A heat-set insert is a small brass nut that you melt into a hole in a 3D printed part, so the screw goes into a metal thread and not into the plastic. That lets you undo and refit the screw many times, where a screw driven straight into plastic soon strips its hole. This one has an M3 thread and is 5mm long, and it is the insert to choose for 3D prints. A smooth collar at one end drops into the hole and keeps the insert upright while it heats. Two bands of slanted knurling, angled opposite ways, then lock into the softened plastic so the insert cannot turn or pull out. If you are casting or moulding an insert into a part, our straight knurl M3 moulding insert is made for that.
You can use it for the lid screws on a printed electronics enclosure, for mounting a circuit board inside a case, or for joining two printed halves. It goes into PLA, PETG and ABS with an ordinary soldering iron, in a hole printed a little smaller than the insert. The inserts are sold loose with a minimum order of 10, and screws are not included.
Specifications
Fitting a heat-set insert
- Print the hole
Model a straight round hole 4.0mm across, with at least 2mm of plastic around it. If the hole stops inside the part, make it 6mm deep so the plastic the insert pushes ahead has somewhere to go. Printed holes come out a little smaller than drawn and every printer is different, so fit one insert in a small test piece before you print the real part.
- Heat the iron
Set the soldering iron 10 to 20°C above the temperature you printed at. That is about 225°C for PLA, 245°C for PETG and 265°C for ABS. A clean cone-shaped tip that sits in the M3 thread keeps the insert centred. An iron with no temperature control runs hotter than this, so work quickly with it.
- Sink the insert
Stand the insert in the hole with the smooth collar down. Rest the tip in the thread, hold the iron upright and let its weight do the work. The insert sinks over a few seconds as the plastic softens. Pushing hard or wiggling the iron widens the hole, so keep it steady. Lift the iron out when the top of the insert is just above the surface.
- Press it flush
Straight away, press the insert level with the surface using something flat and metal, such as the side of a steel ruler. Hold it there for a few seconds while the plastic sets. This squares the insert up and stops it creeping back out while it is hot.
- Check the thread
Once it is cool, run an M3 screw in by hand. It should turn freely to the bottom and the insert should stay put. If the screw binds near the bottom, plastic has been pushed up into the thread, so print the next hole at 4.2mm. If the insert turns or lifts out, the hole was too big, so print the next one 0.1mm smaller.