Ultrasonic cleaning FAQ, the operation principle of ultrasonic cleaning.

  1. What is "cavitation"?

  2. What is "degassing", and why should it be done?

  3. How do I get the best ultrasonic cleaning?

  4. Can ultrasonic cleaning damage my parts? 

  5. Why is a special ultrasonic cleaning solution required for ultrasonic cleaning?

  6. Which cleaning solution should I use?

  7. Which cleaning solution shouldn't I use?

  8. When should the ultrasonic cleaning solutions be changed? 

  9. What is the length of cleaning time?

  10. What is the purpose of the ultrasonic cleaners heater?

  11. How do I know if my ultrasonic cleaner is cavitating properly?

  12. How do I perform the "glass slide" test?

  13. How do I perform the "foil" test? 

  14. What is the optimum cleaning temperature?

  15. Is rinsing required after ultrasonic cleaning cycles?

  16. How does indirect cleaning work?

  17. Principle ultrasonic cleaning.

  18. Advantage?

 


What is "cavitation"?

"Cavitation" is the rapid formation and collapse of millions of tiny bubbles (or cavities) in a liquid. Cavitation is produced by the alternating high and low pressure waves generated by high frequency (ultrasonic) sound. During the low pressure phase, these bubbles grow from microscopic size until, during the high pressure phase, they are compressed and implode.

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What is "degassing", and why should it be done?

"Degassing" is the initial removal of gases present in the solution. Useful cavitation occurs after gasses have been removed from the cleaning solution, leaving a vacuum in the formed bubble. When the high pressure wave hits the bubble wall, the bubble collapses; it is the energy released by this collapse that will assist a detergent in breaking the bonds between parts and their soils.

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How do I get the best ultrasonic cleaning?

There are many considerations important to ultrasonic cleaning. Optimizing these variables will produce the best cleaning. The most important decisions to be made are choosing the proper cleaning solution, cleaning at the right temperature for the correct amount of time, and choosing the right size and type of ultrasonic cleaner.

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Can ultrasonic cleaning damage my parts?

With certain cautions, ultrasonic cleaning is considered safe for most parts. While the effects of thousands of implosions per second is very powerful, the cleaning process is safe since the energy is localized at the microscopic level. The most important cautionary consideration is the choice of cleaning solution. Potentially adverse effects of the detergent on the material being cleaned will be enhanced by the ultrasonics.

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Why is a special ultrasonic cleaning solution required for ultrasonic cleaning?

Soils adhere to the parts... if they didn't, the soil would just fall off the parts! The purpose of the solution is to break the bonds between parts and their soils. Water alone has no cleaning properties. The primary purpose of the ultrasonic activity (cavitation) is to assist the solution in doing its job. An ultrasonic cleaning solution contains various ingredients designed to optimize the ultrasonic cleaning process. For example, increased cavitation levels result from reduced fluid surface tension. An ultrasonic solution will contain a good wetting agent or surfactant.

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Which cleaning solution should I use?

Modern ultrasonic cleaning solutions are compounded from a variety of detergents, wetting agents and other reactive components. A large variety of excellent formulations are available, designed for specific applications. Proper selection is crucial for acceptable cleaning activity and to preclude undesirable reactivity with the part being cleaned.

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Which cleaning solution shouldn't I use?

Flammables or solutions with low flash points should never be used. The energy released by cavitation is converted to heat and kinetic energy, generating high temperature gradients in the solution, and can create hazardous conditions with flammable liquids. Acids, bleach and bleach by-products should generally be avoided, but may be used with indirect cleaning in a proper indirect cleaning container, such as a glass beaker, and appropriate care. Acid and bleach will damage stainless steel tanks, and/or create hazardous conditions.

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When should the ultrasonic solutions be changed?

Cleaning solutions should be replenished when a noticeable decrease in cleaning action occurs, or when the solution is visibly dirty or spent. A fresh batch of solution at each cleaning session is usually not required.

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What is the length of cleaning time?

Cleaning time will vary, depending on such things as soil, solution, temperature and the degree of cleanliness desired. Highly visible removal of soils should start almost immediately after the ultrasonic cleaning action begins. Cleaning time adjustment is the easiest (and most often misapplied) factor used to compensate for process variables. Although new application cycle duration can be approximated by an experienced operator, it usually must be validated by actual use with the chosen solution and the actual soiled parts.

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What is the purpose of the ultrasonic cleaners heater?

