Manufacturing method for battery case lid including explosion-proof valve

A manufacturing method for a battery case lid including an explosion-proof valve, the manufacturing method comprising:

• an extending step of pushing an extending punch from a first side of a metal plate disposed over an extending die having a recess, to form a reduced thickness section constituting the explosion-proof valve,
• wherein the recess has an inner diameter greater than an outer diameter of the extending punch,
• wherein a clearance between a side face of the recess of the extending die and an outer periphery at a top section on a side face of the extending punch is set to be smaller than a thickness of the metal plate, the top section being formed by an end of the extending punch facing the recess,
• wherein in the extending step, the extending punch is pushed while the metal plate is kept unrestrained, and the metal plate is pressed to be extended between a bottom face of the recess and a pressing face of the extending punch, to form the reduced thickness section and a cylindrical wall portion at least until the thickness of the metal plate is smaller than the clearance between the side face of the recess of the extending die and the outer periphery at the top section on the side face of the extending punch, and
• wherein the extending step is followed by a bending step in which the reduced thickness section is pressed by a bending punch smaller in outer diameter than the reduced thickness section from a second side of the metal plate, to bend a cylindrical wall portion formed around the reduced thickness section.