CAN SEAM INSPECTION 101
Various methods and measurements are used to inspect the soundness of a can seam. Below we will discuss some of these somewhat arranged in order of both usefulness and cost.
SEAM THICKNESS
This is simply the thickness of the finished seam. For any particular can type there will be a specified range of seam thickness . This measurement will be an indicator of the overall pressure applied during the seaming operation. Often done with a seam micrometer.
SEAM HEIGHT
Again, a measurement to indicate overall pressure applied during seaming. Done with a seam micrometer.
SEAM IMPRESSION
As the seamer rollers push the cover and body hook materials together, and against the seaming chuck, an impression is left on the inside of the can body. Too much pressure can cause this impression to damage the can liner.
COVERHOOK WRINKLES
A more reliable indication of proper seaming pressure is obtained from inspecting the coverhook wrinkling. Because of the differences in radius, as the various parts of the seam are pressed together, wrinkles will naturally occur in the inside radius of the coverhook. Measuring the depth, type and quantity of these wrinkles will help you to to determine that the correct pressure is being applied, and ensure your seamer is operating properly.
In order to inspect the coverhook for wrinkling, however, it needs to be removed from a completed seam. It is possible to remove it manually with a pair of nippers, but the process can be time consuming, and a bit dangerous. Not to mention the possibility of damaging the very thing you are attempting to inspect.
A much better and safer alternative is known as a Seam Stripper. The seam stripper will slit the top of the coverhook rim, allowing easy removal from the can.
INSPECTION OF THE DOUBLE SEAM
By utilizing a Seam Saw to cut a notch through the seam, it is possible to visually inspect and measure the various components of the double seam itself. A seam saw will cut a notch through the double seam, allowing it to be visually inspected. Normally three points around the can are notched and checked.
A visual inspection can reveal many flaws in the seam including overlap, the body and cover hook lengths, and the verification of proper mating of the parts. These measurements can be done by utilizing vernier calipers. In recent years, however, better and much easier methods have been developed. a much more precise and thorough inspection can be accomplished by utilizing a video inspection microscope.
RECORDING & STORING RESULTS
All of these measurements are normally recorded in a database for future reference, reporting purposes, and to track trends. The OneVision SeamMate® System is an example of a full featured software system designed to do this, and much more. By careful analysis of this data, wear patterns can be uncovered, and maintenance can be scheduled well before an actual operational failure of the seaming operation occurs.
To learn more about can seam inspection best practices, common double seam defects, and quickly reference important calculations, head over to our sister site: Quality Can Seams
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1. Correct Tightness: The seam must be sufficiently tight to ensure the sealing compound is properly held under compression.
2. Correct Body Hook and Cover Hook Length: The geometry of the seam as measured either mechanically or optically includes ensuring the cover and body hook are within the proper tolerance of length and engagement.
3. Overlap: There must be sufficient overlap of the cover and body hooks to ensure an airtight seam.
4. Other Critical Defects: No localized distortions in the seam such as droops, vees, pin lips, or seam fractures.
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