Ultrasonic Cleaning Primer
This paper is based on an original publication by Branson.
Degradation Over Time
Ultrasonic cleaning methods are based on the intense agitation of countless bubbles in a cleaning liquid, called "cavitation". When an item is submerged in this solution, cavitation offers highly effective cleaning for both hidden and difficult to reach surfaces. Higher cavitation frequencies are ideal for particle removal without causing damage to substrates.
There are three main components of any ultrasonic cleaning system: a tank for the cleaning liquid, a transducer to convert electrical energy to mechanical energy, and an ultrasonic generator to provide a high frequency electrical pulse.
Transducer and Generators
Essential to any ultrasonic cleaning system, there are two types of transducers: magnetostrictive (nickel or alloys) and electrostrictive (lead zirconate titanate or ceramics). The dimensions of electrostrictive materials are altered when placed in an electrical field of varying voltage, a phenomenon known as the "piezoelectric effect". Magnetostrictive transducers, on the other hand, are constructed of materials that change physical dimensions when placed in a varying magnetic field.
The common factor between the two types of transducers is the intensity of cavitations. Like any sound wave, ultrasonic energy is actually a series of compressions and rarefactions. If sound energy is sufficiently intense, the liquid will be pulled apart during rarefaction, forming small cavities or bubbles. As bubbles collapse and implode throughout the liquid, an extremely effective cleaning force is created, capable of achieving pressure greater than 10,000 psi and temperatures in excess or 20,000°F. With many millions of bubbles collapsing every second, the cumulative effect of cavitation offers intense scrubbing action and accelerates the rate at which surface films are dissolved.
Cavitation is possible only when liquid pressure is reduced to a value lower than its vapor pressure, and thus there must be sufficient power generated by the transducer. The threshold of cavitation refers to the minimum amount of power necessary to achieve cavitation for a particular type of liquid. Only the ultrasonic energy above the threshold contributes to production of cavitation bubbles and thus the process of ultrasonic cleaning.
Above the threshold level, ultrasonic energy increases will result in higher cleaning efficiency, but only to a point. Once the liquid reaches a certain level, it becomes incapable of accepting power increases and the liquid will become elastic - reducing or eliminating further transmission of energy. This is called surface cavitation.
Likewise, there is a threshold below which cavitation will not occur. However, once cavitation occurs, the energy level can be reduced below this threshold and cavitation will be maintained. The sonic range over which cavitation can exist between threshold and maximum frequency is usually a distance of no more than a ratio of 2 or 3 to 1. A tank with only a small volume of liquid over the transducer, for instance, is subject to surface cavitation at very low levels. On the contrary a deep or heavily loaded tank requires a greater level of power for surface cavitation to occur, as well as to achieve an effective level of cavitation for cleaning purposes.