Boss hole diameters, heat-set tip temperatures and seating force guidance for brass threaded inserts M2–M8 in thermoplastics — from an insert manufacturer.
Typical guidance for heat-set and press-in brass inserts in thermoplastics. Values are a starting point — always confirm with pull-out and jack-out testing on your own host material.
| Thread | Typical insert OD (mm) | Boss hole Ø (mm) | Heat-set tip temp | Typical seating force |
|---|---|---|---|---|
| M2 | 3.2–3.6 | 3.0–3.2 | 200–260 °C | Light hand press |
| M2.5 | 3.6–4.0 | 3.4–3.6 | 200–260 °C | Light hand press |
| M3 | 4.0–4.6 | 3.8–4.2 | 220–280 °C | Light–medium |
| M4 | 5.6–6.0 | 5.4–5.6 | 220–280 °C | Medium |
| M5 | 6.4–7.1 | 6.2–6.5 | 240–300 °C | Medium |
| M6 | 8.0–8.5 | 7.8–8.0 | 240–300 °C | Firm |
| M8 | 9.5–10.5 | 9.3–9.6 | 260–320 °C | Firm (press) |
Boss hole sizes assume amorphous/semi-crystalline thermoplastics; glass-filled grades may need a larger hole. Heat-set tip temperature is set above the polymer melt point, not the insert. Brassland supplies inserts to DIN 16903, IUTB/IUTC/IUTD and customer-specific profiles.
References: DIN 16903 (threaded inserts for plastics) and published heat-set boss-design guidance. Always validate with pull-out / jack-out testing on your own host material. See our brass inserts range.
There is no single “correct” heat-set temperature. The governing rule from established insert-design references is to set the tip/insert temperature to the host resin’s softening temperature plus an offset — start about 28 °C (50 °F) above the initial softening temperature for the plastic in question, and about 83 °C (150 °F) above for filled grades. The aim is to soften the boss so it flows into the knurls, not to melt it (which causes flash). Some glass-filled plastics need the insert as hot as ~371 °C (700 °F).
Set the tip to (resin softening/melt temperature + offset above). The resin figures below are melt/processing windows for the unfilled grade; add the offset, then fine-tune by sectioning a trial insert.
| Resin | Melt / processing temp (unfilled) | Unfilled tip rule | Filled tip rule |
|---|---|---|---|
| PA6 (nylon 6) | melt ~215–225 °C; processing ~240–280 °C | softening temp + 28 °C (50 °F) | softening temp + 83 °C (150 °F); some GF to ~371 °C |
| PA66 (nylon 6/6) | melt ~255–265 °C; processing ~270–300 °C | ||
| ABS | ~200–250 °C processing | ||
| PC (polycarbonate) | ~280–320 °C | ||
| PP (polypropylene) | melt ~160–170 °C; processing to ~200–250 °C | ||
| POM (acetal) | ~163 °C (copolymer) to ~205–225 °C (homopolymer) |
Temperature is the host resin’s softening temperature plus the offset shown — not a fixed insert setpoint. Filler content, moisture, colorant and wall thickness all move the window. Thermosets (phenolic, epoxy) cannot be heat-set — use mould-in, press-in or self-tapping. Figures are typical starting points; validate for your resin, grade and geometry.
| Parameter | Typical starting value |
|---|---|
| Insertion pressure | 0.03–0.10 MPa (5–15 PSI) — as low as possible, just enough to seat the insert as the plastic softens |
| Insertion force (overall) | Low, pneumatically driven — generally under ~220 N (50 lb) |
| Transfer window (pre-heated insert) | Seat within ~5 s of the end of heating (brass) so it retains enough heat to flow the plastic |
| Cool / hold before loading the screw | Let the plastic re-solidify under a positive stop before applying load |
Equipment- and geometry-dependent starting points. As much as ~75% of an insert’s achievable performance comes down to how well it is installed — section a trial insert through its centreline; the plastic should show a negative image of the knurls.
