Technical Guide
7 min read
What Are Moulding Inserts Used For? Types, Installation, and Design Guide
A practical guide to brass moulding inserts used in injection-moulded plastic components. Covers heat-set, ultrasonic, press-in, and mould-in types; pull-out performance; thread standards; and boss diameter design rules.
What Are Moulding Inserts?
Moulding inserts — also called threaded inserts, heat-set inserts, or brass inserts for plastic — are metal fastening elements embedded into plastic components to provide a durable threaded connection point. Plastic alone does not retain threads reliably under repeated assembly cycles or significant load; a brass insert provides a metal-to-metal threaded interface within the plastic body.
They are used wherever a plastic part needs to be bolted, screwed, or connected with a metal fastener: enclosures, housings, PCB standoffs, appliance panels, automotive components, medical devices, and furniture fittings.
Types of Brass Moulding Inserts by Installation Method
| Type | Installation Method | Retention Mechanism | Typical Application |
|---|---|---|---|
| Mould-in insert | Placed in mould before plastic injection | Mechanical keying by plastic flow around the insert exterior | High-volume production; maximum pull-out strength |
| Heat-set insert | Pressed into pre-moulded boss using soldering iron or heat tip at 200–280°C | Plastic re-flows around the insert's knurled/diamond exterior as it cools | Post-moulding assembly; most common for thermoplastics |
| Ultrasonic insert | Driven into boss using an ultrasonic welder (~20–40 kHz vibration) | Same thermal mechanism as heat-set, but faster and more uniform | High-volume post-mould installation; electronics assembly |
| Press-in insert | Cold-pressed into a pre-moulded boss, typically with a serrated or barbed exterior | Mechanical interference; barbs prevent pull-out | Soft or flexible plastics; where heat is not suitable |
External Profiles: Straight, Diamond Knurl, and Custom
- Straight knurl — longitudinal serrations that grip the plastic after installation; used for heat-set and mould-in types
- Diamond knurl — crosshatch pattern providing resistance to both axial pull-out and rotational torque; the most common heat-set profile
- Undercut profiles — circumferential grooves that lock the plastic in place as it flows or re-flows; high pull-out resistance
- Custom profiles — flanged, step, or slotted variants for specific assembly requirements or non-standard boss geometries
Pull-Out Strength and Performance
Pull-out strength (the axial force required to extract the insert from the plastic) depends on:
- Insert engagement length (longer = stronger)
- External surface profile (diamond knurl > straight knurl > smooth)
- Boss wall thickness (thicker walls provide more material to retain the insert)
- Plastic type (high-shrinkage materials grip inserts more firmly; glass-filled grades may behave differently)
- Installation temperature and technique (correct heat produces better reflow than insufficient or excessive heat)
For structural applications, always verify pull-out and torque-out strength through prototype testing in the actual plastic material — insert performance data from supplier catalogues is typically based on test blocks in ABS or PA66.
Thread Standards in Brass Moulding Inserts
The internal thread of a brass insert is usually a standard ISO Metric thread per ISO 68-1. Common sizes:
- M2, M2.5, M3 — miniature electronics, PCB standoffs, consumer devices
- M4, M5, M6 — general-purpose plastic enclosures and panels
- M8, M10 — structural plastic parts, large housings
- UNC (Unified National Coarse) — for American-standard equipment
Thread class is typically 5H or 6H for internal threads in inserts. Confirm the mating screw's tolerance class against the insert specification.
Boss Diameter Design Rules
The boss (the cylindrical protrusion in the plastic part that receives the insert) must be correctly sized. General guidelines for heat-set inserts:
- Boss outer diameter: typically 2× the insert outer diameter minimum; many insert manufacturers recommend a specific boss OD range
- Boss hole diameter: should be 0.1–0.2 mm smaller than the insert OD for heat-set types, creating a slight interference that aids alignment during installation
- Boss depth: at least equal to the insert length plus 1–2 mm; the insert should not bottom out
- Wall thickness: minimum 0.8 mm of plastic around the insert OD; thicker walls improve pull-out strength
- Gate and weld line placement: avoid weld lines in the boss area for mould-in inserts
Materials for Moulding Inserts
- Free cutting brass (IS 319 / CW614N) — standard material; excellent machinability, good corrosion resistance in plastic housings
- Lead-free brass — for applications where the plastic part contacts food, drinking water, or falls under RoHS restrictions
- Stainless steel — where higher torque strength or corrosion resistance is required (less common; harder to install)
Typical Applications
- Injection-moulded enclosures for electronics and controls — panel mounting screws, PCB standoffs
- Consumer appliances — repeated assembly/disassembly points in plastic housings
- Automotive interior components — trim panels, sensor housings, connector bodies
- Medical devices — instrument housings requiring clean-thread assembly
- Furniture hardware — furniture fittings in plastic knockdown (KD) furniture
- Industrial equipment — junction box lids, sensor brackets, cable management components
Frequently Asked Questions
What is the most common type of brass insert for plastic?
Heat-set inserts with a diamond knurl external profile are the most widely used. They are installed post-moulding with a temperature-controlled soldering iron or heat tip, giving a strong pull-out resistance in thermoplastic materials such as ABS, PA6, PA66, PC, and POM.
What temperature is required for heat-set brass inserts?
Installation temperature depends on the plastic. For ABS and PC, a tip temperature of 200–240°C is typical. For PA66 and POM, 250–280°C may be needed. The insert should press smoothly into the boss as the plastic softens — if it requires significant force, the temperature is too low; if the plastic discolours or smokes, the temperature is too high.
Can brass moulding inserts be used with any plastic?
Heat-set inserts work best with thermoplastics — ABS, polycarbonate (PC), nylon (PA6, PA66), acetal (POM), and polypropylene (PP). They are less effective in thermosets (which cannot be re-melted) and very soft or flexible materials (where press-in inserts may be preferable). Mould-in inserts work across a wider range of plastic types.
How do I calculate the correct boss diameter for a brass insert?
Use the insert manufacturer's boss diameter specification, which accounts for the specific insert OD and the target plastic material's shrinkage. A general starting rule is boss OD = 2 × insert OD, and boss hole diameter = insert OD minus 0.1–0.2 mm. Always validate with prototype testing before production.