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Efficient Cleaning with Ultrasound
Not only can coarse contamination be removed by means of ultrasonic cleaning, highly sensitive, finely structured parts can be cleaned as well. Image source: Weber Ultrasonics
During the cleaning bath, the liquid is broken up through exposure to ultrasonic waves caused by the alternating sound pressure intensity during the pulling phase of the oscillation cycle. Cavitation bubbles are formed as a result. Image source: Bandelin
Ultrasonic cleaning systems operated with aqueous media are frequently used for micro-cleaning of sensitive components. Thanks to their modular design, they can be precisely matched to the actual cleaning task. Image source: Amsonic
The working frequency of the sonic transducers is dictated by their dimensions. Image source: Bandelin
As a rule, an ultrasonic power rating of 8 to 10 w/L of bath liquid ensures good cleaning results. Image source: Fisa-Schall
Nearly every manufacturing company needs to do some sort of part cleaning. But regardless of the application, including in the fields of maintenance, repair and operations, selecting the best method of cleaning requires some consideration. Often the process can be executed effectively, economically and ecologically with ultrasound.
Ultrasound technology has been in commercial use for about 100 years. The sinking of the Titanic after it collided with an iceberg led to the development of an ultrasonically supported positioning system for maritime traffic, and a variety of other applications for the use of sound waves evolved thereafter, including ultrasonic cleaning. Its mode of operation allows for environmentally sound removal of particulates and film-like contamination, but also is gentle on the material to be cleaned. The process is effective even for parts with difficult-to-access hollow spaces, such as blind holes, knurling and grooves. Complete subassemblies such as gear units no longer need to be dismantled for cleaning.
Micro Scrubbing Based on Liquid Currents
Ultrasonic waves develop their full cleaning effectiveness in a liquid bath. When a liquid is subjected to ultrasonic sound, the intensity of the alternating sound pressure during the pulling phase of the oscillation cycle breaks up the liquid, and the cohesive forces are overcome. This causes the formation of millions of microscopic bubbles. During the subsequent pushing phase, these cavitation bubbles become unstable and collapse (implode). They generate hydraulic impacts with high-energy densities, thus causing micro-currents in the liquid. When micro-currents strike a surface, they blast off contamination that has been partially dissolved with a cleaning agent and rinse it away.
During the ultrasound process, cavitation occurs at boundary surfaces, which is where contamination tends to adhere to the surface. This cleaning effect is also known as “micro-scrubbing” or “electronic brushing.” Ultrasonic cleaning is just as thorough as it is gentle, because the effects of cavitation, when applied for a short time, leave even the most sensitive surfaces unscathed.
Ideal Oscillation
The sound waves are produced by a generator, which converts normal electrical mains frequency of 50 to 60 Hz into high-frequency oscillation. This electromagnetic oscillation is then transformed into mechanical oscillation of the same frequency by sonic transducers. The dimensions of the oscillator are directly related to the working frequency.
The sound waves spread longitudinally through the cleaning bath, resulting in aphonic and phonic zones. Ultrasonic oscillation can be more evenly distributed in the cleaning liquid with the help of frequency modulation (sweep function).
Effects of Frequency
The ultrasonic frequency has a significant influence on cleaning results. The lower the frequency, the larger the cavitation bubbles and thus the more energy they release. Low frequencies exert powerful cleaning forces on the surface of the part but have only minimal depth penetration for cleaning pores and structures in boundary surfaces. Here are reference values for the selection of ultrasonic frequencies:
- 25 to 35 kHz: removal of particles, grease, oil and other contamination from hard, unpolished surfaces, such as in engine cleaning;
- 40 to 80 kHz: final cleaning tasks, and cleaning of porous and polished surfaces;
- 120 kHz and higher: micro-cleaning tasks, and cleaning of finely porous and high-gloss, polished surfaces;
- 500 kHz: cleaning of finely structured and sensitive surfaces, such as wafers and components from the fields of micro- and nano-technology, LCD technology and photovoltaics.
In the case of components with highly complex shapes, multi-frequency or mixed frequency ultrasonic systems often are used. The mix of larger and smaller cavitation bubbles results in ideal cleaning forces at outside surfaces and boundary surfaces.
Megasound for Sensitive Surfaces
Particles as small as about 4 micrometers can be removed with frequencies as high as 400 kHz. Higher-frequency sound waves (600 kHz to 4 MHz) are used to remove smaller particles down into the nano range. Frequencies beginning at 1 MHz are designated as megasound and generate directional micro-currents, which remove contamination from extremely sensitive substrates and valleys in microstructures in a residue-free, non-destructive manner.
Finding the Right Ultrasonic System
The number of oscillators plays a big role in the performance of an ultrasonic cleaning system. As a rule, quality cleaning results can be achieved with a power rating of 8 to 10 w/L. This means that for a cleaning bath with 100 liters, 800 to 1,000 watts of output power is required, which can be controlled via the generator within a range of 10 to 100 percent, either in steps or in an infinitely adjustable way.
Because sound waves are propagated longitudinally from the sound-emitting surface, the arrangement of the oscillating elements influences cleaning results. If the oscillators are attached only to the floor of the working chamber or cleaning basin, the sound waves are emitted vertically to the surface of the bath and then deflected back down to the floor. However, if the parts being cleaned have hollow spaces and blind holes, and bubbles form in these spaces, the air creates an obstacle for the sound waves, and no cleaning takes place. Therefore, parts should be oscillated or rotated within the bath so that all hollow spaces are filled with cleaning fluid. Because component shapes are becoming more and more complex, ultrasonic cleaning systems are now being equipped with oscillators at multiple locations, such as on the floor and the side walls.
Loading the bath too tightly with parts stacked closely on top of each other or cleaning a large volume of parts at one time prevents the ultrasonic waves from reaching all of the workpiece surfaces to be cleaned. As a rule, surfaces to be cleaned should not exceed the size of the sound-emitting surfaces. At the same time, the mass of the parts should not exceed 50 percent of the bath volume, because the ultrasound would be limited from propagating as required for quality cleaning results.
Compatible with All Cleaning Agents
Ultrasonic cleaning is possible with aqueous cleaning agents or solvents. The cleaning agent is matched to the material and the respective type of contamination. If aqueous cleaning agents are used, rinsing cycles are necessary to remove the active cleaning substances from the surface, and subsequent drying is required. In the case of flammable solvents, the ultrasonic cleaning system must either be explosion proof or be operated under a full vacuum. Solvents that boil at high temperatures necessitate temperature resistant ultrasonic oscillators to match.
Efficient and Ecological
Cleaning time can be reduced by as much as 90 percent by using ultrasound combined with a cleaning agent that is matched to the respective type of contamination. At the same time, cleaning agent consumption also can be reduced.
Doris Schulz has worked as a freelance journalist for more than 15 years. Her specialty is the field of surface treatment, especially parts cleaning. She can be reached at 49 711 854085 or via e-mail at ds@pressetextschulz.de.
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