Technical principle of ultrasonic cleaning
Ultrasonic cleaning is based on cavitation, meaning that numerous bubbles form rapidly in the cleaning solution and quickly implode. The resulting impact peels off the dirt on the inner and outer surfaces of the workpiece immersed in the cleaning fluid. With the increase of ultrasonic frequency, the number of bubbles increases and the blasting impact force decreases. Therefore, high-frequency ultrasonic is particularly suitable for cleaning small particles of dirt without damaging the surface of the workpiece. Bubbles are generated by applying high-frequency (ultrasonic frequency), high-intensity sound waves to the liquid. Therefore, any ultrasonic cleaning system must have three basic components: a tank for cleaning fluid, a transducer that converts electrical energy into mechanical energy, and an ultrasonic generator that generates high-frequency electrical signals.
Transducer and generator
The most important part of the ultrasonic cleaning system is the transducer. There are two kinds of existing transducers, one is a magnetic transducer, made of nickel or nickel alloy, and one piezoelectric transducer is made of lead zirconate titanate or other ceramics. When a piezoelectric material is placed in an electric field with a varying voltage, it will deform, which is called the 'piezoelectric effect'. Relatively speaking, magnetic transducers are made of materials that deform in a changing magnetic field.
No matter what kind of transducer is used, the most basic factor is usually the intensity of the cavitation effect. Ultrasound, like other sound waves, is a series of pressure points, that is, a wave of alternating compression and expansion (as shown below). If the sound energy is strong enough, the liquid is pushed away in the expansion phase of the wave, thereby generating bubbles; while in the compression phase of the wave, these bubbles burst or implode in the liquid instantaneously, generating a very effective impact force, especially Suitable for cleaning. This process is called cavitation.
Theoretically, a burst cavitation bubble will generate a pressure of more than 10,000 psi and a high temperature of 20,000 ° F (11,000 ° C), and the shock wave will quickly radiate outward at the moment of burst. The energy released by a single cavitation bubble is very small, but millions of cavitation bubbles burst at the same time every second. The cumulative effect will be very strong, and the powerful impact force will peel off the dirt on the workpiece surface. This is the characteristic of all ultrasonic cleaning.
If the ultrasonic energy is large enough, cavitation will occur everywhere in the cleaning fluid, so ultrasonic waves can effectively clean tiny cracks and holes. Cavitation also promotes chemical reactions and accelerates the dissolution of surface films.
However, cavitation will only occur in the area when the liquid pressure in the area is lower than the gas pressure in the bubble, so this condition can be met when the amplitude of the ultrasound generated by the transducer is sufficiently large. The minimum power required to produce cavitation is called the cavitation critical point. Different liquids have different cavitation critical points, so the ultrasonic energy must exceed this critical point to achieve the cleaning effect. In other words, only when the energy exceeds the critical point can cavitation bubbles be generated for ultrasonic cleaning.
The importance of frequency
When the working frequency is very low (within the range of human hearing), noise will be generated. When the frequency is lower than 20kHz, the working noise not only becomes very loud, but also may exceed the limits of the safety noise prescribed by the Occupational Safety and Health Act or other regulations. In applications that require high power to remove dirt without considering the damage to the workpiece surface, a lower cleaning frequency in the range of 20kHz to 30kHz is usually selected. The cleaning frequency in this frequency range is often used to clean large and heavy parts or high-density materials. 20KHz magnetic transducer and 25KHz piezoelectric transducer.
High frequency is often used to clean smaller, more precise parts, or to remove tiny particles. High frequency is also used in applications where the surface of the workpiece is not allowed to be damaged. Using high frequency can improve cleaning performance from several aspects. As the frequency increases, the number of cavitation bubbles increases linearly, resulting in more and denser shock waves that allow them to enter smaller gaps. If the power remains unchanged, the cavitation bubble becomes smaller, and the energy released by it decreases accordingly, which effectively reduces the damage to the workpiece surface. Another advantage of high frequency is that it reduces the viscous boundary layer (Ponuri effect), allowing ultrasound to 'discover' very small particles. This situation is similar to the small pebbles at the bottom of the stream when the Water level in the stream drops.
40kHz, 80kHz, 120kHz and 170kHz. When cleaning very small particles, the product with a frequency of 350kHz can be used. The MicroCoustics system for such occasions has recently been launched with a frequency of 400kHz.
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