Patent ID: 8297096

Claim:
A hydroforming method comprising: placing a metal tube having tube ends in a lower mold of a mold comprising the lower mold and an upper mold in a manner such that the tube ends stick out from the lower mold, injecting pressurized fluid into the metal tube through an inside of a seal punch while pressing seal punches against the tube ends to apply a predetermined pressing force, filling the inside of said metal tube with a pressurized fluid to apply a predetermined internal pressure, then, while applying said internal pressure and pressing force, lowering the upper mold to close the mold, deforming the tube along with the tube ends and finishing a forming operation with said tube ends sticking out from the mold, wherein said method is characterized in that when a sectional area of a metal part of said metal tube in a cross-section vertical to an axial direction of said metal tube is S 1 [mm 2 ], a sectional area of an inside of said metal tube is S 2 [mm 2 ], a yield stress of said metal tube is YS [MPa], and said predetermined internal pressure is P 1 [MPa], a force F 1 [N] pressed by said seal punches when closing the mold satisfies formula (1): P 1 ·S 2 +0.3 YS·S 1 ≦F 1 ≦P 1 ·S 2 +0.7 YS·S 1 (1).