The crystallization behavior of polypropylene (PP) and poly(lactic acid) (PLA) during foam extrusion processing was investigated via visualization techniques. In the experiments, a tandem extrusion system equipped with a conical converging flow channel and a high temperature/pressure visualization chamber were employed. Using the developed system, effect of processing parameters on crystallization was elucidated. These parameters included the processing temperature, the carbon dioxide content, the flow rate, and the strain rate. The visualized crystallites were quantified based on their light intensities and by the definition of the Crystallinity Index (CI) and the Crystallite Size (CS), where the CI and CS corresponded to the relative crystallinity and the size of crystallites, respectively. Moreover, a correlation was made between the content of the induced crystallites inside the extruder and the cellular structure of the extruded foams.

It was verified by visualization that both PP and PLA crystallize during the process even at a temperature above their nominal melting peak point (T_m). The formation of crystallites at a temperature above the T_m was asserted to the strain-induced crystallization. Moreover, a procedure was proposed in order to decouple the effect of the flow rate (corresponded to the strain rate) and the cooling history. A significant enhancement in crystallization kinetics was observed by increasing the strain rate. This crystallization enhancement caused by the increased flow rate became more pronounced at lower temperatures, most probably due to the higher degree of molecular orientation. Adding the CO₂ shifted the crystallization temperature to a lower processing temperature due to its plasticization effect, but it accelerated the crystallization kinetics. This clearly explained the decrease in the optimal processing temperature for foaming with an increased CO₂ content. It was also confirmed that the induced crystallites during the process had a great effect on the cellular morphology of the resultant foam product.