Insert Molding

What is Insert Molding

Insert Molding is the process of placing metal inserts (nuts / terminals / pins / springs / mesh) into the mold cavity, then injecting plastic to partially or fully encapsulate the insert, forming an integrated part.

Compared to traditional “molding then assembling nuts / terminals”:

• Reduced assembly stations, unit cost down 20-40%
• Thread pull-out strength 30%+ higher than ultrasonically inserted nuts
• No relative sliding between metal and plastic, no loosening over long-term use
• Better sealing (IP54+ friendly for electronics)

Common Insert Types

• Nuts / Threaded Inserts: Brass PEM inserts, stainless steel internal threads (connectors, fasteners)
• Terminals / Pins: Metal terminals (connectors, sensors, battery contact strips)
• Magnets: NdFeB / Ferrite (motors, seals)
• Metal Mesh / Copper Sheets: Heat dissipation, shielding (electronics, medical devices)
• Pre-molded Plastic Parts: Transparent lens + plastic frame, two-shot preforms
• FPC / PCB Flex Boards: Low-pressure insert molding to protect circuits

Design Guidelines

• Inserts should have knurling, flanges, or blind holes as mechanical locking features to prevent rotation and axial pull-out
• Insert dimensional tolerance ±0.05mm, cavity locating pin accuracy ±0.02mm to avoid misalignment
• Large difference in thermal expansion coefficients between metal and plastic; leave 0.1-0.2mm elastic clearance at critical hole positions
• Preheat inserts to 80-120°C to reduce cold stress cracks (mandatory for PA / PC)
• When nut insert thread direction is perpendicular to the mold opening direction, angled core pull is required

Typical Applications

• Electrical connectors (USB / Type-C / industrial connectors)
• Automotive sensor housings (with threaded interfaces)
• Consumer electronics structural parts (laptop hinges, phone frames)
• Battery tab and connection strip encapsulation
• Motor parts (copper wire / magnet encapsulation)
• Medical devices (stainless steel catheter + plastic handle)