Can Seam Inspection 101

A leaking double seam must be avoided to ensure canned product quality. There are several methods and measurements used to inspect the soundness of a can seam.

SEAM THICKNESS

The thickness of the finished seam. There are specified ranges of seam thickness for all can types. The seam thickness measurement is an indicator of the overall pressure that is applied during the seaming operation. Seam thickness is often tested with a seam micrometer.

SEAM HEIGHT

Seam Height is another measurement that indicates overall pressure applied during the seaming operation. Like seam thickness, a seam micrometer is used to measure.

SEAM IMPRESSION

The seam impression is left inside of the can body as seamer rollers push the cover and body hook materials together and against the seaming chuck. The can liner can become damaged if too much pressure is applied.

COVER HOOK WRINKLES

Inspecting the cover hook wrinkling is a more reliable indication of proper seaming pressure. As the various parts of the seam are pressed together due to the differences in radius, wrinkles naturally occur in the inside radius of the cover hook. You can ensure your seamer is operating properly and determine that the correct pressure is being applied by measuring the depth, type and quantity of cover hook wrinkles.

The cover hook must be removed in order to inspect for wrinkling. Removal can be accomplished through manual removal with a pair of nippers or through utilization of a Seam Stripper.

REVERSE WRINKLE
A Reverse Wrinkle is a non-tightness type of wrinkle that projects towards the can body wall or the center of the can. A reverse wrinkle is formed in the first operation seam and cannot be ironed out regardless of the tightness of the second operation seam. Reverse wrinkles typically appear when seaming harder alloy and/or lighter gauge end stock.

Reverse wrinkles, in themselves, are an appearance issue. They do not attribute to leaker spoilage unless the material folds over on itself (Pucker or Pleat) and/or cracks vertically. Refer to Page 17 in the Seam Defect section.

One should not try to impose a grading system for this condition. Individual situations must be assessed, and decisions made on each separate case.

SEAM IMPRESSION
Seam Impression (Pressure Ridge) is caused by the force of the seaming rolls pushing the cover and body hook material against the bottom of the seaming chuck, resulting in an impression on the inside of the can body.

This feature cannot and should not be taken on its own as an indication of adequate tightness.
**Some roll profiles and chuck designs will not yield a significant seam impression. This is also true of seams produced with light gauge metal, aluminum and double reduced (DR) body and end plate.
**Heavy seam impression can fracture coatings that could lead to leaker spoilage. Make sure seam is not overly tight.

VISUAL INSPECTION

Visually inspecting the external double seam is a valuable tool in maintaining double seam quality. In food canning processes, visual inspection of each container is performed from each seaming station at least every 30 minutes. The inspector is looking for deviations in the double seam as she traverses around the seam. Typical problems found visually include seam vees and droops. Seam bumps are difficult to see because the deviation is small. In practice, visual inspections identify 80% to 90% of seam quality problems.