Addition of nucleating agents (e.g., talc) is a common way to promote the cell density in polymeric foaming process. It is widely believed that such enhancement is caused by the decrease in free energy barrier for the nucleation to initiate heterogeneously as well as the reduction in gas loss with the existence of the inorganic fillers. In this paper, in situ visualization of the cell formation phenomena during polymeric foaming processes of polystyrene-talc composites blown with carbon dioxide revealed that the expansion of nucleated cells triggered the formation of secondary cells around them. Subsequently, the expansion of the secondary cells also promoted the formation of tertiary cells around them similar to a chain reaction. These observations provided evidences to support the theoretical simulation of stress-induced cell formation around expanding bubbles. A series of parametric studies were conducted to correlate the stress-induced cell formation and various processing and material parameters. The elucidation of the aforementioned cell formation mechanism with the presence of nucleating agents would provide additional guidelines for polymeric foam manufacturers to control the cell morphologies of their products in order to optimize and tailor the desired physical properties. (C) 2011 Elsevier B.V. All rights reserved.