An ultrasonic cleaner, sometimes called a sonicator, is a cleaning device that uses ultrasound (usually from 20–400 kHz) and an appropriate cleaning solvent (sometimes ordinary tap water) to clean delicate items. The ultrasound can be used with only water but solvent is advised; it enhances the effect of a solvent appropriate for the item to be cleaned and the soiling.
Ultrasonic cleaners are often used to clean jewellery, lenses and other optical parts, watches, dental and surgical instruments, fountain pens, industrial parts and electronic equipment. They are used in many jewellery workshops, watchmakers' establishments, and electronic repair workshops.
Process characteristics
Ultrasonic cleaning uses high frequency sound waves to agitate in an aqueous solution or organic compound. Cavitation bubbles induced by the agitation act on contaminants adhering to substrates like metals, plastics, glass, rubber, and ceramics. This action also penetrates blind holes, cracks, and recesses. The intention is to thoroughly remove all traces of contamination tightly adhering or embedded onto solid surfaces. Water or other solvents can be used, depending on the type of contamination and the workpiece. Contaminants can include dust, dirt, oil, pigments, grease, polishing compounds, flux agents, fingerprints, soot wax and mold release agents, biological soil like blood, and so on. Ultrasonic cleaning can be used for a wide range of workpiece shapes, sizes and materials, and may not require the part to be disassembled prior to cleaning.
In an ultrasonic cleaner, the object to be cleaned is placed in a chamber containing a suitable solution (in an aqueous or organic solution, depending on the application). In aqueous cleaners, the chemical added is a surfactant which breaks down the surface tension of the water base. An ultrasound generating transducer built into the chamber, or lowered into the fluid, produces ultrasonic waves in the fluid by changing size in concert with an electrical signal oscillating at ultrasonic frequency. This creates compression waves in the liquid of the tank which ‘tear’ the liquid apart, leaving behind many millions of microscopic ‘voids’ or ‘partial vacuum bubbles’ (cavitation). These bubbles collapse with enormous energy; temperatures and pressures on the order of 5,000 K and 20,000 lbs per square inch are achieved [citation needed]; however, they are so small that they do no more than clean and remove surface dirt and contaminants. The higher the frequency, the smaller the nodes between the cavitation points, which allows for cleaning of more intricate detail.
Transducers are usually made of piezoelectric material (e.g. lead zirconate titanate or barium titanate), and sometimes magnetostrictive (made of a material such as nickel or ferrite). The often harsh chemicals used as cleaners in many industries are not needed, or used in much lower concentrations, with ultrasonic agitation. Ultrasonics are used for industrial cleaning, and also used in many medical and dental techniques and industrial processes.
Cleaning solution
Ultrasonic activity (cavitation) helps the solution to do its job; plain water would not normally be effective. The cleaning solution contains ingredients designed to make ultrasonic cleaning more effective. For example, reduction of surface tension increases cavitation levels, so the solution contains a good wetting agent (surfactant). Aqueous cleaning solutions contain detergents, wetting agents and other components, and have a large influence on the cleaning process. Correct composition of the solution is very dependent upon the item cleaned. Solutions are mostly used warm, at about 50–65 °C (122–149 °F), however, in medical applications it is generally accepted that cleaning should be at temperatures below 38 °C (100 °F) to prevent protein coagulation.
Water-based solutions are more limited in their ability to remove contaminants by chemical action alone than solvent solutions; e.g. for delicate parts covered with thick grease. The effort required to design an effective aqueous-cleaning system for a particular purpose is much greater than for a solvent system.
Some better machines (which are not unduly large) recycle the hydrocarbon cleaning fluids. Three tanks are used in a cascade. The lower tank containing dirty fluid is heated causing the fluid to evaporate. At the top of the machine there is a refrigeration coil. Fluid condenses on the coil and falls into the upper tank. The upper tank eventually overflows and clean fluid runs into the work tank where the cleaning takes place. Purchase price is higher than simpler machines, but such machines are economical in the long run. The same fluid can be reused many times, minimising wastage and pollution. Carbon tetrachloride (CCl4, also formerly used in fire extinguishers for electrical fires) was used in the past, but is now prohibited as dangerous. If CCl4 fumes are inhaled through a lit cigarette, carbonyl chloride (COCl2, also called phosgene, a poison gas used in warfare) could be produced.
Uses
Most hard, non-absorbent materials (metals, plastics, etc.) not chemically attacked by the cleaning fluid are suitable for ultrasonic cleaning.
Industrial ultrasonic cleaners are used in the automotive, sporting, printing, marine, medical, pharmaceutical, electroplating, disk drive components, engineering and weapons industries.
Ultrasonic baths are also used to experimentally determine the elastic constants of many anisotropic materials. Ultrasonic waves can usually only be sent through a material at right angles to the material's surface (normal incidence). In water the angle of incidence for a longitudinal wave can be set, inducing both longitudinal and transverse waves in the material. Then, by measuring the time of flight for both waves, the elastic constants can be determined