Mechanical switches have stems that allow for aftermarket keycaps to be installed. Replaceable keycaps allow for novelty keycaps, full keycap set replacement, and general customization of keyboards without doing irreversible modding.
Keycaps are usually made out of ABS or PBT plastics, but can be made out of almost anything. ABS plastic is most commonly used, and is soft compared to many other plastics. ABS plastic has a low melting point, and if placed in boiling water, will warp/melt rapidly. PBT plastic is tougher and more brittle than ABS and has a sandier/grittier feel, but is also less commonly used. PBT also has a high melting point, so they can be boiled without problems. PBT is more expensive to manufacture, so it is less commonly used. Other materials such as heavy metal keycaps, POM (another plastic), and wood are used, but less commonly.
The legends on keycaps are usually placed by pad-printing, laser etching, dye-sublimation printing, or double-shot molding.
Pad printing uses legends that are printed on the top of the keys, and oftentimes has a clear count placed over it to increase durability. Pad printing is the least durable option, but is also the cheapest, which is why it is commonly used.
Laser etching uses lasers to burn the legend onto the keycap, and many manufacturers then fill the legend with color to have a certain aesthetic. Laser etched keycap legends last longer than pad printed, but are slightly more expensive.
Dye sublimation is an interesting process as it is when manufacturers use heat to place the dye into the keycap in the shape of the desired legend. The advantage to this is that the dye is integrated into the keycap, so it is harder to wear off, but dye sublimation can result in fuzzy/blurry legends. Dye sublimation is not common.
Double-shot keycaps are by far the longest lasting and most expensive type of keycaps. Double shot molding is done by shaping plastic around inserts (usually plastic). This mills the legend through the entire keycap, which makes it impossible to wear away and also creates a high-contrast legend. The main disadvantage of double shot keycaps is that it is the most expensive method, so it is rarely used.
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