The principle of operation of an ultrasonic bath - how to choose and use in production or at home. When should the cleaning solution be replaced? Effective cleaning: simple, inexpensive and effective

14.10.2021Heading: Payment

Among all technological processes taking place in liquid media with the effect of ultrasound, the cleaning of the surfaces of solids has received the greatest application.

Ultrasonic cleaning- a cleaning method based on the use of nonlinear effects arising in a liquid under the action of ultrasonic vibrations. Among these effects, cavitation is of prime importance. Other effects: acoustic currents, sound pressure, sound capillary effect.

Cavitation is called the process of formation of cavities and bubbles in an ultrasonic field during the stretching phase, which is available in an alternating sound pressure. During the compression phase, these cavities and bubbles collapse.

Cavitation accelerates a number of physical and chemical processes. The reason for the exceptional efficiency of cavitation is that the collapse of the bubbles begins at the surface to be cleaned. Cavitation is accompanied by the emergence of very high instantaneous hydrostatic pressures, which tear off the dirt particles adhering to the cleaned surface.

Cavitation is heard as a hissing noise generated in a liquid at a certain value of the ultrasonic field intensity.

The introduction of ultrasonic vibrations into washing solutions allows not only to speed up the cleaning process, but also to obtain more high degree surface cleanliness. In this case, in most cases, it is possible to exclude fire hazardous and toxic organic solvents and use exclusively aqueous solutions of technical detergents. This undoubtedly leads to an improvement in the working conditions of workers, an increase in the culture of production, and also allows to partially solve the issues of environmental safety.

Ultrasound is used for cleaning from contaminants arising both during the manufacture of products and parts, and during their operation. Ultrasonic cleaning is especially useful in surface preparation prior to coating application and when cleaning complex cavities and channels in products.

Ultrasound is widely used for cleaning wire, metal tape, nozzles, cables, etc. For special applications of the technology ultrasonic cleaning can be attributed to the cleaning of powders, radioactively contaminated surfaces, regeneration of ceramic filters.

The effectiveness of ultrasonic cleaning depends on the choice of many parameters, including the physicochemical properties of the washing liquid. For the correct choice of solutions, it is necessary to take into account the nature of the contaminants: the degree of their adhesion to the surface to be cleaned, chemical interaction with the cleaning solution, the ability to withstand micro-shock loads (cavitation resistance). Preliminary classification of contaminants is important in order to determine by which of the signs it is easier to remove them from the surface. Having determined this feature, you can choose the right ultrasonic cleaning technology (cleaning media and sound field parameters).

Considering the nature of pollution and the nature of their connection with the surface, the following main types of pollution are distinguished:

Inorganic pollution:

mechanically weakly bonded to the surface (dust, sawdust, metal and non-metal shavings, soot, etc.);
mechanically caricatured into the surface (abrasive grains, mineral or metal particles);
deposited on the surface (salt crusts after treatment in salt baths, scale, etc.).

Contaminants and coatings organic or organic bonded:

mechanically weakly bonded to the surface (dust, plastic sawdust and shavings, soot, coal, coke);
with a low degree of adhesion to the surface (fatty and oil films and lubricants, grinding, polishing and lapping pastes);
firmly adhered to the surface (resin, varnish, glue, paint, etc.).

Ultrasonic cleaning equipment

Ultrasonic cleaning requires a container with a cleaning liquid in contact with the surface to be cleaned, and a source of ultrasonic vibrations, called ultrasonic emitter... The surface of the ultrasonic transducer most often acts as such an emitter. There are also options when the transducer is attached to the tank wall or to the object to be cleaned, which become emitters.

Types of equipment used for ultrasonic cleaning:

The most common and varied devices for ultrasonic cleaning of individual parts are ultrasonic baths. We produce bathtubs of various sizes (from 0.6 to 19,000 liters) and shapes. Depending on the purpose, the baths can be equipped with a variety of additional equipment: heating, timer, overflow pocket, jet cleaning, circulation and filtration of the washing solution, etc.

