Overmolding

Overmolding bonds a second molded material over a rigid substrate to add soft touch, sealing, protection, or color without separate assembly.

Overview

Overmolding is an injection molding process where a second material is molded over a pre-formed substrate, creating a single bonded part. It’s commonly used to add soft-touch grips, integrated seals, impact protection, or cosmetic accents onto a rigid plastic base. The process can run in a single multi-shot tool or as two separate molding steps with reloaded inserts.

Use overmolding when you need multiple material properties in one part at medium to high volumes: rigid structure plus soft grip, hard housing with integral gasket, or color contrasts without painting. It reduces assembly, improves sealing, and can lock components in place. Tradeoffs include higher tooling cost, more complex mold design, longer cycle times, and tighter process control to ensure adhesion and alignment. Not every material pair bonds well, and thick overmold sections can shrink, warp, or peel if the interface and wall thickness aren’t designed carefully.

Common Materials

  • ABS
  • Polycarbonate
  • Nylon 6/6
  • Polypropylene
  • TPE
  • TPU

Tolerances

±0.005" to ±0.010" on critical dimensions, looser on soft overmold surfaces

Applications

  • Hand tool and power tool grips
  • Toothbrush and razor handles
  • Electronics housings with integrated seals or bumpers
  • Automotive interior knobs, switches, and handles
  • Medical device handles and overmolded strain reliefs
  • Vibration-damping mounts and overmolded bushings

When to Choose Overmolding

Choose overmolding when you need soft-touch, sealing, or impact protection directly bonded onto a rigid substrate in one integrated part. It suits medium to high production volumes where eliminating secondary assembly or adhesives offsets higher tooling and setup costs. It’s ideal for handheld products, sealed enclosures, and parts requiring both structural stiffness and localized flexibility.

vs Standard Injection Molding

Choose overmolding instead of standard injection molding when the part must combine rigid structure with soft-touch, sealing, or contrasting cosmetic regions in one piece. Overmolding reduces secondary operations like installing grips, seals, or pads, but only makes sense if the volume justifies more complex tooling and process control.

vs Insert Molding

Choose overmolding instead of insert molding when both substrate and overmold are plastics and you want a bonded interface for grip, sealing, or protection rather than embedding metal or preformed inserts. Overmolding focuses on multi-material surfaces and ergonomics, while insert molding focuses on capturing dissimilar inserts inside the molded part.

vs Thin Wall Molding

Choose overmolding instead of thin wall molding when part performance depends more on local grip, sealing, or protection than on minimizing wall thickness and cycle time. Overmolding tolerates more typical wall thicknesses but demands careful interface design, while thin wall molding targets lightweight, fast-cycling parts with uniform thin sections.

vs Liquid Silicone Rubber (LSR) Molding

Choose overmolding over stand-alone LSR molding when you need a soft, rubber-like interface bonded onto a rigid thermoplastic substrate rather than a fully elastic part. Overmolding can use TPE/TPU or LSR as the overmold layer, allowing rigid/soft combinations in one assembly where pure LSR would be too flexible or fragile.

vs Multi-shot/Co-injection

Choose conventional overmolding (two-step process) over fully integrated multi-shot/co-injection when volumes are moderate or when you want flexibility to run the substrate and overmold in separate tools or presses. Multi-shot tools are faster at scale but require higher up-front tooling investment and more specialized equipment than a staged overmolding setup.

Design Considerations

  • Select material pairs with proven chemical compatibility and adhesion; get your molder’s input before locking in substrate and overmold resins
  • Design mechanical interlocks (undercuts, holes, ribs, texture) at the interface so the overmold is retained even if chemical bond weakens
  • Maintain consistent wall thickness in the overmold and avoid very thick sections to reduce sink, warpage, and long cooling times
  • Add generous draft on both substrate and overmold surfaces to allow reliable ejection without tearing soft material
  • Clearly define critical interface areas, coverage limits, and shutoff surfaces in the CAD model and drawings so the tool can be built and quoted accurately
  • Plan gate locations and flow paths for the overmold shot to avoid knit lines, air traps, and weld lines in high-stress or cosmetic regions