An ultrasonic cleaner is a cleaning device that uses ultrasound (usually from 20–400 kHz) and an appropriate cleaning solvent (sometimes ordinary tap water) to clean delicate items. The ultrasound can be used with just water, but use of a solvent appropriate for the item to be cleaned and the soiling enhances the effect. Cleaning normally lasts between three and six minutes.
Ultrasonic cleaning uses cavitation bubbles induced by high frequency pressure (sound) waves to agitate a liquid. The agitation produces high forces on contaminants adhering to substrates like metals, plastics, glass, rubber, and ceramics. This action also penetrates blind holes, cracks, and recesses. The intention is to thoroughly remove all traces of contamination tightly adhering or embedded onto solid surfaces.
Water or other cleaners can be used, depending on the type of contamination and the material being sonicated. Contaminants can include dust, dirt, oil, pigments, rust, grease, algae, fungus, bacteria, lime scale, polishing compounds, flux agents, fingerprints, soot wax and mould release agents, biological soil like blood, and so on.
Ultrasonic cleaning can be used for a wide range of workpiece shapes, sizes and materials, and may not require the part to be disassembled prior to cleaning. Objects must not be allowed to rest on the bottom of the device during the cleaning process, because that will prevent cavitation from taking place on the part of the object not in contact with water, hence the need for a rack or basket to hold the object above the bottom.
Today you can get ultrasonic systems with frequencies ranging from 20 to 950 kHz, which one to choose depends on what your cleaning job is, what type of soil is to be removed and how clean your part needs to be. In reality, most systems to day incorporate more than one frequency of ultrasonics. They may use 40 / 70 / 170 for a graduated cleaning process. A general list of frequencies that are most common follows.
20-40 kHz Heavy duty cleaning for things like engine blocks heavy metal, heavy greasy soils. The Grant range of
XUB ultrasonic baths have operating frequencies of 32-28 KHz.
40-70 kHz General cleaning of machine parts optics etc. very good at removing small particles. The Industrial cleaning baths from Elma can have dual frequencies and sweep function for optimal cleaning.
70-200 kHz ultra fine gentle cleaning of optics, semiconductor wafers disk drives, etc.
How should I choose the correct power level for an ultrasonic tank?
The average Watts per gallon of ultrasonics should be between 70 to 100 Watts for critical cleaning applications. This is the average rating and can be adjusted dependent on the cleaning application. To calculate the power requirements use the following formula;
L x(in) W(in) x. (H -2″) /231*100=Avg. Watts Power.
It is important to remember that ultrasonic companies can rate the watts of ultrasonic energy in two ways; Peak & Average. Peak watts are the start up requirement and Average Watts are the continuous operating wattage. Base all calculations on Average Watts.
When you are using extremely large tanks, power levels can be reduced to 50 watts per gallon. [Especially in non-critical cleaning applications. ]
Most companies will offer a power intensity control as an option. This control will lower the Wattage of the ultrasonics to any desired level on the top of the power curve. Below 50% there is not enough energy to activate the transducers. (i.e.: if you have a 100 watt avg. ultrasonic tank you will be able to adjust the power from 50 watts to 100 watts below 50 watts the transducers will not operate.)
The power intensity control is a good option where you will be cleaning delicate parts that may be subject to cavitation erosion or in a situation where the ultrasonics is being used in a plating operation or in other chemical processes.
What are the effects of different frequencies and multiple frequencies?
For a given cleaner the lower frequencies are much stronger because they concentrate the available power in fewer bands of cleaning. The higher the frequencies the more the available power is evenly distributed throughout the tank area. Most companies that claim to have multiple frequencies vary the power to the transducers to push them off of their natural frequency. This only works to a limited degree as any transducer has a natural frequency that at which it will resonate best. If they are pushed to far off the natural frequency they will dissipate the power in the form of heat, and will not generate cavitation bubbles. Any ultrasonic unit will generate more than one frequency; most units will generate sufficient high frequencies in addition to the fundamental low frequency to provide a practical balance for general cleaning applications. Higher frequencies such as 65 – 70 or 170 kHz generate much smaller cavitation bubbles and will remove smaller particles more evenly than lower frequencies. There is some loss of efficiency when you run a 40 kHz transducer at 170 kHz resonant point. The loss is only about 5% of the total power of the system and for all practicable purposes is irrelevant.
It is important to remember that power in an ultrasonic cleaner is usually rated in total Watts/Avg. or Peak Watts. Watts average is what the cleaner will draw during continuous operation. Peak Watts is what the cleaner draws on start up. Watts average is the better determination of cleaning power.
There are specific cases where tanks are constructed with 2 or three types of transducers thus enabling true multi frequency operation. These require different generators to run each set of transducers. Some Transducers can be operated at more than one frequency. These transducers have a natural harmonic at a higher frequency such as 40 kHz and 170 kHz, by including 2 different frequency driving boards in a single generator the tank can be switched from one frequency to the other.
Ultrasonic cleaners are also available in a wide and varied selection for different applications.
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