Foam injection molding (FIM) is one of the most promising manufacturing methods for thermoplastics, owing to its capability to produce low-density and lightweight products with high dimensional precision at fast manufacturing rates. However, achievement of a uniform fine cell structure is very challenging in this technology. The issue arises from the lack of clear understanding of cell formation mechanisms and complicated bubbles’ dynamics in FIM. Here I employed an in-situ mold visualization technique to closely and continuously monitor foaming phenomena, occurring inside the mold and during injection, in order to uncover underlying mechanisms of cell nucleation and growth in FIM. In this context, an innovative visualization mold is designed and manufactured. Using the visualization mold, I first studied cell nucleation and growth mechanisms in high-pressure FIM, and proposed the strategy of using a melt packing pressure to remove the nucleated cells during mold filling and switch nucleation mechanism from “gate-nucleation” to “shrinkage-induced” nucleation. Then, the application of a gas-counter pressure (GCP) with FIM, and cell nucleation mechanisms in this technology, was thoroughly explored. The investigation on cell nucleation and growth mechanisms was then extended to high-pressure FIM technology followed by precise mold-opening (or high expansion FIM). Inasmuch as composite thermoplastics foams comprise a large percentage of FIM parts, I extended this research to a comprehensive study on the mutual interaction of nucleated cells and carbon fibers dispersed in a polystyrene matrix in high-pressure FIM. The latter was successfully done using the designed visualization mold and the proposed high-pressure FIM protocol.