Design Considerations For Molded Keypads

Molded keypads made from silicone rubber offer a variety of features traditional keypads don't have

. However there are some important decisions to make to create the best interface for your users.

Tactile Feel

Lack of tactile feedback is a common complaint when using custom silicone keypads. If an operator doesn't get some kind of response it's hard to tell whether a key press has been accepted. The tactile characteristics of the keys are controlled by actuation force and contact force.

Actuation force is the effort required to press the key and make initial contact. Contact force is the effort required to maintain that contact and will always be less than actuation force. The difference between these forces is the "click" that the user will feel as the key is depressed. The larger the difference in force, the greater the tactile feedback. When this force difference is expressed as a percentage of activation force, it is called the snap ratio.

Finding the correct snap ratio for molded keypads is a matter of balancing the tactile feel with the life of the keys. A higher ratio gives stronger feedback but reduced key life. Typically the snap ratio is 50% of the actuation force but this will vary depending on the application.

Markings and Coatings

Custom keypads typically have markings directly on each key. There are three common methods of marking the rubber.

The markings can be screen printed on to the silicone with special inks designed for the material. This method offers a variety of colors and finishes depending on what kind of final appearance you desire. It is a very economical marking solution.

Laser etching involves spraying a translucent rubber key with an opaque topcoat. The topcoat is etched away with a laser, revealing a base coat or the rubber itself. This method is commonly used in molded keypads that will be backlit. It is also effective on curved keys not suited for traditional screen printing.

Hard plastic key caps can be placed over the soft rubber keys. These are more durable than the soft silicone and are ideal for keys that will see heavy use.

Silicone is already highly resistant to temperature, liquid and many environmental hazards, however molded keypad manufacturers offer coatings that improve this resistance further. Coatings can increase the rubber's resistance to abrasion or chemical corrosion, extending the life of the keys.

Although these are some of the primary design decisions for molded keypads, there are often other considerations depending on the specific application the keypad is intended for. Talk to your keypad manufacturer because they have probably designed controls for similar situations and can give advice on keypad design.

by: Christine Harrell

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