Overmolding is the injection molding process where one material (usually a TPE) is molded onto a second material (typically a rigid plastic). If properly selected, the overmolded TPE will form a strong bond with the plastic that is maintained in the end-use environment. The use of primers or adhesives is no longer required to achieve an optimum bond between the two materials.
Overmolding Process Selection
A processor will weigh numerous factors when choosing the appropriate manufacturing method for the application. The most critical decision factors are production scale economics, local labor costs, available equipment and the materials selected.
Generally, insert molding is the process of choice when annual production volumes and local labor costs are low. For higher volume production programs (over 250,000 units annually) or areas with increased labor costs, multi-shot molding operations are the method of choice.
With any overmolding application, the challenge is in achieving maximum adhesion between the TPE and the substrate. For some overmolding TPEs, there may be a significant difference in bond strength between multi-shot and insert molding. Even if an excellent bond is achieved with two-shot molding, the same material may have poor bond strength when insert molded. Thus, a complete understanding of the TPEs, engineering plastics, and associated details about molding these materials is essential to produce high-quality finished products.