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What is ultrasonic anyway...?

Ultrasonic cleaning is a further cleaning method and is used successfully today in many industrial sectors, in production and in service. Especially since the use of solvents had to be restricted for environmental protection reasons, cleaning with ultrasonic by using aqueous cleaning agents has turned out to be one of the best and safest options.

Fig. 1 PZT vibrating element on basin bottom

Our standard design ultrasonic cleaning devices of 0.8 to 200 litre capacity in conjunction with the appropriate heated rinsing tanks and dryers have proven themselves in cleaning individual and serial parts. Ultrasonic cleaning always occurs in an aqueous cleaning bath, where the cleaning additives are adjusted to the contamination and the material to be cleaned. Ultrasonic frequencies of 33 kHz or 40 kHz are generated electrically and transferred to the cleaning tank by means of vibrating elements. As can be seen in Figure 1, the vibrators are usually attached to the basin bottom from the outside. The ultrasonic vibrator shown in the figure is a lead-zirconate-titanium element, also called a PCT vibrator (Fig. 1). The utilised ceramic material changes its thickness when an electrical potential with alternating polarity is applied, the piezo-electrical principle in reverse. This transfers the electrically generated frequency to mechanical vibrations that are transmitted through an aluminium connection element to the basin bottom which then becomes a mechanically vibrating ultrasonic membrane.

Figures 2-4: Drive shaft BEFORE... and AFTER... ultrasonic cleaning/clock cycle approx. 3 minutes

The capacity of an ultrasonic vibrator is specified in watt or kilowatt. The larger the tank, the more watt or kilowatt are obviously required to achieve a consistent ultrasonic energy density in the entire bath. A rule of thumb is often approx. 10 watt per 1 litre cleaning agent.
The ultrasonic energy introduced to the bath generates the so-called cavitation effect. The first easily identifiable signs of ultrasonic in the bath are:

  • Intensive liquid movements
  • Rapid stirring and mixing
  • Intensive turbulences and swirling  


which are helpful properties enough in supporting the basic cleaning process. But it is the cavitation effect that actually causes the extraordinarily thorough cleaning of the parts that are introduced to an ultrasonic bath. Contaminations are removed even from the smallest cavities or tiniest drill holes - practically blasted off - as a direct outcome of the cavitation effect.

What is cavitation...?

Ultrasonic waves generate periodic pressure and expansion phases in the liquid. (33 kHz = 33,000 times/second.) During the compression phase, the pressure in the liquid presses the molecules together. During the expansion phase, under negative pressure, the molecules are pulled apart. When the ultrasonic energy is high enough, a BUBBLE develops during the expansion phase. In other words: If the negative pressure during the expansion phase of the ultrasonic wave is great enough, the “tensile strength" of the liquid is overcome and a near vacuum is developed in the form of a multitude of cavitation bubbles that are filled with liquid vapour and oscillate in the ultrasonic bath. By nature, these cavitation bubbles are instable. During the ongoing fast expansion and compression phases, the bubbles change their size accordingly. But since the quantity of the “gas exchange” between bubbles and surrounding liquid mainly depends on the size of the interface between bubble and liquid, the gas diffusion into the bubble during the expansion phase is always somewhat greater than the diffusion out of the bubble in the compression phase, because the bubble interface is now smaller. So for every ultrasonic wave, the bubble “soaks up” additional energy and grows further during every expansion phase as compared to the reduction in size during the compression phase. Finally, the bubbles reach their “critical” size in which they are able to very efficiently absorb the ultrasonic energy. This “critical” size depends on the utilised sound frequency. At a frequency of, for example, 20 kHz, the “critical” bubble diameter is roughly 0.170 mm. At 40 kHz, the “critical” diameter is below 0.140 mm. In this “critical” stage, the bubble can grow dramatically during the next sonic wave so that it achieves its energy absorption limit and collapses during the next compression phase:

The bubble implodes. The cavitation effect is the continuous implosion of a large number of bubbles in the entire ultrasonic bath and at all surfaces of the immersed parts to be cleaned. During the implosion of a cavitation bubble, the gases in the bubbles heat up to approx. 5,500°C, called “hotspot”, but for such a brief moment only that the surrounding area is not actually affected. Sooner or later the bath temperature will rise as the ultrasonic energy introduced to the bath is converted 100% to heat.
 
Fig. 5: Photo of a cavitation bubble with a diameter of 0.150 mm during implosion. The asymmetrical collapse is clearly visible.

Simplified, ultrasonic cleaning can be called a microscopic brush. This can be illustrated when cleaning a pair of spectacles or an articulated bracelet in the ultrasonic bath. The cavitation bubbles not only implode at all surfaces, they also implode in the smallest gaps, between glass and the metal frame of the spectacles. Contaminants are removed cloud-like from the gaps.

Fig. 6: Schematic illustration of an implosion. During the “collapse”, liquid flows of speeds up to 400 km/h develop briefly. This is what "blows away" the contaminants during ultrasonic cleaning.

Chemical cleaning additives:

Ultrasonic cleaning (“microscopic brush”) requires suitable chemical cleaning additives, depending on the type of contaminant. The natural and perpetual mixing by the ultrasonic usually requires less quantities of cleaning additives as compared to cleaning without ultrasonic. Biodegradable “green” cleaning agents are available for most cleaning tasks.

Note: Solvents and liquids with a low combustion point may not be used for ultrasonic cleaning because, as explained above, the bath is heated constantly due to the implosions even when the heater is switched off.

Please don’t hesitate to contact us should you have any further questions!

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