Used for multiple purposes in stud welding. First, the ferrule helps to shield the arc from the operator. Second, it minimizes the atmosphere that the flux must purge. Third, the ferrule forms the molten metal into the fillet. Finally, the fillet vents allow weld gases to escape.
Ferrules are disposable items. As disposable items they are mass manufactured quickly and inexpensively. Ferrules are cast in a mold out of a low grade moldable ceramic. They are taken out of the mold and are dried. Because of this manufacturing method, ferrule tolerances can be greater than machining or other manufacturing processes. Therefore, it is not recommended to fixture off of a ferrule when exact location is required.