Several techniques are used to weld plastic parts together;
Hot gas welding
This is the same technique as used for Gas welding only with lower temperatures. A specially shaped heat gun produces hot air that melts the parts to be joined. The heat gun also has a feed tube for plastic rod. A bead of melted plastic is laid and the parts are joined.
This is a common fabrication technique for manufacturing large chemical tanks, heat exchangers and plumbing fittings. The materials being welded generally have to be of the same or similiar chemistry.
Contact welding
This is the same as Spot welding except that heat is supplied with convection of the pincher tips, instead of electrical conduction. Two plastic parts are brought together and heated tips pinch them, melting and joining the parts in the process.
High frequency welding
Certain plastics with chemical dipoles, such as polyvinyl chloride (PVC), polyamides (PA) and acetates can be heated with high frequency electromagnetic waves. High frequency welding uses this property to melt the plastics for joining. The heating can be localized and the process can be continuous.
This is the same concept as Induction welding.
High frequency sound waves are used to melt the materials to be joined. Localized melting may be accomplished with designing an energy directing tip into the one of the parts being welded.