products of silicone moldingWhen selecting a material for your keypad, silicone rubbers are the premier choice . Any electronic device that has a keypad can be fitted with this material to ensure that it stays protected during its lifespan. The flexibility of silicone is maintains during the molding process allows any shape to be formed. There is no limit on what devices can or cannot be equipped with silicone protection.

Categorizing Silicone Creation

Silicone rubber can be grouped into two different categories, each category being based on the type of reaction used to cure the material during molding. Condensation curing methods release alcohol and use atmospheric moisture, while addition curing methods utilize heat and do not produce waste materials. Each method produces silicone rubber with differing characteristics. The method selected can depend on how you intend to utilize the silicone rubber. Silicone made for keypads is generally heat cured using the addition method.

Two main types of silicone are used for keypads, gum stock and liquid. Gum stock silicone is the type that can be purchased at a store, and it is not built to last forever. It has a shelf life and feels harder, with a consistency similar to clay or putty. Liquid silicone is much softer. It is used for custom work. The hardness of silicone can be measured using a shore-A scale. Lower numbers indicate that more silicone was used in creating the product. This means that it will be softer. Higher numbers mean that the silicone will not be as soft but it will be a bit sturdier.

There are four types of molding processes used to make silicone: compression, transfer, liquid, and extruding. Compression molding uses an open air system where the silicone is hand cut and weighed. Transfer molding uses compression molding and gum stock material to create a mold under low pressures. Liquid molding uses computer controlled equipment, while a closed loop system to create silicone in a pressurized environment. Extruding molding is a gum stock molding process used for silicone items that are a singular large piece.

Molding the Silicone

After a silicone mold has been cured a shell mold is constructed so that the rubber can retain its shape once it is demolded. Shell molds are made from materials such as plaster or fiberglass. It helps to harden a drying mold by using a vacuum chamber so that no air bubbles are left in the mold once it is solid. The curing process normally takes anywhere from 18 to 24 hours. However this process can be sped up through the use of catalysts. The types of catalysts available for the various kinds of silicone vary. These catalysts can reduce curing time down to less than one hour if the catalysts are prepared correctly. The downside of this process is that molds made quickly tend to be more brittle. They do not last as long before wear and tear take their toll.

Before pouring silicone mix, it is best to stir it first. Silicone that has sat in storage for long periods of time is prone to settling. Use a circular stirring motion to release any air bubbles is necessary. Scrapping the bottom of the container to get all of the settled mix loose is also useful for prime stock. Stirring can be done with a paint stick or a similar tools. After mixing pour however much silicone you plan to use into your mixing container and prepare to add the catalyst.

Measure the correct amount of catalyst and add it to the mix. The recommended amount of catalyst – based on its type and the type of silicone you are using – is a good amount to use if you are not worrying about working in a limited time frame, or if you need to speed up the curing process. Do not go overboard when you are adding catalyst. Speeding up the curing too much will make your mold brittle. Aside from the amount of catalyst, your cure time will be influenced by the temperature and humidity your mix is exposed to (higher temperatures will make the curing process go faster). Stir the silicone-catalyst mix for two to three minutes. Like before, make sure you use circular motions to eliminate air bubbles and scrape the bottom. That way none of the mix settles. After you are done stirring it is good practice to pour the mix into a new container. This should eliminate the poorly mixed material and possibly imperfections. Once your mixture is ready you, will have roughly 20 minutes (with most catalysts) to use it before it starts to cure.

When you pour the silicone over the keypad air bubbles will be your number one enemy. There are several options available to you for avoiding air bubbles. The first is to use a silicone with low viscosity. Second, you can use a paintbrush to fill holes and gaps between the keys that air bubbles. If you do not have a paintbrush handy or wish to not use one you can slowly pour a thin stream of your mixture into the holes, flooding them gradually so that air bubbles cannot form. Your keypad can be tilted and you can pour the mixture from the end that is in the air. This allows the mix to slowly run down and cover the keypad. The last option is to use an air gun to move the silicone mixture around in order to fill the holes in. You may need to move the silicone around a bit as it starts to cure. Once it hardens enough, it will remain still by itself. If you do not have enough silicone to complete a mold on the first attempt, it is best to focus on eliminating air bubbles. Then add a second layer of material later.

