WO2017220420A1

WO2017220420A1 - Method and device for ultrasonic cleaning

Info

Publication number: WO2017220420A1
Application number: PCT/EP2017/064645
Authority: WO
Inventors: Oscar GONZALO DE FRANCISCO; Jon Ander SARASUA MIRANDA; Manu GOIOGANA BENGOETXEA
Original assignee: Fundación Tekniker
Priority date: 2016-06-23
Filing date: 2017-06-14
Publication date: 2017-12-28
Also published as: ES2708149B2; ES2708149A2; EP3260209A1; ES2708149R1

Abstract

A method for ultrasonic cleaning of a part (60, 600) which comprises: applying a cleaning solution on a surface (65, 650) of a part (60, 600) to be cleaned; moving (M) a device (10, 10', 100) comprising a sonotrode (4, 4', 40, 50) along said surface (65, 650), maintaining a distance (D) between said surface (65, 650) and the output surface (4e, 4e', 40e, 50e) of the sonotrode (4, 4', 40, 50) closest to said surface (65, 650); applying ultrasonic vibration (V) by the output surface (4e, 4e', 40e, 50e) of the sonotrode (4, 4', 40, 50), wherein the applied ultrasonic vibration (V) has a certain vibration amplitude; as a consequence of said ultrasonic vibration (V), generating a linear homogeneous column (20) of cleaning solution between said output surface (4e, 4e', 40e, 50e) of the sonotrode (4, 4', 40, 50) and said surface (65, 650) and exposing said surface (65, 650) in contact with said linear homogeneous column (20) of cleaning solution to cavitation, thus removing dirt (70) from said surface (65, 650).

Description

METHOD AND DEVICE FOR ULTRASONIC CLEANING

TECHNICAL FIELD

The present invention relates to the field of cleaning, both industrial and home cleaning. More precisely, it relates to methods and systems for ultrasonic cleaning.

STATE OF THE ART

An ultrasonic cleaning is a process that uses ultrasounds (usually between 20 and 40 kHz) and an appropriate liquid to clean items. Ultrasonic cleaners are used to clean many different types of objects, including optical parts, surgical instruments, tools, industrial parts and electronic equipment. Ultrasonic cleaning can be used for a wide range of workpiece shapes, sizes and materials. In ultrasonic cleaning, the object to be cleaned is immersed in a metallic tank containing a liquid solution (in an aqueous or organic solvent, depending on the application). An ultrasound generating transducer built into the chamber, or lowered into the liquid, produces ultrasonic waves in the liquid by changing size in concert with an electrical signal oscillating at ultrasonic frequency. These elements usually form a resonant circuit. The electrical signal is produced by a high frequency electric source. Ultrasonic cleaning uses cavitation bubbles induced by high frequency pressure (sound) waves to agitate the liquid. During cavitation, gas bubbles collapse with enormous energy, releasing strong shock waves. When bubbles implode near a surface, such as the surface to be cleaned, asymmetric collapse may occur, releasing strong water jets. Both phenomena contribute to remove dirt and accelerate chemical dissolution processes. In other words, the agitation produces high forces on contaminants adhering to substrates like metals, plastics, glass rubber, and ceramics. As appropriate liquid, water or solvents can be used, depending on the type of contamination and the workpiece. Use of a solvent appropriate for the item to be cleaned and the type of soiling present usually enhances the cleaning effect. Contaminants can include dust, dirt, oil, pigments, rust, grease, algae, fungus, bacteria, lime scale, polishing compounds, flux agents, fingerprints, soot wax and mold release agents, biological soil like blood, and so on.

An important limitation of conventional ultrasonic cleaners is that the part to be cleaned needs to be completely immersed in the tank containing the liquid. Therefore, conventional ultrasonic cleaners are unfeasible for large-sized parts or devices, or for non-detachable structures, such as large-sized window panes, building fronts, floors, and so on.

JP H06 2183337 A discloses a portable device for ultrasonic cleaning of a part that does not require the comple immersion of the part within a tank filled with cleaning solution. Cleaning is performed by blowing water with ultrasonic irradiation towards the object to be cleaned. Water is contained in a reservoir. A water puddle is formed between the ultrasonic device and the surface to be cleaned.

