Updated 2026-10-31
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Quick answer: Buy a brass kit weighted toward M3, size holes from the insert maker data and print a test coupon first. Use the tip and heat guidance in your iron manual, ventilate and let parts cool. In our planning example, a $15 kit of 300 works out at $0.05 per insert.
A heat-set threaded insert is a small metal sleeve, usually brass, with knurls or ridges on the outside and a machine thread on the inside. You press it into a slightly undersized hole in a 3D printed part using a heated tool tip, the plastic around it softens and flows into the knurls, and when it cools you have a real metal thread you can screw into again and again. This guide explains when inserts beat printed threads, the main insert classes, how to size holes and walls from the insert maker's data, how to install them with sensible heat safety, and how to work out what a kit really costs per part.
Inserts vs Printed Threads
There are three common ways to put a screw into a printed part, and it helps to know where inserts fit before you buy a kit.
- Printed or tapped threads: the thread is printed or cut straight into the plastic. This costs nothing extra, but plastic threads wear each time the screw goes in and out, and they can strip if you overtighten.
- Self-tapping screws into plastic: a screw cuts its own path into a plain hole. This works well for parts that are assembled once and rarely opened.
- Heat-set inserts: a metal thread is embedded in the part. It costs a few cents per hole and takes a minute to install, but it handles repeated assembly far better than plastic threads.
As a rule of thumb, choose inserts for anything you expect to open more than a handful of times: enclosure panels, tool mounts, electronics covers, jigs and brackets. For a one-off prototype or a decorative part, printed threads or self-tapping screws are often good enough. Inserts also let you use standard metric machine screws, so you can swap parts or hardware without reprinting.
Insert Classes: The Criteria Table
| Class | Best for | Watch out for | Planning note |
|---|---|---|---|
| Straight knurled inserts | General brackets, covers and mounts | Need an accurate hole and enough wall | Most common class in hobby kits |
| Tapered inserts | Easier alignment when pressing in | Hole should match the taper in the maker data | Self-centring helps beginners |
| Short or low-profile inserts | Thin panels and shallow bosses | Less thread length, less pull-out grip | Check length against part depth |
| Kits with tool tips | First-time buyers | Tips must fit your iron, per its manual | Tip set plus assorted sizes |
Most beginner kits are assorted brass inserts in M2, M2.5, M3, M4 and M5, sometimes with a set of matching tips for a soldering iron. M3 is by far the most common size for hobby printer parts, so a kit weighted toward M3 is usually the best value. The exact outside diameter, length and recommended hole size differ between insert makers, so treat any number you see online as a starting point and use the data sheet or listing for the inserts you actually buy.
How to Size Holes and Walls
Getting the hole right matters more than which kit you choose. Too large and the insert spins or pulls out; too small and you push a lot of molten plastic ahead of the insert, which can bulge the part or block the thread.
- Read the insert data first. Good listings give the recommended hole diameter and depth for each size. Use those as your design numbers.
- Add depth for melt. Make the hole a little deeper than the insert length so displaced plastic has somewhere to go instead of filling the thread.
- Leave enough wall. As a planning assumption, aim for a wall around the insert of at least about one insert diameter of solid material, more for parts that carry load. Use extra perimeters in your slicer around insert holes.
- Print a test coupon. Print a small block with three holes at slightly different diameters, install one insert in each, and keep the size that seats flush and does not spin.
- Note your printer. Hole size varies with nozzle, filament and slicer settings, so record what worked for your setup.
Printing holes with the axis vertical usually gives rounder holes than printing them sideways. If you must print a hole horizontally, expect to adjust the size on the coupon.
Installing Inserts Safely
A soldering iron tip runs far hotter than boiling water, and molten plastic sticks to skin. Treat installation as hot work and follow common workshop practice.
- Read the iron manual. Use the tip, temperature range and stand recommended by the maker of your iron or insert tool. Any temperature you read in a guide, including this one, is a planning assumption, not a setting to copy.
- Use a proper stand and a clear bench. Keep the iron in its stand between inserts and keep cables, paper and filament spools away from the tip.
- Protect your eyes and hands. Safety glasses are sensible, and keep fingers away from the tip and the insert while it is hot.