Once the mold has finished curing and is dry, you can demold (remove) the mold from the keypad. When the outside of the mold seems dry, give it a little time before you demold. By doing so the deeper parts between the keys has enough time to dry sufficiently. Start demolding by gently lifting the edges up and move towards the middle until the mold is wholly removed. If at any time the mold seems too soft, stop lifting and give it more time to cure so that you do not tear or otherwise damage it accidentally.

Marking Your Keypad

The design of the keys on your mold will need to fit over the keypad you are working with. However, keys can be molded into any shape while being about to fit almost any keypad. If your keypad has square-shaped keys, you could have a mold created with silicone keys that are circular (instead of square). When creating custom shapes it is important to remember that your selection of shape will control the function of the key. While a solid shape, such as a circle or square, will distribute pressure evenly. When the key is something more intricate, like a half-circle, may need to be pressed in a certain spot to receive enough force to activate the key. This can increase wear to the spots of the silicone that frequently receive pressure. After the basic shape of the silicone keys has been decided upon the keys can be marked. There are three primary methods for doing this: printing, laser etching, and using plastic key caps.

Printing is the technique usually used for marking silicone. The silicone is put in a fixed position so that the tops of the keys are held flat. The keys are then printed upon. Any color of ink can be selected so the markings will be able to stand out well, regardless the color selected for the silicon. There are a few steps that can be taken to improve the life of the ink on the rubber. An oil, epoxy, drip, or parylene coating can be put over the marks. Oil and epoxy coatings are very versatile, being available in gloss, semi-gloss, or matte finishes. Drip coatings are glossy and can be made hard or flexible depending on your needs. They do have limitations, as drip coatings are subject to cracking if used over larger surfaces. They cannot be used on some keys with sharp angles. Parylene coating offers the best level of protection for non-plastic coatings, bonding to the silicone on a molecular level. Aside from coating, you can also apply clear plastic key caps over the marks. Doing so protects the ink from direct exposure to elements that would cause them to fade.

laser etching machine

An example of a laser etching machine.

Laser etching works well with keypads that are equipped with backlights. The etching process generally involves three steps. First, translucent rubber is sprayed with a coat of translucent ink that will make the marks visible to the user. The rubber can be clear or colored. It is then sprayed with a top coat of ink. This ink will be the primary color of your silicone keypad. Finally, the top coat is laser etched so that the first (bottom) coat of ink becomes visible and takes the shape of the marks. If you think two coats of ink are unnecessary you can use a single coat. The laser etching will reveal the color of the rubber instead of the color of a first ink layer. It is recommended to use colors with contrast so that the marks are easily visible on the keypad.

Plastic key caps last longer than either printing or laser etching. Reason being, it is composed of much sturdier materials. Since the markings for the keys are carved into the plastic the key caps can last indefinitely. This separates it from the ink used in the other two methods. This technique is commonly seen on cell phone keypads with the key caps being placed over the silicone.

Benefits of Using Silicone

Silicone has widely been used on electronic devices for the past several decades due it providing better protection from a host of factors (as compared to other rubbers). The exact specifications of what silicone is capable of are not universal, as there are different ways of making silicone. Those various methods will affect the performance of the rubber.

Environmental factors have a more difficult time damaging silicone compared to other materials. This gives silicone an advantage in outdoor or other extreme settings. Silicone has a much higher tolerance to heat than other rubber materials do, making it ideal for high temperature environments. Along with heat resistance, silicone has a high resistance to the cold. Where other rubbers would start to solidify, silicone remains flexible. Silicone also has the advantage of absorbing almost no water, which allows the material to protect delicate electronics that may be exposed to moisture. While other rubbers can decay and require replacement over time, silicone is much more stable and will not wear down. This is true even after it has been in use for long periods of time.

Beyond the general qualities that make silicone remain efficient for a longer period of time, there are additional factors that make the material ideal for use as the rubber keypad coating. Silicone is highly resistant to electricity and makes an excellent insulator, which can protect users from shocks that can potentially be delivered from an electronic device. At high temperatures, silicone becomes more resistant to oil than most other types of rubbers. Silicon products are resistant to solvents and chemicals at any temperature (though strong enough solvents or chemicals can still damage silicone). Certain types of silicone can be made to resist radiation so that the material is ideal in some environments.

In addition to the protective properties, silicone offers they also allow for various aesthetic features. Silicone keypads can be made in color, with various colors in a single keypad or as a single color unit. Keypads may also be translucent. Beyond all of the safety features and visual bonuses provided, silicone is also a cost effective material. This allows customers to get the best quality products without having to worry about their budget.