The primary purpose of the unit heater is to maintain a solution temperature between cleaning cycles. The tremendous energy released by cavitation will also generate a lot of heat in the ultrasonic cleaner.

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How do I know if my ultrasonic cleaner is cavitating properly?

Most poor cleaning usually results from improper control of one or more process variable(s); such as choosing the wrong detergent solution, insufficient heat, or not allowing enough time for the particular soil to be removed. If you suspect that your ultrasonic cleaner is not cavitating properly, there are two simple tests you can perform: the "glass slide" test and the "foil" test.

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How do I perform the "glass slide" test?

Wet the frosted portion of a glass slide with tap water and draw an "X" with a No. 2 pencil from corner to corner of the frosted area. Making sure that the tank is filled to the fill line, immerse the frosted end of the slide into fresh cleaning solution. Turn on the ultrasonics. The lead "X" will begin to be removed almost immediately, and all lead should be removed within ten seconds.

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How do I perform the "foil" test?

Cut three small pieces of aluminum foil about 4" x 8" each. Fold each piece over a rod that you will use to suspend the foil in the tank. A clothes hanger works well. Your cleaner should be filled with an ultrasonic cleaning solution, degassed, and brought up to normal operating temperature. Suspend the first "square" in the center of the tank and the other two a couple of inches from each end of the tank. Make sure that the tank is filled to the fill line, and turn on the ultrasonics for about ten minutes. Remove the foil and inspect: All three pieces of aluminum foil should be perforated and wrinkled to about the same degree.

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What is the optimum cleaning temperature?

Heat usually enhances and speeds up the cleaning process, and most detergent solutions are designed to work best at an elevated temperature. The best way to find the optimum temperature, which will give you the fastest, cleanest and safest results, is to run tests. Usually, the best results are within the 40°C to 65°C range.

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Is rinsing required after ultrasonic cleaning cycles?

Rinsing is recommended to remove any chemical residue, which could be harmful to the part. Parts can be rinsed in an ultrasonic cleaner, using a clean water bath, or in a separate bath using tap, distilled or deionized water.

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How does indirect cleaning work?

Indirect ultrasonic cleaning refers to the practice of taking an ultrasonic tank, filling it with water or cleaning solution, then inserting a solid container into the tank. The solid container is usually a solid tray or a beaker held in a beaker holder. The container is then filled either with a different liquid, or the same liquid.

The physics of the setup are simple. Ultrasonic energy is transmitted from the transducers which are usually bonded to the bottom of the tank. The wavefront propagates through the water in the tank until it encounters a physical object, the side of the tank, or the air/water interface at the top of the tank. If a solid container filled with another liquid is encountered, the ultrasonic energy will pass through the container wall into that liquid, and sonify that liquid as well.

Makes sense – but why would anyone want to do that?

  • You can fill your main tank with clean water, and keep all the soil inside your solid tray or beaker. 
  • This means your tank will never have to be cleaned. 
  • You can do multiple batches of small parts in different cleaning solutions without cross-contamination. 
  • Using a stainless steel or glass beaker will allow you to clean using caustic or other harsh liquids that you do not want your tank exposed to. 
  • You can clean with expensive solutions and limit the amount of chemistry you need to buy.
  • Clean the same part in multiple solutions as an experiment to determine your most effective cleaning process. 

 As you can see, the reasons to use indirect cleaning are as numerous as ultrasonic applications themselves. If you are not taking advantage of this, you’re passing up a free application research tool, and maybe some savings on chemistry as well.

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Principle ultrasonic cleaning.

ULTRASONIC CLEANING involves the use of high-frequency sound waves (above the upper range of human hearing, or about 18 kHz) to remove a variety of contaminants from parts immersed in aqueous media. The contaminants can be dirt, oil, grease, buffing/polishing compounds, and mold release agents, just to name a few. Materials that can be cleaned include metals, glass, ceramics, and so on. Ultrasonic agitation can be used with a variety of cleaning agents.

In a process termed cavitation, micron-size bubbles form and grow due to alternating positive and negative pressure waves in a solution. The bubbles subjected to these alternating pressure waves continue to grow until they reach resonant size. Just prior to the bubble implosion there is a tremendous amount of energy stored inside the bubble itself.

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Advantage?

Ultrasonic cleaning is powerful enough to remove tough contaminants, yet gentle enough not to damage the substrate. It provides excellent penetration and cleaning in the smallest crevices and between tightly spaced parts in a cleaning tank.

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