Ultrasonic cleaning

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Sonorex ultrasonic cleaner from the 1970s or 1980s
Ultrasonic cleaning of a mobile phone

Ultrasonic cleaning is a process that uses

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 0.5 litres (0.13 US gal), 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.[1]

Ultrasonic cleaners are used to clean many different types of objects, including industrial parts,

gramophone records, industrial machined parts, and electronic equipment, optical lenses, etc. They are used in many jewelry workshops, watchmakers' establishments, electronic repair workshops,[2]
and scientific labs.

History

Ultrasonic cleaning has been used industrially for decades,[when?] particularly to clean complex shape parts and/ or having small intricate holes/galleries, and to accelerate surface treatment processes.[3]

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. US patent 2815193, issued December 1954 , 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. US 2651148 .

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.[4][5]

Process characteristics

Ultrasonic cleaning uses

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.[6]

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.[2]

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;[7][8] 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.

Ultrasonic transducers showing ~20 kHz and ~40 kHz stacks. The active elements (near the top) are two rings of lead zirconate titanate, which are bolted to an aluminium coupling horn.

Transducers are usually piezoelectric (e.g. made with lead zirconate titanate (PZT), barium titanate, etc.), but are sometimes magnetostrictive. 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 are also used in many medical and dental techniques and industrial processes.

Cleaning solution

In some circumstances, ultrasonic cleaners can be used with plain water, but in most cases, a

printed circuit boards
, removing biological material, and so on).

Reduction of

alkaline detergent solution may be specifically recommended. Solutions are typically heated, often around 50–65 °C (122–149 °F), however, in medical applications, it is generally accepted that cleaning should be at temperatures below 45 °C (113 °F) to prevent protein coagulation
that can complicate cleaning.

Some ultrasonic cleaners are integrated with

vapour degreasing
machines using 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 descends into the upper tank. The upper tank eventually overflows and relatively clean fluid runs into the work tank where the cleaning takes place. The purchase price is higher than simpler machines, but such machines may be more economical in the long run. The same fluid can be reused many times, minimising wastage and pollution.

Uses

Most hard, non-absorbent materials (metals, plastics, etc.) not chemically attacked by the cleaning fluid are suitable for ultrasonic cleaning. Ideal materials for ultrasonic cleaning include small electronic parts, cables, rods, wires, and detailed items, as well as objects made of glass, plastic, aluminium, or ceramic.[11]

Ultrasonic cleaning does not sterilize the objects being cleaned, because spores and viruses will remain on the objects after cleaning. In medical applications, sterilization normally follows ultrasonic cleaning as a separate step.[12]

Industrial ultrasonic cleaners are used in the automotive, sporting, printing, marine, medical, pharmaceutical, electroplating, disk drive components, engineering and weapons industries.

Ultrasonic cleaning is used to remove contamination from industrial process equipment such as pipes and heat exchangers.

Limitations

Ultrasonic cleaning is used widely to remove flux residue from soldered circuit boards. However, some electronic components, notably

microphones can become damaged or destroyed by the high-intensity vibrations they are subjected to during cleaning. Piezoelectric buzzers
can work in reverse and produce voltage, which may pose a danger to their drive circuits.

Safety

See also

References

  1. ISBN 0766826600
    .
  2. ^
    ISBN 978-0-8247-5830-1
    .
  3. ^ Phillion, R. (June 2011). "The Application of Industrial Scale Ultrasonic Cleaning to Heat Exchangers" (PDF). Heat Exchanger Fouling and Cleaning.
  4. PMID 26054698
    .
  5. ^ Wahl, Paul (March 1970). "Put Sound Waves to Work in Your Shop". Popular Science. Retrieved 20 December 2011.
  6. ^ Robert H. Todd, Dell K. Allen, and Leo Alting; Manufacturing Processes Reference Guide
  7. doi:10.1021/j100103a027
    .
  8. ^ Azar, Lawrence (February 2009). "Cavitation in ultrasonic cleaning and cell disruption" (PDF). Controlled Environments.
  9. ^ William, Brooke (Feb 2001). "End of Lease Cleaning - Optimizing Ultrasonic Cleaning with Solutions". Retrieved November 18, 2012.{{cite web}}: CS1 maint: numeric names: authors list (link)
  10. PMID 26054698
    .
  11. ISBN 9780788114519
    .
  12. ISBN 978-1-4180-3021-6.{{cite book}}: CS1 maint: multiple names: authors list (link
    )
  13. ^ "Ultrasonic Cleaner Operator's Manual" (PDF). Branson. Retrieved November 2, 2018.
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