---
type: process
name: "Insert Molding"
category: "Forming"
subcategory: "Plastics and Rubber"
materials: ["Plastic", "Metal"]
tolerances: "Molded features hold normal injection molding tolerances, about ±0.005 in (±0.127 mm). Insert position relative to molded features is looser and depends on how positively the insert is located — budget the insert's own tolerance plus the locating clearance."
volumes: "1,000–1,000,000+ parts; below 1,000, post-mold insert installation is usually cheaper"
lead_time: "6–12 weeks for tooling including insert locating features and presence sensing; cycles run several seconds longer than the equivalent plain molding"
url: https://manufacturingprocesses.org/processes/forming/insert-molding
---

# Insert Molding

Insert molding loads metal inserts into the cavity and molds polymer around them, capturing threads, terminals or stampings in a single shot.

- **Category**: [Forming](https://manufacturingprocesses.org/processes/forming.md)
- **Family**: Plastics and Rubber
- **Materials**: Plastic, Metal
- **Typical tolerances**: Molded features hold normal injection molding tolerances, about ±0.005 in (±0.127 mm). Insert position relative to molded features is looser and depends on how positively the insert is located — budget the insert's own tolerance plus the locating clearance.
- **Surface finish**: SPI A-1 through D-3 on molded surfaces, as for any injection molded part; exposed insert faces retain their own finish
- **Typical volumes**: 1,000–1,000,000+ parts; below 1,000, post-mold insert installation is usually cheaper
- **Lead time**: 6–12 weeks for tooling including insert locating features and presence sensing; cycles run several seconds longer than the equivalent plain molding

## Overview

Insert molding loads a metal component into the mold cavity before the shot and injects plastic around it, so the finished part comes out of the press already assembled. Threaded brass inserts, stamped terminals, contact pins, shafts, bearings, and reinforcing plates are all routinely captured this way.

The value is assembly elimination. One molding operation replaces a molded part, a separate insert, a press or heat-stake station, and the labor and quality checks around them. It is the standard construction for electrical connectors, encapsulated sensors, gear-and-shaft assemblies, and any plastic housing that needs a metal thread capable of repeated disassembly.

The costs are equally concrete: an operator or robot has to load inserts every cycle, which lengthens the cycle and makes the mold vulnerable to a mis-set insert, and the mismatch in thermal expansion between metal and polymer must be designed around rather than ignored.

## How it works

1. **Prepare the inserts.** Inserts arrive clean and, for critical applications, degreased — mold release or drawing oil on an insert becomes a bond-line failure. Preheating inserts, typically to somewhere near the mold temperature, is common practice: it reduces the thermal shock as melt hits cold metal, which lowers residual stress in the surrounding plastic and improves encapsulation.

2. **Load and locate.** Inserts are placed onto core pins, magnets, vacuum cups, or shouldered pockets in the cavity. Location has to be positive, because injection at 5,000–20,000 psi (35–140 MPa) will move anything that is not properly restrained. This step is the reason insert molding cycles are longer than plain injection molding, and it is where robotic loading pays back.

3. **Close and verify.** Production tools normally carry insert-presence sensing. A missing insert leaves a core pin unsupported; a proud or tilted insert gets crushed when the mold closes, which can damage the cavity — an expensive failure mode compared with a short shot.

4. **Inject.** The melt flows around the insert and welds to itself on the far side, forming a knit line downstream of every insert. That knit line is a genuine weak point, so gate placement should push it away from any loaded region.

5. **Cool and eject.** The polymer shrinks onto the insert, which is exactly what generates the grip. Encapsulated knurls and undercut grooves convert that shrinkage into torque and pull-out resistance.

### What holds the insert in?

Three mechanisms, in order of importance. Shrinkage grip from the polymer contracting onto the metal, mechanical interlock from knurls and undercut grooves, and only rarely chemical adhesion. Straight knurls resist torque, annular undercut grooves resist pull-out, and a diamond knurl combined with a groove resists both — which is why standard molded-in inserts carry that geometry.

