Five installation methods exist for brass inserts in plastics: heat-set, ultrasonic, press-in, self-tapping, and mould-in. Choose by thermoplastic vs thermoset, production volume, pull-out load required, and whether the boss is hot when the insert goes in. Get the boss-hole diameter wrong and the pull-out load drops by 60% — this guide gives the exact numbers per ISO 2768-m.
Threaded brass inserts let you create a strong, reusable metal thread in a plastic component without the strength or fatigue limitations of cutting threads directly into the polymer. The five mainstream installation methods each have a sweet spot — get the right method matched to your plastic, volume and load case and the result is a thread that outlasts the part. Get it wrong and you'll see inserts spinning loose, cracking the boss, or pulling out under torque.
This is a complete engineering reference to specifying and installing the five insert types — written for product designers, manufacturing engineers, and procurement managers buying inserts at scale. Every figure here applies to CW614N brass per EN 12164, the dominant alloy for threaded inserts globally.
Method 1: Heat-set inserts
The dominant installation method for thermoplastics at production volume. A specialised tip heats the brass insert to slightly above the glass transition temperature of the plastic (typically 200–270°C for polyamide, 230–280°C for PEEK). The hot insert sinks into a pre-moulded boss; the plastic locally re-flows around the insert's knurls and undercuts; the assembly cools and the plastic locks the insert in place.
When to specify heat-set
- Thermoplastic only — polyamide (PA6, PA66), polycarbonate (PC), ABS, PEEK, polypropylene (PP, with reduced pull-out).
- Production volume — heat-set tooling is fast (1.5–3 seconds per insert) and well-suited to assembly-cell automation.
- Highest pull-out values per unit length of any insert method (typically 800–2,500 N depending on size and plastic).
Boss design rules
| Thread size | Recommended boss hole Ø | Min boss wall thickness | Min boss depth |
|---|---|---|---|
| M3 | 4.0 mm | 1.5 × insert Ø | 1.2 × insert length |
| M4 | 5.4 mm | 1.5 × insert Ø | 1.2 × insert length |
| M5 | 6.4 mm | 1.5 × insert Ø | 1.2 × insert length |
| M6 | 7.9 mm | 1.5 × insert Ø | 1.2 × insert length |
| M8 | 9.6 mm | 1.5 × insert Ø | 1.2 × insert length |
Method 2: Ultrasonic insertion
Identical end-result to heat-set but the installation is faster and uses friction-heating rather than conducted heat. An ultrasonic horn vibrating at 20–40 kHz drives the insert down into the boss; the friction at the boss wall melts a thin layer of plastic in milliseconds; the insert seats. Cycle time 0.3–1 second per insert — three to ten times faster than heat-set.
When ultrasonic wins
- Very high volume (automotive, consumer electronics)
- Thermoplastics with sharp glass transition (PA, PC, ABS)
- Where the boss is too long for a heat-set tip to reach the bottom in time before the plastic re-solidifies
Method 3: Press-in (cold-press) inserts
The simplest possible method — drive the knurled insert into a slightly undersized boss with a mechanical or pneumatic press. No heat, no ultrasonics. The knurls cut into the plastic; the boss provides interference holding force.
The trade-off: lower pull-out load (typically 40–60% of heat-set), no permanent re-flow of the plastic. Press-in inserts also stress the boss radially and can cause stress-whitening or cracking in brittle plastics (PC, PMMA). Generally avoided in production but useful for prototyping or for thermoset bosses that can't be re-melted.
Method 4: Self-tapping inserts
The insert has cutting flutes at the leading end. Driven in with a screwdriver or pneumatic driver, the flutes cut their own thread into the plastic. No pre-moulded boss is required — just a drilled or moulded straight hole.
Self-tapping is the right choice when:
- The plastic is a thermoset (epoxy, phenolic) and can't be melted
- The part is too large or too complex to fit in a heat-set or ultrasonic press
- Field installation is required (repair, service)
- Cost-per-insert matters more than installation time
Brassland's self-tapping inserts follow the dimensional-equivalent three-flute design and are available in M2 to M10 in CW614N brass with optional nickel plating.
