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Ultrasonic Cleaning; The Layman's Explanation

cleaning environments

Ultrasonic Cleaning has been used in a wide range of applications in both science and industry over the last thirty years. In cases where extreme cleanliness is required, ultrasonics have proven to be more effective than mechanical or chemical means. Cleaning with ultrasonics has the distinct advantage of penetrating complex geometries and removing tightly bonded contaminants from a wide variety of surfaces. Most people that see an ultrasonic cleaning tank for the first time, they tend to see it as a vibrating tank that simply vibrates the contaminants from the part to be cleaned. This is far from the truth. Ultrasonics is much more complex and has little to do with simple vibration. Ultrasonic cleaning is a result of sound waves introduced into the cleaning liquid by means of a series of "transducers" mounted to the cleaning tank. The sound travels through the tank and creates waves of compression and expansion in the liquid. In the compression wave, the molecules of the cleaning liquid are compressed together tightly. Conversely, in the expansion wave, the molecules are pulled apart rapidly. The expansion is so dramatic, that the molecules are ripped apart creating microscopic bubbles. The bubbles are not seen by the naked eye since they are so small and exist for only a split second of time. The bubbles contain a partial vacuum while they exist. As the pressure around the bubbles becomes great, the fluid around the bubble rushes in collapsing the bubble very rapidly. When this occurs, a jet of liquid is created that may travel as fast as 650 miles per hour. As the gases inside the bubble are compressed at this very high rate, they rise in temperature to as high as 5000 degrees Celsius, which is roughly the temperature of the surface of the sun. The liquid immediately surrounding the bubble area is also raised to approximately 2000 degrees Celsius. This extreme temperature, combined with the liquid jet velocity provides a very intense cleaning action in a minute area. Because of the very short duration of the bubble expansion and collapse cycle, the liquid surrounding the bubble quickly absorbs the heat and the area cools quickly.

cavitation diagram

There are many variables that affect the ultrasonic cleaning process. The first variable that must be considered is the matching of power supplies and transducers. Generator output frequency must closely match the design frequency of the transducer. This matching is a part of the design process and is not something that can be changed by the user. The second variable to be considered is the frequency of the ultrasonics. The frequency relates to the number of times that a compression and expansion cycle completed per second. This determines the number of times per second that bubbles can be produced and the size of the bubble itself. In general, the higher the frequency, the smaller the bubble produced. The two typical frequencies used commercially today, are 25 kHz and 40 kHz. 25 kHz has fewer bubbles than 40kHz and are relatively large. The jet produced during bubble implosion is very powerful and very abrasive which can harm delicate items. This frequency is suitable for cleaning larger. massive items with stubborn contaminants. A 40 kHz ultrasonic cleaner generates very small bubbles that can penetrate very small holes and crevices. This frequency is used in most cleaning applications, often where other types of mechanical cleaning equipment cannot reach. Higher frequencies like 68kHz, 132kHz, and 170kHz are also offered at UPC for applications with cleaning specifications in the sub-micron levels, like precision optics and computer hard drive platters. A third variable is the type of cleaning fluid used in the tank. Fluids have a "tensile strength" which holds the molecules together within them. Pure water has such a high tensile strength that most ultrasonic generators cannot produce cavitation in it. Since cavitation involves the separation of fluid molecules to create cavities, the fluids used for ultrasonic cleaning must have relatively low tensile strengths so that cavitation can be produced without over taxing the ultrasonic transducers and generator. Yet another variable in ultrasonic cleaning is the temperature of the cleaning liquid. When liquids are heated, they become more dense and the sound waves travel through them faster. Optimum ultrasonic cleaning in water takes place in the 120 degree - 140 degree Fahrenheit range. Ultrasonic cleaning is indeed a complex science. Consultation with a reputable manufacturer insures the best match of equipment to your particular needs.