Ultrasonic cleaning equipment is often used to clean small, complex and finely structured components, for example, jewellers, goldsmiths, dental technicians, watchmakers and opticians use ultrasonic cleaning baths to clean their products.
The system consists of a liquid-filled, possibly heatable tub, one or more ultrasonic transducers or ultrasonic transducers and a generator that supplies the transducer(s) with high-frequency electrical energy. Sonic oscillators can be glued directly to the outside of the walls as well as under the bottom of the tub, but only if the material thickness of the wall or floor is not too strong. In this design, the ultrasound is then introduced directly into the liquid via walls and/or floor. Another variant are so-called immersion transducers, which are hung in the liquid or attached to racks in the pool. Finally, there are plate transducers that are flanged to a corresponding opening in the pool wall.
The devices are usually manufactured in stainless steel. The arrangement and distribution of the transducers must be carried out in such a way that a uniformly strong, non-static sound field is created. Some manufacturers therefore modulate the frequency to prevent standing waves. A function for degassing the cleaning fluid is also not uncommon. Here, the output power of the device is pulsed to allow any gas bubbles to rise to the surface. The reason for this is the poor formation of cavitation bubbles in the presence of gas bubbles, as these would absorb the power. The minimum sound power required for a good cleaning result is around 35 to 40 W per litre of cleaning fluid. The sound generators require a minimum distance of 1 to 2 cm in order to form a homogeneous sound field. Contact of the user with an active ultrasound bath leads to an attack of the cell structure and should be avoided.
---
Technology Behind Ultrasonic Cleaning
Piezoceramic transducer systems are used to generate the ultrasonic vibrations. These are glued to the vibrating pan and contain a piezoceramic disc that expands or contracts when a voltage is applied. A high-frequency generator is used to generate ultrasound, which converts the mains frequency from 50 Hz to at least 20 kHz. The resonant frequency of an ultrasonic oscillating system is defined by the size and cannot be changed after manufacture.
Frequencies from 20 kHz to 400 kHz are usually used. Low frequencies around 20 kHz produce bubbles of larger diameter with powerful pressure surges, whereas higher frequencies around 35 kHz are better suited for intensive and gentle cleaning of surfaces. The current frequency for particles with a diameter greater than 1 μm is about 200 kHz (ultrasound, “US”). The frequency range above 400 kHz to 1–2 MHz is also referred to as megasonic (“MS”) in the literature. Here, particles with a diameter of less than 1 μm are optimally detached.
The frequency needed is determined by the smallest opening or structure that can be cleaned by sound. The wavelength in the water must be less than half the diameter of the aperture. Otherwise, the sound will not be able to penetrate and the surface would be smooth and closed for the sound effect.
Application of Ultrasonic Cleaning
The use of solvents, acids or alkalis significantly supports the cleaning effect of ultrasonic baths. In particular, the surface tension of the water must be reduced with a suitable cleaning preparation for effective ultrasonic cleaning. No flammable liquids are used as cleaning agents, as the ultrasound always introduces heat into the liquid, which could lead to an ignition of the bath liquid if the ignition temperature is exceeded.
Ultrasonic baths are also used in sample preparation to tear biological substances in the analytical laboratory, i.e. to fragment them. Ultrasonic baths are used for cell disruption or to shear DNA.
In the medical industry, high-performance ultrasound baths are used for the reprocessing of surgical instruments. Blood and tissue residues can be effectively removed from the joints of the instruments. In addition, ultrasound shortens the necessary exposure time of the disinfectant preparations.
In the automotive industry, ultrasonic baths are used to clean carburettors, spark plugs and injectors. Larger, industrial ultrasonic cleaning systems are used for the cleaning of raw parts, e.g. for surface finishing (electroplating).
