dioxide. The mold filling was followed by packing pressure to redissolve the cells nucleated during the mold filling. In experiments with both PS and PP, the cell nucleation occurred in the cooling phase, due to the pressure drop obtained during the melt shrinkage (i.e., shrinkage-induced nucleation). With PP, however, additional cell-nucleation behavior was identified. A myriad of fine cells nucleated rapidly inside the melt; this nucleation mechanism was activated after a certain elapsed time from the mold filling. The final cell structure of the molded PP foams was characterized with large shrinkage-induced cells and ultra-fine cells. We surmise that this mode of cell nucleation, observed only with PP, is associated with PP crystallization and the heterogeneous nucleation of cells on the surface of the formed crystals. The temperature profiles were estimated within the sample thickness and were correlated to samples’ crystallization temperature. A reasonable agreement was concluded between the estimated crystallization time of the sample (at each layer) and the appearance of ultra-fine cells in visualization snapshots, reinforcing the hypothesis of a heterogeneous cell nucleation on the formed crystals. We learned that: (i) cell nucleation due to cavity pressure drop is the major nucleation mechanism in high-pressure FIM of amorphous polymers; (ii) cell nucleation in FIM of semi-crystalline polymers differs from that of amorphous polymers by the distinct contribution of the formed crystals; (iii) the effects of processing conditions on the crystallization behavior of polymers must be considered to control the cell structure in FIM of semi-crystalline polymers.