The double seam structure is judged by measurement and evaluation of specific components comprising the seam. These measurements are based on guidelines provided by the container manufacturer to the low-acid canned food manufacturer to assist in maintaining acceptable seams during production. The final evaluation of the double seam can only be made by a visual inspection of the torn down seam in conjunction with the measurements. The seam measurements that can be performed to evaluate the double seam are as follows.
Countersink: The countersink is the distance measured from the top of the double seam to the end panel adjacent to the inside wall of the double seam.
Seam thickness: Seam thickness is the maximum dimension measured across or perpendicular to the layers of material in the seam. This measurement is one, but not the only indication of the tightness of the double seam.
Seam width (length or height): Seam width is the dimension measured from the top to the bottom of the double seam.
Body and cover hook: These are internal measurements. As previously referenced the body hook is formed from the body flange, and the cover hook is formed from the end curl during the double seaming operation. These structures, observed in a cross section, have an interlocking relationship to each other.
Overlap: The degree or length of interlock between the body hook and cover hook is known as overlap.
Tightness: Seam tightness is judged by the degree of wrinkling at the end of the cover hook. During double seam formation, the cover curl is guided around and up under the body flange. This crowds the cut edge of the curl into a smaller circumference, resulting in a wavy cut edge with accompanying wrinkles around the seam. The second operation in the formation of the double seam presses the body and cover hooks together to such a degree that the wrinkles should be ironed out sufficiently to ensure a hermetic seal.