Liquid Silicone vs. Gum Stock

Liquid silicone is a two-part system that gets mixed as it is used. Gum-stock silicone, on the other hand, is a one-part system that has a shelf life. Both liquid and gum-stock silicones are heat cured. Liquid silicone has the consistency of Vaseline, while gum-stock has a consistency of putty. Gum-stock silicone is normally used for compression molding. The gum-stock is cut into a specific weight and placed in a mold. Compression molding is an open-air system similar to using a waffle iron. Liquid silicone is typically used for Liquid Injection Molding (LIM). Also referred to as liquid silicone injection molding, it is a closed loop system and molded under pressure using computer controlled repeatable equipment. At Si-TECH, we only use LIM for all of our products, allowing us to be more dependable in duplicating our processes.

Insert Molding

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Liquid injection molded part with a clear window and two light pipes

Using LIM has many advantages that compression molding cannot match; one advantage being insert molding. Insert molding is a process that allows for colored keys, clear windows or light pipes to be added to a silicone rubber keypad. Insert molding is achieved by having two molds, one mold to make the inserts such as colored keys, clear windows, or light pipes. Once the inserts are molded, they are placed into the second (keypad) mold. The inserts are held in place by a high clamping force. This ensures that the inserts will stay in place while the material flows into the mold around the inserts, causing a cross-link bond joining the inserts to the part and giving the appearance that it was molded all at once.

Using Inserts

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Parts made using compression molding using two different colored gum-stocks

Compression molding is not able to use inserts because gum-stock has the consistency of clay or putty-like material that is pushed into a mold under low pressure. This causes the inserts to shift around, not allowing them to stay in the correct position for molding. The only way around this would be by putting two different colored gum-stock in a mold to allow for different color keys, but that is an outdated and labor-intensive way to get an inconsistent product just for having different colored keys. You cannot make custom keypads with clear windows or light pipes using compression molding.

Si-TECH is a domestic silicone rubber manufacturer located in Newport News, Virginia. We do everything from custom silicone molding to assembling parts in-house. At Si-TECH we have more than 75 combined years of experience with liquid silicone injection molding and insert molding. Contact us at (757) 887-8488, or request more information or a quote on our home page, www.SiTech-corp.com.