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Core line, represented 25+ years
Insert molding
Insert molding places a preformed part, usually metal, into an injection mold, then injects plastic around it so the two become one part. It is used for threaded inserts, pins, terminals and stamped contacts. It removes a post-mold assembly step and gives higher pull-out strength than pressed-in inserts. Tooling typically runs $8,000–$80,000+, like a standard mold plus insert locators.
A method under plastic injection molding, alongside overmolding.
How does insert molding work?
The schematic plays while it is on screen. Pick a phase to hold it there.
- 1
Make the inserts
Inserts come from screw machining, stamping or a catalog. They must be held to tight tolerances so they seat in the mold.
- 2
Load the inserts
An operator or robot places inserts on locating pins or in pockets in the open mold, often pre-heated to reduce stress in the plastic.
- 3
Close and clamp
The mold closes. Shutoffs seal against the insert so plastic does not flow onto threads or contact surfaces.
- 4
Inject
Molten plastic fills the cavity and flows around knurls, grooves and undercuts on the insert, locking it in mechanically.
- 5
Cool and eject
The part cools, the mold opens, and ejector pins push the finished part out with the insert captured.
What can insert molding hold?
Typical ranges across principals. Your drawing, material and quantity set the real numbers in the quote.
| Insert position | ±0.003–0.005 in (±0.08–0.13 mm) to molded features |
|---|---|
| Load time | Adds 3–10 seconds per cycle hand-loaded; less with robots |
| Press type | Vertical with shuttle or rotary table, or horizontal with robot load |
| Plastic around insert | Wall ≥ 0.5× insert diameter; boss OD about 2× insert OD |
| Tooling cost | $8,000–$80,000+ |
| Tooling lead time | 5–12 weeks |
| Economic volume | 5,000 parts and up |
When is insert molding the right call?
Best for
- Threaded brass inserts, bushings, pins and shafts in plastic parts
- Electrical terminals, contacts and stamped lead frames
- 5,000 parts and up, where post-mold insertion labor adds up
- High pull-out and torque-out strength
Pick something else when
- Low volumes under a few thousand: heat-set or ultrasonic inserts after molding
- Designs where the insert may change: post-mold insertion keeps the mold simple
- Soft-touch grips over a rigid part: overmolding
How does insert molding compare with the alternatives?
Typical values. Pick an alternative to see where each one wins.
Insert molding compared with
Insert moldingPost-mold inserts
- Precisionlooser → tighter
- Volume it pays atone-offs → millions
- Upfront toolingnone → heavy
- Shape freedomnarrow → wide
Insert molding · pick it when
Metal and plastic in one part with the best bond and no post-mold step.
Post-mold inserts · pick it when
Threaded inserts in a molded or printed part at lower volume, or when the insert may change.
MethodTypical toleranceEconomic volumeToolingLead time
- Insert moldingthis pageTypical tolerance±0.003–0.005 in (±0.08–0.13 mm) to the insertEconomic volume5,000 parts and upTooling$8,000–$80,000+, a standard mold plus insert locatorsLead time5–12 weeks for tooling
- Typical toleranceSet by the hole: ±0.002–0.003 in (±0.05–0.08 mm)Economic volumeAny volumeToolingA press, heat-stake or ultrasonic tip, $0–$3,000Lead timeAdds days, not weeks
- Typical tolerance±0.005 in (±0.13 mm) on the overmoldEconomic volume1,000–250,000 parts a yearToolingTwo molds, each $5,000–$60,000Lead time6–12 weeks for tooling
Which design rules keep insert molding affordable?
Rules of thumb principals apply at DFM review. Each one has a reason.
RuleTargetWhy
- Insert retention
- Knurls, grooves or undercuts
- Plastic does not bond to metal. Retention is mechanical, so give the plastic something to grip.
- Wall around insert
- ≥ 0.5× insert diameter
- Thin walls crack as the plastic shrinks onto the metal.
- Insert tolerance
- ±0.001–0.002 in (±0.025–0.05 mm) on shutoff faces
- Loose inserts let plastic flash onto threads; oversize inserts get crushed.
- Sharp edges
- Break every edge on the insert
- Sharp corners are crack starters in the surrounding plastic.
- Resin choice
- Tougher, less notch-sensitive resins
- Glass-filled nylon, PBT and PC hold inserts well; brittle resins crack from residual stress.
What drives the cost of insert molding?
In order of impact.
- 1
Load time
Every insert placed adds seconds to the cycle. Robots cost more up front but cut cycle time and variation.
- 2
Insert cost
The metal insert is often more expensive than the plastic around it.
- 3
Tooling
Locating features, shutoffs and a shuttle or rotary table add tooling cost over a standard mold.
- 4
Scrap
A rejected part scraps the insert too, so first-pass yield matters more.
- 5
Press type
Vertical presses suited to insert loading may run at a different rate than horizontal presses.
Budgetary estimate for the molded body, without the insert, from the same estimator as the part builder. Change a size, the quantity or the material and the price updates.
Housing · Injection molding
Order total $24,973
- Tooling
- $17,029
- one-time, spread over the order
- Piece price
- $0.76
- before setup + tooling
- Lead time
- 6–9 weeks
- to first production
Size
Drag a row to adjust · click to type
Length140140 mm
Width90.090.0 mm
Wall2.202.20 mm
Quantity
Price per part falls as volume rises
Exact quantity1000010000 pcs
Material
Design checks pass
Open in builderBudgetary estimate for a representative part. Firm quotes come from principals after drawing review.
Insert molding questions
What is the difference between insert molding and overmolding?
Insert molding molds plastic around a preformed insert, usually metal. Overmolding molds a second material, usually a soft TPE, over a molded plastic part. Both put two materials in one part; the first is a metal-plastic part, the second a plastic-plastic part.
Is insert molding stronger than heat-set inserts?
Usually. Molded-in inserts see plastic flow into every knurl and groove, which gives high pull-out and torque-out. Heat-set and ultrasonic inserts come close when the hole is sized to the insert maker's spec.
How are inserts loaded?
By hand on low to moderate volumes, often on a vertical press with a shuttle table so one side loads while the other molds. Robots load at higher volume or with many inserts.
Does the insert need pre-heating?
Often. Pre-heating metal inserts to near mold temperature slows the plastic's cooling at the interface, which reduces residual stress and cracking, especially in glass-filled and amorphous resins.
Quote insert molding with a rep.
Send a STEP file and drawing. A rep routes the RFQ to principals that run this method every day, and brings back firm quotes.