Tin Can Seams - What is a double seam?
Smooth Tin Cans in Production Assembly in Can It's factory
Smooth Tin Cans on conveyor belt in Can It's factory

What is a Double Seam?

A double seam is a canning process for sealing a tin can by mechanically interlocking the can body and a can end (or lid).

Originally, the can end was soldered or welded onto the can body after the can was filled. However, this introduced a variety of issues, such as foreign contaminants (including lead and other harmful heavy metals). The double seam was later developed as a cheaper and safer alternative and quickly replaced the welded seam.

The double seam is made using a double seamer, which can have just one or a number of heads or seaming stations. The double seam is formed by mechanically interlocking five layers of material together: three layers of the can end and two layers of the can body. Each seaming head typically consists of two rolls, a first operation roll and second operation roll, and a chuck. Some seaming machines have two first operation rolls and two-second operation rolls and a few machines use a method called “rail seaming” which requires no rolls.

During the seaming operation, the can-end is lowered on to the filled can-body and held down by the chuck, which acts as an anvil to the seaming operation.

The first operation roll then engages the can end against the can body thereby folding the end curl around the flange of the body. In some seaming machines, this is done as the can is turning at high speed. In other seaming machines, the can is stationary and the first operation roll (or rolls) spins around several times to ensure a complete first operation. After the first operation is complete, the first operation roll disengages from the can and the second operation roll then engages the can.

The purpose of the second operation is to iron out the double seam into its final shape and remove the voids between the layers of can and end material. In practice, ironing out all of the cans and end material in a double seam without leaving some voids is impossible without the use of a sealing compound.