Fundamentals of Ultrasonic Cleaning Technology2004#1

China’s exploding ultrasonic cleaning machine market.

  • There are more than 2,000 ultrasonic cleaner manufacturers with a market size of more than 65 billion yen and many manufacturers with more than 1,000 employees.
    May 10, 2012

Introduction.

Here, I would like to discuss the latest ultrasonic cleaning technology.But first, I would like to clarify my position on cleaning technology and environmental issues.One of the biggest challenges facing humanity in the 21st century is the global environmental problem.The environmental problems of China, which has the largest population on the planet and is now developing as the world’s manufacturing factory, are also the environmental problems of the planet.Cleaning technology is also a technology for the concentration and transfer of substances to be removed, i.e. various environmental burdens created by the survival and activities of mankind.Therefore, cleaning technology, which is indispensable for the development of Chinese industry, must also actively contribute to the conservation of the global environment.The cleaning technology required in China today must be a comprehensive technology that helps protect the environment and develop industry, based on the above perspectives.

The historical lessons of the CFC must not be forgotten.

[China does not need the same cleaning technology used in the developed world.]

Among these, I am convinced that ultrasonic cleaning technology, if used correctly and efficiently, can contribute to China’s economic development and help protect the global environment [a developing technology for the future].

What is ultrasonic cleaning?

This is due to a misunderstanding of its basic understanding.Ultrasonic cleaning systems have made rapid progress in accordance with advances in peripheral technologies and user requirements.transport technology, instrumentation technology and sheet metal welding technology.

However, there has been no significant change in the basic content of ultrasonic cleaning technology, with a few exceptions.Even if the appearance and transport technology change, ultrasonic cleaning technology will not be able to meet the demands of the times unless there are fundamental innovations in ultrasonic cleaning technology.

In order to fully utilise innovative ultrasonic cleaning technology for the new era, it is first necessary to have a deep understanding of what ultrasonic cleaning is, why it removes dirt and why it does not, and to dispel misconceptions about its principles.

Ultrasonic cleaning is a cleaning method in which powerful ultrasonic waves are emitted into a liquid and the impact forces generated when cavities are created and extinguished are used.If cavities are not generated, it cannot be said to be ultrasonic cleaning.Ultrasonic cleaning is a technology that uses cavities generated by ultrasonic waves for cleaning. Therefore, the basic requirement for understanding and effectively using ultrasonic cleaning is a correct understanding of cavities and the phenomenon of their generation and disappearance (cavitation).Powerful sound waves above 20 KHz, i.e. ultrasonic waves, are irradiated into the liquid.When sound pressure changes above a certain level in the liquid, so-called cavities are generated.

Cavities consist of a number of vacuum nuclei (microcavities), the overall size of which varies according to the frequency and the magnitude of the sound pressure change, but on a practical level is about 100 microns to a few dozen millimetres.

They come in a variety of shapes and can be broadly divided into gas nebula types [Photo 1] and globular nebula types [Photo 2].In order to distinguish cavities from cavities generated by sound pressure changes other than ultrasound, the author refers to them as cavities (microvacuum nuclei).

Spherical nebulae

gas nebula

Cavities (groups of microvacuum nuclei) are generated and annihilated as follows.At 25 KHz, let’s explain.

  1. [Generation process] Positive shock wave.

    During the initial (1/200,000th of a second) sound pressure decompression process, numerous microvacuum nuclei in the form of a mist are generated in the cavity generating area.The microvacuum nuclei continue to coalesce and grow, reaching their maximum after three to four hundred thousandths of a second and forming a mass of more than a dozen microvacuum nuclei, which are attached to each other like a cluster of grown grapes.This process is a high-speed ejection of liquid from the cavity area, and if the cavity has an outer diameter of 6 mm, this means that liquid moves outwards from the maximum cavity at about 200 m/sec, i.e. a shock wave is generated.This is referred to as a positive shock wave.The size of the shock wave is determined by the size (shape) of the cavity and the speed of movement of the liquid.The liquid (space) trapped between the microvacuum nuclei is subjected to high pressure.This process is responsible for part of the cleaning power of ultrasonic cleaning.However, if it is merely a pressure wave in one direction, dirt can, in some cases, bite through.Ultrasonic cleaning is characterised by the following pressurisation process.

