Overmolding

Overmolding is a manufacturing process in which a material is molded over a previously formed part or substrate so that two or more materials form a single assembly. It is commonly achieved through two main approaches: two-shot molding, where two materials are injected sequentially in one tool, and insert overmolding, where a pre-formed part is placed in the mold and overmolded around it. The substrate is usually rigid; the overmold provides a softer or differently functional surface.

In two-shot molding, the first material forms the substrate and remains in the mold, then a second material is injected to form the overmold in the same cycle. In insert overmolding, the pre-formed part is positioned in the mold, and the overmold material is injected to encapsulate or cover it. Adequate surface preparation and bonding promoters are critical for adhesion.

Common substrates include ABS, polycarbonate, or PC-ABS blends; overmolds often use thermoplastic elastomers (TPEs), silicones, polyurethane, or liquid silicone rubber. Material compatibility, surface energy, and thermal expansion must be considered to prevent delamination or warping.

Applications span electronics housings with a soft-grip zone, medical devices requiring biocompatible elastomer surfaces, consumer products, and automotive components with ergonomic or weather-sealed features.

Advantages include improved grip and tactile feel, enhanced impact resistance, insulation, protection from moisture and dust, reduced assembly steps, and design flexibility. Disadvantages include higher tooling costs, longer cycle times, and potential adhesion or process challenges.

Design considerations include gate location, wall thickness balance, venting, undercuts avoidance, and ensuring reliable interfacial bonding. Proper tolerancing, post-molding inspection, and process control are important for quality.