Overmolding

Overmolding bonds a second molded material over a first substrate to add soft-touch, sealing, grip, or insulation in high-volume production.

Overview

Overmolding is an injection molding process where a second material is molded over a pre-molded substrate (or first shot) to create a single, bonded assembly. It’s commonly used to combine a rigid structural core with a soft-touch elastomer, add seals/gaskets, improve grip, provide electrical insulation, or protect electronics.

Choose overmolding when you need repeatable alignment and a durable bond between materials at production volumes that justify multi-step tooling and process development. It reduces secondary operations like adhesive bonding, staking, or installing separate gaskets.

Tradeoffs are higher tool cost/complexity, longer cycle times, and added scrap risk if bonding or registration is poor. Material compatibility is critical; some pairings need mechanical lock features or surface prep to achieve reliable adhesion. Tight cosmetic requirements can increase cost because knit lines, gate blush, and witness marks are harder to hide on multi-material parts.

Common Materials

  • ABS
  • PC/ABS
  • Nylon 6
  • Polypropylene (PP)
  • TPE
  • LSR silicone

Tolerances

±0.005 in

Applications

  • Soft-grip tool handles
  • Sealed push buttons and keypads
  • Overmolded cable strain reliefs
  • Medical device housings with seals
  • Consumer electronics protective bumpers
  • Encapsulated sensors and small PCBs

When to Choose Overmolding

Overmolding fits parts that need two distinct material properties in one component—typically a rigid core plus a compliant, sealing, or insulating outer layer. It works best for medium to high volumes where repeatability matters and eliminating secondary assembly steps offsets tooling cost. Plan for it early in the design so the interface, gating, and shutoffs support reliable bonding and cosmetics.

vs Standard Injection Molding

Choose overmolding when one resin can’t deliver both structure and touch/seal/insulation requirements in a single shot. It’s also a good fit when you want to eliminate post-mold assembly of gaskets, grips, or protective covers and control alignment within the tool.

vs Insert Molding

Choose overmolding when both materials are polymers/elastomers and you need a continuous outer skin or seal over a plastic core. Insert molding is typically better when the embedded component is metal, a bushing, or a preformed insert that doesn’t need full surface coverage.

vs Thin Wall Molding

Choose overmolding when the value comes from multi-material function (grip, sealing, damping) rather than minimum wall thickness and high-speed filling. Thin wall molding prioritizes flow length and cycle time; overmolding prioritizes interface control, bonding, and cosmetic management across two shots.

vs Compression Molding

Choose overmolding when you need precise placement of a soft material onto a complex rigid substrate with injection-level detail and repeatability. Compression molding can be cost-effective for simpler geometries and thicker elastomer sections, but it’s less suited to fine features and tight registration to a rigid core.

vs Liquid Silicone Rubber (LSR) Molding

Choose overmolding when the end part needs a silicone sealing layer or soft-touch region integrated onto a thermoplastic core. A dedicated LSR-only part is simpler when the entire component is silicone and you don’t need a rigid backbone or integrated assembly.

Design Considerations

  • Pick material pairs with proven chemical adhesion or add mechanical interlocks (undercuts, holes, ribs) to carry peel loads
  • Control wall thickness and transitions in the overmold to prevent sink, read-through, and distortion of the substrate
  • Define cosmetic surfaces and hide gates/parting lines; overmold witness marks are hard to rework
  • Add robust locating features so the second shot registers consistently (datums, pockets, anti-rotation features)
  • Call out bond and peel performance requirements (not just “overmold TPE”) so the shop can select resin, primers, and process windows
  • Keep shutoff edges and seal lands generous; knife-edge shutoffs and long thin seals drive tool wear and flash risk