What are heat-set inserts and how do they work?
Heat-set inserts are knurled brass bushings with a machine thread inside. A soldering iron heats the insert, the plastic around a slightly undersized hole melts, and the knurls sink into the softened wall. Once cool, the plastic locks the insert in place. In CNC Kitchen’s tests, an M3 insert in PLA (polylactic acid) held 181 kg before pulling out.
What they are, and why brass
An insert is a short metal sleeve with an internal thread and an external knurl, a pattern of ridges cut into the outside. Ruthex and CNC Kitchen both make theirs from brass, and Prusa Research’s shop lists the CNC Kitchen version as a lead- and cadmium-free brass alloy. Brass conducts heat well, so the iron warms the whole sleeve fast, and it is soft enough to machine a fine thread and a sharp knurl.
The knurl pattern decides the grip. Ruthex describes its knurl as spiral. CNC Kitchen found that knurling in more than one direction resists pull-out: a cheap insert with only vertical knurling held 157 kg and started rotating in softer ASA (acrylonitrile styrene acrylate).
Heat-set versus tapping, self-tapping screws and captive nuts
A thread cut or driven straight into plastic works until it is reused. DigiKey’s guide puts it plainly: bare plastic can be threaded, but repeated use wears it until the thread fails. CNC Kitchen’s numbers show the trade. A screw driven into a 2.7 mm hole in PLA pulled out at 142 kg, not far below an insert, but gave way at about 1 Nm of torque. All three tested inserts held 3 to 4 Nm until the bolt head sheared.
A captive nut trapped in a printed pocket avoids the melted joint altogether, but it needs a pocket modeled into the part and access from the back. A heat-set insert goes in after printing, from the front, into a blind or through hole.
Size families: M2 to M5, standard and short, straight and tapered
Metric inserts come in the same sizes as the screws they take. Each size has an outer diameter, a length, a hole diameter and a minimum wall around the hole.
| Size | Outer diameter | Length | Hole | Minimum wall |
|---|---|---|---|---|
| M2 | 3.6 mm | 4.0 mm | 3.2 mm | 1.3 mm |
| M3 | 4.6 mm | 5.7 mm | 4.0 mm | 1.6 mm |
| M4 | 6.3 mm | 8.1 mm | 5.6 mm | 2.1 mm |
| M5 | 7.1 mm | 9.5 mm | 6.4 mm | 2.6 mm |
Figures from the Ruthex table as published by 3DJake.
Short variants keep the outer diameter and cut the length: the Ruthex M3 short is 4.6 mm across but 4.0 mm long, for thin walls. The Voron style, named after the printer project whose parts use it, is fatter and shorter: 5.0 mm across, 4.0 mm long, 4.4 mm hole, 1.3 mm wall. The M3 hole figures per maker are in What hole size does an M3 heat-set insert need?.
Some inserts taper at the leading end. UltiMaker notes the taper is there to make starting easier, and Hackaday calls it helpful but not necessary. The hole itself should not taper: CNC Kitchen asks for a straight hole, no chamfer, and a blind hole about 1 mm deeper than the insert.
Choosing between M3 and M4
Pick the screw first, then the insert to match. An M3 insert is 4.6 mm across and needs 1.6 mm of wall, so its boss can be under 8 mm wide and fits in thin printed parts. It is also the size Ruthex makes in a dedicated Voron printer-part variant. An M4 needs a 5.6 mm hole and 2.1 mm of wall, so the boss is nearly 10 mm across. Use M4 where the screw carries load and the part has room for the boss, M3 where space is tight.
Temperature
CNC Kitchen and Prusa Research’s shop page for the same inserts give one rule: set the iron 10 to 20 °C above the material’s printing temperature. That works out to about 225 °C for PLA, 245 °C for PETG (glycol-modified PET) and 265 °C for ABS (acrylonitrile butadiene styrene). The per-material table, with a second maker’s rule, is in What temperature should the iron be for PLA, PETG and ABS inserts?. PETG has its own page, because it strings around the tip: Do heat-set inserts work in PETG?.
Installation
Melt the insert most of the way with the iron, then finish the last stretch with a flat tool and hold it for a moment. CNC Kitchen found that inserts creep back out right after melting. Iron tips with a centering pin keep the insert square. The full procedure is in How do you install heat-set inserts straight without a press?.
When it goes wrong
A tilted insert can be reheated and pressed flush with a flat face, and a wrong-size insert can be pulled with a heated screw. Once the insert is out, drill the hole back to size or up to the next insert; a split boss gets reprinted. Steps and limits: How do you remove or fix a crooked heat-set insert?.
Sources
Questions answered in this guide
CNC Kitchen's measured pull-out loads for M3 heat-set inserts in PLA: 181 kg for Ruthex, 157 kg for eBay inserts, 39 kg for cheap ones, and how hole size changes it.
CNC Kitchen's M3 test in PETG: a bottom-pocket nut held 166 kg against 119 kg for a heat-set insert, but the insert took more torque. When each fastening wins.
Iron temperature per material for heat-set inserts: about 225 °C for PLA, 245 °C for PETG and 265 °C for ABS, from CNC Kitchen's printing temperature plus 10 to 20 °C.
Yes: brass heat-set inserts hold in PETG at an iron temperature of about 245 °C, and the part stays rigid to a glass transition near 80 °C. Expect stringing from the tip.
A soldering-iron procedure for square, flush heat-set inserts: iron 10 to 20 °C above printing temperature, melt about 90 % of the way, finish with a flat tool.
Reheat a tilted insert at 20 °C above printing temperature (240 °C for PLA) and press it flush, or pull it out on a screw in about 10 seconds; drill 5.6 mm for an M4.
Hole diameter, hole depth and minimum boss wall for standard, short and Voron M3 heat-set inserts, taken from the Ruthex and CNC Kitchen size tables.
- Threaded Inserts for 3D Prints - Cheap VS Expensive, CNC Kitchen (accessed September 21, 2026)
- Tips & Tricks for Heat-Set Inserts used in 3D printing, CNC Kitchen (accessed September 21, 2026)
- ruthex M2/M3/M4/M5 Threaded Insert Assortment Box, specification table (3DJake) (accessed September 21, 2026)
- ruthex Threaded Insert M3 Voron, specification table (3DJake) (accessed September 21, 2026)
- ruthex Threaded Insert M3S short, specification table (3DJake) (accessed September 21, 2026)
- ruthex M3 thread insert RX-M3x5.7 product page (accessed September 21, 2026)
- Heat Set Inserts - M3 standard 100 pcs (Prusa Research e-shop, CNC Kitchen inserts) (accessed September 21, 2026)
- Tips and Tricks: Heat-Set Inserts (DigiKey Maker.io) (accessed September 21, 2026)
- Threading 3D Printed Parts: How To Use Heat-Set Inserts (Hackaday) (accessed September 21, 2026)
- How to use heat set inserts to securely fasten 3D printed parts (UltiMaker) (accessed September 21, 2026)