This study is an effort to modify conventional batch processes to be able to produce polymeric foams with high cell density and small cell size, which cannot be reached by conventional batch foaming processes. This has been attained by controlling the foaming temperature and controlled stabilization of the cellular structure. The method was tested for both with and without addition of nanosized particles in polymeric matrix. The desired morphologies were obtained using a novel apparatus with the capability of instantaneous pressure drop and controlling stabilization of the foam structure. The design of the said apparatus was based on the idea that in a foaming process, nucleation is the predominant mechanism that determines the final foam structure. The produced foam products have uniform structures without any unfoamed skin. Results show that the control of the foaming temperature and the cell stabilization are the predominant factors in adjustment of the final foam morphology. A wide range of microcellular structures with cell densities between 10(7) and 10(12) bubbles/cm(3) and average cell sizes of 500 nm-20 mu m were produced. Foaming of polystyrene-nano-silica nano-composites with the same method showed that nanoparticles act as nucleating agent and increase the cell density in the final foam products compared with that of neat polystyrene. POLYM. ENG. SCI., 50:1558-1570, 2010. (C) 2010 Society of Plastics Engineers