Method 5: Mould-in inserts
The brass insert is placed in the moulding cavity before plastic injection. The plastic flows around the insert and cools, encapsulating it permanently. The DIN 16903 standard defines 14 standardised forms (A through U) for mould-in inserts; Brassland stocks all 14 forms.
Mould-in wins on pull-out load (maximum of any method — the plastic fully encapsulates the insert's anti-rotation features), eliminates a secondary assembly step, and gives the cleanest aesthetic finish (no visible boss). The trade-off: it slows the moulding cycle (the operator has to load each cavity) and risks insert dislodgement during injection if the insert isn't held positively.
Pull-out load comparison (M5 insert in PA6, room temperature)
| Method | Pull-out load (N) | Torque-out (N·m) | Cycle time |
|---|---|---|---|
| Heat-set | 1,800–2,400 | 3.2–4.5 | 2 sec |
| Ultrasonic | 1,800–2,400 | 3.2–4.5 | 0.5 sec |
| Press-in (knurled) | 900–1,400 | 1.8–2.5 | 1 sec |
| Self-tapping | 1,200–1,800 | 2.5–3.5 | 3 sec |
| Mould-in (DIN 16903 Form D) | 2,500–3,200 | 4.5–6.0 | n/a — at moulding |
Common installation defects
- Insert protruding above boss face — heat insufficient for the plastic to re-flow; increase temperature or dwell time. Drive depth gauge stop helps.
- Plastic flash around insert head — heat too high or dwell too long; plastic boils, cools as flash. Reduce temperature 15–20°C.
- Spinning insert on first thread engagement — boss-hole oversized or knurls polished smooth. Check insert lot for visible knurls.
- Boss cracking radially from insert — boss wall too thin (less than 1.5× insert Ø); brittle plastic; or press-in interference too high.
- Insert pulled out under torque — anti-rotation feature insufficient. Switch from straight-knurl to diamond-knurl insert; increase insert length.
How to specify a brass insert correctly
A clean drawing callout on a plastic component should specify:
- Standard — DIN 16903 Form D, or a heat-set form, or a self-tapping form
- Thread size and tolerance class — M5 6H (medium fit, internal)
- Material — CW614N or CW724R (lead-free)
- Plating — none / nickel-plated / tin-plated
- Pull-out load minimum if it's a safety-critical assembly
- Installation method — heat-set / ultrasonic / press / mould-in
RoHS / REACH considerations
Standard brass inserts use CW614N (CuZn39Pb3, ~3% Pb). This is under RoHS Exemption 6(c) until 30 June 2027. For EEE applications expecting to remain in production past 2027, specify CW724R lead-free silicon brass — pull-out load is 10–15% lower but the alloy is RoHS-free without exemption.
Branded inserts, distributors, and made-to-drawing equivalents
Many drawings call out a branded insert — a branded heat-set, a Helicoil-type, a self-tapping — or the part is bought through a C-parts distributor such as a C-parts distributor. Brassland machines the dimensional equivalent to your drawing, direct from the factory: specify the insert to DIN 16903 or its published OD, length and thread, and we produce a drop-in part in CW614N (or lead-free CW724R) — removing single-source supply risk and the distribution margin. See the insert range.
FAQ: installing brass inserts
Which insert type should I use for thermoplastic versus thermoset?
Can Brassland supply an equivalent to a branded insert?
Which brass alloy is used, and is it RoHS compliant?
What hole size and installation torque should I use?
Sources & references
- EN 12164:2016 — Copper and copper alloys — Rod for free machining
- DIN 16903 — Brass moulding inserts for plastics (14 standardised forms)
- VDI 2230 — Bolted joint design (referenced for pull-out load calculations)
- Copper Development Association — brass machinability and properties
- CW614N alloy datasheet · CW724R lead-free datasheet