## Design guidelines

### Boss geometry around the insert

The boss outside diameter should be at least 2× the insert outside diameter. Too little plastic around the insert and the hoop stress from cooling shrinkage — plus the hoop stress from any screw driven into it — cracks the boss, often not immediately but weeks later in the field. Keep the boss wall at 0.5–0.6 × the nominal part wall, and connect it to adjacent walls with ribs or gussets rather than a thick blend, exactly as tabulated on [/charts/injection-molding-design-guidelines](/charts/injection-molding-design-guidelines).

### Thermal expansion mismatch

This is the failure mode most often missed. Steel expands at about 6.5 µin/in·°F (11.7 µm/m·K) and brass at about 10 µin/in·°F (19 µm/m·K), while unfilled thermoplastics run several times higher. Over a thermal cycle the polymer moves and the metal does not, which loads the interface every time. Keep encapsulating walls thick enough to carry that stress, avoid long metal inserts fully bonded along their length, and prefer glass-filled resins where the part sees wide temperature swings — the filler pulls the polymer's expansion much closer to the metal's.

### Insert support

Support every insert against the injection pressure on at least two features, and shut off cleanly on any surface that must stay free of plastic — a thread, a contact face, a sealing land. Flash on a molded-in thread is a scrap part; there is no rework.

### Keep inserts off the parting line

An insert that straddles the parting line will flash and will resist ejection unevenly. Set it fully within one half.

### Thread selection

Molded-in inserts are specified by the screw they accept, so start from the fastener. Thread sizes and their pilot dimensions are on [/charts/thread-size-chart](/charts/thread-size-chart) and [/charts/tap-drill-chart](/charts/tap-drill-chart); size the boss from the screw's major diameter, not from the insert catalog number.

### When not to insert mold

If the insert count is high, the parts are small, or the program is short, post-mold installation — heat staking or ultrasonic insertion — is often cheaper and lower risk. It keeps the molding cycle fast, keeps the tool simple, and lets the same tool run parts with and without inserts.

| Feature | Recommended | Limit | Why |
| --- | --- | --- | --- |
| Boss OD | 2.5 × insert OD | 2 × insert OD | Below this the boss cracks from hoop stress |
| Boss wall | 0.5–0.6 × nominal wall | — | Same sink-mark rule as any molded boss |
| Insert location | Fully within one mold half | Never across the parting line | Straddling inserts flash and eject unevenly |
| Insert retention | Diamond knurl plus undercut groove | Knurl alone | Knurl resists torque, groove resists pull-out |
| Resin for thermal cycling | Glass-filled | Unfilled | Filler brings polymer expansion nearer the metal's |
| Knit line | Placed away from loaded regions | — | Melt rejoins downstream of every insert |

## Cost drivers

Insert molding trades assembly cost for cycle cost. The saving is real — one operation instead of three or four, no separate insertion press, no assembly inspection — but every second spent loading inserts is a second added to a cycle that would otherwise be 20–40 seconds. On a high-volume part, that arithmetic decides the process.

Volume breakpoints:

- Under 1,000 parts: mold plain and install inserts afterward by heat staking or ultrasonics. Tooling stays simple and there is no loading fixture to build.
- 1,000–100,000: manual insert loading with a shuttle or rotary table so the operator loads one half while the other runs.
- Over 100,000: robotic loading with insert-presence verification, which removes both the cycle penalty and the crushed-insert risk.

Cost reduction:

1. **Cut the insert count.** Every insert is loading time on every cycle. Two well-placed inserts often carry a joint that four were specified for.
2. **Use a shuttle or rotary tool.** Loading time overlaps the molding cycle instead of adding to it.
3. **Standardize on one insert size.** A single insert across a product family simplifies loading fixtures, reduces mis-set risk, and improves purchasing leverage.
4. **Add insert-presence sensing.** A crushed insert can damage a cavity, and the repair plus lost production dwarfs the cost of the sensor.
5. **Reconsider post-mold installation for short programs.** Heat-staked inserts reach most of the pull-out strength of molded-in ones without the cycle penalty or the tool risk.

