BULGE FORMING METHOD AND BULGE FORMING APPARATUS

A bulge forming method includes arranging a raw material tube inside a die having an inner surface shape, in which plural crest portions and root portions are alternately formed, and providing a rod inside the raw material tube in the axial direction of the raw material tube, and the rod has a pressurized liquid supply opening. The bulge forming method further includes providing a pair of sealing portions so as to have the pressurized liquid supply opening therebetween, and the sealing portions seal a space between the raw material tube and the rod. The bulge forming method further includes moving the rod in the axial direction while applying compressive stress to the raw material tube in the axial direction and supplying a pressurized liquid from the pressurized liquid supply opening into the raw material tube, whereby the raw material tube is formed into the shape of the inner surface of the die. The sealing portion is in an anterior side of a forming direction is applied with liquid pressure of the pressurized liquid in a direction from the rod toward the raw material tube so as to be followable to bulging of the raw material tube. The distance between the sealing portions is set to have a length of at least two crest portions of the inner surface shape of the die, and a length in the axial direction of the sealing portion in a posterior side of the forming direction is set to have a length of at least one crest portion of the inner surface shape of the die.