|This article requires additional photographic illustration — need photos of QWERkeys UV printing and mechanical engraving, and imsto/WASD non-infilled laser|
Over the years, manufacturers of keyboards have devised different methods of marking keycaps with characters. There is no perfect technique, and various trade-offs have to be made.
Pad printing has been the most popular form of keycap printing since the 1990s, which uses pads dipped into ink to transfer lettering onto the keycaps.
Pad printing now accounts for virtually all commercial (office and consumer) legend printing across the world.
Laser marking is the use of a laser beam to mark the legends onto the keycaps. Laser marking works well with straight lines, but has difficulty achieving clean coverage of solid fill areas such as arrowheads.
Laser etching uses a laser to change the appearance of keycaps with minimal surface texture alteration. Light keycaps can be darkened, and dark keycaps lightened. Traditionally, laser etching on dark keycaps yielded pale, grey or yellow-grey legends. However, recent innovation has led to the ability to produce bright white legends on black keycaps; Matias use this technique with the Quiet Pro, and both WASD and QWERkeys offer white-on-black laser-etched keycaps.
Laser engraving burns deep grooves into the surface of the keycaps. These can be left empty for aesthetic effect, or filled with colourant, termed "infill". Cherry keyboards with black keycaps are laser engraved with white infill, which protrudes above the surface of the keycap. Infill tends to stain quickly.
Laser engraving is often used in the creation of transparent keys: the transparent plastic is covered in a (usually rubber) coating, and a laser burns off the coating to expose the transparent plastic in the shape of the required character. In this instance, the durability of the material used in the coating is the deciding factor in longevity.
Dye sublimation is a process where heat is used to impregnate a material with a dye. This is different to printing in that printing forms a layer of paint on top of the plastic, whereas dye sublimation causes the dye to sink into the plastic. Dye sublimation was a popular method for keycap printing in the 1980s and 1990s. Because the dye permanently stains the plastic, it cannot be worn off like paint, and unlike laser printing, it can be used to easily print a mixture of different colours.
However, in addition to being more expensive than pad printing or laser etching, dye sublimation requires that the dye must be darker than the material it is being used to dye. This means that you cannot use dye sublimation to print white lettering (as nothing is brighter than white) or to print onto black keycaps (as nothing is darker than black). Given the popularity of white-on-black keycaps since the early 2000s, dye sublimation has become non-existent in mainstream keyboards. Today, it is largely confined to Unicomp, who use it to print onto white, grey or coloured keycaps. Topre uses it to print onto white keycaps, but also to print black lettering onto dark grey keycaps.
Prominent users of dye-sublimation include IBM and Unicomp, Apple, and Topre. Acer used dye-sublimated legends on their 6310 series keyboards. Cherry used dye sublimation keycaps on some keyboards of their 3000 series keyboards in the 1990s.
IBM Model M dye-sublimated legends
Double-shot injection molding
Double shot injection molded keycaps (often known as "double shots") are often considered the best type of keycap. Rather than print the legend onto the keycap, the legend is molded from one colour of plastic, and then the remainder of the keycap is molded around it in another colour of plastic. This process has the advantage of producing markings that cannot be worn off, as they are physically part of the keycap. The process also provides high contrast as the legend is not affected by the colour of the surface of the keycap.
Double shot keycaps were de facto standard in the 1970s and early 1980s. However, they are expensive to produce and inflexible (given that a mold is needed for each possible symbol to be printed). Double shot keycaps are limited to two colours per keycap. Triple or even quadruple shot molded keycaps have been made, but these would involve even more expense and complication. Manufacturers often used pad printing or dye sublimation to add additional colours to double shot keycaps.
Engraving follows the same process used to cut custom name badges: the writing is engraved mechanically, and the grooves are inlaid with paint. This time-consuming process is not widely used, being reserved for custom products. Examples of companies providing mechanically engraved keycaps include RAFI and QWERkeys. One disadvantage of this process is that dirt will accumulate in the grooves, which does not wipe clean.
The extent to which silkscreen printing is used, is unclear.
Edgar Matias has indicated that their silkscreen process involves mixing the UV curing substance directly with the paint before application. He also indicate that silkscreen is used in Asia for (presumably) secondary colours on lasered keycaps.
This is a proprietary printing technology from QWERkeys which permits printing any colour directly onto even darker keycaps; the ink used is cured with ultra-violet light, hence its name. The keycap is then coated with lacquer for protection.
In a more general sense, the mixture of a UV curing substance can apply to silkscreen printing, and may also be part of the non-coated pad printing process.