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What is ultrasonography?

Ultrasonic cleaning is another cleaning method and is now successfully used in many industrial sectors, in production and in service. Especially since the use of solvents had to be restricted to protect the environment, ultrasonic cleaning using aqueous cleaning fluids emerged as one of the best and safest alternatives.

Our ultrasonic cleaning units in standard design from 0.8 to 200 litres capacity, in connection with suitable heated rinsing tanks and dryers have proven themselves in the cleaning of individual and series parts. Ultrasonic cleaning is always carried out in an aqueous cleaning bath, whereby the cleaning additives are matched to the contamination and the material to be cleaned. Ultrasonic frequencies of 33kHz or 40kHz are generated electrically and transmitted to the cleaning tank by means of oscillator elements. As can be seen in Figure 1, the transducers are usually attached to the bottom of the tub from the outside. The ultrasonic transducer shown in the diagram is a lead-zirconate-titanium element, also called a PZT transducer (Fig.1). The ceramic material used changes its thickness when an electrical voltage with alternating polarity is applied, in inversion of the piezo-electric principle. In this way, the electrically generated frequency is converted into mechanical vibrations, which are transmitted via an aluminium connecting part to the tank bottom, which thus becomes a mechanically vibrating ultrasonic membrane.

Abbildung 2-4: drive shaft BEFORE… and AFTER… ultrasonic cleaning/cycle time approx. 3 minutes

The power of an ultrasonic transducer is expressed in watts or kilowatts. Thus, the larger the tank, the more watts or kilowatts are obviously required to achieve a consistent ultrasound energy density throughout the bath. As a rule of thumb, approx. 10 watts per 1 litre of cleaning liquid is often given.
The ultrasonic energy introduced into the bath creates the so-called cavitation effect. The initially easily recognisable signs of ultrasound in the bath are:

  • intense fluid movements
  • fast mixing and blending
  • intense turbulence and swirls

helpful properties in themselves to support the basic purification process. But it is the cavitation effect that actually brings about such extraordinarily thorough cleaning of the parts that are placed in an ultrasonic bath. Even in the smallest cavities, in tiny holes, impurities are removed, blasted away, as it were, as a direct result of the cavitation effect.

What is cavitation…?

Ultrasonic waves generate periodic pressure and expansion phases in the liquid. (33kHz = 33,000 times/second). During the compression phase, the molecules are pressed together by the pressure in the liquid. During the expansion phase, under negative pressure, the molecules are pulled away from each other. If the ultrasound energy is high enough, then a bubble is formed during the expansion phase. In other words, if the negative pressure is sufficiently high during the expansion phase of the ultrasonic wave, the “tensile strength” of the liquid is overcome and a near-vacuum is created in the form of a multitude of liquid vapour-filled cavitation bubbles oscillating in the ultrasonic bath. By nature, these cavitation bubbles are unstable.

During the more advanced, rapid expansion and compression phases, the bubbles change size accordingly. However, since the quantity of the “gas exchange” between the bubble and the surrounding liquid depends mainly on the size of the separation area between the bladder and the liquid, the gas diffusion into the bubble during the expansion phase is always somewhat greater than the diffusion out of the bubble during the compression phase, because the bubble separation area is now smaller. Thus, with each ultrasound wave, the bladder “absorbs” additional energy and grows a little more during each expansion phase, compared to the reduction in size during the compression phase. Eventually, the bubbles reach their “critical” size where they can absorb the ultrasound energy very effectively.

This “critical” value depends on the sound frequency used. At 20kHz frequency, for example, the “critical” bladder diameter is about 0.170mm. At 40kHz the “critical” diameter is below 0.140mm. At this “critical” stage, the bladder can grow dramatically during the next sound wave, so that it reaches its energy absorption limit and collapses during the next compression phase:

Fig. 5: Photo of a 0.150 mm diameter cavitation bubble during implosion. The asymmetrical collapse is clearly visible.

The bladder implodes. Cavitation effect means the continuous implosions of a multitude of bubbles in the entire ultrasonic bath and on all surfaces of the immersed parts to be cleaned. During the implosion of a cavitation bubble, the gases in the bubble heat up to about 5,500ºC, called a “hotspot”, but for such a short moment that the immediate surroundings are not actually affected. Über kurz oder lang wird die Badtemperatur jedoch ansteigen, da letztlich die ins Bad eingebrachte Ultraschallenergie zu 100% in Wärme umgesetzt wird.

Fig. 6: Schematic representation of an implosion. During the “collapse”, liquid flows of speeds up to 400km/h occur for a short time. In this way, impurities are “blown away” during ultrasonic cleaning.

In simplified terms, ultrasonic cleaning can be described as a microscopic brush. This can be illustrated when cleaning a pair of glasses or a link bracelet in an ultrasonic bath.The cavitation bubbles not only implode on all surfaces, they also implode in the smallest gaps, between the glass and the metal frame of the glasses. Impurities are removed from the interstices like clouds.

Chemical cleaning additives

For ultrasonic cleaning (“microscopic brush”), suitable chemical cleaning additives are required, depending on the type of contamination. Due to the natural and constant mixing by ultrasound, smaller quantities of cleaning additives are usually required compared to cleaning without ultrasound. Environmentally friendly, biodegradable, “green” cleaning agents are available for most cleaning tasks.

Note: Solvents and liquids with a low focal point must not be used for ultrasonic cleaning because, as explained above, the implosions cause the bath to heat up continuously even when the heating is switched off.

 

Please do not hesitate to contact us if you have any further questions!