Everyone has been exposed to cleaning, which seems to be very ordinary and ordinary, but it is not an easy task for some special cleaning objects. Because of its cavitation in liquid, ultrasonic wave can achieve outstanding cleaning effect and has a unique position in some occasions with difficult cleaning and high quality requirements.
The transduce converts electric energy of ultrasonic frequency into mechanical vibration, and radiate ultrasonic waves to that cleaning liquid contained in the clean tank through the wall of the cleaning tank. Microbubbles (called cavitation nuclei) existing in liquid vibrate under the action of sound waves. When the sound pressure or intensity reaches a certain value, the bubbles grow rapidly and then suddenly close. When the bubble closes, a shock wave is generated, and the pressure and local high temperature around the bubble are1012 ~10/3pa. This physical phenomenon is called ultrasonic cavitation. The great pressure generated by cavitation can destroy insoluble pollutants and disperse them in solution. On the one hand, the direct and repeated impact of steam cavitation on the dirt layer will destroy the adsorption of dirt and clean the surface of parts, on the other hand, it will also cause fatigue damage and separation of the dirt layer. The vibration of bubbles scrubs the solid surface. Once the dirt layer has cracks to drill, bubbles can also "drill into" the cracks to vibrate, so that the dirt layer falls off. Due to cavitation, the two liquids quickly disperse and emulsify at the interface. When solid particles are wrapped by oil and attached to the surface of cleaning parts, the oil is emulsified and the solid particles fall off by themselves. When the ultrasonic wave propagates in the cleaning liquid, it will produce alternating sound pressure and impact the cleaning part. At the same time, due to the nonlinear effect, acoustic flow and micro-acoustic flow will be generated, while ultrasonic cavitation will produce high-speed micro-jet at the solid-liquid interface. All these actions can destroy dirt, remove or weaken the boundary dirt layer, increase stirring and diffusion, accelerate the dissolution of soluble dirt, and strengthen the cleaning effect of chemical cleaning agents. It can be seen from the introduction that all places where liquid can be soaked and sound field exists have cleaning function, with fast cleaning speed and high quality. It is especially suitable for careful cleaning of cleaning parts with complex surface shapes such as cavities and slits, and it is easy to realize cleaning automation. In some cases, water agent can be used instead of organic solvent for cleaning, or the concentration of acid and alkali can be reduced. For some cleaning that is harmful to human health, such as cleaning radioactive dirt, remote control or automatic cleaning can be realized. Ultrasonic cleaning also has its limitations, for example, it has a good cleaning effect on materials with strong sound reflection such as metal, ceramics and glass, but it has a poor cleaning effect on materials with high sound absorption such as cloth, rubber and dirt with high viscosity.
Ultrasonic cleaning began in the early 1950s. With the development of technology, its application scope is also expanding. At present, it has been widely used to clean semiconductor devices, electron tube parts, printed circuits, relays, switches, filters and so on in the electronic and electrical industries. Used to clean gears, bearings, oil pump nozzles, fuel filters, valves and other mechanical parts in the mechanical industry, such as engines and missile parts, as small as watch parts; Used for cleaning all kinds of lenses, glasses and frames, medical glassware, needles and surgical instruments in optical and medical instruments; Used in textile industry to clean spinnerets, food bottles, lids, molds and carving crafts.
The above-mentioned example of ultrasonic cleaning generally uses low-frequency ultrasonic waves of 20 ~ 40 kHz. In recent years, the United States has developed a technology called megasonic cleaning, that is, using 1MHz high-frequency ultrasonic to clean large-scale integrated circuits, which can remove pollutants smaller than1μ m. At this time, the cleaning mechanism is not ultrasonic cavitation, but mainly the vibration speed and sound flow of particles. In 1990s, American Crest Company claimed that they invented the 68kHz ultrasonic cleaning floppy drive, which was twice as efficient as 40kHz. ?
Ultrasonic cleaning in China has achieved fruitful results in the following aspects:
1) Cleaning of mechanical parts before and after electroplating or before spraying, and cleaning of disassembled and repaired parts. Require high cleanliness, such as the cleaning of oil pump nozzle fittings and bearings.
2) Cleaning of signal control relays, components, picture tubes and electric vacuum devices of printed circuit boards, silicon wafers and railway systems.
3) Cleaning and sampling glass plates of optical systems such as microscopes and telescopes.
4) Cleaning of oral cavity, surgical instruments and other medical instruments, and cleaning of medicine bottles and some food bottles, dispensing rooms and bottles and cans used in biochemical laboratories.
5) Cleaning of spinnerets, spectacle frames, molds, carving crafts, etc.
Recently developed ultrasonic cleaning of automobile chassis frame, combined with special cleaning solution, can complete rust removal, oxide film removal and phosphating at one time, and can be painted after drying, which overcomes the characteristics of manual rust removal and strong acid cleaning in the past, not only improves working conditions, but also reduces environmental pollution.
There are two kinds of ultrasonic cleaning equipment: one is equipment using aqueous solution as cleaning agent, including ultrasonic frequency power supply, ultrasonic transducer, corresponding ultrasonic container and electric heater; One is single tank type, and the other is multi-tank (multi-station) continuous cleaning equipment, often with timing and automatic conveying devices. The other is to use volatile organic solutions, such as trichloroethylene, trichloroethane and freon, as cleaning agents. In addition to the above components, this kind of equipment needs to be equipped with condensation circulation, oil-water separation and organic solution filtration and recovery. At present, all kinds of special cleaning equipment and complete sets of equipment produced online are mainly produced and developed at home and abroad. It also produces desktop and compact cleaning equipment, such as medical cleaners and small household cleaners for jewelry and glasses.
The frequency of ultrasonic wave is generally 20 ~ 50 kHz, the electric power of small equipment is only a few days, and the large complete equipment reaches tens of kilowatts to hundreds of kilowatts. From 1950s to 1960s, magnetostrictive transducers were mostly used as ultrasonic transducers, and at present, piezoelectric transducers with high efficiency are mostly used. In the past, electronic tube devices were used as the power source of ultrasonic frequency, but now they have been eliminated and solid-state devices are used. The equipment made of this device has high efficiency and small volume. The combined structure can increase the power and create good conditions for the popularization and application of high-power ultrasound.
There are more than 40 companies producing ultrasonic cleaning equipment in the United States, of which about 10 is well-known. Western European countries mainly produce cleaning equipment in Britain, France, Germany and Switzerland. There are about 10 major manufacturers of ultrasonic cleaning equipment in China, and there are countless small-scale individual companies.
Foreign manufacturers attach great importance to the supporting supply of chemical cleaning agents and the consulting service of cleaning technology, which most domestic manufacturers lack. Domestic manufacturers mainly consider the production of ultrasonic cleaning equipment, and few specialized departments are engaged in the development of cleaning agents to meet the needs of different cleaning objects. If ultrasonic cleaning equipment and chemical cleaning agent can be well combined, better results will be achieved, because the main physical mechanism of ultrasonic cleaning is ultrasonic cavitation, and the cavitation intensity is related to the viscosity coefficient, surface tension, vapor pressure, temperature and other parameters of cleaning solution besides the ultrasonic power density and frequency. At present, from the perspective of environmental protection, water-based cleaning agents are required to replace organic solvents for ultrasonic cleaning, but many domestic manufacturers have not been able to adapt to this challenge and need cooperation and development in many aspects.