Ultrasonic amplitude and energy are equipment- and geometry-tuned; the figures below are published starting points, not hard settings. Inserting uses less amplitude than staking or welding. Semi-crystalline resins (nylon, acetal, PP, PBT) need more energy than amorphous ones (ABS, PC).
| Parameter | Inserting starting value | Note |
|---|---|---|
| Amplitude (inserting) | 25–65 µm (1.0–2.6 thou) | Lowest of the mechanical bands (staking is 75–125 µm) |
| Weld (ultrasonic-on) time | 0.02–2.0 s | Size/geometry dependent |
| Hold time | ≈ half the weld time | — |
| Trigger / weld pressure | 20–40 PSI | Inserting sits at the higher end (metal into plastic) |
| Frequency | 15 / 20 / 30 / 40 kHz | 20 kHz is the workhorse; 30/40 kHz for small parts |
Amplitude is meaningful only for its stack frequency (a 20 kHz converter outputs ~20 µm before the booster/horn scale it). Reference band only — validate for your welder, insert size and fixturing.
Anchor to the resin, then bias toward the lower end and within the 25–65 µm inserting band.
| Resin | Class | 20 kHz amplitude (µm) |
|---|---|---|
| ABS | Amorphous | 30–70 |
| ABS / Polycarbonate | Amorphous | 50–125 |
| Polycarbonate (PC) | Amorphous | 50–100 |
| Acetal / POM | Semi-crystalline | 75–125 |
| Polyamide / nylon | Semi-crystalline | 60–125 |
| Polyester (PBT) | Semi-crystalline | 60–125 |
| Polypropylene (PP) | Semi-crystalline | 70–125 |
These are welding-reference amplitudes; for inserting, bias low and inside the 25–65 µm band, then tune. Glass fibre/talc/carbon fillers raise the energy required, especially for semi-crystalline resins.
| Method | Key parameters | Force / temperature |
|---|---|---|
| Press-in (cold press) | Retention is entirely the interference fit — knurl OD must exceed the hole. Needs a larger boss/wall than heat-set; press while the part is still warm from moulding to lower force. Piloted punch to a positive stop. | Press force is set by interference × length × resin modulus, so it is specified per insert/resin — on request (no generic force-per-size table is published). |
| Mould-in | Placed on a core pin before injection; pin meets the tightened minor-thread Ø so plastic doesn’t flash onto the threads. Straight holes ≤1° included, tapered 8° included. Highest retention of all methods. See DIN 16903 mould-in inserts. | Insert preheat mitigates stress-cracking, but a specific preheat temperature is on request (not verifiable to a single value — depends on resin/insert mass). |
Press-in suits softer/ductile plastics (PP, HDPE); hard, glass-filled or thermoset materials give high install forces and low retention — move to mould-in or self-tapping. All methods: validate on your own boss, resin and process.
Hole Ø from the table (smaller end for unfilled polymers, larger end for glass-filled), hole depth = insert length + 0.5–1 mm melt reservoir, boss wall ≥ one hole-radius of plastic around the insert, and a slight lead-in chamfer. The knurl and undercuts anchor against both torque-out and pull-out — two separate failure modes, tested separately.
Moulded-in (DIN 16903) — highest retention, for injection moulding at volume. Heat-set — the flexible post-mould standard; tip temperature 40–80 °C above polymer melt point. Ultrasonic — fast in-line installation. Press-in/expansion — cold installation into existing holes. Full family overview: threaded inserts, moulding inserts and knurled & expansion inserts.
Values here are starting points from established insert-design references. Qualify with pull-out and jack-out tests on your actual moulding — polymer, filler content, gate position and regrind all shift results. We supply inserts to DIN 16903 and customer prints with EN 10204 material certificates; send your drawing.
Last reviewed: June 2026. Calculator outputs are estimates; verify against the governing standard and your drawing for procurement-critical work.
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