Insert molding places a metal component into the tool before every shot so the plastic flows around it and locks it in place. An insert molding service therefore sells a combined part, not a moulding with hardware bought later: the finished component leaves the press assembled. That difference decides torque strength and failure mode, which is why connector, appliance and power-tool programs specify molded-in inserts rather than hardware fitted after moulding. This page explains the process and what to confirm with an insert molding service before a program starts.
Where Insert Molding Wins
Post-moulding assembly, pressing or ultrasonically driving an insert into a moulded boss, works and can be cheaper at very low volumes. A molded-in insert cannot loosen, because the plastic was formed around its knurls and undercuts; a pressed insert depends on boss diameter and wall friction, and creep relaxes both over years. Insert molded components also allow thinner bosses, because the plastic-to-metal key carries the load instead of a boss wall. When a threaded connection will see repeated assembly or torque, insert molding is usually the cheaper answer.
Insert Types and Holding Features
- Threaded inserts: brass and stainless knurled inserts for machine screws, the most common molded-in hardware.
- Pins and shafts: hardened pins acting as bearing surfaces, pivot points or contact electrodes.
- Bushings and sleeves: precision bores for sliding or rotating members, held concentric by the tool itself.
- Electrical terminals: stamped metal contacts on connector and sensor housings, positioned by the cavity.
Inserts are located on diamond or straight knurls, an undercut groove or a milled flat, which give the plastic something to key against; the tool carries them on spring-loaded pins.
Resin Choice Around the Insert
The resin has to bond to metal and hold a thread. Glass-fibre-reinforced PA66 and PBT are the workhorses for metal insert molding because they combine stiffness with the creep resistance a threaded boss needs. Filler content is a trade: more glass raises strength but roughens flow around the insert and can trap weld lines.
Process Control and Quality Checks
Because an insert sits in the tool every cycle, press discipline matters more. Inserts arrive clean, the tool carries a presence check that stops the cycle if an insert is missing, and the first shots of every setup are checked before the rest run. Quality evidence should cover pull-out force and breakaway torque, positional accuracy against the drawing datums, and a cross-section record showing the plastic encapsulated behind the insert head.
What to Confirm With an Insert Molding Service
Ask for the insert drawings the tool was built around, the datum scheme that positions each insert, and the retention test values with pass criteria written down. Confirm who supplies the inserts, and how insert lots are traced to finished part lots. Background reading: the custom molding category, the plastic part design guidelines and the injection molding defects guide; the China plastics company directory lists verified custom molders.
Related product categories
The materials and part types discussed above are grouped in our sourcing directory:
