Silicone overmolding is an injection molding process that molds silicone over, under, or through a substrate material to complete the final part. By allowing product designers to combine the best of two materials, this manufacturing process supports variations in durometer and rigidity. For example, a soft silicone rubber can be molded over a handle or cover that’s made of a hard plastic. The plastic provides strength and rigidity while the silicone provides a soft-touch surface.
Known also as in-mold assembly, silicone overmolding supports ergonomics, color variations, and brand identification in applications such as healthcare and infant care. As an overmold material, silicone can also impart functionality such as impact resistance, vibration damping, and environmental resistance. Silicone overmolding adds value to product designs, but it’s important to start with some fundamentals. Designers also need to understand the role of techniques and tooling in successful projects.
Silicone overmolding is more just a layering process. Rather, the overmold is assembled with the substrate material. The strength of this chemical or mechanical bond is critical, but it’s difficult to bond a silicone to a non-silicone material. With especially challenging material combinations, the substrate may require flame, corona discharge, or plasma treatment. Typically, however, bonding is achieved with mechanical undercuts, protrusions or indentations that prohibit part ejection from a mold.
Undercuts support overmolding, but these part features add costs and complexity to the mold. Plus, non-drafted areas can be difficult to remove. For product designers, it’s also important to remember that few injection molders are experts in overmolding liquid silicone rubber (LSR) to plastics. Overmolding an LSR with one durometer and color to an LSR with a different durometer and color is also a specialized discipline. There are differences in molding techniques to consider, too.
Silicone overmolding can be achieved with multi-shot molding or insert molding. With double-shot molding, the first material shot is the substrate and the second is the overmold. A press with multiple barrels shoots two different resins into the same injection molding tool. The entire operation uses one machine in a continuous process, but double-shot molding is better-suited for larger volumes since the molds and machinery are more expensive.
Insert molding is performed by placing the substrate into the mold and then overmolding some or all of the substrate. To achieve optimum bond strength, the injection molder may need to preheat the substrate so that its surface temperature is closer to the melt temperature of the overmold. Across lower volumes, insert molding is more cost-effective than double-shot molding; however, insert molding requires a tool for the substrate and a different core and cavity to create the volume for the overmolded material.
The cost of tooling isn’t all that designers need to consider, however. The shutoff between the overmold and the substrate needs a sharp transition. The overmold also needs to avoid thinning or feathering that can cause delamination. With gate sizing, the ratio of flow length to wall thickness is critical for adhesion. To minimize flow length, gates are located in the thickest wall areas. Because air entrapment can interfere with bonding, the mold needs vents that are deep enough to avoid flash.
Texture is another important design consideration since it enhances the feel of the part and can minimize surface imperfections. Often, a part with a textured surface feels softer to the touch than its durometer actually measures. Textured surfaces provide better part releases than polished surfaces, too. A wide variety of textures are possible with injection molded parts, and designers can even incorporate images or logos for branding or product identification.
Choose Extreme Molding for Silicone Overmolding
Extreme Molding of Watervliet, New York is one of the few custom molders in the United States that offers plastic and silicone molding at the same location. Importantly, we are experts at overmolding silicone onto plastic substrates – including BPA-free plastics like Eastman Tritan™ co-polyester. Extreme Molding is also expert at combining various types of silicone for performance and durability. Our experience includes overmolding electronics with high-performance plastic or silicone.