The morphology and properties of the extruded polymeric foams are affected by the geometry of extrusion die. In this paper, the flow in the slit die was simulated using POLYFLOW software package to predict the pressure distribution and residence time. Then the pressure drop rate in the die was calculated. The results for seven dies with different length (L) and gap (H) of the straight section were compared. The effects of the L and H on the nucleation position were analyzed. Based on the critical foaming pressure (Pf), the minimum length and maximum gap providing required pressure drop were determined under a constant gap and a constant length, respectively.

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