The 2nd Operation seaming process flattens the seam, pressing the folds of metal together tightly enough to compress the compound which fills voids and areas of the seam not occupied by metal. The ideal 2nd operation seam should be well rounded with no sharpness in the upper portion of the countersink area, no distortion, or scratches partway or all around the seam.
Seam specifications are normally set by the can manufacturer or quality control personel in each individual company. The double Seam thickness generally will be approximately three times the thickness of the tin plate in the cover, plus two times the thickness of the tin plate in the can body and plus three to five thousandths of an inch. .003 to .005 of an inch is normal allowance for sealing compound in food container covers or .010 of an inch in beer or beverage can covers. Plastic Bowls with Aluminum covers will compress slightly tighter than the actual compiled five layers to insure that the plastic is compressed properly.
The Pressure Ridge, which is an impression around the inside of the can body directly opposite the Body Hook and caused by the 2nd operation seaming roll, should be closely examined to properly anaylize the seam. Visual inspection based on good judgement is the only way to determine the proper pressure ridge.
The important thing is that there must be a distinctly visible pressure ridge to be assured of a good 2nd operation seam, even though the seam measures within allowable tolerances. Visual seam inspection is important and the appearance of the finished seam is some guide to quality; however, visual inspection alone is not adequate to determine the quality and integrity of the finished 2nd operation seam in all instances. A cross section of a double seam made with a normal 1st operation and normal 2nd operation will show a good overlap with very small voids at the end of both the cover hook and body hook filled with the compound.