Insert Molding

Insert molding bonds metal or plastic inserts inside an injection-molded part, creating strong, repeatable hybrid components in high-volume production.

Overview

Insert molding is an injection molding process where pre-made inserts (threaded bushings, pins, contacts, magnets, sleeves) are placed in the mold and plastic is molded around them. The result is a single, mechanically robust assembly with the insert captured and aligned by the tool, eliminating secondary staking, adhesive bonding, or fastener installation.

Choose insert molding when you need durable threads in plastic, reliable electrical terminations, wear surfaces, or precise positioning of a hard component inside a molded housing—typically at medium to high volumes where tooling cost is justified. Key tradeoffs are higher tool complexity, longer cycle time from insert placement, and added quality controls to prevent insert shift, flash, and voids. Inserts must tolerate melt temperature and clamp forces, and part geometry needs enough plastic thickness and features to lock the insert against torque, pull-out, and creep.

Common Materials

  • ABS
  • PC
  • PA66 (Nylon)
  • PBT
  • PEEK
  • LCP

Tolerances

±0.003" to ±0.005"

Applications

  • Threaded inserts in plastic housings
  • Electrical connector bodies with metal contacts
  • Sensor housings with molded-in magnets
  • Medical device handles with metal reinforcement
  • Automotive under-hood connectors
  • Valve bodies with wear sleeves

When to Choose Insert Molding

Insert molding fits parts that need a hard feature captured inside a molded body—threads, contacts, shafts, magnets, or wear components—with consistent location controlled by the tool. It works best when volumes justify dedicated tooling and you want to remove manual assembly steps. Designs should allow positive retention so the insert can’t rotate or pull out under load.

vs Standard Injection Molding

Choose insert molding when the molded part needs integrated metal/plastic features—durable threads, electrical contacts, reinforcement, or wear surfaces—that would otherwise require secondary assembly. Standard injection molding is better when a single-material plastic part meets requirements and you want the simplest tool and shortest cycle.

vs Overmolding

Choose insert molding when the embedded component is a discrete insert placed into the tool (metal bushing, contact, magnet) and must be mechanically locked in position. Overmolding is typically used to add a second polymer layer onto a substrate for grip, sealing, or soft-touch, often with different material compatibility constraints.

vs Thin Wall Molding

Choose insert molding when insert retention, torque resistance, or pull-out strength matter more than minimizing wall thickness. Thin wall molding prioritizes fast fill and minimal sections; inserts increase thermal mass, restrict flow paths, and can drive local thickening that fights thin-wall targets.

vs Compression Molding

Choose insert molding when you need high feature detail, tight insert location control from a closed tool, and automated high-volume cycles. Compression molding can embed inserts, but it’s generally better for larger, thicker parts and materials that don’t flow like thermoplastics, with less emphasis on fine molded detail.

vs Transfer Molding

Choose insert molding when you want thermoplastic performance and high-rate production with integrated inserts. Transfer molding is often used with thermosets for encapsulation and electronics; it can be more forgiving around inserts but won’t match typical thermoplastic properties or cycle economics for many housings.

Design Considerations

  • Add anti-rotation features (flats, knurls, undercuts, holes) so inserts resist torque without relying on plastic friction alone
  • Maintain sufficient plastic wall around the insert to prevent sink, cracking, and pull-out; avoid sharp section jumps near the insert
  • Provide lead-ins, nests, or pockets that positively locate the insert in the tool and prevent tipping during injection
  • Specify insert material/finish and cleanliness requirements; oils and plating choices affect bonding and cosmetic flash
  • Gate and vent to avoid trapping air behind the insert; trapped gas drives burn marks and voids at the insert interface
  • Call out critical insert location dimensions and datums; measuring from molded surfaces without defined datums causes inspection ambiguity