There are multiple means of inscribing the legends (letters, labels) onto keycaps. There is no perfect choice: each method has its disadvantages as well as its benefits. The optimal choice depends on the requirements and available budget.
Injection moulded legends have been in use at least as far back as the 1960s. Each legend is moulded from solid plastic, after which it is placed into a second mould that forms the remainder of the keycap using a different colour of material. The solid plastic legends are wear resistant and there is no limit to the combinations chosen. Typically, there are two injection shots for each keycap: one for the legend and one for the remainder of the keycap, giving rise to the terms “double-shot injection moulding” and “two-shot injection moulding”. As many as four moulding passes (shots) have been known, allowing a single keycap to have different legends in three separate colours.
The chief disadvantage is the high tooling cost for each legend mould. Manufacturing limitations also limit the detail level of the legends: small details are difficult to machine and can restrict the flow of plastic during moulding. Double-shot moulding favours straightforward typefaces and large, clear writing.
Injection moulding works well with ABS plastic, making that the obvious choice for keycaps. The disadvantages of ABS are yellowing and poor surface wear resistance: ABS keycaps can develop an unsightly shine and uneven surface (a little like that of cellulite) quite rapidly. Other materials are available; see the materials page for more details.
Injection moulding can be used to create backlit keycaps, although the large and complex form of the first shot tends to provide dim and uneven lighting.
Pros: Durable; unlimited colour choice
Cons: Expensive; high set-up costs for new legends
Suitable for: Large-scale production
Keycap houses maintain a stock of pre-existing moulds for common and not-so-common legends. However, customers frequently desire non-standard legends, and for small production runs it’s not always financially viable to have fresh moulds cut. The standard alternative for custom legends was mechanical engraving. The legends are cut into the keycaps and then infilled with paint.
Setting aside the time-consuming nature of the manufacturing process, engraved legends are liable to fill with dirt. Dark legends on light backgrounds should be OK, but bright legends will turn dark and will be difficult to clean.
Pros: Very flexible, all the way down to one-off legends
Cons: Time consuming
Suitable for: Small production runs
Dye sublimation was used by several major manufacturers, notably IBM, Apple and Hi-Tek. Hi-Tek chose dye sublimation because it allowed for more intricate legends than double-shot moulding, with lower set-up cost. Dye sublimation is a printing technique but the dye is absorbed into the keycap to provide excellent wear resistance. Dye sublimation allows for more intricate typography than injection moulding.
Dye sublimation requires a plastic with a high melting point: PBT is the standard choice. Because PBT is notoriously difficult to mould, the space bar on such keyboards is normally ABS, and this is evident when the space bar is the only key to have yellowed with age. Even extremely high end products such as the Topre Realforce do not use PBT space bars due to poor moulding yields, although the IBM Model M is a notable exception of a dye sublimated keyboard where every key is free from yellowing.
Since dye can only darken the keycap, light-on-dark colourways are conventionally impossible with dye sublimation. When the beige era ended and keyboards returned to the 70s and 80s aesthetic of white-on-black, dye sublimation’s value diminished greatly. Light-on-dark is in fact possible, by placing dye everywhere except the legend; this means that the sides of the keycap must also be coloured with the dye. This technique is unlikely to be used outside of enthusiast products.
Dye sublimation does not have the perfectly sharp edges of injection moulding, and it seems that legends can become more blurred with age. This process does not seem to be well understood; it could be due to physical wear or diffusion of the dye within the plastic.
Pros: Durable; good typographic detail level; the requisite wear-resistant plastic ensures good surface texture retention
Cons: Dark-on-light colourways only
Suitable for: Large-scale production
Most keycap legends in recent decades have been printed. The various processes are poorly understood. Some manufacturers use Tampo printing (pad printing) where a rubber pad (called a cliché) is used to transfer the ink from a printing plate to a curved surface such as a keycap. (The term “cliché” itself originates from the printing trade.) At least one manufacturer claims to use screen printing instead, although it’s unclear how the screen is made to confirm to the curved upper surface of a traditional spherical or cylindrical keycap. Screen printing would however work fine with flat-top chiclet keycaps.
Legends can be formed of either thin or thick ink; thin ink with a UV-cured clear coating is the most common approach, but a thick ink that may have the UV-cured substance mixed in was the older technique that can still be found. Diatec favour spraying the entire keycap with a protective coating.
Printing offers the best choice for typographic detail of any method, but it has by far the worst wear resistance. Legend longevity depends heavily on the user’s skin: a keyboard can survive for over a decade with no legend wear, or the legends can start to wear away far sooner depending on usage. Printing offers unlimited colour choice. Cherry used pad printing as a means to add secondary legends in a different colour on their double-shot moulded keyboards, as this was far simpler and cheaper than the rarely-used option of tooling up for triple-shot moulding.
The keyboard of the R2E Portal computer uses a combination of injection-moulded and printed legends. It appears that printing was used for all the non-stock legends. The photos below—courtesy of Frederic Stark / Arthur Dauzat—show how the injection-moulded legends have yellowed but remained intact, while the printed legends have ended up in a very poor state, due to both wear and abuse:
Pros: Cheap; unlimited colour choice; unlimited typographic detail level
Cons: Very poor wear resistance
Suitable for: Large-scale production
Laser marking is quick, cheap and visually underwhelming. The laser beam can be used to engrave, char or foam the plastic. Laser-engraved legends can be filled with paint or with a solid filler. Laser foaming heats the plastic into a froth that sets hard as a dark cream or white layer. Solid filler can shrink, crack and fall out, and both the solid filler and the foam are highly prone to discolouration from use as they each have a rough surface that picks up dirt readily. Laser charring simply discolours the plastic; this was widely used in the 1990s and 2000s with beige keyboards, giving low-contrast legends. None of these options are good aesthetic choice, and legends are very limited in colour, with the only options being charred plastic, cream, dark gold or white. Topre opted for dark gold for the Realforce and this is one of the rare examples of laser marking that looks reasonably good. Apple use (or at least used) grey on white, and their marking technique achieved a relatively good surface coverage of solid areas, something not easily achieved with laser charring.
Laser is also used for backlit keycaps: the whole keycap is sprayed with paint and then the laser beam removes the paint in the form of the legends. When the keyboard is illuminated, light shines through where the legends were cut.
Cons: Almost no colour choice; poor aesthetic longevity for white legends
Suitable for: Large-scale production and small production runs