Heat-Set Inserts for 3D Printing
Threads tapped straight into printed plastic strip after a few uses. The fix that pros reach for is a heat-set insert: a small brass sleeve you melt into the part with a soldering iron, leaving a strong metal thread that takes a screw again and again. The catch is that the part has to be designed for it — here is the hole to model and why.
Model a hole, not a thread
The most important thing: you do not model a threaded hole for a heat-set insert. You model a plain straight hole sized so the insert grips as it melts in. The insert itself provides the threads.
Hole size = the insert’s spec
Each insert lists a recommended boss hole diameter — usually slightly smaller than the insert’s outside diameter so the melted plastic flows into its knurls. Model the hole to that spec. As a rough guide, an M3 insert often wants a hole around 4 mm and an M4 around 5.6 mm, but always use the number from the insert you are buying.
To choose between an insert, a tapped hole, and a plain clearance hole in the first place, see how to add threads to a part.
Give the insert a boss to live in
An insert pushes outward on the surrounding plastic as it seats, so the hole needs enough material around it — a small raised boss. Two rules:
- Wall around the hole. Leave a healthy ring of plastic, not a thin shell, or the boss splits when the insert goes in. Thin walls are the most common failure — see wall thickness for 3D printing.
- Depth a bit deeper than the insert. Make the hole slightly deeper than the insert is long so it can seat flush (or just below the surface) without bottoming out.
A small chamfer at the mouth of the hole helps the insert start straight, and a chamfer at the boss base reduces the stress riser.
Why inserts beat tapping plastic
Tapped plastic threads are fine for a one-time assembly that never comes apart. But for anything you open repeatedly — a battery lid, a serviceable enclosure, a panel you remove to access a board — the plastic threads wear and strip. A heat-set insert gives you a metal thread that survives many cycles. They are a natural fit for electronics enclosures, where a lid screws on and off a lot.
Design insert-ready bosses fast
You do not need to model knurls or threads. Describe the part to PartWork.ai and ask for a boss with a straight hole at your insert’s recommended diameter — then adjust the hole and wall if a test fit needs it. See modifying parts to tune the hole, and exporting for a print-ready file.
Start with 2 free AI generations — no card required
Open the studio, describe a boss with an insert hole, and export a print-ready part. More credits: 100 for $4.99 (~5¢ each).