Ultrasonic Welding Explanation
This is the much simplified explanation. If you really want to learn all there is on the subject try
The Ultrasonics Book
Ultrasonics are high frequency sounds, like the ones bats use for navigation and hunting.
For welding we work on two assumptions:-
1/ If you fire sound into something solid, the item will vibrate. You can test this with the fabled tin and string telephone, if you speak into the tin and touch the string very gently you can feel the string vibrate as you speak.
2/ When two parts are rubbed together hard enough they heat up and ultimately melt. Again easy to test, the easiest is by sawing a length of polypropelene cord across another piece.
In an ultrasonic welder the two parts are subjected to high frequency sound from a piezo generator (from about 7.5KHz for plastic). This causes resonant vibration in the parts, effectively rubbing them together at very high speed. Where they meet (the interface) they become hot enough to melt. Press them together firmly, remove the vibration and the material sets again, giving a bond.
Because it is only the very surface of the part that melts, there is no heat generated in the rest of the component, so there is no damage and the setting time is minimal, because it is conducted away almost instantly.
May be worth noting that ultrasonics are heavily used for a range of other tasks in industry, including inspection of hard to reach places, forming, automatic feeding and non-mechanical pumps