Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 101 of 707

energy to compensate for load attenuation. In most general degreasing applications 40 to 60 W/gal seems adequate; however, in applications such as removing burned-on carbon in turbines or removing a diamond lapping compound, Watt densities of 100 to 150 W/gal are common. Often the number of transducers seems to be used as a yardstick to measure how much power is in a tank. This is not an accurate method and should not be used to compare manufacturers. Most types of crystal transducers can range from 20 to 50 Watts per transducer. Magnetostrictive transducers can range from 50 to 1,000 Watts per transducer. You would need twice as many of one type to get the same energy in a given volume as another. This is why the W/gal approach is a more reliable method of determining if the power available will be adequate. Piezoelectric and magnetostrictive devices vary in their construction, power out- put per transducer, and methods of attachment, i.e., epoxy bonded or silver brazed. The efficiencies in general—considering all parameters—are similar. Typically, you find piezoelectric in lower power applications and magnetostric- tive in higher power ranges. It is safe to say that the more power, the more expensive; therefore, if you are cleaning lightly soiled parts or printed circuit boards a low Watt density is adequate. But, if you are decarbonizing jet engines or large textile spinnerettes, a high Watt density tank is a much better choice. Trying to determine the size of the tank can be difficult. Trying to scale up any type of manufacturing process can be challenging. If it is possible you should test clean production lots in the laboratory or in equipment to closely approximate the manufacturing environment. Choose a tank size that will adequately accept your part or basket with enough clearance to allow for easy placement and extraction. The other consideration is liquid displacement. As a general rule your part volume should not displace more than 25% of the total tank volume. The reason for this is to maintain enough liquid level to cover the part. You do not want to displace solu- tion out of the overflow weir every time you put a basket of parts in the tank. You will have to constantly add water to maintain a consistent level. There are two things that are usually overlooked that will greatly enhance any equipment selection. Make sure that the ultrasonic tank has adequate filtration and a sparge/overflow system with oil separation when oil is being removed from the parts. This keeps the floating oil from redepositing on the parts and keeps the particulate level down in the tank. It also has the additional benefit of enhanc- ing the cavitation because high amounts of suspended solids also attenuate (reduce) the ultrasonic activity. The money spent on filtration will be recouped by way of reduced chemical consumption, cleaner parts, more consistent quali- ty, and reduced cycle times. Removing as much oil as possible will give similar ben- efits. The preferred type of oil separation is usually a coalescing system. Oil wheels work but do not remove oil fast enough in a production situation. The next thing to consider is an adequate rinse system. "You cannot get clean dishes out of dirty dishwater." The same applies to clean parts. Your part will only be as clean as your final rinse. As a minimum a two-tank cascading system should be considered. WHEN SHOULD I CONSIDER AN ULTRASONIC RINSE? Ultrasonic rinsing typically is not considered because of cost considerations. It adds 30 to 40% increased cost to most clean lines; however, if the holes in your part 100

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