M3 and M4 screws are a very easy and strong way to connect 3d printed plastic parts that possible need to be taken apart again later. Gluing parts is another very good option, but only when you never need to take them apart any more.
The nuts can be either left on the surface of the plastic like here:
A bit more convenient is to embed the nuts in nut traps: either horizontal of vertical. They are then hold in place and don’t move when mounting the screws.
The drawbacks if the nut traps are various:
- More work to design them.
- Sometimes need to add extra support layers to hold the layers on top of them. Particularly is this needed when the nut trap is on the bottom of an object.
- The horizontal ones do take away a lot of material, weakening the object. They can only be used in larger objects.
- Need to have access to either the side or the back of the object.
- Depending on the object the screws needed can get very long, because they have to go through part or whole of the object.
Then I came across these nuts you melt in the plastic. They solve lot of issues.
One is inserted into the plastic part. They only need very little space and material, but still hold the plastic very strongly.
They come if lots of all kinds different lengths, diameter and screw sizes. The ones shown in the picture are M3 x 4mm high x 4mm diameter. They are mounted in the plastic using a soldering iron set to 250 degree for ABS.
Here is an example of use of them: hold the spring screw.