The most cost effective and most robust key top legends are embossed or engraved directly in the key-top's silicone. The biggest advantage (besides cost) is that the marking will not wear off. The keys can even be backlit (by using clear silicone). Insert molded or flow molded keys can also be used to add different colored keys to the keypad.
However, embossed/engraved keys are more prone to dirt being trapped in their corners or recesses. Also, because the legend is part of the mold, any changes would require a costly tool modification.
Embossed/Engraved Design Rules
Because the tooling must form complex structures, there are design limits to the embossed/engraved size, depth and line thickness. See dimensions at left.
Embossed legends are better to tool than engraved legends.
Paint can be used to print legends onto the key-top surface. The paint is squeezed through a silkscreen which has tiny holes that match the legend's pattern. The silkscreen ensures precise and repeatable legend application.
Almost any color can be mixed and printed - a separate silkscreen is needed for each color so the more colors used, the higher the cost of the keypad.
Position tolerance for screen applications is 0.2mm from center and legends must be more than 0.3mm from any edge.
Minimum Legend Thickness
For the most part, the minimum achievable line thickness with silkscreen printing is too thin to be legible. As a general rule, Abatek recommends legends with line thickness of at least 0.25mm for best effect.
Backlighting with Silkscreens
If the keys need to be illuminated, positive or negative silkscreen printing can be used over clear or milky-white silicone. This is a cost effective way of achieving backlighting compared to laser etching.
Keytop Shape Limitations
The silkscreen process is best performed when the screen is laid flat on the keytop surface. For this reason, convex and concave keytop shapes are not recommended. Maximum key top radius shown at left.
Spray Paint & Laser Etch Basics
The basic one layer spray paint laser & etch process has the following steps:
1. First the silicone keypad is molded from light colored silicone
2. Secondly the entire top surface of the keypad is spray painted in a dark color.
3. Lastly the legends are added by burning off the dark spray paint with a laser and uncovering the light colored silicone underneath
Clear silicone can be used if the legends will be constantly illuminated - however the legends may be difficult to see if the silicone is not backlit.
Position tolerance for laser etching is +/- 0.1mm and legends must be more than 0.2mm from any edge. Minimum light thickness should be 0.25mm for best visibility of legends and a spacing of 0.2mm between legends.
For some keypads, it is important to have legible, non-illuminated keytops for daytime use and then have the ability to backlight the legends for nighttime use (these keypads are typically used in automotive applications). The following steps are used to achieve this type of legend:
1. First the silicone keypad is molded from lightly colored silicone
2. Secondly the entire top surface of the keypad is spray painted in a lightly colored, slightly translucent color (called the illumination layer)
3. Third the keypad is again spray painted, this time in a dark color (called the masking layer).
4. Lastly the legends are added by burning off the dark spray paint masking layer with a laser and uncovering the lightly colored, slightly translucent illumination layer underneath.
Here the white paint provides a nice contrast to the dark masking layer during daytime, and because it is translucent, the legends can easily be backlit for legibility during nighttime.
Laser Etching Process Limitations
Position tolerance for laser etching is ±0.1mm and legends must be more than 0.2mm from any edge. Minimum light thickness should be 0.25mm for best visibility of legends and a spacing of 0.2mm between legends.
Keytop Shape Limitations
The laser process is performed by securing the position of the keypad in a fixture and having an overhead laser burn off the spray paint. For this reason some limitations are present.
Spray Paint Limitations
The spray paint process is performed with an automated spray booth using an overhead spray nozzle.
In order to prevent light leakage due to insufficiently sprayed key-tops, Abatek recommends that the height of keys be no more than half the distance between keys.
Without some sort of coating (protection), the silkscreen printing and spray paint layers can wear off with use and over time. The coatings help protect the legends by providing a seamless, clear layer over the paint. Abatek provides different types of coatings available for various industries. For most applications Abatek recommends coatings in a matt finish.
Sealplast: used primarily in the automotive industry for maximum wear resistance, this exclusive Abatek technology requires the key webs to be masked so as to no affect the actuation forces.
PU Coating: standard coating used in a variety of applications. This coating offers a great performance to cost ratio.
Super UV Resistance PU Coating: used in applications where keypads will be primarily left outdoors. Helps protect the coating, spray paint and silicone for yellowing while retaining the same abrasion resistance performance of standard PU coating.
Epoxy: very thick and glossy coating applied to keytops to make them appear plastic like. While Epoxy is extremely durable, it is also very expensive as it must be applied by hand and yields a high scrap rate.
The abrasion resistance of coatings is tested using a standardized test-set up and tester. The tester runs a strip of paper across the top surface of a key. The strip of paper is pressed down against the key with a standard weight. The abrasion resistance is calculated by how much (length) of paper must be run across the surface of the key before legend wear is noticed.