Why printed threads are not enough on their own
Cutting or printing threads directly into ABS, PLA, or PETG works for a screw or two, but it does not hold up to repeated assembly and disassembly. Plastic threads strip easily, and once a screw has been overtightened even once, the threads it cut are often ruined for good. Anything that gets opened for maintenance, like an electronics enclosure or a part with a removable panel, needs something sturdier than plastic-on-plastic threading.
Heat-set threaded inserts solve this by putting a small brass sleeve with real metal threads inside the printed part. You melt it into a printed hole rather than screwing it in, so the plastic locks around the insert’s textured outer surface instead of trying to hold a thread pattern on its own. Once it is in, you get a metal thread that can be assembled and disassembled dozens of times without wearing out the printed part around it.
What the insert actually is
A heat-set insert is a short brass tube, threaded on the inside for a machine screw and knurled, ribbed, or barbed on the outside. That outer texture is the whole trick: when the insert is hot enough to soften the surrounding plastic, gentle pressure sinks it into a printed hole, and the plastic flows into the grooves and barbs as it cools. Once solid again, the insert cannot spin or pull straight out, because the plastic has physically locked around its shape rather than just gripping a smooth surface.
They come in a wide range of sizes, generally M2 up to M12, though M3 is where most hobby projects live. It is a good match for the screws already sitting in most parts bins, and small enough to fit into a wide range of wall thicknesses without needing a huge boss around the hole. M4 and M5 show up on larger, load-bearing parts where a bit more thread engagement and pull-out resistance matters.
What you need to install one
A soldering iron with a tip small enough to sit inside the insert is the standard tool, and it does not need to be expensive: a basic 40-watt iron is enough for the smaller sizes. Dedicated insert-installation tips, shaped to match a specific insert size and sold separately or as part of a heat-set insert tool, give more even heat transfer and are worth it if you are doing this often, but a plain iron tip used correctly on M2 to M4 inserts works fine for occasional use.
Temperature depends on the plastic. PLA generally softens enough somewhere around 200 to 225°C. PETG needs more heat, typically in the 240 to 250°C range. ABS and nylon need the most, generally 250 to 270°C. A reasonable starting point on any non-adjustable iron is to aim roughly 10 to 20°C above the temperature you print that material at, then adjust from there based on how easily the insert sinks in.
Sizing the pilot hole
The hole needs to be smaller than the insert’s outer diameter so the plastic actually grips it, but not so small that the insert cannot seat cleanly. As a general rule, aim for a hole about 0.3 to 0.5mm smaller than the insert’s measured outer diameter, then check the specific insert’s datasheet, since knurl patterns and wall thickness vary between brands and sizes even at the same nominal size.
For M2 inserts, a pilot hole in the 3.0 to 3.2mm range is a common starting point. For M3, the most widely used size, 4.0mm is the figure most manufacturers list, though plenty of people land on results closer to 4.2mm depending on how their specific printer holds tolerance on small holes. Print a quick test block with a few hole sizes in 0.1mm steps before committing to a final part; a few minutes of test printing saves you from either a loose, spinning insert or one that will not seat without cracking the boss around it.
Keep the hole straight rather than tapered, and add a very small chamfer at the top edge. That chamfer gives the plastic the insert displaces somewhere to go as it flows, instead of forcing it to squeeze out messily around the rim. If the hole is blind rather than going all the way through the part, make it roughly 1mm deeper than the insert itself, so there is somewhere for that displaced plastic to settle without pushing the insert back out before it cools.
Installing it without cracking the part
Heat the iron to temperature first, then lower it straight down onto the top of the insert, applying only light, steady pressure rather than pushing. As the plastic around the hole softens, the insert should sink in under its own weight and the iron’s, sinking slowly rather than dropping in. Forcing it before the plastic has actually softened is the most common way to knock it in crooked, and a crooked insert is much easier to fix in the first few seconds, while everything is still soft, than after it has cooled.
A widely used technique is to only melt the insert down to about 90% of its final depth with the iron, then finish seating it the rest of the way with a flat tool once the iron is off, rather than holding the hot iron on it until it is fully flush. This avoids overheating the plastic right around the insert, which weakens the exact area that is supposed to be holding it in place.
Once it is seated, let the part sit untouched for a minute or two so the plastic can fully re-harden around the insert before you thread a screw into it. Threading a screw in while the plastic around the insert is still soft can push the insert slightly and undo some of the grip you just built.
What goes wrong, and why
An insert that spins freely once cool almost always means the pilot hole was too large, or the plastic never got hot enough to properly flow into the knurling before it was pushed the rest of the way in by force. A hole that is too small in the other direction creates a burr of displaced plastic that either stops the insert from sitting flush or later blocks a screw partway down the thread; if you feel resistance turning a screw in, that is usually the fix needed on the next part, not a fault with the insert itself.
Cracking around the boss comes down to wall thickness. An insert needs enough surrounding plastic to actually grip, and a hole positioned too close to the outer wall of a part, or a boss printed too thin around it, gives the plastic nowhere to go under stress and it splits instead. Design a bit more material around the hole than feels strictly necessary, especially on parts that will see repeated tightening.
Threaded inserts are one piece of a bigger picture: the post-processing overview on this site covers where drilling, tapping, and inserts fit relative to sanding, priming, and painting on the same part.
Inserts make sense when a joint needs to come apart again. For a permanent bond instead, see our guide to gluing 3D prints, which covers picking the right adhesive by material.