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Science Notebook

The surprising science behind the aluminum soda can

Most of us interact with soda cans every day, so it's easy to forget that their design is the result of brilliant engineering and manufacturing carefully honed over decades. The modern soda can can hold beverages at pressures up to six atmospheres, yet is less than a tenth of a millimeter thick.

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    Bill Hammack, a chemical engineer who goes by the YouTube handle "engineerguy," explains how soda cans are manufactured from flat metal disks.
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We take for granted that soda cans are cylindrical – the shape is easy to hold and the cans stack well on top of each other. But how did today’s can design become standard? After all, cylindrical cans don’t pack together as well as cube-shaped cans would, and they use more metal than spherical cans would.

A new video from Bill Hammack, a professor of chemical engineering at the University of Illinois who has a YouTube channel called “engineerguy,” explains the science of how the modern soda can came to be.

A spherical can may use the smallest amount of packaging, but of course it would roll off the table – so that’s out. A cube-shaped can wouldn’t work because the edges are weak points, and the walls would have to be made much thicker to withstand the pressure of the carbonated beverage within. (It’s not particularly easy to hold or drink from, either.)

A cylindrical can combines the best qualities of a sphere and a cube. When packed in a box, cylinders take up about 90 percent of the available space, and their round shape is able to withstand a good amount of pressurization. Modern aluminum cans are less than a tenth of a millimeter thick, yet hold liquid at up to 90 pounds per square inch (about six times regular atmospheric pressure).

The aluminum or tin-plated can begins life as a flat disk a few inches across, and is mechanically pressed into a shallow cup shape and then into a taller cup that’s the same diameter as the final can. The bottom of the cup is then pressed into a concave dome shape, which allows the can to withstand greater pressures than if it were flat. The whole process takes only a seventh of a second, allowing a single machine to produce about 100 million cans in a sixth-month period.

Finally, the outside of the can is decorated, and the inside is sprayed with a coating that keeps the soda from taking on a metallic taste. The still-open top of the can is tapered in, and once the can is filled with soda or juice, a separate machine immediately puts the top on the can and seals it to the body. Soda, Dr. Hammack explains, is pressurized with carbon dioxide, while juice is pressurized with nitrogen. That internal pressure allows the can to be relatively strong in spite of its thin walls – think about how easy it is to crush an empty can with your hand versus how hard it would be to do the same to an unopened can.

The modern soda can also incorporates a small tab that opens the top of the can without detaching itself. Today this feature is ubiquitous, but until the 1970s, cans featured a pull-tab that came off of the can, and beaches were often littered with discarded pull-tabs.

Most of us interact with modern beverage cans every day, but it’s easy to forget that they’re carefully designed and manufactured with an incredible degree of precision. The beverage industry makes about 100 billion cans each year, thanks to a design that results in strong, reliable, efficient cans.

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