The overlap is a critical internal measurement of the double seam and needs to be measured by cutting the double seam using a seam saw and inspecting it with double seam evaluation software SEAMetal & SEAMscan. The overlap is often referred to as the “secondary seal” in the double seam (the “primary seal” being the tightness and free space)
In the past, prior to computerized seam inspection, the only way to evaluate overlap was to calculate it based on cover hook and body hook. This often results in incorrect measurements, due to the high subjectivity in measuring coverhook with a micrometer, the affects of the stripping process and the calculation being done on different areas of the seam.
A bad overlap means that the seam is open and susceptible to contamination, leaks or hazardous materials.