4008 Cell Phone Shell Welding
Ultrasonic welding of mobile phone metal shells: a thin-wall shell is pressed against an internal structure under the horn, and high-frequency vibration breaks the surface film to form the bond — no screws, snap fits or adhesive.
Such parts are thin-walled and the visible face cannot be marked, so the horn is usually an insert with local contact and pressure acts only on the weld zone rather than the whole surface. Bond quality is checked by pull test plus visual inspection, not by a whitened spot alone.