Small baths with one ultrasonic emitter: UZV-1, UZV-1.1.
Small baths with several emitters, automatic heating and a timer: UZV-2, UZV-4, UZV-7.
Baths with overflow pockets: MO-46, MO-55, MO-197, MO-229, MO-207.
Baths with additional jet cleaning: MO-12.
Baths for cleaning large and very large items: MO-21, MO-92, MO-93.
Special baths for cleaning spray nozzles, plunger bushings, etc.

Ultrasonic modules are used to improve existing washing equipment. They can be embedded in containers, immersed in them, or float on the surface of a liquid.

For cleaning long products (wire, tape, pipes), we offer special installations that can be built into production lines (

What is ultrasound?

Ultrasound (US) - elastic vibrations and waves, the frequency of which is higher than 15 ... 20 kHz. The lower boundary of the region of ultrasonic frequencies, separating it from the region of audible sound, is determined by the subjective properties of human hearing and is conditional. The upper limit is due to the physical nature of elastic waves, which can propagate only in a material environment, that is, provided that the wavelength is much greater than the mean free path of molecules in gases or interatomic distances in liquids and solids Oh. Therefore, in gases, the upper limit of ultrasonic frequencies is determined from the condition of approximate equality of the sound wavelength and the free path of molecules. At normal pressure, it is 10 9 Hz. In liquids and solids, the decisive factor is the equality of the wavelength to the interatomic distances, and the cutoff frequency reaches 10 12 -10 13 Hz. Depending on the wavelength and frequency, ultrasound has specific features of radiation, reception, propagation and application, therefore, it is convenient to subdivide the region of ultrasonic frequencies into three sub-regions:

Low - 1.5–10 ... 10 5 Hz;

Average - 10 5 ... 10 7 Hz;

High - 10 7 ... 10 9 Hz.

Elastic waves with frequencies of 1 · 10 8 ... 1 · 10 13 Hz are commonly called hypersound.

Sound wave theory

Ultrasound as elastic waves

Ultrasonic waves by their nature do not differ from elastic waves of the audible range, as well as from infrasonic waves.

The propagation of ultrasound obeys the basic laws common to acoustic waves of any frequency range, usually called sound waves. The basic laws of their propagation include the laws of reflection and refraction of sound at the boundaries of various media, diffraction and scattering of sound in the presence of obstacles and inhomogeneities in the medium and irregularities at the boundaries, the laws of waveguide propagation in limited areas of the medium.

Specific features of ultrasound

Although the physical nature of ultrasound and the basic laws governing its propagation are the same as for sound waves of any frequency range, it has a number of specific features that determine its importance in science and technology. They are due to its relatively high frequencies and, accordingly, a small wavelength.

So, for high ultrasonic frequencies, the wavelengths are:

In the air - 3.4⋅10 -3 ... 3.4⋅10 -5 cm;

In water - 1.5⋅10 -2 ... 1.5⋅10 -4 cm;

In steel - 1⋅10 -2 ... 1⋅10 -4 cm.

Such a difference in the values of ultrasonic waves (USW) is due to the different speeds of their propagation in different media. For the low-frequency region, ultrasonic wavelengths do not exceed, in most cases, several centimeters and only near the lower boundary of the range reach several tens of centimeters in solids.

USW decays much faster than low-frequency waves, since the sound absorption coefficient (per unit distance) is proportional to the square of the frequency.

Another very important feature of ultrasound is the ability to obtain high intensity values at relatively small amplitudes of the vibrational displacement, since at a given amplitude the intensity is directly proportional to the square of the frequency. The amplitude of the vibrational displacement is in practice limited by the strength of acoustic emitters.

The most important nonlinear effect in an ultrasonic field is cavitation - the appearance in a liquid of a mass of pulsating bubbles filled with vapor, gas, or their mixture. The complex movement of bubbles, their collapse, merging with each other, etc., generate compression impulses (microshock waves) and microflows in the liquid, cause local heating of the medium, ionization. These effects affect the substance: the destruction of solids in the liquid occurs (cavitation erosion), various physical and chemical processes are initiated or accelerated (Fig. 1).