Therefore, there is a need for developing a new method and device for ultrasonic cleaning that overcome the drawbacks of conventional ones, that is to say, that are suitable for ultrasonic cleaning of large-sized parts or devices and for non-detachable structures.

DESCRIPTION OF THE INVENTION

It is an object of the present invention to provide a new method and device for ultrasonic cleaning which does not require immersing the part to be cleaned within a tank filled with liquid. Based on the same principles of conventional ultrasonic cleaning, the method and device manage to remove dirt from large surfaces (such as metals, bricks, tiles...) without immersion.

In a first aspect of the invention, it is provided a method for ultrasonic cleaning of a part. The method comprises: applying a cleaning solution on a surface of a part to be cleaned; moving a device comprising a sonotrode along said surface, maintaining a distance between said surface and the output surface of the sonotrode closest to said surface; applying ultrasonic vibration by the output surface of the sonotrode, wherein the applied ultrasonic vibration has a certain vibration amplitude; as a consequence of said ultrasonic vibration, generating a linear homogeneous column of cleaning solution between said output surface of the sonotrode and said surface and exposing said surface in contact with said linear homogeneous column_of cleaning solution to cavitation, thus removing dirt from said surface.

In a particular embodiment, the vibration amplitude applied at the output surface of the sonotrode depends on the vibration amplitude at the input surface of said sonotrode and on the ratio between the input cross-section of the sonotrode and the output cross- section of the sonotrode.

The output surface of the sonotrode is preferably curved, the output surface being defined by a radius r, wherein r = w / 2, w being the width of the output cross-section of the sonotrode.

In a particular embodiment, the vibration amplitude applied at the output surface of the sonotrode does not exceed a threshold above which said cleaning solution atomizes, said threshold being dependent on the type of cleaning solution and on the working frequency.

In a particular embodiment, the device is moved along said surface maintaining a substantially constant distance between said surface and the output surface of the sonotrode closest to said surface.

In a particular embodiment, the cleaning solution is applied on the surface to be cleaned prior to starting operation of the sonotrode, in such a way that a layer of cleaning solution is disposed on said surface.

In an alternative embodiment, the cleaning solution is applied on the surface to be cleaned as the sonotrode moves along said surface to be cleaned, in such a way that said linear homogeneous column of cleaning solution is disposed between said surface and the output surface of the sonotrode. The cleaning solution may be supplied externally to the sonotrode by means of a syringe or a spraying nozzle; or it may be supplied internally to the sonotrode along a channel disposed within the sonotrode, said channel being designed to take cleaning solution to the output surface of the sonotrode closest to the surface to be cleaned.

The cleaning solution may be reused by the sonotrode sucking up dirty droplets, filtering said dirty droplets and supplying filtered droplets while the sonotrode moves forward over said surface to be cleaned.

In a particular embodiment, the cleaning solution is selected from the following group: water and aqueous solutions comprising at least one chemical agent.

In a second aspect of the invention, it is provided a device for ultrasonic cleaning a part with the aid of a cleaning solution to be disposed on a surface of the part. The device comprises: an ultrasonic wave oscillator configured to convert a standard electric signal working at 50-60 Hz into electrical energy working at a frequency comprised within the ultrasonic frequency; an ultrasonic converter for converting said electrical energy provided by the ultrasonic wave oscillator into mechanical vibration; an ultrasonic sonotrode coupled to said ultrasonic converter and configured to generate at its output surface ultrasonic vibration having certain vibration amplitude; wherein in use of the device, said sonotrode is configured to, as a consequence of said ultrasonic vibration at its output surface, generate a linear homogeneous column of cleaning solution between said output surface of the sonotrode and said surface and to expose said surface in contact with said linear homogeneous film of cleaning solution to cavitation, thus removing dirt from said surface.

Additional advantages and features of the invention will become apparent from the detail description that follows and will be particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

To complete the description and in order to provide for a better understanding of the invention, a set of drawings is provided. Said drawings form an integral part of the description and illustrate an embodiment of the invention, which should not be interpreted as restricting the scope of the invention, but just as an example of how the invention can be carried out. The drawings comprise the following figures:

Figure 1 shows a device for ultrasonic cleaning according to an embodiment of the present invention.

Figures 2(a), 2(b) and 2(c) show the behavior of water under different working conditions during operation of the method according to the invention.

Figure 3 shows a sonotrode or horn according to a particular embodiment of the invention.

