In typical plastic foaming processes, plastics are subjected to various amount of stress/strain, which can induce cell nucleation and affect the final cell morphology of plastic foams. Despite many valuable insights in previous work on stress-induced foaming, further studies are needed to clarify the subject. While in-line observations of foaming processes provide useful cell growth and interaction data, a controllable stress/strain field cannot be easily induced in a plastic sample to evaluate its effect. This study presents a novel batch foaming in-situ visualization system that was developed to capture the foaming process of a plastic specimen under controllable extensional strain. Using the in-situ visualization system, this study explored the effects of extensional stress/strain on the foaming behaviors of polystyrene blown by carbon dioxide under various processing conditions. It was observed that cell density increased with the application of extensional strain and that this effect was more apparent at low processing temperatures and with the addition of talc. (C) 2010 Elsevier Ltd. All rights reserved.