What exactly are heat set inserts?
Heat set inserts are brass bushings with internal threads and a coarse knurled exterior. You heat the insert and press it into a hole that is just slightly too small. The surrounding plastic melts, flows into the knurling, and locks the insert in place once it cools.
This works because inserts are suitable for almost all thermoplastics, meaning any material you use on an FDM printer. A thermoplastic softens as soon as it exceeds its glass transition temperature and hardens again when it cools down.
By the way, the plastic doesn't actually stick to the brass. High-speed camera footage shows that the plastic locks into the knurling purely through mechanical force. That grip is substantial. In a pull-out test by CNC Kitchen, M3 inserts held an average of about 1,400 newtons—roughly 150 kilos—before pulling out.
When a threaded insert is the smart choice
Not every screw hole needs an insert. Are you screwing something in once and leaving it there? Then a screw directly into the plastic is fine. You simply design a hole slightly smaller than the bolt, and the bolt will tap its own thread into the softer plastic.
A threaded insert is useful in these situations:
- You open and close the part regularly, such as an enclosure with a lid or a component you need to remove for maintenance.
- The screw needs to be tightened firmly and must not strip the plastic.
- You want to use metric machine screws, for example, to attach your print to a metal part.
There are two other alternatives. You can embed a standard nut into a hexagonal recess in your model. That’s cheap and strong, but the nut takes up space and must remain accessible. If you want to join parts permanently, you can also glue them together. If you want to be able to disassemble them later, an insert is better.
What you need to install inserts
You probably already have half of this lying around.
- A soldering iron with temperature control. You can get by without it, but these irons often get hotter than necessary, which can cause the plastic to break down.
- An insert tip. This is a separate soldering tip that you attach to your iron. The centering pin ensures the insert sits straight on the tip and goes into the hole perfectly, while the flat shoulder presses against the edge of the insert instead of the surrounding plastic.
- Inserts in the correct size. M2 to M5 are the most common. M3 is the standard for enclosures and brackets.
- Something flat to press with, such as a screwdriver, tweezers, or a piece of aluminum.
- A wooden board as a base, so you don't burn anything.
Installing dozens of inserts at a time? Then an insertion station is handy. Such a station is essentially a soldering iron on a vertical guide, making it much easier to press the inserts in straight.
How to design the hole for your insert
The melting process rarely goes wrong; the problem usually lies in the design.
The right hole size: start with the manufacturer
Every insert has a recommended hole size on the packaging or in the datasheet. Use that as a baseline, because inserts vary significantly in shape. There is no universal hole size for an M3 insert. The correct size depends on the outer diameter of the insert, not the internal thread. For example, CNC Kitchen recommends a 4.0 mm hole for their own M3 insert, while another brand with a thicker outer wall might require a larger hole.
Then, add a little extra. Printed holes almost always turn out smaller than you draw them. In CNC Kitchen's tests, the holes were about 0.25 mm too small. A 4.2 mm M3 hole in the design proved to be the best balance: the insert fits in beforehand, no burr forms at the bottom, and you retain about 90% of the maximum strength. Their rule of thumb: for smaller inserts, add 0.2 to 0.3 mm to the diameter in your design.
Don't go too big. A larger hole makes installation easier, but the strength drops quickly because the knurling has too little plastic to grip into. Still unsure? Print a test block with a range of hole sizes first. CNC Kitchen has one for free on Printables for every insert size.
Hole depth: give the melted plastic room
Make a blind hole about 1 mm deeper than the length of the insert. For a through-hole, the length of the insert is sufficient. Design the hole straight rather than tapered; you usually don't need a chamfer at the edge. That extra millimeter is there for a reason: the insert pushes a small amount of molten plastic ahead of it during installation, and that needs somewhere to go.
Enough plastic around the insert
An insert pushes plastic aside when installed. When you tighten the screw later, it pulls on its surroundings. If the wall around the hole is too thin, it will crack. Therefore, stick to this lower limit: at least 1.5 mm of plastic around the hole for M3, 2 mm for M4, and 2.5 mm for M5 and up.
Does the connection need to withstand significant force? It's better to make the boss around the hole about twice as wide as the insert. For a 4.6 mm M3 insert, that's a diameter of about 8 to 9 mm. Also, don't place inserts too close to an edge. The risk of cracking increases there, especially with small parts and thin-walled housings.
Horizontal holes behave differently
You print a hole in a side wall horizontally. With such horizontal holes, the top can turn out slightly smaller because the material sags a bit. Such a hole sometimes requires a slightly different size than a vertical hole in the same part.
Print settings: walls matter more than infill
Many people increase the infill to make the insert sit more securely. That barely helps. The internal fill pattern contributes little to how well an insert stays in place; the pull-out strength depends primarily on the number of solid walls around the hole. You can read more about what infill actually does in the knowledge base.
A good guideline is at least 3 to 4 walls around the hole, and 4 to 6 walls if the part is under real load. Are you having your part printed via the price calculator? Then you choose the number of walls yourself, from 1 to 6. Set it to 4 or higher for parts with inserts.
