Die capable of being opened, manufacturing method thereof, and mold

A die which is structured by a polyhedron having a multiple planes of a similar shape and which is selectively opened into a continuous polygon area on a plane and closed into an assembled state, wherein, in the assembled state of said polyhedron, said die has outer planes (11o through 16o) positioned on the outer side and inner planes (11i through 16i) positioned on the inner side, and has inclined boundary areas (Rb) having a predetermined inclination angle at boundaries of said polygonal area of said inner side, while predetermined inclined boundary areas (Rb) of said opened polygonal area have engaging portions (21a, 21b, 24a, 24b, 25a, 25b, 26a, 26b) of a size not exceeding each boundary area (Rb);

• the die comprising:
• bridge portions (Br) which are intermediate between the remaining inclined boundary areas (Rb) except the predetermined inclined boundary areas (Rb);
• the bridge portions (Br) linking two adjacent ones of the planes to each other to provide a linear extended groove which is left on the outer plane sides of the two adjacent planes and which is placed on an outside of the remaining inclined boundary areas (Rb).