This research investigated the expansion behavior of extruded polycarbonate (PC) foams blown with supercritical (sc) CO2 to achieve low-density microcellular foams. The expansion behavior of PC foam was interpreted by the amount of gas retained in the cell structure in consideration of cell opening, cell-to-cell diffusion, and melt stiffening. The expansion ratio curve plotted against the die temperature showed a typical mountain shape, confirming our previous results. In addition, because the expansion behaviors and foam properties are strongly dependent on the cell density, the cell nucleation behaviors were also thoroughly investigated. Three filamentary dies were designed to investigate the effects of the die geometry. The pressure-drop rate had some effect on the cell density of PC foams as expected, i.e., a higher pressure-drop rate resulted in a higher cell density of PC foams. A high pressure-drop rate was also favorable for a high expansion ratio. By controlling all of these parameters, an expansion ratio of over 14 with a cell density of over 10(10) celIs/cm(3) could be achieved.