3-pc Welded Food Can-Making
Low acid foods have to be sterilized after filling to prevent microbial development, Microbes spoil the food and may also develop toxins, which are deadly when consumed. One knows from personal experience that food cans are not completely filled, they have a headspace of maybe 5 mm to 10 mm of space between the top of the fill good and the top of the can. This head space aids product agitation inside the can during the process cycle, reducing the sterilization time (conduction vs. convection of heat) and allowing for steam in the head space to collapse during cooling, creating a vacuum inside the can.
For the can, this means it has to stand up to inside pressure and vacuum. To compensate for increased pressure caused by the inside temperature and corresponding expansion of the vapor, the ends of the can are designed to flex outward into a convex shape. This increases the inside space and reduces over pressure.
During the cooling cycle, the steam in the head space collapses and the ends flex back into a concave shape. Now the sidewall of the can is in danger of being pulled in, to panel. To resist that, the sidewall of food cans are thus beaded. Beads are corrugated areas, mostly used in groups. Depending on materials used, the beads have a certain number, can be grouped and have a certain depth.
To make the beaded can, the can body is first flanged, then beaded and then one end is seamed on. In many cases the top or the bottom of the can body, or both, are necked in (diameter reduced). This allows to use smaller ends (material savings) and cans can be nested during transport and on supermarket shelves. As the cans are shaped, they are measured and tested after each operation, or, in case a combination machine is used, the finished can is measured either with individual gauges or with a combination gauge.
Figures 1-3 show manual gauges to measure the flange width (all measurements are taken at 3 points of the circumference of the can), the Factory finished can height ('Factory' means can making plants in this case), and the bead depths.
Figure 4 shows a combination gauge to measure Bead Depths, Can Height and Flange Width in one gauge, also rotating the cans automatically.
The dimensional measurements give an idea on the expected performance of the cans. To be sure though, finished cans are also subjected to an Axial Load test and an Implosion (Panelling) test. If the beads are too deep, they might cause the cans to collapse below the specified Axial load. If the beads are too shallow (only one has to be bad) , or if the beads were in a spiral configuration, the can side wall could implode (panel) during the sterilization process. (Figure 5)