Method for manufacturing short fiber oriented rubber or synthetic resine and expansion die for extrusion molding

A method of manufacturing short fiber oriented rubber or synthetic resin by extrusion molding, said method comprising:

• providing an expansion die including:
• an outer die having an inner surface;
• an inner die having an outer surface, the inner die being arranged with respect to the outer die such that an annular mold opening is formed between the inner surface of the outer die and the outer surface of the inner die, the mold opening having an expansion portion with a diameter expanding in an extrusion direction, the mold opening having an inlet portion with a radius Ri and a width Wi, and the mold opening having an outlet portion with a radius Ro and a width Wo, each of the inlet portion, the radius Ri, the outlet portion, and the radius Ro being coaxial with the mold opening;
• a first annular center block;
• and
• a second annular center block, an inner diameter of the first center block being smaller than an inner diameter of the second center block, the first center block and the second center block being interchangeably and removably mountable between the outer die and the inner die for adjusting a radius Ri of the inlet portion of the mold opening;
• wherein the outer die, the inner die, the first annular center block, and the second annular center block are configured to allow adjustment of a shape of the mold opening by interchanging the first center block and the second center block, and by removing both the first center block and the second center block, such that the shape of the mold opening meets a ratio Wo/Wi<Ro/Ri, and such that an expansion ratio Ro/Ri of the mold opening is within a range of 2≦Ro/Ri≦8;
• extruding rubber or synthetic resin mixed with short fibers from the mold opening of the expansion die;
• molding a cylindrical body of the rubber or synthetic resin such that the short fibers are oriented in a predetermined direction;
• and
• inserting, interchanging and removing the first center block and the second center block between the outer die and the inner die as necessary to adjust the radius Ri of the inlet portion of the mold opening during said extruding so as to thereby obtain a correct orientation of the short fibers in the predetermined direction within the cylindrical body.