Insert Molding

Insert molding encapsulates pre-placed metal or other inserts in molded plastic, creating strong, integrated assemblies with improved threads, wear surfaces, and mounting features.

Overview

Insert molding combines standard injection molding with pre-placed metal or other inserts, molding plastic directly around them in a single cycle. The process produces plastic parts with integrated threads, bushings, contacts, shafts, or structural cores, eliminating secondary fastening and improving strength and durability. Typical volumes range from low to high, depending on whether inserts are hand-loaded or robot-loaded.

Choose insert molding when you need metal-strength features in a plastic part: robust threads, high-wear locations, electrical terminals, or precise mounting points. It cuts assembly steps, improves alignment, and reduces the risk of loose hardware backing out. Tradeoffs include higher tooling and fixturing complexity, longer cycle times due to insert loading, and tighter requirements on insert precision and cleanliness. Poorly designed insert features or excessive metal mass can cause sink, voids, or warpage. When designed correctly, insert molding delivers reliable hybrid parts that are hard to achieve with separate molding and assembly.

Common Materials

  • ABS
  • Polycarbonate (PC)
  • Nylon 6/6
  • PBT
  • PEEK

Tolerances

±0.002" to ±0.005" on molded features relative to inserts

Applications

  • Electronics housings with molded-in threaded brass inserts
  • Automotive sensor and connector bodies with metal terminals
  • Medical device handles with metal cores or rods
  • Knobs and dials with molded-on metal shafts or hubs
  • Plastic levers and gears with hardened metal bushings
  • Appliance components with integrated mounting studs or brackets

When to Choose Insert Molding

Use insert molding when you need metal-strength threads, wear surfaces, or electrical contacts integrated into a plastic part without secondary assembly. It fits best for small to medium-sized parts where alignment of metal and plastic features is critical and production volumes justify a repeatable loading process. Hand-loaded inserts suit low to mid volumes; automated loading supports high-volume production.

vs Standard Injection Molding

Choose insert molding instead of standard injection molding when your plastic part requires durable metal threads, terminals, or mounting features that would otherwise need secondary hardware or assembly. Insert molding improves alignment and pull-out strength compared to screws or heat-set inserts added after molding, at the cost of more complex tooling and molding operations.

vs Overmolding

Choose insert molding over overmolding when the substrate is a rigid metal or pre-formed component that must be fully or partially encapsulated by plastic in one shot. Overmolding is better for plastic-on-plastic or soft-grip layers, while insert molding excels at locking metal inserts, contacts, or bushings into a primary plastic structure.

vs Thin Wall Molding

Choose insert molding instead of thin wall molding when the main requirement is integrating metal features or stiffeners, not driving wall thickness to a minimum for weight or cycle-time reduction. Insert molding can still use relatively thin walls, but part geometry and flow must accommodate the inserts and avoid knit-line weaknesses around them.

vs Compression Molding

Choose insert molding over compression molding when you need tight positional accuracy between metal inserts and plastic features, complex 3D geometry, or higher production rates with thermoplastics. Compression molding is better for large, thick, or thermoset parts; insert molding shines in precise, smaller components with integrated metal details.

vs Metal Injection Molding (MIM)

Choose insert molding instead of MIM when only certain features need metal properties and the rest of the part can be plastic to save weight and cost. Insert molding lets you localize metal where strength, threads, or conductivity are needed while leveraging lower-cost plastic for the bulk geometry.

Design Considerations

  • Design mechanical retention features on inserts (knurls, undercuts, through-holes) so plastic can lock in and resist pull-out and torque
  • Keep insert mass reasonable and avoid very thick plastic around inserts to reduce sink, voids, and differential shrink warpage
  • Specify realistic positional tolerances between inserts and molded features; over-constraining will drive tooling cost and scrap rates
  • Standardize insert geometry and material where possible so suppliers can automate loading and reduce part cost
  • Ensure inserts are clean, dry, and dimensionally controlled; contamination or plating burrs can cause poor bonding or flash
  • Provide clear access for insert loading and ejection in the mold design, and call out any orientation-critical features in your drawings