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, and 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 105 cells/cm3) were obtained at die temperature profile of 150-145 ⁰C. It was also observed that, 100 RPM, revealed the highest void fraction (around 32 %) with cell density of around 3x105 cells/cm3. The increase of chemical blowing agent content increased the void fraction from around 10 % to 31 % where the cell densities were also increased, respectively, from around 8 x 104 to 2 x 107 cells/cm3.