The global demand for polymers especially Polypropylene (PP) foams is increasing rapidly. Foam structures can be very beneficial for producing structural components which can be significantly larger than the raw material formed by volume expansion. In this context, the objective of this work is to develop uniform fine-cell and low-density polymer foams with improved mechanical properties. In order to promote a deeper understanding of the low-density (<0.1 g/cc) and microcellular structure (10^8 cells/cm^3), a novel foaming-visualization system was developed. This novel custom-made system captures in situ crystallization-induced foaming behaviors of polymers. The shear effect on bubble and crystal growth processes were investigated independently in an isolated manner. Based on data observed from the visualization system, a two-dimensional model of the foam nucleation process was developed. The model was extended to account for the simultaneous cell nucleation, growth, and collapse processes of the foaming bubbles. By means of connection among neighboring bubbles, secondary nucleation behaviors emerged from multi-bubble interactions were attempted in simulations. Finally, the effects of gas pressure, temperature, additive content, and shear stress were thoroughly investigated for the sake of optimizing the processing conditions and foamed products. Potential applications from these researches lie in the analysis of the resulting micro-/nano-cellular structures and the development of innovative plastic foaming technologies and foams.