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Cut away illustration of a double seam
Cut away illustration of 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.[1] 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 can and end material in a double seam without leaving some voids is impossible without the use of a sealing compound.

Defects

Incorrect (left) and correct (right) seams
Incorrect (left) and correct (right) seams

The production of a high-quality double seam is dependent on several factors, including conformity to the set can and end specifications, the quality of the seamer tooling used and its compliance with the can and end being used, the condition of the seaming machine and the setup of the seaming rolls, lifter pressure and other components. When the machinery is set up correctly and the incoming materials (cans, ends, tooling, etc.) comply to the set specifications, the result should be[2] ideal first and second operation seams.

A problem in any one of these factors and others can contribute to seam defects that have an adverse effect on the ability of the can to withstand foreign contamination and keep the product from leaking or reduce its shelf life. Below is a list that can be used as a reference.[3]

Government regulations

In the United States, the production and quality of double seams is regulated by the US Department of Health and Human Services - Food and Drug Administration (FDA) and the Department of Agriculture (USDA). The plant's responsibility towards the government depends on the product being canned. The FDA regulates all components of canning low acid canned foods, including handling of empty containers, glass, metal and plastic containers. FDA regulations require that at least one double seam can per seamer must be visually inspected every 30 minutes. Additionally one can per line must be cut open and inspected with a micrometer or a seam scope, which projects a magnified image of the seam, at least every four hours.[4] The USDA regulates canning of animal products.[5] Additionally, X-rays may be used to inspect the seam as part of the visual inspection

References

  1. ^ "Complete History of the Can". Can Manufacturers Institute. Retrieved 10 October 2012.
  2. ^ "Ideal First & Second Operation of Double Seam" (PDF). Shoreline Packaging and Processing Machinery. Archived (PDF) from the original on 2020-11-28.
  3. ^ "Double Seam Defects". ShoreLine PPM. Retrieved 11 October 2012.
  4. ^ "US FDA - Guide to Inspections of Low Acid Canned Food Manufacturers: Part 3". US Food and Drug Administration. Retrieved 17 October 2012.
  5. ^ "USDA - 9 CFR 381.301 - Containers and Closures" (PDF). US Department of Agriculture. Retrieved 17 October 2012.

Further reading