Method of forming captive can closure

A method of integrally forming a sliding closure member in a can lid by sequential punching and die forming in four steps;

• firstly, punching said sliding portion, displacing it partially from the container lid, with complete shearing of the metal taking place;
• and secondly, forming the sheared edge of the closure member between two dies to round the inner and outer edges and extrude the metal, slightly increasing the projected area of the closure;
• thirdly, repositioning the sliding portion to its original position in the opening while clamping it between two rounded dies to reduce its projected area to that of the opening;
• and fourthly, forming the edges of the opening between two dies to conform to the rounded edges of the closure, causing extrusion of the edges of said opening, increasing its projected area and causing it to overlap and tightly seal against the edges of the closure while forming guide grooves for the sliding member, and restoring the plane surface of the container top, further increasing sealing pressure.