[1][2] The process uses a welding head that has a nozzle for hot air and an extruder that pushes filler material out.
The process applies extrudate with pressure to ensure good bonding, and then allows the part to cool.
This is typically achieved by using the hot air nozzle on the welding gun and fanning the starting area.
Once welding has commenced, proper travel speed and gun angle will ensure that the upcoming faying surface has been sufficiently preheated.
The welding shoe will also have guide nipples that prevent the molten extrudate from flowing in an undesired direction.
Automation or skill by an experienced welder, coupled with properly set parameters will result in consistent and reproducible welds.
If the travel speed is too fast, there won't be enough filler material deposited, resulting in a small weld bead.
Conversely, if the travel speed is too low, then too much filler material will be deposited, resulting in an unaesthetic weld and potentially flash formation which will then need to be removed.
[1] Studies by have found that airflow rate and temperature are the two biggest factors in the development of creep strength of the welded parts.
This is because airflow rate and air temperature are the two biggest factors in development of a melt layer prior to welding.
Along with extrudate temperature and airflow rate/temperature, extrusion rate must be balanced in order to achieve ideal weld properties.
Conversely, if too much filler is added, then there is potential to create flash or sharp point at the toes of the weld cap.
These would act as stress risers and dramatically decrease the strength of the weld both in dynamic and fatigue loading.
[5] The main advantage of extrusion welding is that it can achieve very high deposition rates of filler material into a joint, thus cutting down on cycle time.