Crystallization has been reported as one of the important phenomena when foaming of semi-crystalline thermoplastics. In this study, the effect of processing conditions on crystallization kinetics of polylactic acid (PLA) and polypropylene (PP) was investigated in a tandem extruder by means of direct visualization. It was observed that crystallites formed for both PLA and PP by decreasing the cooling extruder barrel temperature. Using the concept of the axiomatic design approach, the effect of cooling and flow-induced crystallization was decoupled during extrusion. It was achieved by processing the melt at a low flow rate (identical cooling) followed by a sudden increase in the flow rate (higher strain rates). A significant crystallization enhancement was visualized and quantified when the molten polymer experienced higher strain rates while maintaining the identical cooling history. In addition, the extrusion foaming behavior of PLA and PP was investigated in terms of verifying and elaborating the role of induced crystals on the cellular structure. A correlation was made to better understand the effect of crystals on foaming by comparing the images in-situ taken from the crystallites to the ex-situ characterization of the foam samples.