What you should know about Ultrasound........

Ultrasonic cleaning is widely used in many fields of industrial production, in manufacture and in service industries. Especially since the use of solvents had to be restricted for environmental reasons, ultrasonic cleaning and aqueous cleaning liquids offered themselves as one of the safest and best alternatives. Our standard range of ultrasonic cleaning units from 0.8 to 300 litres together with rinse tanks and dryers are well suited to cover most batch applications whenever cleaning of objects or parts is required.

Ultrasonic cleaning is always carried out in a fluid, specially selected to suit both the material of the goods to be cleaned and the particular type of contamination  to be removed. Ultrasound frequencies of 33kHz or 40kHz are electrically generated and transferred into the cleaning tank by transducers. As the enclosed schematic picture shows, transducers are often bonded from outside to the tank bottom. The transducer type shown schematically in Picture 1 is a lead-zirconate-titanate (also known as PZT)  transducer. It uses ceramic material, which changes its thickness according to the reversed piezo-electric principle, when a voltage with alternating polarity is applied. The mechanical movements thus obtained are transferred via a transfer block of aluminium, which in turn oscillates the tank bottom at the electrically generated frequency

Transducers bonded to tank bottom

  Component shaft

 Before....        and  After .....             Ultrasonic Cleaning   

• (Cycle Time approx.: 3 min)

The output of ultrasonic transducers is measured in Watts or Kilowatts. The bigger the tank, the more Watts or Kilowatts are obviously required to obtain the necessary energy density throughout the cleaning bath. An average specification often  used is approx. 10 Watts per litre of cleaning fluid.

The ultrasonic energy introduced into the cleaning fluid causes the so-called cavitation effect. The obvious signs of ultrasound being introduced into the liquid are:

 What is Cavitation......?

Ultrasound waves create a periodic compression and expansion in the fluid. During the compression phase a positive pressure acts on the fluid, which moves the molecules together. During the expansion phase, a negative pressure pulls the molecules apart. If the intensity of the ultrasonic energy is high enough, then, during the expansion phase a BUBBLE is created. In other words, if during the expansion phase of the ultrasonic wave the negative pressure is high enough, the "tensile strength" of the fluid is overpowered, a near vacuum in form of a multitude of liquid-steam filled CAVITATION BUBBLES are created and oscillate in the liquid agitated by ultrasound. These cavitation bubbles are by nature unstable.

During the continuing rapid expansion and compression phases they change accordingly in size. Since, however, the quantity of "gas" being exchanged between bubble and surrounding liquid depends mainly on the outer surface size of the bubble, the diffusion into the bubble during the expansion phase is a little higher than the diffusion during the compression phase, because now the outer bubble surface is smaller. The bubbles therefore "suck" in additional energy during every ultrasonic wave cycle and thereby always grow in size a little more during the expansion phase of the bubble compared to the size reduction during the compression phase. Finally the growing bubbles reach their "critical" size, which allows them to absorb the ultrasonic energy very efficiently. This critical size depends on the ultrasound frequency  employed.. At 20kHz frequency for example, the "critical" bubble Diameter is about 0.170 mm. At 40kHz the critical size is reached at below 0.140 mm. At this "critical" stage, one further wave cycle allows the bubble to grow dramatically in one step, and unable to absorb any further energy it collapses during the

• onset of the compression phase:  An implosion takes place.
• The cavitation effect is this continuing implosion of a multitude of bubbles through- out the ultrasonic bath and on all surfaces of immersed goods to be cleaned.
• During the implosion of a cavitation bubble the gases inside the Bubble heat up to about 5500º C, called a hot-spot, but for only such a short time, that the surrounding liquid or the submersed surfaces are not really affected.  Over a period the bath temperature will however rise, as basically 100% of the ultrasound energy activated in the liquid will become heat.

• The asymmetrical caving-in is clearly visible.

• Colloquially ultrasonic cleaning is compared with a microscopic brush.  This can be illustrated for example by cleaning a pair of steel rimmed spectacles in an ultrasonic cleaning bath. The cavitation bubbles implode not only on the main surfaces, they also implode in the tight space between the glass and the steel rim. Clouds of dirt are blasted out from under the steel rim vigorously.

• Picture 6:
• The schematic drawing  is a further illustration of what happens during the implosion. The caving- in results in a tiny amount of jetted liquid at  400km/hour speed. This is how ultrasonic cleaning "blasts" particles, dirt, contamination off during the cleaning process.

Whilst ultrasound offers the mechanical brush effectively, it still requires chemical cleaning agents in the fluid, to dissolve grease or other contamination which a mechanical brush alone could not remove. There is an increasing variety of "green", environmentally safe cleaning agents or chemicals available.

Usually, because of the natural agitation of the liquid by ultrasound, the amount of chemicals needed is considerably lower than in a cleaning process without Ultrasound.

Note:

• Because of the heat generated by ultrasound as explained, solvents or liquids with low flash point must not be used in Ultrasonic Cleaning operations..
• Please ask for further Information on our wide range of Ultrasonic Cleaning Units, Rinse Tanks and Hot Air Condenser Dryer Units.