超声波清洗 -外文文献
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Ultrasonic CleaningUltrasonic cleaning is a good fit for a wide range of applications, from removing swarf and grinding and polishing residue to treating parts covered in oil, grease, or layers of paint. Ultrasonics can be used to clean miniature watch parts or to support the overhaul of jumbo jet engines. And from an industry perspective, the fields of electrotechnics, precision mechanics and light engineering, optics, metal processing, and medical equipment have proven particularly receptive to the ultrasonic concept.So the impact of ultrasonic cleaning is clearly recognizable. But to truly understand the value of ultrasonics, one must understand how ultrasonic cleaners really work.Ultrasonic Cleaning ExplainedThe cleansing effect of ultrasound is the product of a phenomenon called cavitation. Billions of minute gas bubbles implode, causing shock waves that undermine dirt and blast it off a parts surface. One of the key advantages of this process is that it allows users to clean part surfaces that are completely inaccessible to a manual cleaning process.Ultrasound frequencies generally range between 20 kilohertz and 50 kilohertz, depending on application requirements. Ultrasonic cleaning is typically performed at temperatures between 122 F and 176 F .In an ultrasonic cleaning system, cavitation is produced by introducing sound waves into a cleaning liquid by means of a series of transducers mounted to a cleaning tank. The sound travels throughout 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 contain a partial vacuum. As the pressure around the bubbles becomes greater, surrounding fluid rushes in and collapses the bubble. When this occurs, a jet of liquid is created, resulting in temperatures as high as 9,032 F (roughly the temperature of the surface of the sun). The extreme temperature, combined with the liquid jets velocity, provides a very intense cleaning action. However, because the bubble expansion and collapse cycle is so short, the liquid surrounding the bubble quickly absorbs the heat, preventing the tank and cleaning liquid from becoming overly hot during the cleaning process.Secrets to Ultrasonic SuccessThere are seven major concerns related to successful ultrasonic cleaning:1. Time 2. Temperature 3. Chemistry 4. Part Fixture Design5. Ultrasonic Output Frequency6. Watts Per Gallon 7. Loading TimeCleaning times can vary tremendously in an ultrasonic process, depending largely on how dirty the part is and how clean is clean. A normal trial period is two to 10 minutes, since very few parts are sufficiently clean in a shorter period of time.Precleaning may be required to remove gross contamination or to chemically prepare the parts for a final clean. Some applications require more than one ultrasonic treatment to complete the required cleaning. Ultrasonic rinsing may also be required in some cases to more thoroughly remove wash chemicals.Temperature & ChemistryTemperature and chemistry are closely related. Generally, ultrasonic cleaning in an aqueous solution is optimized at 140 F . Some high pH solutions require higher temperatures. The chemical pH is a good place to start; but a thorough examination of chemistry is beyond the scope of this article.In brief, the following should be considered the main components of aqueous ultrasonic cleaning chemistry:A. Water (hard, soft, DI, or distilled) B. pH C. Surfactants Wetting agents Dispersants Emulsifiers Saponifiers D. Optional Ingredients SequestrantsInhibitors Buffering agents Defoamers The chemical formulation must consider all of the above characteristics. Some chemicals designed for spray cleaning or that include rust inhibitors are not suitable for ultrasonic cleaning.Part Fixture DesignThe procedure for ultrasonic cleaning is generally as follows: Put parts in basket and place basket through three or four process steps (i.e., ultrasonic wash, spray rinse (optional), immersion rinse, dry). Some parts loaded in baskets can mask or shadow from the radiated surface of the ultrasonic transducers. Most ultrasonic cleaning systems are designed for specific applications. Bottom-mounted transducers or side-mounted transducers are important considerations during the process design stage. Automated systems must specifically address the location of the transducers to ensure cleaning uniformity. Some parts require individual fixturing to separate the part for cleaning or subsequent processes. Some parts require slow rotating or vertical motion during the cleaning to ensure critical cleanliness.Ultrasonic Output FrequencyThe majority of the ultrasonic cleaning that is done in industrial applications today uses 40 kHz as a base frequency. Lower frequencies, such as 20-25 kHz, are used for large masses of metal where ultrasonic erosion is of little consequence. The large mass dampens or absorbs a great amount of the ultrasonic cleaning power.WattsPer GallonIn general, smaller parts require higher watts per gallon to achieve the desired level of cleanliness. Most industrial ultrasonic cleaning systems use watt density from 50 to 100 watts per gallon. However, tanks over 50 gallons usually require only about 20 watts per gallon because ultrasonic processes traditionally have shown diminishing returns in large tanks sizes.LoadingLoading of the parts to be cleaned must be considered when developing an ultrasonic cleaning process. A large dense mass, for example, prevents internal surfaces from being thoroughly cleaned (i.e., metal castings). A rule of thumb is that the load by weight should be less than the weight of half the water volume. So, for example, in five gallons (approximately 40 lbs .) of water, the maximum workload should be less than 20 lbs . In some cases, it is better to ultrasonically clean two smaller loads rather than one larger load.Each of the factors outlined here must be considered when specifying an ultrasonic application to ensure a high level of cleaning success. Neglecting any single factor can have a negative impact on the overall cleaning process.- 配套讲稿:
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