by Chris Devers
More manufacturers are turning to overmolding as they learn how it adds value to their product lines. Not only does it improve functionality and performance and aesthetics, it lowers total production costs–a rare combination these days.
Overmolding is a one-shot or two-shot process that bonds a thin layer of TPE (thermoplastic elastomer) to the substrate of the product. This exterior layer is what creates the soft-touch grip that is popular for a wide range of products, ranging from personal care products like toothbrushes to power tools. Overmolding also reduces shock and vibration, dampens sound, provides electrical insulation, boosts chemical/UV resistance, and can be a barrier to oxygen and moisture.
As a result, more consumers are demanding this feel to the products they purchase and thus overmolding applications continue to expand. The availability of this technology also gives product engineers more options for creating attractive and functional products and staying on the leading design edge of their markets.
Not every injection-molder can overmold, however; it requires extensive training, specialized equipment, and precise control of multiple variables throughout the production run. Even the slightest variations can affect quality, so skilled operators are a must. Much of this knowledge relates to the type of TPE and substrate being used, so a deep understanding of material science and chemical behavior is also required. New resin blends are constantly being engineered to provide expanded properties, including chemical and heat resistance, UV resistance, hardness, scratch resistance, clarity, and UL standards.
A key step in overmolding is determining the best TPE/substrate combination for optimal bonding, followed by designing the appropriate mold and tools. Maintaining the melt temperature of the TPE (which determines how easily the TPE flows, as well as its bondability) is absolutely critical. The surface of the substrate must be extremely clean and free of contamination including dirt, dust, excessive moisture, or skin oil (gloves should be used at all times when handling)–if the substrate is even slightly contaminated the bond between the TPE and the substrate could be weakened or even fail. Because the moisture level of both the TPE and the substrate can negatively impact the chemical bond, both materials must be carefully dried before molding.
For tooling, vents must be properly sized and placed in the mold to permit the release of air from the mold cavity during overmolding; trapped air can weaken the bond between the TPE and the substrate. The shutoff must also be properly designed for the materials involved to eliminate the risk of delamination or curling. Gate size is determined according to the type of material, viscosity, flow rate, and the amount of shear that may be required.
It’s rare when an advanced technology improves product viability and customer satisfaction, yet still reduces overall production costs. Total costs are lower because overmolding can reduce the number of steps involved in making the product. For example, the TPE layer forms such a strong bond with the substrate that it eliminates the need for some secondary finishing operations such as priming, painting, or coating. By combining these steps into one injection molding process, overmolding increases quality, reduces waste, boosts throughput, and decreases overall production cost.