Ultrasound is known from the second half of the 19th century, but practical applications were discovered only in the First World War, with detection systems, submarines and sonar. This was one of the first cases that used the piezoelectric effect for their generation. The discovery of this special property has easy to use sources of ultrasonic waves, setting the technology for a major expansion of its use.
Piezoelectric materials have, in fact, the characteristic increase or decrease depending on the voltage they are subjected to this allows transforming a voltage signal oscillating at the same mechanical vibration frequency. The intensity and frequency of mechanical vibration are dependent on the intensity and frequency of the electrical signal.
The main applications of ultrasound technology are:
- Sensors (proximity sensors, diagnostic structural);
- Industrial cleaning (ultrasonic cleaning baths);
- Welding metals;
- Welding or plastics processing
The ultrasonic welding is a technology widely used for assembling parts made of thermoplastic material, making use of a technology most widely known and frequencies ranging from 15 to 40 KHz, with variable amplitude according to the material to be welded .
The welding technology by ultrasound is based on the piezoelectric principle, to transform an electrical signal into mechanical vibrations 4 elements are needed:
- An ultrasonic generator to control and provide power to the ultrasound head.
- A mechanical unit consists of a converter, a booster and a sonotrode
Ultrasound generator converts the power source (100-250 volt, 50-60 Hz) in 15 to 30 kHz, 800-1000 Volts electrical signal. This signal is applied to piezoelectric ceramics (including the converter) that will convert the signal into mechanical vibrations
These fluctuations are amplified by the converter and booster, creating a hammer. The converter converts electrical energy into high frequency mechanical vibration. The active elements are generally piezoelectric ceramic. The booster (optional) works like an amplitude transformer. To increase or reduction of amplitude is possible by certain design characteristics or the geometric shape of the booster.
The sonotrode is the active part of the ultrasound unit. It is in contact with the product and works by transmitting mechanical vibration and pressing against the support tool (Fixture). This system will allow the fusion of the molecules inside the product by mechanical action, allowing obtaining a melting of the plastic. The design of the sonotrode and the fixture can be adapted to the operation to be performed.