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By changing the conditions for the occurrence of cavitation, it is possible to enhance or weaken various cavitation effects. For example, with an increase in the frequency of ultrasound, the role of microflows increases and cavitation erosion decreases; with an increase in hydrostatic pressure in a liquid, the role of microshock effects increases. An increase in frequency usually leads to an increase in the threshold intensity value corresponding to the onset of cavitation, which depends on the type of liquid, its gas content, temperature, etc. For water in the low-frequency ultrasonic range at atmospheric pressure it is usually 0.3-1 W / cm 3.

Sources of ultrasound

In nature, ultrasound is found in many natural noises (in the noise of the wind, waterfall, rain, in the noise of pebbles rolled by the sea surf, in the sounds accompanying lightning discharges, etc.), as well as in the world of animals that use it for echolocation and communication.

Technical ultrasound emitters used in the study of RAS and their technical applications can be divided into two groups. The first includes emitters-generators (whistles). Oscillations in them are excited due to the presence of obstacles in the path of a constant flow - a jet of gas or liquid. The second group of emitters is electro-acoustic transducers: they convert already given electrical vibrations into mechanical vibrations of a solid body, which emits acoustic waves into the environment.

Application of ultrasound

Multiple applications of ultrasound, in which various of its features are used, can be conditionally divided into three directions. The first is associated with obtaining information through RAS, the second - with an active effect on the substance, and the third - with the processing and transmission of signals (directions are listed in the order of their historical formation).

Ultrasonic cleaning principles

The main role in the effect of ultrasound on substances and processes in liquids is played by cavitation. The most widely used ultrasonic technological process is based on cavitation - cleaning the surfaces of solids. Depending on the nature of the contamination, various manifestations of cavitation, such as microshock impacts, microflows, and heating, may be of greater or lesser importance. By selecting the parameters of the sound field, the physicochemical properties of the washing liquid, its gas content, external factors (pressure, temperature), it is possible to control the cleaning process within wide limits, optimizing it in relation to the type of contamination and the type of parts to be cleaned. A type of cleaning is etching in an ultrasonic field, where the action of ultrasound is combined with the action of strong chemical reagents. Ultrasonic metallization and soldering are actually based on ultrasonic cleaning (including from the oxide film) of the surfaces to be joined or metallized. Brazing cleaning (Fig. 2) is caused by cavitation in the molten metal. In this case, the degree of purification is so high that compounds of materials that cannot be soldered under normal conditions are formed, for example, aluminum with other metals, various metals with glass, ceramics, and plastics.

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In the processes of cleaning and metallization, the sound-capillary effect is also essential, providing the penetration of the cleaning solution or melt into the smallest cracks and pores.

Cleaning and washing mechanisms

Cleaning in most cases requires that impurities be dissolved (in the case of dissolving salts), scraped off (in the case of insoluble salts) or both dissolved and scraped off (as in the case of insoluble particles fixed in a layer of fatty films). The mechanical effects of ultrasonic energy can be useful both to accelerate dissolution and to separate particles from the surface to be cleaned. Ultrasound can also be effectively used in the rinsing process. Residual detergent chemicals can be quickly removed by ultrasonic rinsing.

When removing contaminants by dissolution, the solvent must come into contact with the contaminating film and destroy it (Fig. 3, a). As the solvent dissolves the contamination, a saturated solution of contamination in the solvent arises at the solvent – contamination interface, and dissolution stops, since there is no delivery of fresh solution to the contamination surface (Fig. 3, b).

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Exposure to ultrasound destroys the saturated solvent layer and ensures the delivery of fresh solution to the contamination surface (Fig. 3, c). This is especially effective when cleaning is performed on “irregular” surfaces with a labyrinth of sinuses and surface relief, such as printed circuit boards and electronic modules.

Some contaminants are a layer of insoluble particles firmly adhered to the surface by the forces of ionic bonding and adhesion. It is enough only to separate these particles from the surface in order to break the forces of attraction and transfer them into the volume of the cleaning medium for subsequent removal. Cavitation and acoustic currents rip off contaminants such as dust from the surface, wash off and remove them (Fig. 4).

