Opening polygonal rib structure and polygonal rib structure

A mold for molding an opened polygonal rib structure, including an upper-lower pair of molds, comprising:

• one of said molds has a structure in which a tapered tetragonal pyramidal male mold being rhombic in cross-sectional shape and tapered toward a tip end thereof is located at a center of each outer peripheral surface of a virtual regular polygonal columnar surface, and a longitudinal section of said tapered tetragonal pyramidal male mold is disposed along said virtual regular polygonal columnar surface;
• and
• the other of said molds has a structure in which a regular polygonal groove with a fixed width is formed, with said virtual regular polygonal columnar surface as a center, and a V-shaped recessed portion gradually reduced toward a groove depth is formed at a center in a longitudinal direction of said regular polygonal groove.

A method of molding a polygonal rib structure comprising the steps of:

• charging a space between a pair of molds with a molding material;
• and
• bringing opposed wall surfaces of tetragonal pyramidal opening portions formed by a tapered tetragonal pyramidal male mold into contact with each other to constitute a polygonal rib structure.