How Ultrasonic Cleaning Works

What is an Ultrasonic Cleaner?

An ultrasonic cleaner is a cleaning device that uses ultrasound (usually from 20–400 kHz) and an appropriate cleaning medium (sometimes ordinary tap water) to clean delicate items. The ultrasound can be used with just water, but use of a cleaning medium (e.g. solvent, degreaser, etc.) appropriate for the item to be cleaned enhances the effect.

Ultrasonic cleaners are used to clean many different types of objects, including jewellery, lenses and other optical parts, watches, dental and surgical instruments, tools, coins, fountain pens, golf clubs, firearms, industrial parts and electronic equipment. They are used in many jewellery and watch workshops, and electronic repair workshops.

The Ultrasonic Cleaning Process

Ultrasonic cleaning uses high frequency sound waves to create cavitation bubbles in a liquid. These cavitation bubbles release energy which have a scrubbing effect on contaminants adhering to substrates like metals, glass, and ceramics. This action also penetrates blind holes, cracks, and recesses, and is able to thoroughly remove all traces of contamination tightly adhering or embedded onto solid surfaces. 

Water or other cleaning solutions (e.g. solvent, degreaser, etc.) can be used, depending on the type of contamination and the item to be cleaned. 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, blood and other biological material, and so on.

Ultrasonic cleaning can be used for a wide range of shapes, sizes and materials. There must be sufficient space around the object in the ultrasonic cleaner so that the cleaning medium can reach all the areas to be cleaned.

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 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; 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.

Low Frequency vs. High Frequency

The lower the frequency, the larger and more powerful the cavitation bubbles. This provides stronger cleaning at the expense of being able to reach very narrow spaces.

Some examples of machines that use (relatively) low frequency ultrasound are the Elma Xtra Basic series and the Elma XL series. These machines can have frequencies as low as 25 kHz and are good for powerful cleaning of stubborn dirt.

The higher the frequency, the smaller and more gentle the cavitation bubbles. This provides very fine and thorough cleaning at the expense of speed and power.

Some examples of machines that use (relatively) high frequency ultrasound are the Elmasonic P series (up to 80 kHz) and the Elma TI-H series (up to 130 kHz). These machines are good for fine cleaning of small components such as electronic parts. They are also good to use for sensitive materials that may be damaged by more powerful cleaning forces.

Cleaning Solution

The often harsh chemicals used as cleaners in many industries are not needed, or used in much lower concentrations, with ultrasonic cleaning, as the cavitation does much of the cleaning work. However, cleaning solutions contain 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).

Some better machines recycle the cleaning solutions. These machines use filters (with pumps) to recycle the cleaning solutions, and some of them are arranged in a cascade in multi-tank systems. Purchase price is higher than simpler machines, but such machines are economical in the long run, as the same cleaning solution can be reused many times, minimising wastage and pollution.