Ultrasonic Cleaning Technology and Application Areas

Before explaining the "Ultrasonic Cleaning" technique, it is necessary to understand some scientific information about sound. This will enable a much better understanding of the fundamental principles of ultrasonic cleaning.

Sound

Sound is the pressure within the hearing range of humans from mechanical waves that propagate in solid, liquid and gas environments. Sound travels at different speeds in different media. For example, its speed in air at 21°C is 340 m/s. Sound propagates independently of frequency; the factors determining the propagation speed of sound are air temperature and density. The science that studies sound is called acoustics. The human ear can hear sound frequencies in the range of 20 Hz to 20 kHz. Sound waves above 20 kHz are called "Ultrasonic – Ultrasound". Ultrasonic cleaning terminology originates from here. The reason is that frequencies of 28 kHz and 40 kHz are commonly used in ultrasonic cleaning.

Figure 1: Bell Telephone Laboratories

Ultrasonic Cleaning

Ultrasonic cleaning, as mentioned above, is the process of mechanically generating high-frequency sound waves and applying them within a liquid-filled tank to be cleaned, thereby producing the effect known as "cavitation" in physics. From this explanation, it is clear that what is important here is the "cavitation" effect. Ultrasonic energy is used solely and exclusively to produce the cavitation effect. The continuation of this article provides a detailed explanation of what "cavitation" is.

Figure 2: Vacuum bubble

Cavitation

Vacuum voids formed as a result of successive mechanical impacts applied within a liquid, depending on the intensity and frequency of the mechanical impact and the density of the liquid, and this physical phenomenon occurs through a series of events. This phenomenon is explained in the continuation of the topic with images and explanations. As shown in Figure 2, the first phase of the reaction is initiated by applying a positive mechanical impact within the liquid (this effect is achieved through crystals that we use in our devices), and a vacuum bubble is formed and is in its most unstable state. The pressure generated during this process has been measured in very precise and advanced tests to reach approximately 7,000 atm. Temperature rises to as high as 5,000°C. However, since these occur at micro-molecular scale, they cause no damage to the material in the liquid. Following this phase, the second phase of the reaction begins. This is the application of a negative pressure pulse within the liquid. With this negative pulse, the pressure around the "cavitation bubble" decreases. At the same time, the "cavitation bubble" enters a cycle of returning to a stable state and collapses inward at extremely high speed. Following the high-speed inward collapse of the cavitation bubble, a shock wave called "micro jet" is produced. Based on tests conducted, the shock wave propagates at speeds reaching 400 km/h depending on the conditions within the liquid. This is precisely the moment when the cleaning effect occurs, and all foreign substances not belonging to the product are removed from the material surface. This phenomenon is termed "Ultrasonic Cleaning" in terminology.

Figure 3: The moment of cavitation bubble collapse.

Ultrasonic Cleaning

Following the detailed explanation of the cavitation phenomenon, we can move to more general information about ultrasonic cleaning. In the context of global climate change, through agreements such as the Montreal Protocol (1987) and the Kyoto Protocol (1997), countries have begun efforts to reduce the emission of greenhouse gases (trichloroethane, hydrochlorofluorocarbons, hydrobromofluorocarbons and methyl bromide, etc.). As the use of these chemicals, which are good aids in cleaning, has decreased due to their environmental damage, ultrasonic cleaning systems have been rapidly introduced into factories for cleaning purposes in manufacturing processes. What makes ultrasonic cleaning attractive is its low investment cost and the fact that the chemicals used in it are water-based, which does not incur significant costs for companies in waste management. Ultrasonic cleaning technique is the simultaneous occurrence of the cavitation phenomenon explained earlier with hundreds of thousands of vacuum bubbles within a tank. Mechanical vibrations arriving at specific frequencies create vacuum bubbles in very large numbers within the liquid; the diameters, quantities and power of these bubbles depend on the frequency and the amplitude of the applied signal. As shown in Figure 6, at approximately 28 kHz frequency, the vacuum bubble diameter is ~150–200 microns. Successive mechanical impacts/vibrations create continuous cavitation energy, thereby performing the cleaning. Frequencies may vary depending on the cleaning requirements of the parts. For example, in industrial applications, general and rough cleaning can be performed at frequencies between 28 kHz and 40 kHz. However, much more delicate materials, such as semiconductor wafers, can be cleaned at 200 kHz frequencies.

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

Manufactured in power ratings based on requirements. This unit drives the crystals and converts them to ultrasonic energy. The starting power is 30 W and can be manufactured up to the kilowatt range.

Transducer

The unit that converts the signal from the generator into mechanical energy and directly affects the quality of the device. Its manufacture requires a high level of expertise, and incorrectly calibrated crystals directly affect the service life of the device. They are manufactured at various frequencies; some models can operate at dual frequencies.

Figure 9: The transducer that generates mechanical energy.

Liquid Tank

The unit in which the liquid where the cleaning process takes place is stored. It is manufactured from stainless steel (AISI304, AISI316). While standard dimensions are used in desktop products, special designs are used in the manufacturing of facilities with capacity reaching hundreds of liters.

Figure 10: Liquid tank

Applications of Ultrasonic Cleaning

It can be used in virtually all production facilities and factories wherever cleaning is needed; it is more commonly preferred for cleaning hard objects. Appropriate chemicals are selected for each cleaning process. Water-based alkaline chemicals are used for cleaning oils from sheet metal, iron, steel and similar products, while blood-dissolving and germicidal chemicals are used in hospitals for disinfection of surgical equipment.

Some Sectors That Prefer Ultrasonic Cleaning

Manufacturing industry, automotive industry, automotive maintenance and repair workshops, jewelry sector, healthcare sector, defense industry, ship maintenance and repair industry, energy sector, textile industry, coating industry, aerospace industry, electronics industry, food industry, mining industry. Bilgehan Mehmet Tireli / General Manager / Amber Elektromekanik San. Tic. Ltd. Şti.