Where insert molding fits
Insert molding puts a metal component into the cavity before the shot, so the plastic forms around it and the part leaves the tool as a single assembly. Threaded bosses, bushings, terminals, blades, magnets and stamped brackets all end up this way.
It is not the same process as overmolding, which molds a second plastic over a first plastic, nor two-shot molding, which runs both materials in one machine on a rotating tool. The distinction matters commercially: insert molding needs an operator or robot to place the insert every cycle, so labour and placement accuracy — not injectable material — usually set the piece price.
Substrate selection starts with thermal expansion
The plastic and the metal expand at different rates. A metal insert constrained inside a plastic boss develops hoop stress as the part cools, and that stress either cracks the boss or relaxes into a loose fit. Matching the pair is the first design decision.
| Substrate | CTE (×10⁻⁵ /°C) | Pairing notes |
|---|---|---|
| PA66-GF30 | 2–3 | Best all-round match for brass and steel; absorbs moisture, so dry before molding |
| PBT-GF30 | 3–4 | Low moisture uptake, stable for electrical terminals |
| PPS-GF40 | 2–3 | High-temperature and chemically resistant; brittle, needs generous radii |
| POM | 10–12 | High shrinkage; avoid thick sections around inserts |
| PC | 6–7 | Tough but stress-crack prone with brass; anneal after molding |
| PP | 12–18 | Cheapest option but the worst CTE mismatch; use only for low-load inserts |
Glass-filled grades reduce the mismatch because the mineral fraction lowers expansion, which is why PA66-GF30 and PBT-GF30 dominate terminal and connector work.
Insert preparation decides whether the bond holds
Mechanical interlock, not adhesion, carries the load. Three preparations do most of the work:
- Knurling or grooving. A diamond or straight knurl, or a machined undercut, gives the plastic something to grip. A smooth pin relies on shrink fit alone and pulls out at a fraction of the load.
- Preheating. Warming inserts to 60–120 °C before placement reduces the thermal shock to the melt and closes the gap that opens during cooling. It also drives off condensation on cold metal.
- Degreasing. Cutting oil on a brass insert is a release agent. Aqueous or solvent cleaning before molding is normal for structural parts.
Inserts that will be plated or passivated after molding must be specified before the tool is cut, because post-plate chemistry attacks some alloys inside a sealed boss.
Geometry rules that survive production
| Feature | Rule of thumb |
|---|---|
| Wall around insert | 0.8–1.2 × insert diameter, minimum 1.5 mm |
| Boss height above insert | At least 0.5 mm plastic cover, or the insert shows through |
| Insert depth | 60–80% of boss length; leaving too little material under the insert causes sink |
| Draft on boss | 0.5–1° per side |
| Radius at boss base | 0.5 × wall thickness, never a sharp corner |
| Knurl engagement length | At least 1.5 × insert diameter |
| Gate position | Not directly opposite the insert; a jet of melt displaces it |
Two failure patterns dominate prototype reviews. The first is a boss that cracks at the base during ejection because the radius was drawn sharp. The second is a weld line running straight through the boss wall, which halves pull-out strength and only shows up in a burst test.
Tooling: vertical machines and fixed inserts
Insert molding is usually run on a vertical clamp with a rotary table or a shuttle, so the operator loads inserts on one station while the other molds. That keeps the machine productive without a robot.
Three tooling decisions carry the cost:
- Insert location. Pins or nests locate the insert to ±0.05 mm. Loose fits produce flash around the insert and short shots on the far side.
- Shut-off at the insert. The tool must seal against metal, not plastic. Hardened shut-off inserts that contact the metal wear, and the wear allowance has to be designed in.
- Ejection. Ejector pins cannot push on the insert. Plan ejector placement on plastic surfaces only, or the insert will move on the way out.
For high-volume programs, a robot with a vibratory bowl feeder replaces manual loading and pays back within a year on parts above roughly 500,000 units, but only if the insert geometry can be oriented reliably.
Process settings that matter
- Melt and mold temperature. Run the upper half of the substrate's window so the melt wets the knurl before it freezes. Too hot and the insert shifts; too cold and the plastic shorts around it.
- Injection speed. Fast fill around an insert risks displacing it. Profile the fill so the melt reaches the insert at moderate speed and packs afterwards.
- Packing pressure. Higher packing around a boss reduces sink but raises hoop stress. Structural parts often need a short pack followed by a longer hold at lower pressure.
- Cycle time. Inserts add cooling mass. A metal insert at 20 °C inside a 250 °C melt acts as a heat sink and can add 3–8 seconds to the cycle.
Validation: what to test and how
| Test | Method | Typical acceptance |
|---|---|---|
| Pull-out force | Tensile test, axial, 5 mm/min | Per drawing; specified as a minimum, not an average |
| Torque resistance | Torque driver to failure or a set value | Thread strip or spin, whichever comes first |
| Push-out / push-in | Axial compression | For bushings loaded in both directions |
| Leak test | Air decay or pressure hold | For fluid-path inserts |
| Thermal cycling | −40 °C to +125 °C, 100+ cycles | No cracking, no torque loss beyond 20% |
| Cross-section | Microtomy at the boss | Knurl fully encapsulated, no voids |
| X-ray or CT | Non-destructive | Insert position and concentricity in critical parts |
Pull-out values quoted from a single cavity are misleading. Cavity-to-cavity variation of 15% is normal, and the specification should be set on the weakest cavity.
Cost drivers and how to reduce them
| Driver | Effect | Lever |
|---|---|---|
| Insert unit cost | Often 20–40% of part cost | Standard catalog inserts instead of custom machining |
| Manual loading | 1 operator per machine | Robot + bowl feeder above ~500k units |
| Cycle time | Insert heat sink | Preheated inserts, conformal cooling at the boss |
| Scrap | Misplaced inserts, cracked bosses | In-mold sensing, generous radii, dried resin |
| Tooling | Vertical tool with nests | Share insert families across one tool |
RFQ information the supplier needs
- Insert drawing with material, finish, and the load case (pull-out, torque, fatigue)
- Substrate grade and glass content, plus any flame-retardant or food-contact requirement
- Annual volume and the expected tool life in shots
- Whether inserts are supplied free-issue or purchased by the molder
- Test standard and acceptance values for pull-out and torque
- Post-molding operations: annealing, plating, ultrasonic welding, leak test
- Traceability requirement — batch-level or unit-level
Related product categories
The materials and part types discussed above are grouped in our sourcing directory:
Standards referenced
- ISO 527 — tensile properties of plastics, used for pull-out fixtures
- ISO 898 — mechanical properties of fasteners, for thread engagement sizing
- IEC 60512 — connector tests, for terminal insert retention
- ISO 16750 — environmental conditions for electrical equipment in vehicles
