A moulding insert is a brass nut that sits inside a plastic part, so the screw goes into a metal thread and not into the plastic. This one has an M3 thread and is 5mm long, and it is the insert to choose when the plastic or resin sets around it. Each end has a band of straight knurling, with teeth that run along the insert like the teeth of a gear, and between the bands is a narrower plain waist. The teeth stop the insert turning and the plastic that fills the waist stops it pulling out. Both ends are the same, so it fits either way up. If you are melting inserts into 3D prints with a soldering iron, our M3 heat-set insert has a guide collar that makes that job easier.
You can cast it into epoxy or polyurethane resin, mould it into an injection moulded part, or glue it into a resin 3D print. It can also be heated into a hole in a printed or moulded plastic part. It has no collar to guide it into a hole, so the steps below show how to hold it square. The inserts are sold loose with a minimum order of 10, and screws are not included.
Specifications
Fitting the insert
- Casting or moulding it in
Wind the insert fully onto an M3 screw that passes through the wall of your mould, so it is held where the thread is needed and the resin cannot run into it. A smear of mould release or grease on the screw makes it easy to remove later. Pour the resin, let it set, then unscrew the screw and open the mould. In an injection mould the insert sits on the core pin in the same way.
- Gluing it into a resin print
Resin prints do not melt, so the insert is glued. Make the hole so the insert pushes in with light finger pressure. Start at 4.6mm and adjust in 0.1mm steps. Wind the insert onto the end of an M3 screw to use as a handle, coat the knurls with thick superglue or two-part epoxy, and push it in level with the surface. Leave the screw in until the glue has set, so the insert stays square and glue stays out of the thread.
- Heating it into plastic
For PLA, PETG, ABS and other plastics that soften with heat, make a straight hole 4.0mm across. If the hole stops inside the part, make it 6mm deep. Set a soldering iron to about 225°C for PLA, 245°C for PETG or 265°C for ABS. Sit the insert on the hole, steady it with tweezers and rest the iron tip in the thread until the insert has sunk nearly level. Then press it flush with something flat and metal and hold it for a few seconds while the plastic sets.
- Check the thread
Once the part has set or cooled, run an M3 screw in by hand. It should turn freely to the bottom and the insert should stay put. If the screw binds after heating in, plastic has been pushed up into the thread, so make the next hole 4.2mm. If the insert turns or lifts out, make the next hole 0.1mm smaller.