Patent ID: 8025497

Claim:
An expansion die for extrusion molding, comprising: an outer die having an inner surface; an inner die having an outer surface, said inner die being arranged with respect to said outer die such that an annular mold opening is formed between said inner surface of said outer die and said outer surface of said inner die, said mold opening having an expansion portion with a diameter expanding in an extrusion direction, said mold opening having an inlet portion with a radius Ri and a width Wi, and said mold opening having an outlet portion with a radius Ro and a width Wo, each of said inlet portion, said radius Ri, said outlet portion, and said radius Ro being coaxial with said mold opening; a first annular center block; and a second annular center block, an inner diameter of said first center block being smaller than an inner diameter of said second center block, said first center block and said second center block being interchangeably and removably mountable between said outer die and said inner die for adjusting a radius Ri of said inlet portion of said mold opening; wherein said outer die, said inner die, said first annular center block, and said second annular center block are configured to allow adjustment of a shape of said mold opening by interchanging said first center block and said second center block, and by removing both said first center block and said second center block, such that the shape of said mold opening meets a ratio Wo/Wi<Ro/Ri, and such that an expansion ratio Ro/Ri of said mold opening is within a range of 2≦Ro/Ri≦8.