Automating Beverage Can Seam Inspection
In the past the seam dimensions were taken manually on the individual components of the double seam. Today the vast majority of processors use a computerized seam inspection system, where the seam in sectioned with a double bladed saw and the cross section is viewed with a seam scope. The image is enlarged on a computer screen and analyzed using double seam measurement software, measuring all the values discussed and calculating the remainder. Following that, the seam is stripped and the coverhook is inspected by the operator and the wrinkles are interpreted. As the strength of a chain is determined by the weakest link, the hermetic properties of a double seam is determined by the largest wrinkle. Large refers to the wrinkle with the longest extension from the edge of the coverhook and the bend making up the lower part of the outside double seam. Comparisons of operator evaluations or this most important feature of the tightness of the double seam show a measurement uncertainty from 30% to 50%. (Figure 1)
Many efforts are thus ongoing to develop alternatives to the subjective evaluation. A most promising one is an X-ray method. Here the density of the coverhook material gives a good representation of the wrinkles. With computer visualization methods coverhook wrinkles can now be displayed and measured better than with human interpretation.
Double Seam Measurements on Filled vs. Empty cans
There are differences in the seam construction in filled vs. empty can. In a beverage can, the seam gap should be kept rather small, particularly when the can body is made from steel. A seam gap may pass inspection on an empty can as being good. On a filled can though, which is normally subject to internal pressures, it might not pass because of the seam gap, that may now be too large. The opposite observation applies to food cans that are often subject to vacuum in the normal state of use. A vacuum tends to decrease the gap in the seam.
Measuring cans in the condition of actual use reflects reality, but it means that the existing specifications of can makers and fillers might have to change - an arduous process in an industry that does not like change. But the increased speed and accuracy that comes with measuring filled cans have caused a number of filling plants to start using the new X-ray methods.
Destructive vs. Non-Destructive Inspection Methods
Traditional seam inspection requires a destruction of the double seam, as described above. The disadvantages are high labor cost, experienced QA inspectors to evaluate the seam and can components, loss of product and cans during inspection and measurements that do not reflect the condition of the seam in the active use of the can.
As non-destructive method for a full seam inspection, only X-ray systems are known today. They are available as semi-automatic or fully-automatic systems and are normally paired with contact gauges to measure countersink and seam thickness, to comply with industry regulations.
In-line monitoring to control external seam dimensions
High Speed cameras can take and process upwards of 45 images / sec. As such, they are being used in line to monitor a number of parameters on cans.
At the can maker, cans are monitored for physical changes, inside coating condition. The label (print) is checked for color, bar code, production code etc. The ends are scrutinized just as thoroughly. The motivation is the high cost of returned shipments and losing a customer totally. The inspection process costs a small amount compared to a legal claim by the filler.