Patent ID: 7368680

Claim:
A method of producing a plate stack preform useable upon hydrostatically forming a product including a frame and a reinforcement which is disposed within the frame and has an X-shape or cross shape in section, the plate stack preform including a flat tubular body and a reinforcement stack within the tubular body, the method comprising: preparing the reinforcement stack that has a flattened X-shape or cross shape in section and includes a first reinforcing plate and a second reinforcing plate overlapped with each other; overlapping a first plate larger in size than the first reinforcing plate of the reinforcement stack on the first reinforcing plate thereof and a second plate larger in size than the second reinforcing plate of the reinforcement stack on the second reinforcing plate thereof; inserting a weld-preventing conductive plate between side peripheries of the overlapped first and second reinforcing plates of the reinforcement stack, the weld-preventing conductive plate preventing the side peripheries of the overlapped first and second reinforcing plates from being welded to each other; after the inserting operation, simultaneously conducting joining the first plate with the side periphery of the first reinforcing plate and joining the second plate with the side periphery of the second reinforcing plate by lap resistance welding; and after the simultaneously conducting joining operation, joining side peripheries of the first plate with side peripheries of the second plate by continuous welding, to form the flat tubular body within which the reinforcement stack is disposed.