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Pollution, as a rule, is multicomponent and may contain both soluble and insoluble components in a complex. The effect of ultrasound is that it emulsifies any components, that is, it transfers them into a washing medium and, together with it, removes them from the surface of products.

To introduce ultrasonic energy into the cleaning system, an ultrasonic generator, a converter of the generator's electrical energy into ultrasonic radiation and an acoustic power meter are required.

An electric ultrasonic generator converts electrical energy from the grid into electrical energy at an ultrasonic frequency. This is done by known methods and does not have any specificity. However, it is preferable to use a digital generation technique, when the output is rectangular pulses of alternating polarity (Fig. 5). The efficiency of such generators is close to 100%, which makes it possible to solve the problem of the energy consumption of the process. The use of a rectangular waveform results in acoustic radiation rich in harmonics. The advantages of a multifrequency cleaning system are that no “dead” zones are formed in the interference nodes in the volume of the cleaning medium. Therefore, multi-frequency ultrasound irradiation makes it possible to locate the cleaning object practically in any zone of the ultrasound bath.

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Another technique for getting rid of the "dead" zones is to use a swept generator (Fig. 6). In this case, the nodes and antinodes of the interference field move to different points of the cleaning system, without leaving any areas for cleaning without irradiation. But the efficiency of such generators is relatively low.

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There are two general types of ultrasonic transducers: magnetostrictive and piezoelectric. They both perform the same task of converting electrical energy into mechanical energy.

Magnetostrictive converters (Fig. 7) use the effect of magnetostriction, in which some materials change their linear dimensions in an alternating magnetic field.

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The electrical energy from the ultrasonic generator is first converted by the winding of the magnetostrictor into an alternating magnetic field. The alternating magnetic field, in turn, generates mechanical vibrations of the ultrasonic frequency due to the deformation of the magnetic circuit in time with the frequency of the magnetic field. Since magnetostrictive materials behave like electromagnets, the frequency of their deformation vibrations is twice the frequency of the magnetic, and, therefore, the electric field.

Electromagnetic converters are characterized by an increase in energy losses for eddy currents and magnetization reversal with increasing frequency. Therefore, powerful magnetostrictive transducers are rarely used at frequencies above 20 kHz. Piezo transducers, on the other hand, can emit well in the megahertz range. Magnetostrictive transducers are generally less efficient than their piezoelectric counterparts. This is primarily due to the fact that a magnetostrictive converter requires a double energy transformation: from electrical to magnetic and then from magnetic to mechanical. Energy losses occur at every transformation. This reduces the efficiency of magnetostrictors.

Piezo transducers (Fig. 8) convert electrical energy directly into mechanical energy through the use of the piezoelectric effect, in which some materials (piezoelectrics) change their linear dimensions when an electric field is applied. Previously, piezoelectric emitters used such piezoelectric materials as natural quartz crystals and synthesized barium titanate, which were fragile and unstable, and therefore unreliable. More durable and highly stable ceramic piezoelectric materials are used in modern converters. The vast majority of ultrasonic cleaning systems today use the piezoelectric effect.

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Ultrasonic cleaning equipment

The range of ultrasonic cleaning equipment used is very wide: from small tabletop modules in dentistry, jewelry stores, the electronics industry, to huge systems with volumes of several thousand liters in a number of industrial applications.

Right choice necessary equipment is of paramount importance to the success of ultrasonic cleaning. The simplest ultrasound cleaning application may require as little as heated washing liquid. More complex purification systems require a large number of baths, the latter of which must be filled with distilled or deionized water. The largest systems use submersible ultrasonic transducers, the combination of which can irradiate baths of almost any size. They provide maximum flexibility and ease of use and maintenance. Ultrasonic baths heated detergent solution is most often used in laboratories, medicine, jewelry.

Lines of ultrasonic cleaning (Fig. 9), used in large-scale production, combine in one building electric ultrasonic generators, ultrasonic transducers, a transport system for moving objects to be treated in the baths and a control system.