Aluminum Beverage Can-Making
Today, aluminum is the most widely used material in food packaging. It is estimated that close to 20% of the available aluminum world wide is used for can making, particularly for beverage cans. Added to that is the use in flexible packaging, from aluminum trays to a variety of foils. For this discussion, we will only look at cans.
The aluminum coils can be measured similarly to ETP coils. Normally this is done at the rolling mill. At the can-maker, the aluminum coils are fed directly into the multi tool cupping press or the end stamping press. The thickness of the manufactured beverage can is then measured, as the material is thinned down during the production process (DWI). The beginning of the beverage can production line, is called the Front End. Manual, semi-automatic or automatic gauges are used here to measure wall thicknesses (mid wall and top wall), the trimmed can height, and the dome depth. The wall thickness measurement normally employs highly accurate contact gauges, as shown in this semi-automatic gauge.
As with ETP cans, the DWI process is followed by the trimming, necking (sometimes multiple necking) and flanging process. Front End (Figure 1) and Back End gauges (Figure 2) are the same as used on ETP cans. The same is true with Axial Load (Figure 3) and Buckle with Dome Growth (Figure 4).
Reduction in material thickness may require changes in the design of the beverage can so it will survive the process and transportation to market. Axial Load and internal pressure resistance are part of that.
Marketing efforts strive to differentiate packages. Preferred changes involve shaping of the beverage can side wall. The thinned sidewall of cans with standard aluminum is very hard and can barely be expanded (under 1% expansion). For shaping, either the wall thickness has to be increased, or the alloy changed or an annealing step needs to be introduced. Any changes need to be subjected to appropriate testing with gauges shown in Figures 1 - 4.
A recent phenomena is the so-called Dome growth. During increase in internal pressure of a filled beverage can and prior to the buckle value (reversal of bottom dome to the outside), some cans will experience a deformation in the bottom dome, referred to as dome growth. This will not greatly influence the performance of the can, but it has happened that the internal coating cracks, thus creating metal exposure, an unacceptable condition for any beverage filler. Today, the Buckle Tester with Dome Growth measurement is increasingly used by beverage can-makers.
Increased production speed on beverage can-making and filling demands tighter specifications on dimensional conformity of the package. Beverage can-making lines are now approaching speeds of 3000 cpm. The trend is therefore to increase inspection rates off-line, apply SPC, and apply more and more sophisticated monitoring systems.
Off-line measurements can employ very high resolution tools, such as optical projections with on-screen measurement, contact gauging, capacitor systems for coating thickness, sensitive pressure sensors and the like. The available in-line monitoring systems can compare and even read (OCR) at speeds of 64 images per second. Many deviations from a given norm can be detected and eliminated from the can line. Optical systems are already employed to inspect for damages to the exterior of beverage cans, look for inside contamination, check for correct color and print copy, etc. A combination of in-line monitoring and off-line measurements brings us closer to the goal of 0-defect manufacturing.
Safety - Regulatory requirements on the integrity of the package demand specific testing of the double seam of beverage cans.