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Every day, around the world, individuals interact with millions of bottle caps. Some are on child-proof pharmaceutical bottles that contain life-saving medications, while others are on bottles of water needed to quench someone’s thirst. Each one of those bottle caps had to be made somewhere — and a surprising amount of effort goes into their manufacturing.

Most plastic bottle caps are manufactured through a process known as injection moulding. In this process, thermoplastic polymer pellets are heated in a metal hopper to the point that they become very soft and flexible. (It would be at this stage that any custom colouring would already be added). The softened plastic is then injected under very controlled, high pressure into a complex, detailed steel mould. The mould forms the softened polymer into the desired shape for the bottle caps. Once the polymer material has cooled enough to harden, the mould is opened and the bottle caps are ejected. If several caps are being made from a single mould (as is often the case), they will be carefully separated from each other and prepare for further processing.