Melting in threaded inserts: a guide for 3D printing
are explained briefly when you hover over or tap them.
Screws driven directly into printed plastic hold at first, but after a few rounds of screwing and unscrewing, the thread is worn out. For housings, brackets and anything that is opened more often, there are threaded inserts made of brass. They are melted into the part with a soldering iron and give you a metal thread that survives many assemblies.
What options are there?
| Method | How it works | Good for | Limits |
|---|---|---|---|
| Heat-set inserts | Brass sleeve with knurling is pressed in hot, the plastic flows into the structure | almost everything, frequently opened joints | soldering iron needed |
| Press-in nuts | Sleeve is pressed or hammered in cold | quick assembly without heat | hold considerably less, part can crack |
| Embedded nuts | Hex nut is placed in a pocket during a print pause and printed over | high tensile loads, no tool other than the printer | pause has to be set in the slicer, nut cannot be changed later |
| Nut in an open pocket | Nut is slid in from the side or from below | simple designs | pocket has to be provided in the model |
| Directly cut thread | Print or drill a core hole, cut the thread with a tap, or use plastic screws (self-tapping) | rarely opened joints | wears with frequent loosening |
For most projects, heat-set inserts are the best compromise between hold, effort and price. The rest of this article is about them.
Hole diameter: follow the manufacturer’s table
The hole must be slightly smaller than the outer diameter of the insert so that the displaced plastic flows into the knurling. The correct dimensions are in the manufacturer’s table, because inserts differ in length and outer diameter.
Example M3: According to manufacturer figures, common inserts with a 4.6 mm outer diameter and 5.7 mm length need a hole of about 3.9 to 4.0 mm. Designs with a 5 mm outer diameter, as used in some printer kits, need a larger hole, about 4.2 to 4.4 mm. M2, M4 and M5 have their own values.
Important: printed holes usually come out slightly smaller than in CAD. Before the actual part, print a small test block with holes in 0.1 mm steps and try out which size works cleanly.
More rules for the hole:
- Straight, not tapered. A chamfer on the edge is normally not necessary.
- Blind holes about 1 mm deeper than the length of the insert. The displaced plastic collects there.
- Holes printed perpendicular to the build plate come out rounder than horizontal ones.
Wall thickness around the hole
There must be enough material around the hole, otherwise the wall bursts or the insert turns when tightened. For M3 inserts with a 4.6 mm outer diameter, manufacturers state a minimum wall thickness of 1.6 mm. More does no harm.
In the slicer it helps to increase the number of walls, for example to 3 to 4. The area around the hole then consists of solid perimeters instead of infill. How walls and top layers work together is explained under Walls and top layers. A higher infill helps in addition, but does not replace walls.
Soldering iron temperature by material
CNC Kitchen recommends setting the soldering iron 10 to 20 °C above the filament’s printing temperature. This gives enough heat to melt without decomposing the plastic.
| Material | Soldering iron guide value |
|---|---|
| PLA | about 225 °C |
| PETG | about 245 °C |
| ABS, ASA | about 265 °C |
The starting point is always your actual print temperature. For nylon, PC or fibre-reinforced filaments the same rule applies, with correspondingly higher values. For large inserts from M5 upwards, use a thick tip and raise the temperature slightly.
If the soldering iron is too hot, plastic swells up and the hole becomes too wide. If it is too cold, you have to push, and the insert sits crooked.
Tips and tools
An ordinary soldering tip works, but slips easily and sets the insert crooked. Special brass insertion tips are better. They have a pin that engages in the thread of the insert and a shoulder that presses evenly. They are available as sets for M2 to M8, to fit common soldering stations. Before buying, check whether the tips fit your soldering iron.
Typical prices:
- Threaded inserts M3, 100 pieces: about €10
- Insertion tip set: about €20 to €30
- Temperature-controlled soldering iron: from about €30
Helpful but not necessary: a soldering iron holder or a stand that guides the iron vertically. With many inserts in particular, they then all sit the same.
Step by step
- Lay the part down firmly. The hole faces up, the part does not wobble.
- Place the insert. Many inserts have a narrower side. This one points into the hole.
- Heat the soldering iron to the temperature from the table, using a suitable tip.
- Apply vertically and let it sink in without pressure. The weight of the iron almost suffices. If you have to push, it is too cold.
- Melt in only about 90 % of the depth, then take the soldering iron away.
- Press flush. Press in the rest with a flat, cold surface, for example a screwdriver shaft or a metal plate. This way the insert sits level with the surface.
- Hold for a few seconds until the plastic is solid again.
- Check. Screw in a screw and see whether it sits straight. Remove swollen plastic at the edge with a craft knife.
If an insert sits crooked, you can heat it again with the soldering iron, pull it out and reset it. The hole usually becomes slightly wider as a result, though.
Common mistakes
| Mistake | Cause | Solution |
|---|---|---|
| Insert turns | wall too thin, hole too large | more walls, smaller hole |
| Wall bursts or bulges | hole too small, wall too thin | hole according to the manufacturer’s table, print a test block |
| Plastic in the thread | blind hole too shallow, too hot | deeper hole, lower the temperature |
| Insert sits crooked | too much pressure, no special tip | hotter, without pressure, insertion tip or stand |
Continue with: Post-processing 3D prints or Tools for 3D printing.