Patent ID: 7401486

Claim:
A die having a through hole for use in an extrusion process to reduce a diameter of a metal pipe or tube, the through hole having an inside surface including a bell portion, an approach portion, and a bearing portion from the entrance side of said die formed in a continuous manner, wherein a diameter of the through hole at said bell portion gradually decreases from an entrance part of said bell portion to an exit part of said bell portion, a diameter of said approach portion is D 1 on an entrance part of said approach portion and D 2 on an exit part of said approach portion, the diameter of said approach portion gradually decreasing from the entrance part of said approach portion to the exit part of said approach portion to satisfy Equation (1): 0.7 ≦D 2 /D 1<0.97 (1), a die half angle of a first part of said approach portion having a diameter D 3 is not less than a die half angle of a second part of said approach portion nearer to the exit part of said approach portion than the first part, the diameter D 3 being D 2 /0.97, an axial length LR between the first part and the exit part of said approach portion satisfies Equation (2): 20 ≦LR /(( D 3 −D 2)/2)≦115 (2), a diameter of said bearing portion is fixed at D 2 , and a length of said bearing portion is LB, the length LB satisfying Equation (3): 0.3 ≦LB/D 2≦10 (3).