Metal inserts molded in. Strong threads. Single-piece function.

Plastic threads tapped directly into plastic: pull-out strength 50-100 N typical for M4 thread. Strip out on 5-10 assembly cycles of high-cycle use. Brass heat-set insert in plastic: pull-out strength 500-1000 N. Survives 100+ assembly cycles. For parts requiring repeated fastener installation (service covers, maintenance access), brass inserts are essential. Use brass inserts for: any thread experiencing > 1 removal cycle, any thread under significant load.

Heat-set inserts: installed after molding via heat stake or ultrasonic. Flexible — same plastic part can accept different inserts depending on application. Insert molded: installed during molding — inserts placed in mold before injection. Stronger bond between insert and plastic (chemical bond). More expensive due to mold complexity and cycle time. For production: insert molding if volume justifies; heat-set for flexibility.

Challenges: (1) Hot plastic can degrade electrical plating — specify plated-after-molding or plastic-compatible plating. (2) Electrical contacts must be exactly positioned in mold — automated placement preferred at volume. (3) Interface between metal contact and plastic needs seal to prevent moisture intrusion. (4) Thermal expansion mismatch can cause delamination at high temperature. All manageable with appropriate design and materials.

Insert molding tooling cost: 1.2-1.5× standard injection tooling. Extra complexity: insert placement fixtures, possibly pneumatic inserts that retract during plastic injection. Automation of insert placement: important for high volume. For 10,000+ annual production, fully automated insert placement and handling integrated with injection molding cell.

Key rules: (1) Minimum wall thickness around insert 2× insert diameter to prevent shrinkage voids. (2) Insert diameter should be 80-100% of hole diameter in plastic. (3) Knurled or undercut insert surface improves plastic-to-metal bond. (4) Insert should not sit flush with surface — slight recess prevents flash issues. (5) Consider differential thermal expansion in service — brass expands more than plastic.

Insert molding adds $0.05-0.30 per insert depending on volume and automation. Manual insert placement: $0.20-0.50 per insert. Automated placement: $0.05-0.15 per insert. For threaded inserts, cost comparable to secondary operation (heat-set) at mid volumes. Savings: eliminated secondary assembly operation + improved joint strength + cleaner appearance (no heat-set marks).