What is Ultrasonic Cleaning
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. Temperature inside a cavitating bubble can be extremely high, with pressures up to 500 atm. The implosion event, when it occurs near a hard surface, changes the bubble into a suction jet about one-tenth the bubble size, which travels at speeds up to 400 km/hr away from the hard surface. With the combination of pressure, temperature, and velocity, the jet frees contaminants from their bonds with the substrate. Because of the inherently small size of the jet and the relatively large energy, ultrasonic cleaning has the ability to reach into small crevices and remove entrapped soils very effectively.
For a Wikipedia guide to ultrasonic cleaning click here
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 where they are compressed and implode.
What is Degassing?
"Degassing" is the initial removal of gases present in the solution. Non removal or "trapped Gas" in the cleaning solution leads to gaseous cavitation where bubbles cannot effictivly form, 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.
How do I "Degas" my ultrasonic Cleaner?
A. Once filled with water the ultrasonics should be left operating for a few minutes so that excess air is driven from the liquid. Tiny bubbles will be seen to rise to the surface but this is gaseous cavitation and not suitable for cleaning. At the end of the de-gasing period, a hissing sound and a churning of the liquid surface (cold boiling) should occur indicating vaporous cavitation.
At this point switch off the ultrasonics add chemical as required and repeat this cycle. Once cold boiling returns the liquid is now in the correct mode for ultrasonic cleaning.
How do I get the best ultrasonic cleaning?
There are many considerations important to ultrasonic cleaning. Optimising 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.
Is a special solution required for 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 optimise 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.
What 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. A crucial part of Hilsonic's service is that we work with the client offering advise on cleaning solutions as necessary.
What cleaning solution shouldn't I use?
Flammables or solutions with low flash points should only be used on a suitable Hilsonic's ATEX EX rated system. 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.
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.
When should 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.
Why must I keep solution at a high level?
The solution level should always be maintained. Maintaining the proper solution level provides optimum circulation of solution around parts, and protects heaters and transducers from overheating or stress.
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.
What is the purpose of the unit heater?
The primary purpose of the unit heater is to maintain a solution temperature between cleaning cycles. The tremendous energy released by cavitation will generate the heat for cleaning.
How do I know if the unit 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.
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.
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 one minute. Remove the foil and inspect: All three pieces of aluminum foil should be perforated and wrinkled to about the same degree.
Why must baskets be used?
Items being cleaned should never be placed directly on the tank bottom. Transducers (which produce the ultrasound) are sometimes bonded to the bottom of the tank. Items resting directly on the tank bottom can damage the transducers and/or reduce cavitation. Additionally, a tray or beaker will position the item within the optimal cleaning zone of the tank. The tray or beaker will also hold the load together and allow for easy, no-touch removal, draining and transport of the items to the next step in the cleaning process.
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 50°C to 75°C range.
Is rinsing required after cleaning cycles?
A. Rinsing is recommended to remove any chemical residue, which could be harmful to the part. Parts can be rinsed right in your ultrasonic cleaner, using a clean water bath, or in a separate tub containing tap, distilled or deionised water.
Why shouldn't I leave my cleaner on constantly?
Low solution levels can seriously damage your cleaner. Running your unit continuously runs the strong risk of lowered levels as the solution evaporates, especially when heated. Getting into the habit of shutting off the ultrasonics when not in use, and monitoring the solution level when in use, will yield many years of trouble free service from your ultrasonic cleaner.