Main printing technology for keyboard keycap print are pad printing, screen printing with uv coat, laser engraving, laser etching, laser etching with paint filled in, dye sublimation and double shot injection molding.
 
 

Pad printing

Pad printing has been the most popular form of keycap printing since the 1990s. The idea is very simple - pads are dipped into ink, and then pressed onto the keycap to form a layer of ink on top of the keycap. This method has the advantage of being flexible (the process can be adapted easily to print any character in any color required) but the ink is prone to wear.

Disadvantage of pad print is the low efficiency and thus leads to high cost. Pad print is usually used in some high end keyboards. Those factories who can afford the laser print machine may also use this kind of print because the machine needed is cheap to buy.

Screen printing with uv coat

Screen printing can is also called as silk screen or silk screen printing. Letter of keyboard will be hollowed in a specially made woven mesh. When the paint is brushed, paint will filled the hollow parts and print letters on keycaps.

Screen printing is the most widely used printing technology in keycap printing. Similar to pad printing, it can also print various colors. To keep the letters from being scratching, uv coating will be added on top of letters. 

Laser engraving

Laser engraving is often used in the creation of transparent keys - the transparent plastic is covered in some coating, and a laser burns off the coating to expose the transparent plastic in the shape of the required character.

Laser etching

A laser is used to burn the required letters into the keycaps. For this kind of keycap, letters are impossible to wear off. Track of burning will leave on the keyboard, and laser will create black letters because of burning. So, this printing is mainly used for white keycaps or grey keycaps. If on black keycaps, letters are invisible.

Bamboo keycaps can also be laser etching.

Laser etching with paint filled in

As mentioned above, laser etching is mainly used for white or grey keycaps. It is not suitable for keycaps of other colors. Filling contract color paint from the keycaps can solve this problem. Like for some black keycaps, white paint can be filled into the etched letters and make the letters visible.

Dye sublimation

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, and it can be used to easily print a mixture of different colors.

However, in addition to being more expensive than pad printing or laser engraving, 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 colored keycaps. Topre uses it to print onto white keycaps, but also to print black lettering onto dark grey keycaps.

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 characters onto plastic, the character and underside of the keycap are molded on one piece of plastic, and the top part of the keycap is molded from another. They are then fused together to form a dual layer keycap. This process has the advantage of producing markings that cannot be worn off as they are physically part of the keycap, and they provide high contrast as the printed character is not affected by the color 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 colors 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 colors to double shot keycaps.

Some content is retrieved from http://deskthority.net/wiki/Keycap_printing