HYDROFORM PROCESS

PROBLEM TO BE SOLVED: To provide a hydroform process capable of preventing a tube stock from being buckled. SOLUTION: The tube stock 1 is set on forming dies 2A and 2B, and both ends thereof are held by a pressing cylinder 4, and the compression is applied thereto along the axial direction of the tube stock. The internal pressure is applied to the tube stock 1 by introducing the high pressure water into the tube stock 1 from a water passing hole 5 opened in a tip of the pressing cylinder 4, and the external pressure is applied to the tube stock 1 by introducing the high pressure water between the tube stock 1 and the forming dies 2A and 2B from a water passing hole 6 in an expansion part 3 to form the tube stock 1 into a predetermined form along forming surfaces of the forming dies 2A and 2B. Generation of the buckling can be avoided by the internal pressure and the external pressure applied to the tube stock 1.