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 Types
Two injection molding processes dominate the manufacture of overmolded products: insert molding and multi-shot injection molding. Insert Molding The most widely used process is insert molding, where a pre-molded insert is placed into a mold and the TPE is shot directly over it (Figure 1). For molders, the advantage of insert molding is that conventional single shot IM machines can be used (new machinery expenditures are not necessary), and the tooling costs associated with insert molding are lower than with multi-shot processing. Multiple Material Molding Multiple material, also known as two-shot (or multi-shot), molding requires a special injection molding machine that is equipped with two or more barrels, allowing two (or more) materials to be shot into the same mold during the same molding cycle. A molder will choose multi-shot molding to reduce cycle times, achieve superior part quality and reduce labor costs.
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.