Did you know that ultrasonic cleaning is connected with early studies at the Kiev Institute of Cinema? Lazar Davidovich Rozenberg (1908-1968) was a researcher in acoustic cavitation and has been credited with laying the scientific foundation for the development of ultrasonic cleaning. He founded the Department of Acoustics and Acoustoelectronics, originally as part of the Kiev Institute of Cinema; it is now part of the National Technical University of Ukraine.1
Ultrasonic cleaning — cleaning with high frequency sound waves — has become a staple of the laboratory and of the manufacturing facility for generations. For years, we have understood and “taught” the impact of temperature on cavitation based on studies by Rozenberg published in 1960.2 Specifically, the “Rozenberg graph” (see Fig. 1) shows the impact of temperature on cavitation for a number of solvents; there is a peak at which cavitation is most effective for each solvent.
Components and product have evolved, and so have ultrasonic cleaning systems. Ultrasonic cleaning is used with new materials of construction and of engineered coatings, and the miniaturization of components. Ultrasonics may have value to exploit the promise and to meet the challenges of additive manufacturing. In the “olden days,” there was ultrasonic cleaning with 18KHz. Now, ultrasonic cleaning equipment with many options such as ever-higher frequencies and controllable power level, has been developed.
In Controlled Environments columns that appeared a decade ago, we emphasized that independent studies are a must to determine optimal approaches to ultrasonic cleaning.3, 4 Therefore, we have been evaluating the impact of ultrasonic variables on cleaning performance. Some of these variables include the shape and materials of construction of the substrate, the soils, the chemical or chemical blend used for cleaning, the ultrasonic frequency, power density, temperature, and time.
Our experiments included revisiting the Rozenberg study. Attention: we do not intend to revisit the study using low flashpoint solvents, and we strongly recommend that you not play with fire either. The potential for fire posed by heat and ignitions sources is real.
We evaluated the impact of temperature on cavitation in 40 KHz systems with sweep operated at about 18 watts/L (80 percent of maximum power). We tested water and two commercially-available aqueous chemistries.5 The chemistries were tested at the manufacturer’s recommended dilution, ranging from 1 to 10 percent, for 3 minutes at ambient temperature. To monitor cavitation, commercially-available 0.015 mm thick aluminum foil was immersed in the chemistry being testing. The effectiveness
of ultrasonic cavitation was judged visually by the characteristic orange peel deformation pattern of the foil. While ultrasonic monitors have been developed, the aluminum foil test remains the most widely-used test for cavitation, perhaps because it is readily accessible, simple to perform, and cost-effective. Be aware that at higher frequencies, commercial aluminum foil becomes a less effective indicator of cavitation.
Our results are summarized in Fig. 2, and they are somewhat different than the Rozenberg results. While we considered calling this the “Kanegsberg, Kanegsberg, Norris graph,” in the interest of not inflicting polysyllabic torture we will mercifully refer it to as the KN graph. In both the KN and the Rozenberg results, as the temperature of the cleaning agent approaches the boiling point, cavitation decreases. The most likely explanation is that cavitation involves implosion of vacuum voids (“bubbles”) that result from tears in the liquid. As liquids approach the boiling point, the voids in the liquid become filled with vapor; the more vapor, the less forceful the implosion. Manufacturers involved in critical cleaning of high-value product may increase the temperature of an ultrasonic system to near the boiling point. Cleaning effectiveness may actually decrease due to decreased cavitation. If cleaning effectiveness is effective primarily near the boiling point, it may be due to melting and solubilization of the soil; you have very likely lost the benefit of ultrasonics.
However, in contrast with Rozenberg, the KN results show a sort of cavitation “tail” at ambient temperature, with a valley as the temperature increases before then increasing to a maximum. However, those involved in cleaning heat-sensitive product might consider testing ultrasonics cavitation at ambient temperature versus at somewhat elevated temperature. We do not know if this phenomenon occurs in 40 KHz ultrasonic systems from other suppliers. Further, initial studies at 132 KHz indicate that yet a different pattern of cavitation versus temperature may emerge.
Many people take ultrasonic cleaning systems for granted.6 Manufacturers may use default settings and practices based on historical experience. Or they may rely solely on recommendations from equipment suppliers. We all probably make assumptions that are logical but may not be correct.
It is impractical to test all the variables, particularly in a busy manufacturing environment. However, investing in some initial effort to test and optimize the ultrasonic system is likely to yield benefits in terms of cleanliness, surface quality, and contamination control.
The authors wish to convey thanks and appreciation to Plasma Technology Inc. of Torrance, Calif., for providing test facilities. They also wish to thank Crest Ultrasonics for providing ultrasonic cleaning equipment, and The Brulin Co., Crest Ultrasonics, e-Chem, and Mirachem for supplying aqueous chemistries.
References
1. Department of Acoustics and Acoustoelectronics, National Technical University of Ukraine, http://kpi.ua/en/node/203.
2. Rozenberg, L.D. “On the Physics of Ultrasonic Cleaning,” Ultrasonic News, 4, p. 16 (1960).
3-4. Kanegsberg, B. and E. Kanegsberg, “Ultrasonics Cleaning: Parts 1 and 2,” Controlled Environments Magazine, November 2005 and December 2005.
5. Kanegsberg, B. and E. Kanegsberg, “New Studies: Improve Your Ultrasonic Cleaning Process,” presented at SUR/FIN, Rosemont, Ill., June 10, 2015.
6. Kanegsberg, B, E. Kanegsberg, and Steve Norris, “Ferris and Ultrasonics,” Spraytime Magazine, Third Quarter, 2014.
Barbara Kanegsberg and Ed Kanegsberg (the Cleaning Lady and the Rocket Scientist) are experienced consultants and educators in critical and precision cleaning, surface preparation, and contamination control. Their diverse projects include medical device manufacturing, microelectronics, optics, and aerospace. They can be reached at [email protected].
Steve Norris is New Project Director at Plasma Technology Inc. with locations in Torrance, Calif. and Hartford, Conn. He has had 13 years of experience in studying the effect of ultrasonic cleaning on metal surfaces to be used with engineered coatings. He can be reached at [email protected].
This article appeared in the November/December 2015 issue of Controlled Environments.