## FAQ

### How much plastic do I need around a molded-in insert?

Design the boss outside diameter to at least twice the insert outside diameter, and keep the boss wall at 0.5–0.6 times the nominal part wall. Less material than that and hoop stress from cooling shrinkage plus screw installation cracks the boss, frequently weeks later in service rather than at assembly.

### What holds a metal insert in a molded part?

Mainly the polymer shrinking onto the metal as it cools, reinforced by mechanical interlock. Straight knurls resist torque, annular undercut grooves resist pull-out, and standard molded-in inserts combine a diamond knurl with a groove to resist both. Chemical adhesion between metal and thermoplastic contributes very little.

### Should I mold inserts in or install them afterward?

Mold them in above roughly 1,000 parts, where eliminating a separate insertion operation pays for the longer cycle. Below that, or when insert counts are high, heat staking or ultrasonic insertion after molding is usually cheaper: the tool stays simple, the cycle stays fast, and a mis-set insert cannot damage a cavity.

### Why does thermal expansion matter in insert molding?

Steel expands at about 6.5 µin/in·°F (11.7 µm/m·K) and brass around 10 µin/in·°F (19 µm/m·K), while unfilled thermoplastics expand several times faster. Every thermal cycle loads the plastic-to-metal interface. Glass-filled resins bring the polymer's expansion much closer to the metal's and are the standard answer for parts that see wide temperature swings.

### Where do knit lines form in an insert molded part?

Directly downstream of every insert, where the melt splits around the metal and rejoins on the far side. Knit lines carry a fraction of the base material's strength, so gate placement should push them away from loaded regions, mounting points, and anywhere the part sees impact.

### What happens if an insert is loaded incorrectly?

A proud or tilted insert gets crushed when the mold closes, which can damage the cavity steel — a repair far more expensive than the scrapped part. A missing insert leaves the core pin unsupported. Production tools therefore carry insert-presence sensing, which pays for itself the first time it prevents a cavity strike.

## Alternative processes

- [Staking](https://manufacturingprocesses.org/processes/joining/staking.md): Heat staking softens a molded plastic boss with hot air or ultrasonic energy and reforms it into a head that traps the mating part.
- [Mechanical Fastening](https://manufacturingprocesses.org/processes/joining/mechanical-fastening.md): Mechanical fastening joins parts with screws, inserts, clinch hardware or snap fits so the joint can be assembled and taken apart again.
- [Overmolding](https://manufacturingprocesses.org/processes/forming/overmolding.md): Overmolding molds a second material, usually a soft elastomer, directly onto an already-molded rigid substrate so the two bond permanently.
- [Injection Molding](https://manufacturingprocesses.org/processes/forming/injection-molding.md): Injection molding forces molten thermoplastic into a steel mold under high pressure, producing high-volume identical plastic parts.

## Related processes

- [Overmolding](https://manufacturingprocesses.org/processes/forming/overmolding.md): Overmolding molds a second material, usually a soft elastomer, directly onto an already-molded rigid substrate so the two bond permanently.
- [Injection Molding](https://manufacturingprocesses.org/processes/forming/injection-molding.md): Injection molding forces molten thermoplastic into a steel mold under high pressure, producing high-volume identical plastic parts.
- [Staking](https://manufacturingprocesses.org/processes/joining/staking.md): Heat staking softens a molded plastic boss with hot air or ultrasonic energy and reforms it into a head that traps the mating part.
- [Liquid Silicone Rubber (LSR) Molding](https://manufacturingprocesses.org/processes/forming/liquid-silicone-rubber-lsr-molding.md): LSR molding injects two-part liquid silicone into a heated mold where it cures, producing flexible parts with a very wide service temperature range.

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*Source: [Manufacturing Processes](https://manufacturingprocesses.org/processes/forming/insert-molding)*

*Last updated: August 11, 2026*