Brass threaded inserts provide a great way to fasten plastic parts together. All you need is a soldering iron and a steady hand.

Sometimes, you want to 3D print an object in several parts. Printing a complete object is often not desirable, or difficult because of a complex shape. One alternative for connecting the pieces together is just to glue them with Superglue. This works great if you never want to take them apart.

I can use a variety of methods to fasten 3D printed parts. These are summarized聽 here and here. To this point, I have mainly used M2 or M3 metric fasteners. My favorite has been the “captive nut” method. This is where you extrude a space to capture a nut and hold it firm. Subsequently, your screw just fits through the nut.

For my latest project, I tried a new approach – new for me at least. This approach involves sinking threaded inserts into the plastic. These are made of brass. Threaded inserts allow you to repeatedly screw and unscrew the parts. I wanted to use this method for attaching a plastic lid to a plastic project box.

Brass Threaded Inserts – Soldering Iron Required

My local 3D printing supplier, Spool3D had some of these inserts in stock. An M3 insert is shown above top left. This brass threaded insert contains an M3 thread. It is 4 mm wide and available in various lengths.

To make this work, you print a hole the width of the insert. Then, you heat the brass insert. The plastic around it melts during the heating and the insert “sinks into the plastic”. When you remove the heat, the surrounding plastic hardens again. The end result is a brass thread solidly contained in your plastic part.

Two factors need attention. First, the hole must be the right size for the insert. You probably want a hole that is a bit longer than the insert. This way, there is room to “screw through” for a good connection. But most important, the insert must be perpendicular to the surface. You want the screw to run straight so everything fits together. I found the best way to accomplish this was to attached the insert onto the end of a long screw. Holding the screw vertically, the insert pressed in straight when heat was applied.

Heating the soldering iron to聽300 degrees and pressing the tip against the brass threaded insert did the trick. A solid connection. The inserts only cost around 10 cents, although there are more expensive models available. This short video demonstrates the process.