Brass Insert Installation Methods Compared — Heat-Set, Ultrasonic, Press-In, Self-Tapping & Mould-In (2026)

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 heated tip brings the brass insert to the softening temperature of the host plastic plus an offset — as a starting point about 28 °C above for unfilled grades and about 83 °C above for filled grades (for orientation: PA6 melts at roughly 220 °C, PA66 at roughly 260 °C). The tip temperature is set by the resin, not by the insert; the full per-resin table is in our insert installation reference. 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 (a few seconds per insert) and well-suited to assembly-cell automation.
• High pull-out and torque-out for a post-moulding method — second only to mould-in inserts. Indicative ranges per thread size are on the brass inserts page.

Boss design rules

These are the same figures as in the boss-hole table on our brass inserts page. There is no ISO or DIN standard for the body of a heat-set insert — the outside diameter differs from design to design, so the hole must always be checked against the drawing of the insert actually used. Open the hole slightly for glass-filled grades, and validate on your own moulding.

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. The ultrasonic pulse itself typically lasts well under two seconds, which makes this the fastest post-moulding method.

When ultrasonic wins

• Very high volume (automotive, consumer electronics)
• Amorphous thermoplastics (ABS, PC) respond best; semi-crystalline resins (PA, POM, PP) need more energy
• 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: markedly lower pull-out and torque-out than heat-set, because retention is pure interference — the plastic does not re-flow into the knurls. 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 machines self-tapping inserts with a three-flute cutting end to your drawing, in M2 to M10, in CW614N brass; nickel plating is carried out by plating partners where the drawing calls for it. See the insert range.

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 offers 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.

How the five methods compare

Relative selection guidance, not test values — absolute pull-out and torque-out depend on the resin, the boss and the installation, and must be validated on your own moulding. Indicative ranges for heat-set and ultrasonic inserts are published on the brass inserts page.

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 too small (outer diameter below about 2× 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:

1. Standard — DIN 16903 Form D, or a heat-set form, or a self-tapping form
2. Thread size and tolerance class — M5 6H (medium fit, internal)
3. Material — CW614N or CW724R (lead-free)
4. Plating — none / nickel-plated / tin-plated
5. Pull-out load minimum if it's a safety-critical assembly
6. 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 — the alloy is RoHS-compliant without exemption; validate insert performance in your own moulding as for any material change.

Branded inserts, distributors, and made-to-drawing equivalents

Many drawings call out a branded insert — a heat-set, a press-in, a self-tapping — or the part is bought through 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

Sources & references

• EN 12164:2024 — Copper and copper alloys — Rod for free machining
• DIN 16903 — Brass moulding inserts for plastics (14 standardised forms)
• Copper Development Association — brass machinability and properties
• CW614N alloy datasheet · CW724R lead-free datasheet