Thin-wall parts exist in many products and are frequently manufactured by injection molding. The recognition and separation of inner and outer surfaces on the computer-aided design (CAD) model of a thin-wall part has many applications. One of its applications is for extrusion-faces recognition on the CAD model. The main idea of the proposed method was to separate those faces that are obviously visible and invisible from the mold opening direction. The former belong to the inner surface, while the latter belong to the outer surface. A series of region-growing procedures were then implemented to expand the faces on the inner surface and finally yield the desired faces on the inner surface. With all inner faces available, an extrusion recognition algorithm was also proposed to recognize and group each set of extrusion faces automatically. The recognition of extrusion faces on a thin-wall model can enhance the generation of solid meshes with better quality for mold flow analysis.