There are three main ways of making these legends: pad printing, screen printing, and laser etching.
Pad printing transfers an image from a photoetched plate to a soft, thick silicone pad. The pad then gets pressed against the switch face or against an insert for the switch face. Pad printing is often the choice for marking switches that have an unusual shape. Print quality is high and the legends tend to last a long time. Most materials and textures can take a pad print and the process is generally cost competitive with other methods.
Screen printing forces ink through fabric stretched across a frame. The fabric carries a stencil in the shape of the applied legend. Screen printing most commonly gets the nod for membrane switch circuits and graphic overlays. It is also known for giving a high-quality appearance and is considered durable. But screen printing works best on flat surfaces. It can still be used on switch faces that are not flat by printing on a flat insert that goes behind the key face, most typically with illuminated pushbuttons.
Laser etching refers to a process where a laser heats up the surface of the switch face to a level that causes surface melting. The melting discolors the plastic to form the legend. The melting usually penetrates the surface of the switch face only slightly, on the order of a thousandth of an inch. The etching process can be speedy and less labor intensive than either pad or screen printing, which makes laser etching potentially inexpensive. Features can be etched with a great deal of precision, which makes the technique a high-quality process.
Though pad and screen-printed legends are durable, they can still wear off if the switch is in heavy use on equipment that has a long useful life. In this case laser-etched legends are the method of choice. A typical application example is on illuminated pushbuttons found in commercial audio and video equipment.
Information for this article came from NKK Switches, nkkswitches.com