Ultrasonic cleaning is a process that uses ultrasound (usually from 20 to 40 kHz) to agitate a fluid, with a cleaning effect. Ultrasonic cleaners come in a variety of sizes, from small desktop units with an internal volume of less than , to large industrial units with volumes approaching 1,000 litres (260 US gal).
The principle of the ultrasonic cleaning machine is to convert the sound energy of the ultrasonic frequency source into mechanical vibration through the transducer. The vibration generated by the ultrasonic wave is transmitted to the cleaning liquid through the cleaning tank wall so that the micro-bubbles in the liquid in the tank can keep vibrating under the action of the sound wave, destroying and separating the dirty adsorption on the surface of the object.
Depending on the object being cleaned, the process can be very rapid, completely cleaning a soiled item in minutes. In other instances, cleaning can be slower, and exceed 30 minutes.
Ultrasonic cleaners are used to clean many different types of objects, including industrial parts, jewelry, scientific samples, lenses and other optical parts, watches, dental and surgical instruments, tools, coins, fountain pens, golf clubs, fishing reels, window blinds, firearm components, car fuel injectors, musical instruments, gramophone records, industrial machined parts, and electronic equipment, optical lenses, etc. They are used in many jewelry workshops, watchmakers' establishments, electronic repair workshops, and scientific labs.
History
Ultrasonic cleaning has been used industrially for decades, particularly to clean complex shape parts and/ or having small intricate holes/galleries, and to accelerate surface treatment processes.
It appears that ultrasonic cleaners developed as a natural evolution of several earlier inventions that used vibrations to agitate and mix substances, and thus there is no clear "inventor" of ultrasonic cleaning. , is the earliest patent on record that specifically uses the term "Ultrasonic cleaning" although earlier patents refer to the use of ultrasound for "intense agitation," "treatment" and "polishing," e.g. .
By the mid-1950s there were at least three ultrasonic cleaner manufacturers established in the United States and two in the United Kingdom; and by the 1970s ultrasonic cleaners were widely established for industrial and domestic use.
Process characteristics
Ultrasonic cleaning uses cavitation bubbles induced by high-frequency pressure (sound) waves to agitate a liquid. The agitation produces high forces 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, rust, grease, algae, fungus, bacteria, lime scale, 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.
Objects must not be allowed to rest on the bottom of the device during the cleaning process, because that will prevent cavitation from taking place on the part of the object not in contact with solvent.
Design and operating principle
In an ultrasonic cleaner, the object to be cleaned is placed in a chamber containing a suitable solution (in an aqueous or organic solvent, depending on the application). In aqueous cleaners, surfactants (e.g., laundry detergent) are often added to permit dissolution of non-polar compounds such as oils and greases. 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'/'partial vacuum bubbles' (cavitation). These bubbles collapse with enormous energy; temperatures and pressures on the order of 5,000 K and 135 MPa are achieved; 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.