THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

490 17. Commercial and Medical Ultrasound ApplicationsIn contrast with ultrasonic cleaning, ultrasonic plastic welding requires muchgreater power densities, typically at hundreds of watts per square centimeter at theweld and at the contact of the tool with the workpiece. Modern plastic weldersoperate predominantly around 20 kHz at power outputs below 1 kW, lock automaticallyon the horn resonance, and keep vibrational amplitude constant for varyingmechanical loads.Commercial gear for ultrasonic metal welding was introduced in the late 1950sand originally used in the semiconductor industry. Later advances in equipmentdesign and the need for joining high-conductivity metals spurred greater interestin the process. Standard equipment now available can weld parts up to0.32 cm (1/8 in.) thick and larger, depending on the material and part configuration.Ultrasonic metal welds are characterized by low heat (the material doesnot melt as in the process of arc welding) and relatively low distortion. Becausewelding temperatures are well below the melting temperatures, embrittlementand formulation of high-resistance intermetallic compounds are avoided.The equipment for ultrasonic metal welding range from low power microbonders(used in semiconductor industry) operating between 40 and 60 kHz to machinesof several kilowatt output capacity operating between 10 and 20 kHz forwelding of larger parts. It is interesting to note that on high-conductivity materialsultrasonic welding can be over 20 times more energy efficient than resistancewelding. A5kWwelder may be equivalent to a 100-kVA resistancewelder.AgricultureOver the last decade, ultrasound technology has considerably impacted the meatindustry, with a commanding role in the rating of product value. Ultrasound is beingused to measure the thickness of fat layers in pigs and cattle as part of livestockmanagement. It is utilized to predict carcass traits such as fat cover, ribeye area,and intra-muscular fat (marbling) in live animals. Ultrasound also has been usedto improve the quality of homogenized milk. A related agricultural application ispest control that includes killing of insects.Extraction ProcessesHigh-intensity cavitation is widely used for biological cell disruption in researchand low volume processing. Near field cavitation breaks down cell walls, causingrelease of the cell contents into the surrounding fluid. This method is appliedto extract active antigens for making vaccines and as a general tool forstudying cell structure. Other extraction uses include the extraction of perfumefrom flowers, essential oils from hops, juices from fruits, and chemicals fromplants. The equipment is normally in the range of 100 –500 W, operating at around20 kHz. High amplitude horns are used to produce power densities of around80 W/cm 2 .