Plastic bottles are made by blow-molding, a beautiful, elegant process that can create a full-bodied shape with a narrow little neck. But how the heck do they make scuba tanks, fire extinguishers and gas canisters, which have similarly narrow necks, but are of course made of non-blow-moldable metal?
My first thought was that they make them in two halves via metal drawing, then weld them together—but obviously that makes no sense. So I looked into and found that yes, a scuba tank comes from a single flat disc of metal alloy.
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And while drawing is still the initial production method, I wasn't aware that what they call "hot spinning" is how you get the bottleneck: