Silicone Rubber Explained – Part II

Silicone is a remarkable material. Without silicone’s properties, rubber keypads would not have moving keys, be able to close electrical switches or have self positioning features. Part II of our Silicone Rubber Series reveals many of silicone’s basic properties that allow it to be molded into a multi-functional keypad.

Silicone Raw Material

Raw silicone starts off with the consistency of clay. The raw silicone is first mixed together with a catalyst which assists in the molding processes. Different base raw silicones are mixed together in specific ratios and formulas to achieve a very specific silicone hardness.

Silicone Hardness

Silicone rubber keypads can be made with different hardness ranging from 30 shore A to 80 shore A durometer. Rubber bands have durometer of 20 shore A, plastic is about 95 Shore A or higher. Raw silicone comes in base hardness of 30, 50, 70 and 80 Shore A. The typical durometer of a molded rubber keypad is 60 Shore.

Two raw silicones can be mixed together to achieve a specific hardness. (Example: a mixture with 50% silicone at 40 Shore A and 50% at 60 Shore A will result in a final material with a Durometer of 50 Shore.)  The hardness or durometer is based on the purpose of the rubber such as sealing requirements, tactile force requirements, insert molded keytops and pull through tabs.

Silicone Color

Silicone without pigment is clear or a slightly milky white color. Pigments can be added to the raw silicone mixture to make parts in virtually any color. Rollers are used to integrate the pigments into the raw silicone. If a silicone rubber keypad has multiple silicone colors, each color must be prepared separately.

Matching Silicone Color

The color of the silicone rubber keypad is controlled by the amount and type of pigments used in the molding process. CSI uses the Pantone system to match silicone colors, but we can also color match parts to plastic samples or color chips. Because of silicone’s unique texture, it is possible that the keypad will not color match in certain lighting conditions. This is called metamerism. In this case, the pigment formula can be slightly changed (example: made darker or more blue). Once the color is approved, the pigment formula is locked in assuring consistent color throughout production and life of the rubber keypad product.

Transparent & Tinted Silicone

Raw silicone without pigments will appear transparent (clear) or slightly milky white after molding. This is perfect for backlighting nomenclature or symbols. Using a tiny amount of pigment, the silicone can be made mostly transparent with a slight color tint. By varying the pigment color and amount, the tint can also be varied from highly transparent to virtually solid.

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