Insert Molding Guide — Metal Inserts in Plastic
Process
What insert molding does
Insert molding places a pre-formed metal (or sometimes ceramic/plastic) insert into the mold cavity, then injects plastic around it — producing a single part with molded-in threads, bushings, bearings, electrical terminals or contact pins. It replaces a post-mold threading, a press-in bushing or a solder terminal with one shot that cannot loosen.
This guide covers insert types, how the insert is positioned and held, the wall rules that stop sink and flash, and the numeric tolerances real programs are held to.
The Snapshot
- Real automotive connector housing: glass-filled PPS / PA66 with insert-molded terminals, held to ±0.005 mm critical / ±0.02 mm general, 2M units/year, 10-week production tooling, IATF 16949.
- Real EV busbar: C11000 copper terminals encapsulated in PA6 GF30, 250,000+ units/year, 8 weeks DFM-to-SOP, heavy-duty commercial EV Tier-1.
- Insert seating tolerance is the number that matters: ±0.02–0.05 mm position; the plastic part around it runs ±0.1–0.2 mm general.
- Wall around the insert: ≥1.5 mm typical (min 0.8 mm) to avoid sink; uniform wall stops the insert reading as a blemish.
- Insert molding beats post-mold tapping: no chips, no tap wear, no loose fastener — and a stronger joint under load.
Insert types and when to use them
| Insert | Material | Carries | Example |
|---|---|---|---|
| Threaded insert | Brass / stainless | Screw load | Housings, covers |
| Bushing / sleeve | Steel / bronze | Bearing, pin | Hinges, pivots |
| Terminal / contact | Copper / brass (plated) | Current | Connectors, busbars |
| Nut / standoff | Steel | Structural clamp | Mounting bosses |
| Pin / shaft | Steel | Rotation/link | Gears, levers |
The automotive connector uses brass/phosphor-bronze terminals in glass-filled PPS for heat and dimensional stability; the EV busbar uses C11000 copper (≥99.9 % Cu, high conductivity) leads in PA6 GF30 for mechanical and thermal load.
Positioning, holding and wall uniformity
The insert must sit exactly where the cavity expects it, every shot. How it is loaded decides repeatability and cost:
- Manual load: operator places the insert in the open cavity; fine for low volume or large/awkward inserts. Risk: mis-seat if the fixture is loose.
- Robotic / vibratory feed: automated placement for volume (the busbar and connector programs run automated loading at 250K–2M units/year). Removes the human variable.
- Mold-located seating: the cavity has a precision seat (±0.02–0.05 mm) the insert drops into; the plastic then locks it. Seating, not the operator’s eye, sets the position.
- Clamp force: the machine tonnage must hold the insert against melt pressure (300–1,400 bar injection) so it does not float or tip during fill.
The plastic around the insert is where parts fail cosmetically and structurally. Rules:
- Min wall to surface: ≥0.8 mm, preferred ≥1.5 mm so the metal doesn’t show as a sink or read-through.
- Uniform wall: keep the stock even around the insert; a thick lobe on one side sinks and warps the seat.
- Gate location: gate away from the insert face so the flow doesn’t push the insert off its seat; use a balanced fill.
- Thermal: metal inserts pull heat — a steel bushing chills the local melt, so raise local mold temp or slow the fill to avoid short-shot around it.
- Flash at insert: a poorly seated insert leaks plastic at the parting; verify seating before every run on automated lines.
Material and process windows
The plastic around the insert runs at its own temperature; the insert is inert but conducts heat.
| Material | Melt temp | Mold temp | Shrinkage | Why here |
|---|---|---|---|---|
| PPS (GF40) | 300–340 °C | 120–160 °C | 0.2–0.8 % | Connector heat/class |
| PA66 (GF30) | 260–300 °C | 60–100 °C | 0.2–0.8 % | Structural, moisture-sensitive |
| PA6 GF30 | 240–280 °C | 60–100 °C | 0.4–1.0 % | Busbar mechanical |
| PC+ABS | 240–280 °C | 60–100 °C | 0.4–0.7 % | Housing with inserts |
Glass-filled semi-crystalline resins (PPS, PA66, PA6) shrink 0.2–1.0 % and need controlled gate/rib design to avoid warp around the rigid insert. PPS at 120–160 °C mold temp is the connector’s choice for under-hood-adjacent heat.
Tolerances: seating is the spec
With insert molding the critical dimension is often the insert’s position relative to the molded features, not the plastic thickness.
- Insert seating position: ±0.02–0.05 mm; the connector program held ±0.005 mm on critical terminal locate — the tightest we run.
- General plastic features: ±0.1–0.2 mm band.
- Mold (tool) tolerance: ±0.02 mm on the steel seat.
- Shrinkage split: PPS 0.2–0.8 %, PA6 GF30 0.4–1.0 % — the mold is cut oversize to land the part, and the insert seat is cut to the ±0.005 mm it must hold.
Specifying ±0.005 mm on the plastic body is meaningless; put that number on the terminal locate that actually carries the electrical contact.
Compliance for automotive / EV programs
Insert-molded electrical parts ship into regulated supply chains, so the documentation gate is real:
- IATF 16949 — the automotive QMS gate; without it a molder cannot supply a Tier-1 line (the connector program runs under it).
- PPAP — first-article proof: material cert, dimensional report, FMEA, capability study at Cpk ≥ 1.33 on critical characteristics.
- Lot traceability — resin lot, machine and cycle map back to each part; a copper/PA6 busbar defect must trace to its lot.
- UL94 — flame class on the encapsulating resin where the program requires it (e.g. V-0 for under-hood-adjacent).
Real programs we have run
- Automotive connector housing — glass-filled PPS / PA66, insert-molded terminals, ±0.005 mm critical / ±0.02 mm general, 2M units/year, 10 weeks, IATF 16949.
- 800V EV busbar — C11000 copper + PA6 GF30, 250,000+ units/year, 8 weeks DFM-to-SOP, heavy-duty commercial EV Tier-1.
- Speaker components — PP / ABS / ABS+PC with insert features, ±0.02 mm parts / ±0.05 mm mold, 150+ mold sets / 30 models, 8 weeks.
Where insert molding goes wrong
- Loose insert seat → terminal floats, electrical contact fails; seat must be ±0.02–0.05 mm.
- Thin wall over insert → sink / read-through on the cosmetic face; keep ≥1.5 mm.
- Gate pushes insert → off-center seat; gate away from the insert face.
- No IATF / PPAP → cannot enter Tier-1; the audit blocks the program.
- Chilled insert short-fill → raise local mold temp or slow fill around metal.
Compliance pass
Send the insert spec (material, thread/class, seating tolerance) and the plastic resin, plus any IATF 16949 / PPAP / UL94 requirements. We return a seating design, a wall-uniformity plan and a tolerance split — held to ±0.005 mm on critical programs — with a tooling path of 8–10 weeks to production.