Ultrasonic welding profitably replaces the traditional assembly methods, which use solvents and adhesives: welding times are extremely short (equalling tenths of second) outer perfection of the welded item remarkable savings in labour watertight assemblies, which are stable in time welding can be performed even if foreign bodies (for example, dust or liquids) are present no risks for the operator, as the welding device (or sonotrode) is cold excellent mechanical seal low energy consumption extremely simple and flexible operation mould design is very simple.