- Ventilate. Heating plastic can release fumes. Work in a ventilated area and check the safety data sheet (SDS) for your filament, which the filament maker publishes, for handling advice.
- Let parts cool. The insert stays hot for some time after the tip is removed. Let it cool before you touch it or drive a screw.
- Switch off and unplug. Turn the iron off and unplug it when you finish, and let the tip cool in the stand before storing it.
The installing motion itself is simple: place the insert square on the hole, rest the heated tip in it, and let the heat and light pressure sink it in slowly. Stop just flush with the surface, remove the tip, and if needed press a flat, cool surface on top to keep the insert level while the plastic sets. Pushing hard and fast is the most common cause of crooked inserts.
Common Problems and Fixes
- Insert spins in the hole: the hole is too large or the plastic was overheated. Reduce hole size on the next print or let the part cool between inserts.
- Insert goes in crooked: use a tapered insert, a tip that centres in the insert, or a simple guide such as a drill press style stand made for insert tools.
- Plastic fills the thread: make the hole deeper and avoid pushing the insert below the surface.
- Part bulges or cracks around the insert: the wall is too thin. Add perimeters or make the boss larger.
- Insert pulls out under load: use a longer insert, more wall, or spread the load across more fasteners.
What to Look For When Buying
- Size range: pick a kit weighted toward the screw sizes you use, usually M3.
- Data sheet: prefer listings that state hole diameter and depth for every size.
- Material: brass is the usual choice for hobby work because it transfers heat well and is easy to install.
- Tool fit: if the kit includes tips, confirm they fit your iron using the iron manual before buying.
- Storage: a labelled compartment box saves time and stops mixing sizes.
- Returns: buy from a seller with a clear return policy in case the sizes do not match the listing.
Cost Example: What an Insert Really Costs
These are planning assumptions for a worked example, not quoted prices. Check current prices before you buy.
- Kit price: assume an assorted kit of 300 inserts costs $15.
- Cost per insert: $15 / 300 = $0.05 per insert.
- One bracket: a bracket with 8 inserts uses 8 x $0.05 = $0.40 of inserts.
- A small project: an enclosure with 6 panels and 4 inserts per panel needs 24 inserts, so 24 x $0.05 = $1.20.
- Tip set: if a separate tip set costs $10, the first project costs $1.20 + $10 = $11.20, and later projects cost only the inserts.
Compare that with reprinting a part whose plastic thread stripped. If a bracket uses 40 g of filament at $0.02 per gram, a reprint costs 40 x $0.02 = $0.80 in filament plus print time, which is double the $0.40 of inserts that would have prevented it. For parts you open often, inserts usually pay for themselves the first time they save a reprint.
Who Should Buy Heat-Set Inserts
Inserts make sense if you design your own parts, print enclosure panels or tool mounts, or regularly take printed assemblies apart. They are less important if you mostly print models and display pieces. A beginner can start with a small M3-heavy kit and a tip that fits the soldering iron they already own, then expand once they know which sizes they use most.
FAQ
Q: Do I need a special tool to install heat-set inserts? A: Many hobbyists use a temperature-controlled soldering iron with an insert tip. Check the iron manual for compatible tips and temperature guidance before you start.
Q: What size insert should I buy first? A: M3 is the most common size for hobby printer parts. A kit weighted toward M3 with some M2.5, M4 and M5 covers most projects.
Q: How big should the hole be? A: Use the hole diameter and depth from the insert maker's data, then print a small test coupon with a few sizes to find what works on your printer.
Q: Can I use inserts in PLA, PETG and ABS? A: Yes, inserts are used in all common filaments. Softer or lower-temperature plastics need gentler heat, so follow the iron manual and the filament SDS for handling.
Q: Are heat-set inserts better than printed threads? A: For parts you assemble repeatedly, usually yes. For one-off or decorative parts, printed threads or self-tapping screws are often enough.
Q: Is installing inserts safe for beginners? A: Yes with care. Treat the tip as hot work, use a stand, wear eye protection, ventilate, let parts cool, and unplug the iron when finished.
Check the insert data sheet and your iron manual, print a test coupon, then buy a kit weighted toward the screw size you use most.
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