This research investigated the expansion behavior of polycarbonate (PC) foams blown with CO_2 to achieve a low-density microcellular foam. The expansion behavior of PC foam was interpreted by the amount of retained gas 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. Three filamentary dies were designed to investigate the effect of the die geometry. A high pressure drop rate was favorable for a high expansion ratio. By controlling all these parameters, an expansion ratio over 14 times with a cell density over 10~(10) cells/cm~3 could be achieved.
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