A seamer head is a completely independent “machine” inside the seamer which closes seams. A seamer can contain between 1 and 24 heads. Some machines are able to close both the top and bottom (typically aerosol), so they contain both up and down heads in the same machine.
The configuration of a seamer is:
The Seaming chuck holds the end in place, while the body of the can is pushed into it.
As the chuck holds the lid in position, the first operation roll will roll around and perform the first operation – creating the first operation seam.
Next, the second operation roll comes in and tightens up the seam. The end will have compound material on it, which acts almost like a a gasket, ensuring that the closure is completely sealed and air-tight. This allows the seam to prevent loss of internal pressure as well as insuring that no hazardous materials can enter the sealed double seam.