Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 97 of 707

chemical surface preparation ULTRASONICS—A PRACTICAL APPROACH BY KENNETH R. ALLEN TECHSOL LLC, ROXBURY, CONN. Ultrasonic cleaning is the introduction of high-frequency sound waves into a liquid, usually between 20 and 80 kHz. The resulting action is called "cavita- tion." Cavitation is created by high- and low-pressure areas produced in the solu- tion as the sound waves pass through it. In low-pressure areas microscopic vapor bubbles form. The pressure rises rapidly as the next sound wave passes through the area, violently imploding the minute bubbles, releasing the energy that does the cleaning. The Transducer This device converts high-frequency electrical energy into mechanical motion. The transducer physically oscillates at its resonant frequency. There are two basic types, magnetostrictive and piezoelectric. Magnetostrictive transducers are metallic, usu- ally made of laminated nickel and are typically silver brazed to a plate. Piezoelectric transducers are composed of man-made crystals and are mounted in various ways, the most common being epoxy bonding. The Generator The generator converts low-frequency line power at 50 to 60 cycles per second to high-frequency power (20-80 kHz) that matches the resonant frequency of the transducer. Generators designed to drive piezoelectric transducers often have automatic tuning or frequency controls to compensate for fluctuations in the resonant frequency of the crystal transducer. Typically, magnetostrictive gen- erators do not have automatic tuning because the resonant frequency of the magnetostrictive transducer is more stable. THE APPLICATION OF ULTRASONICS FOR CLEANING In general ultrasonics is used in precision-cleaning applications of more complex parts. It has the ability to clean in narrow crevices and small holes that would not be easily accessible by a spray washer or other methods of cleaning. Difficult soils, such as buffing compounds and baked-on carbon, are also good candidates for ultrasonics. In other words the more complex the part configuration and more stubborn the contaminant, the more likely ultrasonics will be of benefit to the cleaning process. There are also many other factors that contribute to the cleaning process equation such as the type of soil, chemical limitations, temperature, cycle time, piece part volume, etc. I attempt to illustrate how these parameters interrelate to ultrasonic cleaning. There are three types of energy available in any given cleaning application: (1) ther- mal energy, (2) chemical energy, and (3) mechanical energy. The success of a cleaning application depends on the balance and relationship of these three types of energy. The fourth parameter is time, which increases the effectiveness of all three energies. Ultrasonics is only one type of mechanical energy. It is only one part of the equa- 96

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