LTRASONIC WELDING TECHNIQUE
The typical profile of a joint design for ultrasonic welding has two functions:
1. Its shape of a groove and tongue eases positioning during assembly and maintains in place the parts during the welding cycle
2. The energy director in the joint concentrates the energy to a small area and supplies at the same time the necessary material for an optimal welding joint. The profile of the joint area depends largely on the purpose of the application; it can therefore be shaped quite differently. The settings of the machine, like welding pressure and amplitude, are adjusted to produce a movement of the horn front in phase with the part to be welded. The second part placed in the fixture must stay passive. The motional interference produces friction in the contact area of the two parts. Due to this highly intensive ultrasonic vibration the joint area melts together immediately. Under the pressure applied simultaneously and some fractions of a second longer than the ultrasonic impulse lasts the melted area solidifies and an inseparable joint is produced.
Ultrasonic staking is used to assemble injected plastic parts with parts of another material. Studs on the molded plastic part penetrate the second article to be mounted. The projecting part of these studs is formed into riveting heads with a specially shaped horn tip. The cavity in the horn is big enough to hold the displaced material and form it nicely. The amplitude of the horn has to be adjusted for maximum motional interference between oscillating horn tip and plastic stud. Heat is then produced by friction between the plastic part and the horn tip, which allows an immediate melting in this area. The pressure controlled by the Welding Machine lets the riveting head solidify after a short hold time. Multi-tip horns are often used for simultaneous staking of different areas on the same part.
Ultrasonic spot welding is used whenever large plastic parts cannot be placed on a traditional machine. Therefore, the welding head designed as a handgun is brought to the part; either manually or with a robot. The advantage of ultrasonic spot welding is the fact that the parts do not need any special preparation (no energy director required). Therefore large size vacuum formed parts can also be assembled easily.
The Ultrasonic Horn is built to produce high amplitude. The tip of the horn is shaped in order to penetrate the two layers of material, leaving a typical ring pattern on the workface, while the backside rests smooth and clean.
INSERTION OF METAL PIECES:
A metal piece such as a threaded insert or bolt has to be embedded into an existing hole in a plastic part. The metal piece is contacted by the oscillating horn, which creates friction between the two parts melting the plastic material. The machine pressure displaces the now melted material into the structure on the metal part surface. Once the displaced material has solidified the inserted piece remains tightly seated and resists to tensional as well as to torsion forces.