Ultrasonic cleaning

An ultrasonic cleaner is a cleaning device that uses ultrasound (usually from 15-400 kHz) and an appropriate cleaning solution to clean delicate items. The ultrasound is not effective without the cleaning solution; it enhances the effect of a solution appropriate for the item to be cleaned and the soiling.

They are often employed for cleaning of jewellery, lenses and other optical parts, coins, watches, dental and surgical instruments, fountain pens, industrial parts and electronic equipment. In everyday use such devices may be found in use in most jewelry workshops, watchmakers establishments, or in cellular phone repair workshops (where it could be used for cleaning a phone that has been exposed to enough moisture to hinder its operation).

Design and operating principle

In an ultrasonic cleaner, the object to be cleaned is placed in a chamber containing a suitable ultrasound conducting fluid (an aqueous or organic solvent, 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 of 10,000 K and pressures of 50,000 lbs per square inch have been reportedFact|date=December 2007; 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 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

Dirt adheres to the parts, otherwise it would fall off. The cleaning solution should be basically suitable for the job without ultrasonics. 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 should contain 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. The solution should not react in an undesirable way with the item being cleaned. A warm solution is best, at about 50-65°C.

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.

Flammable solutions should not be used, as they may be hazardous with the high local temperatures generated. Acids and bleaches should only be used if they will not react either with the item being cleaned or the cleaning tank. Solutions should be replenished when ther are noticeably spent.

To test for effectiveness, draw an "X" with an HB (normal) pencil across a wet frosted glass slide. Immerse it, and turn on. The "X" should start to fade immediately, and should be all gone within 10 seconds.To test for evenness of cleaning, take 3 pieces of aluminium foil of about 10×20 cm. Suspend the 3 pieces in the tank, one in the middle and the othe two near opposite edges. Run the ultrasonics for about 10 minutes. The 3 pieces should be about equally perforated and wrinkled.

After cleaning, items should be cleaned with water, either in the cleaner or separately. [http://www.coleparmer.co.uk/techinfo/techinfo.asp?htmlfile=ultrasoniccleaner_faq.htm&ID=792]

If the item cleaned may be damaged by leaving it wet after rinsing, it can be warmed or centrifuged to hasten drying.

Uses

Industrial ultrasonic cleaners are used in the automotive, sporting, printing, marine, medical, pharmaceutical, electroplating, disk drive components, engineering and weapons industries. Cleaners 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 longidunal 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 determinedFact|date=September 2008.

Suitable materials for ultrasonic cleaning are stainless and mild steel, aluminium, copper, brass and other alloys, wood, plastics, rubber, and cloth.

Devices for home and hobby use are readily available from about US$20 up as of January 2007.

ee also

* Parts cleaning
* Acoustic cleaners
* Washing machine

External links

* [http://www.prosysmeg.com/technology/articles/megasonics_cleaning.php A supplier of cleaners at exceptionally high frequency, commercial name Megasonic Cleaning]
* [http://www.nationaljewelerssupplies.com/page/NJS/CTGY/ultrasonics A commercial supplier of ultrasonic cleaning equipment for jewellers]
* [http://www.pprc.org/pubs/techreviews/aqueous/aqtech.html Technical Issues and Aqueous Cleaning Systems (not limited to ultrasonic only)]


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