Site with more details form this video is down.
Here is more industrial ultrasonic plastic welder:
And when you need some quick and dirty plastic welding and you have a soldering iron, this is a short video about it, with interesting tip on how to use a paperclip as aid :-)
I don't know how applicable those methods are for 3d printed models. I know some people "weld" ABS with acetone (toxic) with varying results. It is interesting information and good to know skills.
If you want to weld broken plastic with soldering iron and you are aware of some risks and limitations here is a detailed guide:
here is a new post about plastic friction welding with 3d printing filament inserted into Dremel: