This paper presents the optimization of a continuous foam extrusion process to manufacture high impact polystyrene (HIPS) foams through a twin-screw extruder. The effects of chemical blowing agent type and content as well as; process parameters: e.g., die temperature profile and screw speed (RPM) were investigated. After determination of the optimum die temperature profile, effect of chemical blowing agent type and content (i.e., 1, 3, 5 wt. %) and screw RPM (i.e., 100, 150, 200, and 250) were investigated. Results show that; the best void fraction (around 22%), with the highest cell density (around 6 x 10(5) cells/cm(3)) obtained at die temperature profile of 150-145 degrees C. It was also observed that 100 RPM, revealed the highest void fraction (around 32%) with cell density of around 3 x 10(5) cells/cm(3). The increase of chemical blowing agent, increased the void fraction from around 10% to 31% where the cell densities were also increased, respectively, from around 8 x 10(4) cells/cm(3) to 2 x 10(7)cells/cm(3).