Silicone over-molding is a process that is used to cover, bond or encapsulate an existing part with silicone material. For example, a silicone over-molding process may be used for providing a grip surface to a smooth plastic handle, a flexible septum on a hard plastic surface or for encapsulating an electronics assembly for mounting or protection from the environment.
The motivation for over-molding silicone may include a means of final assembly, enhancing physical surface characteristics and feel, providing a self-healing internal access (ie, an integrated septum), creating the finished product enclosure (such as a key-ring LED light), creating a thermally conductive path for heat-sinking or perhaps applying elastic properties to mounting components,
Two significant issues must be considered prior to developing a silicone over-molding bonding:
a.) Silicone adhesion to the surface to be over-molded
b.) The temperature required to cure the silicone
Surface compatibility and the ability to enhance surface adhesion characteristics is fundamental to the success or failure of bonding to the over-molded part. Feasibility testing should be done to verify silicone bonding to the given substrate. The typical silicone curing temperature is from 220o F to 350o F (100o C to 160o C). At a first pass, the over-molded part must be able to withstand the silicone curing temperature.
Here are 10 important things you should know about Silicone Over-Molding:
1. Other applications for silicone over-molding include: Gaskets onto thermal plastic and metals, soft tips for catheters, encapsulation of circuit boards, sensors, motors and others.
2. Over-molding of silicone to silicone with different durometers is an easy opportunity. Silicone bonds well with silicone.
3. Normal curing temperature is in the range of 350 F (160 C) but process curing may be driven down to 220 F (100 C).
4. If significant mechanical bonding is required, the silicone over-mold may be mechanically interlocked with the part.
5. Selection of the material of the part to be over-molded, the use of primers and the use of bonding-enhanced silicones may allow enhanced chemical bonding of the silicone to the over-molded part.
6. Over-mold bonding strengths typically increase over time.
7. Polymers that are ideal for over-molding include polycarbonate, nylons and other high temperature resins.
8. A two step, two shot continuous polymer molding and silicone over-molding process may be developed. First the thermoplastic polymer piece is injection molded and then the silicone may be over-molded in a continuous operation.
9. In order to have a clean silicone over-mold, the over-molded part must be made with the requisite tight tolerances and part samples of exact size are needed. A stable-dimensioned part is needed to develop and maintain the desired clean over-mold outcome.
10. For encapsulation, silicone over-molding provides a seamless environmental barrier. However, silicone is gas permeable and, over time, the internal gas partial pressures will equilibrate with external gas partial pressures – including the partial pressure of water vapor. If the over-molded part must be isolated from moisture or other environmental gas partial pressures, other barrier means besides silicone must be used.