How to weld plastic

1. With a soldering iron.
1. Fix two parts of the broken thing together. E. g. with a clamp, vise, tape or just with your hands.
2. Heat up your soldering iron.
3. Make Frankenstein stitches (transverse stitches) with soldering iron at the broken connection.
4. Make same stitches at the opposite side of the connection.
5. Use metal wire or thin metal mesh. Melt it into the plastic connection. It will become a fitting.
6. Melt and weld external plastic into the connection to fill it up. E. g. zip tie could serve as a filler rod. You will cover metal mesh and fill in the connection with additional plastic.
7. Sand the result with sandpaper to make it look nice.
2. With heat gun or iron
1. If the 2 parts are even, we can merely heat them with a heat gun or with an iron. And then press and stick together. Attention: this way was not always successful for me.