Hydroforming method and mold used for the hydroforming method

A mold used for a hydroforming method in which a slot with an aspect ratio of 3 or more is pierced onto a metal tube blank, the metal tube blank held by a pair of molds, and pressure is applied onto a working fluid inside the metal tube blank for hydroforming and piercing, with a piercing punch contacting an outer surface of the metal tube blank during application of the pressure during hydroforming and retreating from a surface of the hydroformed metal tube for piercing, the mold comprising:

• a die port for enabling a piercing punch to move slidably;
• an opening with the aspect ratio of 3 or more at a bottom of said die port;
• and
• a concave segment disposed in a length-wise direction on a top face of said piercing punch;
• wherein a concave depth Hg of the concave segment disposed on the top face of said piercing punch satisfies the relationship expressed by the equation [1] as below, where t=thickness of the metal tube blank at the bulge-formed portion:
• 0.1t<Hg3t [1].

A mold used for a hydroforming method in which a slot with an aspect ratio of 3 or more is pierced onto a metal tube blank, the metal tube blank held by a pair of molds, and pressure is applied onto a working fluid inside the metal tube blank for hydroforming and piercing, with a piercing punch contacting an outer surface of the metal tube blank during application of the pressure during hydroforming and retreating from a surface of the hydroformed metal tube for piercing, the mold comprising:

• a die port for enabling a piercing punch to move slidably;
• an opening with the aspect ratio of 3 or more at a bottom of said die port;
• and
• a concave segment disposed in a length-wise direction on a top face of said piercing punch;
• wherein a concave width Wg of the concave segment disposed on the top face of said piercing punch satisfies the relationship expressed by the equation [2] as below, where Wp=punch width:
• 0.4<Wg/Wp<0.95 [2].