Patent ID: 8408034

Claim:
A hot bulge forming die apparatus for forming a preheated tubular workpiece, comprising: a first die having a cavity surface, wherein an elongated hole is formed in a circumferential edge portion of the cavity surface of the first die, and the elongated hole extends in an outward direction; and a second die having a cavity surface, wherein a projection is formed on a circumferential edge portion of the cavity surface of the second die, and the projection is received in the elongated hole thereby creating circumferential spacing around the projection and allowing for thermal expansion of the first and second die when the first and second die are clamped together, wherein an end of the elongated hole in the outward direction is positioned further outwards than a position where the projection is located when the first die and the second die are clamped together in such a state that the first and second dies are deformed due to thermal expansion, and the outer end of the elongated hole is positioned in a position such that the projection is brought into abutment with the outer end of the elongated hole when the first die and the second die are deformed due to an internal pressure therein after the first die and the second die are clamped together in such a state that the first die and the second die are deformed due to thermal expansion.