Why do tin cans have those extra creases/lines? Why aren’t they smooth like an aluminum beverage container?
In: Engineering
Tinned food is cooked in the can, those ridges allow a small amount of expansion so the cans don’t deform or explode.
The “ribs”or corrugations on these cans are there to strengthen the side walls and lid. This helps them resist denting. The cans are filled with food, then flash cooked with steam, the lid is then quickly assembled and then crimped with a special machine which produces a mechanical seal, while the food is still near boiling. The cans are then cooled in a cold water bath for several minutes.
The hot steam in the top unfilled space excludes air and causes a partial vacuum to develop inside the can as it is cooled.
This fact is actually important as the design of the joint of the lid is such that the force of the vacuum inside clenches the lid against the sides, improving the tightness of the seal.
However, the partial vacuum make the can prone to buckling or denting. This doesn’t usually affect the seal or the quality of the food but is unsightly. The Corrugated ribs where added to improve denting resistance.
2) On the other hand, aluminum drink cans don’t usually have such ribs because they’re usually *pressurized*, unlike food cans. The positive internal pressure is very effective in resisting denting and crushing.
Even if you manage to temporarily dent them, the internal pressure causes the dent to spring back out as soon as the force is removed.
Final note: Despite the colloquial name, food cans are usually zinc plated *steel*, and usually have an additional coating of paint or lacquer on the inside to prevent corrosion from food acids. Modern cans contain no actual tin.
Food vessels made of tin or tin alloys such as pewter have been made by skilled artisans since at least the iron age. Early attempts at hermetically sealed food were made with glass bottles and either cork stoppers or tin lids. But these were easily broken. Inventors switched to using cans formed of tin sheets, and employed skilled plumbers to fabricate them by hand.
As thin steel sheets became widely available, canners switched to steel because it was much cheaper, stronger, and lighter in weight. The unfilled lidless cans along with the lids were then dipped in a bath of molten tin or tin-based solder. Hence the term “tinned cans.” The tin coating served to protect against rusting and also filled ocassionall gaps in the bottom joint. Finally, the lids were sealed on the cans by flowing more tin solder around the head joint, a process also sometimes known as “tinning.”
In the Late 1800 a process was invented to seal the lids by mechanically crimping the joint instead of soldering. This saved a great deal of labor.
The public had long used the term tins or tin-cans by this time despite the switch to steel construction.
However zinc is much cheaper than tin and offers similar corrosion protection, so using zinc plating became industry standard in the 1930’s and the use of tin was abandoned.
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Any time you see a corrugation, it’s usually entirely for one reason – if it was a flat sheet, it would bend. By corrugating it, you are preventing that bend from occurring, as it’s difficult to bend a corrugation.
Don’t forget that drinks cans are pressurised, so can’t bend. Tins aren’t, they are just sealed.
Structural strength – Less likely to dent. Aluminum has different structural properties, and I believe those beverage containers have a smaller diameter, so are stronger.
Those cans of food have to pressured cooked, the ribs allow for the cans (previously sealed) to expand without bursting and exploding in the pressure canner.
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