We used an in-situ mold visualization technique to explore foaming phenomena in foam injection molding of semi-crystalline polymers. A polypropylene/carbon dioxide (PP-CO2) mixture was used as the sample foaming system. We employed a high-pressure protocol such as using a full-shot injection followed by melt packing. Two major cell nucleation mechanisms were identified as follows: (i) nucleation of cells due to the melt shrinkage during the melt-cooling phase. This was similar to the cell nucleation that occurs in high-pressure foam injection molding of amorphous polymers, and produced relatively large cells inside the melt; (ii) heterogeneous nucleation of cells on the surface of solidified crystals in the polymer melt. The latter produced a myriad of fine cells, rapidly, and was only observed in foam injection molding of the semi-crystalline polymers. We examined the effect of various packing conditions on the cell nucleation in high-pressure foam injection molding of PP-CO2. We learned that the cell nucleation mechanism and the overall cell structure can be governed by adjusting the packing length under a constant packing pressure magnitude. A short packing time promoted the cell nucleation upon melt shrinkage, whereas a lengthy packing promoted the cell nucleation occurring on the surface of the solidified crystals.