the shape and conformation of the finished seam are determined by the design of the seaming rolls and the taper of the chuck. Roll shapes may be changed to accommodate different thicknesses of the can material. The roll shapes, the pressure adjustments of the rolls and the base plate will determine the shape, the dimensions and the integrity of the double seam. At first sight, the shape of the double seam may appear to be satisfactory, however, one or more of the internal structures may be out of limits. As a result, a hermetic seal will not be produced. The commercial sealing machine may close 1000 cans each minute, so a defect, if not recognized soon, may result in thousands of defective closures. To prevent this from happening, a trained mechanic is continuously inspecting the production.
A Visual Inspection of closures shall be performed at sufficient intervals which may discover more obvious defects. The interval between visual inspections cannot exceed 30 minutes of continuous sealing machine operation.
In addition, USDA/FSIS regulations require Double Seam Teardown examinations which shall be performed at intervals of sufficient frequency, not to exceed 4 operating hours.