Before explaining the “Ultrasonic Cleaning” technique, some scientific knowledge on sound needs to be known. In this way, the basic principles of ultrasonic cleaning can be better understood.
Sound; is the pressure within the range of hearing of mechanical waves which can be emitted in solid, liquid and gas environments. Sound diffuses at different speeds in different environments. For example, the air velocity is 340m / sec at 21°C.
The velocity of sound propagates independently of the frequency, the factors determining the velocity of sound propagation are air temperature, density. The science of sound is called acoustics. The human ear can hear audio frequencies in the range of 20Hz to 20KHz.
Sound waves above 20Khz are called “Ultrasonic - beyond sound”. Ultrasonic washing terminology comes from here. The reason is that 28Khz and 40Khz frequencies are widely used in ultrasonic cleaning.
Image 1: Bell Telphone Laboratories
Ultrasonic Cleaning
Ultrasonic cleaning, as mentioned above, is to reveal the effect called “cavitation” in physics by applying high frequency sound waves mechanically to the tank filled with liquid to be cleaned. As can be understood from this explanation, what is important here is the “cavitation” effect.
Ultrason ic energy is an energy used exclusively and only to reveal the effect of cavitation. In the rest of our article, “cavitation” is explained in detail.
Figure 2: Vacuum bubble
Cavitation
These are the vacuum gaps that occur depending on the intensity, frequency and density of the mechanical impact as a result of successive mechanical impulses applied to the liquid, and this physical event occurs with a series of events.
This event is explained in the following. As shown in Figure 2, the positive mechanical impact applied to the liquid (this effect is achieved by the crystals used in our devices) initiates the first stage of the reaction in the liquid and creates a vacuum bubble and is the most unstable.
The pressure generated during this process was measured to reach 7000 atm in very sensitive and advanced decks. The temperature rises to 5000°C. However, since they are of micro-molecular size, they do not harm the material in the liquid. After this step, the second step of the reaction begins.
This is to send the negative pressure pulse into the liquid. With this negative pulse, the pressure around the cavitation bubble decreases. At the same time, the cavitation bubble enters a stabilizing rotation cycle and collapses into itself too quickly. After the cavitation bubble collapses rapidly, a shock wave called micro jet is formed.
As a result of the tests carried out, it spreads rapidly up to 400Km/h depending on the conditions in the liquid. This is the moment when the cleaning effect is sent and all foreign materials that do not belong to the product are discarded. This is called as “Ultrasonic Cleaning” in terminology.
Figure 3: The moment when the cavitation balloon collapses into itself.
Ultrasonic Cleaning
After a detailed explanation of the cavitation event, we can proceed to more general information on ultrasonic cleaning.
Global climate changes have forced the world states to take common steps and in this context, countries such as Montreal Protocol (1987) and Kyoto Protocol (1997) have started to work on reducing emissions of greenhouse gases (Trichloroethane, hydrochlorofluorocarbons, hydrobromofluoro carbons and methyl bromide etc.).
While these chemicals, which are a good helper in cleaning, reduce their use as a result of their harm to nature, ultrasonic cleaning systems have been rapidly put into use in factories for cleaning purposes.
What makes ultrasonic cleaning attractive is the low investment cost and the fact that the chemicals used are water-based, so that they do not incur huge costs to firms in waste management.
The ultrasonic cleaning technique is the process of cavitation described previously, in a tank with hundreds of thousands of vacuum bubbles.
Mechanical vibrations at certain frequencies produce very high numbers of vacuum bubbles in the liquid, the diameter, number and strength of these bubbles depends on the frequency and the amplitude of the applied signal.
As shown in Figure 6, the vacuum bubble diameter at a frequency of about 28 kHz is ~ 150-200 microns. Consecutive mechanical shocks / vibrations produce a continuous cavitation energy and perform cleaning. The frequencies may vary depending on the cleaning requirements of the parts.
For example, general and rough cleaning in the industrial area can be performed at frequencies between 28kHz and 40kHz. However, much more sensitive materials, such as semiconductor discs (wafer), can be washed at frequencies of 200 kHz.
Figure 4: Propagation of the shock wave
Components of Ultrasonic Cleaning Units
Basically an ultrasonic cleaning device consists of the following components;
• Ultrasonic Signal Generator,
• Transducer,
• Liquid tank.
Ultrasonic Signal Generator
It is manufactured in the forces depending on the need. This unit drives transducers and is converted into ultrasonic energy. 30W is the starting power and can be manufactured up to KW.
Transducer
It is the unit that converts the signal from the generator into mechanical energy and affects the quality of the device. Its production requires a high level of knowledge and the correctly calibrated transducers directly affect the life of the device. They are produced in various frequencies, and some models can operate at dual frequency.
Figure 9: Transducer generating mechanical energy.
Liquid Tank
This is the unit where the cleaning process takes place. Made of stainless steel (AISI304 AISI316). While standard sizes are used in desktop products, special designs are used in the production of facilities up to hundreds of liters.
Ultrasonic Cleaning Applications
It can be used in almost all production facilities and factories where cleaning is needed, it is preferred for cleaning hard objects.
Chemical suitable for every cleaning process is preferred. Water-based alkaline chemicals are used for cleaning the oils on sheet, iron, steel etc. products, and blood solvents and germicidal chemicals are used for disinfection of post-operative equipment in hospitals.
Some of the Sectors That Prefer Ultrasonic Cleaning
Manufacturing industry, automotive industry, automotive maintenance repair shops, jewelery industry, health sector, defense industry, ship maintenance and repair industry, energy sector, textile industry, coating industry, aviation industry, electronic industry, food industry, mining industry.
