Rimmed three-piece can construction involves several stages;
· Forming a tube and welding or soldering the seam of the sides
· Joining the bottom end to the tube
· Printing or attaching labels to the can
· Filling the can with content; sterilization or retorting is required for many food products
· Joining the wall and top "end".
Double seam rims are crucial to the joining of the wall to a top or bottom surface. An extremely tight fit between the pieces must be accomplished to prevent leakage; the process of accomplishing this radically deforms the rims of the parts. Part of the tube that forms the wall is bent, almost at its end, turning outward through 90 degrees, and then bent further, toward the middle of the tube, until it is parallel to the rest of the tube, a total bend of 180 degrees.
The outer edge of the flat piece is bent against this toward the middle of the tubular wall, until parallel with the wall, turning inward through 90 degrees. The edge of bent portion is bent further through another 90 degrees, inward now toward the axis of the tube and parallel to the main portion of the flat piece, making a total bend of 180 degrees. It is bent far enough inward that its circular edge is now slightly smaller in diameter than the edge of the tube. Bending it yet further, until it is parallel with the tube's axis, gives it a total bend of 270 degrees. It now envelops the outward rim of the tube.
Looking outward from the axis of the tube, the first surface is the unbent portion of the tube. Slightly further out is a narrow portion of the top, including its edge. The outward-bent portion of the tube, including its edge, is still slightly further out. Furthest out is the 90-degree-bent portion of the flat surface.
The combined interacting forces, as the portion of the flat surface adjacent to the interior of the tube is indented toward the middle of the tube and then outward forward the axis of the tube, and the other bent portions of the flat piece and the tube are all forced toward the axis of the tube, drives these five thicknesses of metal against each other from inside and out, forming a "dry" joint so tight that welding or solder is not needed to strengthen or seal it. Illustrations of this process can be found here.