Insert molding: what it is and which parts use it

Insert molding places a pre-made metal (or other material) part into the mold cavity, by hand or by robot, and injects plastic directly around it. The result is a single assembly where metal and plastic are joined into one piece. Common examples: threaded brass inserts, electrical connector pins, the grounding plate inside a power supply housing.

Why use it?

• No separate assembly step: The insert-fitting or welding operation disappears; a finished part comes out of a single operation.
• Thread durability: Screwing directly into a plastic thread wears it out over time (especially with repeated disassembly); a molded-in brass insert makes that durability permanent.
• Electrical/mechanical continuity: A metal pin molded into the plastic body keeps the risk of loosening under vibration lower than a separate fastened joint.

Practical rules

1. Securing the insert in the mold: The insert must be held in a pocket or by a magnet/pin so it can't shift under injection pressure; shifting causes flash and dimensional errors.
2. Thermal expansion mismatch: Metal and plastic have different expansion coefficients; as the part cools, the plastic shrinks around the insert and builds up stress. Avoiding sharp corners on the insert geometry (rounding, knurled/grooved surfaces) reduces cracking risk.
3. Insert preheating: Large/thick cold metal inserts can rapidly chill the surrounding plastic, causing short shots or weak bonding; preheating the insert may be necessary.
4. Cycle time: Manual insert placement (without a robot) adds directly to cycle time; at high volumes, automated feeding/robotic placement should be considered.

When should you choose it?

Insert molding makes sense when a threaded joint will be assembled/disassembled repeatedly, electrical conductivity is required, or volumes are high enough that assembly savings justify the mold/insert cost. For one-time fixed joints, simpler alternatives such as self-tapping screws or ultrasonic welding may be sufficient.