  2. ［Extinction process] Negative shock wave.

    During the depressurisation process, the growing microvacuum nuclei group shrinks without changing its position from the cavity centre.After approximately 200,000/6 seconds, the microvacuum nucleus group is annihilated.To be precise, the annihilation time is faster than the time from the generation to the growth maximum.Contrary to the onset process, there is a high-speed movement of liquid towards the centre of the cavity.Observations have measured a speed of about 220 m/sec.The liquid between the microvacuum nuclei is subjected to a concentration of shock waves and turbulence from the liquid at high velocity and high pressure during a period of 200,000 parts per million or two seconds, first through depressurisation and expansion, and then at high velocity and high pressure.In this process, the shock wave towards the centre of the cavity is called a negative shock wave.This is the source of the cleaning power and a feature of ultrasonic cleaning.

  3. [Cavity migration] Cleaning and diffusion

    The biggest misconception in ultrasonic cleaning technology is the illusion that the visible bubbles, which are generated when ultrasonic waves are irradiated into a liquid, are cavities (groups of microvacuum nuclei) generated by the ultrasonic waves described above.Most cleaning solvents other than water contain large amounts of air.In all alcohols, chlorinated solvents, hydrocarbon solvents and other solvents that can and are used for cleaning, the oxygen dissolved content is more than 20 mg/ℓ.When irradiated with powerful ultrasonic waves, the pressure change causes the dissolved air to deflate and form bubbles, which burst at the rising liquid surface!It does not disappear or shrink in the liquid.This is called ultrasonic gas aeration.The air that has escaped from the liquid is re-dissolved from the liquid surface and the ultrasonic bubble generation phenomenon (ultrasonic bubbling or ultrasonic gas aeration) is perpetuated.The good news is that the tiny air bubbles generated in this case originate from the ultrasonic vibration surface and serve to effectively block and absorb the ultrasound waves.They do not generate the cavities that should be generated by ultrasound.Without measures to prevent these bubbles, it would be sneaky to compete on the frequency and oscillation method of ultrasonic oscillators.

  4. Biggest misconceptions in ultrasonic cleaning.

    The biggest misconception in ultrasonic cleaning technology is the illusion that the visible bubbles, which are generated when ultrasonic waves are irradiated into a liquid, are cavities (groups of microvacuum nuclei) generated by the ultrasound described above.Many cleaning solvents other than water contain large amounts of air.In all alcohols, chlorinated solvents, hydrocarbon solvents and other solvents that can and are used for cleaning, the oxygen dissolved content is 20 mg/ℓ or more.When the liquid is irradiated with powerful ultrasonic waves, the dissolved air is defoamed by the pressure change and bubbles rise to the surface of the liquid and burst!They do not disappear or shrink in the liquid.This is called ultrasonic gas aeration.The air that has escaped from the liquid is re-dissolved from the liquid surface and the ultrasonic bubble generation phenomenon, ultrasonic bubbling, (or ultrasonic gas aeration), is permanent.The good news is that the tiny air bubbles generated in this case originate from the ultrasonic vibration surface and serve to effectively block and absorb the ultrasound waves.They do not generate the cavities that would otherwise be generated by ultrasonic waves.It is absurd to compete on the frequency and oscillation method of ultrasonic oscillators without taking measures against this bubble.

In non-water ultrasonic cleaners without measures to reduce dissolved air, more than 99% of the ultrasonic energy is lost on the vibrating surface!(measured by the author) The reason this was not a problem in non-water cleaning is that the cleaning agent itself had a certain amount of cleaning power, which was overlooked, but these are not rough times.Cavities generated by ultrasound are a high-speed phenomenon that repeats generation and disappearance more than 20 000 times per second, and are not a phenomenon visible to the human eye as bubbles?The bubbles are not a phenomenon visible to the human eye as bubbles, etc.When visible bubbles are observed during ultrasonic oscillation, the ultrasonic tank should be considered to have disappeared, as most of the ultrasonic energy is not used.

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