The heating of joining layers at the joint sides is carried out by thermal gas or, occasionally, by thermal radiation.

The basic material is superficially melted at the surface of the joint sides. During the welding process, it mixes with the extrudate that has been added.

In addition to the welding parameters of the temperature and volume flow of thermal gas, it is the geometry and the position of the jet as technological components that influence the preheating to a great extent.