Another design tip: make the wall thickness around the hole a multiple of your line width, so the slicer creates completely solid lines without infill gaps.
Installing heat set inserts in 7 steps
- Heat your soldering iron to 10–20 °C above the printing temperature of your material.
- Place the insert in the hole with the narrow end facing down.
- Place the tip of the iron on top of the insert and wait a few seconds for it to heat up.
- Apply slow, steady, and even pressure, and keep the iron straight. Don't force it.
- Stop at about 90% of the depth and remove the iron.
- Push the last bit in with a screwdriver or tweezers until the insert is flush with the surface.
- Hold the insert in place for a few seconds until the plastic hardens, as inserts often creep back up after melting.
Don't test it immediately. Let the insert cool down completely first, as the plastic needs time to harden around the knurling.
What temperature should you use for each material?
As a general guideline, use about 225 °C for PLA, 245 °C for PETG, and 265 °C for ABS. You can find the details for each material in our knowledge base on PLA, PETG and ABS. For ASA, nylon, or PC, the same rule of thumb applies: take your filament's printing temperature and add 10 to 20 degrees.
Larger inserts require more heat. For an M5, for example, use a thick tip, a slightly higher temperature, and as much contact with the brass as possible. Just stay within the recommended range. If the iron is too cold, the knurling won't grip properly once it cools. If it's too hot, the plastic will off-gas and leave voids behind.
A note on PLA. Inserts in PLA work fine, but PLA is less suitable for long-term mechanical stress or higher temperatures. If your part gets hot, for example in a car or next to an appliance, choose a more heat-resistant filament.
How to troubleshoot insert issues
There is plastic under the insert
Is your screw getting stuck at the end of the insert? The hole was likely too small, causing a burr to form underneath. Make the hole slightly larger next time. For this print, you can carefully drill away the residue using a drill bit the size of the screw.
The insert is sticking out or is crooked
An insert that sticks out has almost always been melted in completely in one go. Stick to the 90% rule and press the rest in cold. For a crooked insert, a tip with a centering pin is the best remedy. It keeps the insert straight from the very first moment.
The insert spins or pulls loose
This indicates a lack of grip. A hole that is too large increases the chance of it spinning, because the surrounding plastic cannot properly handle the tightening torque. Too few walls have the same effect. You can remove such an insert by reheating it. For M2 to M3, there are special extractors that you use in combination with a soldering iron.
The wall cracks during installation
A crack usually occurs because there is too little plastic around the hole. If the wall is too thin, it will split during installation. Make the boss wider or move the insert further away from the edge.
Inserts in resin prints: gluing instead of melting
Resin does not soften after curing, so melting won't work. In resin prints, you glue the insert in place, preferably in a stepped hole, using thick superglue or two-component epoxy. Not sure which printing technique suits your part? Read about the difference between FDM and SLA.
Start with the design, not the soldering iron
You can learn the installation process itself in an afternoon. The mistakes you can't fix later are the ones in the model. Think of a hole that is 0.2 mm too tight, a 1 mm wall, or an insert placed right next to the edge. You only notice those once the print is already finished.
So, check those points before you print. Read how to make a CAD model print-ready, or have your file checked for free with the printability check. Don't have a model yet? You can also have your part modeled, with the holes and bosses drawn to size for your inserts right from the start.
Frequently asked questions about heat set inserts
What temperature do you need for heat set inserts?
Set your soldering iron 10 to 20 °C above your material's printing temperature. This puts you at around 225 °C for PLA, 245 °C for PETG, and 265 °C for ABS.
What hole size do you need for an M3 insert?
For most M3 inserts, the starting point is between 4.0 and 4.2 mm, depending on the outer diameter, the knurling, and the material. Always check your insert's datasheet first. Then, add 0.2 to 0.3 mm, as printed holes tend to come out smaller.
Can you install heat set inserts without a special tip?
Yes. You can get most inserts in place with a regular, clean soldering tip. A special tip just makes it easier, faster, and more precise. It really helps prevent crooked inserts, especially when doing a lot of them.
Can you remove a heat set insert?
Yes. Reheat the insert and pull it out, for example by turning a screw into it a few times. There are special extractors for smaller sizes. Keep in mind that the hole will be a bit wider afterward, so a new insert won't be as secure.
Are inserts stronger than threading directly into plastic?
For repeated use, yes. If you drive a screw directly into plastic, the threads deform a little bit every time until you're left with a stripped hole. With a brass insert, you can screw and unscrew indefinitely.
The blog is approximately 2,050 words long. The "Installing heat set inserts in 7 steps" guide is formatted as a numbered list to increase the chances of getting a featured snippet.
There are 14 internal links and one external link to the hole size test model on Printables. The CNC Kitchen test is cited but not linked in the text. If you want a direct link to that source, I can add it. There are no tables (Webflow), and there is no mention that PixelPrints installs inserts themselves.
Since there was no GSC data: filter in Search Console for queries containing "insert" in a month or two. You'll see whether "threaded insert" or "heat set inserts" gets more impressions, and you can adjust the title accordingly.
Would you like me to prepare the blog as a doc so you can easily paste it into Webflow?