Figure 4 shows a detailed view of the output surface of the sonotrode of figure 3. An output cross section is also shown.

Figure 5 shows an ultrasonic horn being moved along the surface of a part to be cleaned, according to an embodiment of the method for ultrasonic cleaning of the invention. Figure 6 illustrates an ultrasonic horn during operation of the method for ultrasonic cleaning according to the invention.

Figure 7 shows a brick, half of which surface has been successfully cleaned applying the method of the present invention.

DESCRIPTION OF A WAY OF CARRYING OUT THE INVENTION

In this text, the term "comprises" and its derivations (such as "comprising", etc.) should not be understood in an excluding sense, that is, these terms should not be interpreted as excluding the possibility that what is described and defined may include further elements, steps, etc.

In the context of the present invention, the term "approximately" and terms of its family (such as "approximate", etc.) should be understood as indicating values very near to those which accompany the aforementioned term. That is to say, a deviation within reasonable limits from an exact value should be accepted, because a skilled person in the art will understand that such a deviation from the values indicated is inevitable due to measurement inaccuracies, etc. The same applies to the terms "about" and "around" and "substantially".

The following description is not to be taken in a limiting sense but is given solely for the purpose of describing the broad principles of the invention. Next embodiments of the invention will be described by way of example, with reference to the above-mentioned drawings showing apparatuses and results according to the invention.

Ultrasonic cleaning without immersing the part to be cleaned into a tank filled with liquid can be carried out using the device 10 illustrated in figure 1. The part to be cleaned is also shown (part 60). In the embodiment, the surface 65 to be cleaned of part 60 has been depicted as being substantially flat, but the surface 65 may be single curved; for example, a cylinder. It may have irregularities, such as cavities, pores, etc. The device comprises an ultrasonic wave generator 1 plugged to a conventional line voltage (electric grid) 5. The line voltage 5 usually works at 50-60 Hz, depending on the country. The ultrasonic generator 1 converts the standard electric signal working at 50- 60 Hz into electrical energy working at ultrasonic frequency, that is to say, above 20,000 cycles per second (20 KHz) approx. In other words, ultrasonic wave generator 1 generates high-frequency (ultrasonic) electrical energy. The electrical energy provided by the ultrasonic wave generator 1 is converted at an ultrasonic converter (also referred to as ultrasonic transducer) 2 into mechanical vibration. It is a harmonic vibration whose frequency is the same one as the one generated by the wave generator 1. The amplitude of the vibration (maximum displacement) is related to electric power, and it can be mechanically amplified by other components like boosters 3 or sonotrodes 4. Specific characteristics of this mechanical vibration are given later on in this description. The device 10 may optionally have a booster 3 (also referred to as amplifier 3). A booster 3 is required in applications demanding high power (that is to say, power higher than the one provided by the ultrasonic wave generator 1 ). Thus, in applications requiring lower power, the booster 3 may be removed. When present, the booster 3 is connected to the ultrasonic converter 2. The booster 3 amplifies the ultrasonic vibration input from the ultrasonic converter 2. The booster 3 also serves as a mount for a sonotrode or horn 4.

The sonotrode or horn 4 is connected to the booster 3 and receives the ultrasonic vibration from the booster 3. In the absence of booster, the sonotrode 4 is directly connected to the ultrasonic converter 2. The material of which the sonotrode 4 is made may vary on the power requirements, which in turn depend on the application. Preferably, in applications requiring low power (for example, power < 400 Watt) the sonotrode 4 may be made of aluminium, while in applications requiring high power (for example, power > 400 Watt) the sonotrode 4 may be made of titanium.

A possible implementation of a sonotrode 4 is illustrated in figure 3. The ultrasonic sonotrode or horn 4 includes an input-side end section or main body 4a having an input surface 4c that receives the ultrasonic vibration form the booster 3 (or from the ultrasonic converter 2, as the case may be), and an output-side end section 4b having an output surface 4e (also referred to as sonotrode's tip) disposed spaced apart from the input surface 4c by a distance corresponding to the half-wavelength (½λ) of the ultrasonic vibration that is input to the input surface 4c, and which outputs ultrasonic vibration to the liquid disposed on surface 65. The input surface 4c of the main body or input-side end section 41 has a certain width W and length L. The output-side end section 4b is a thinner body 42 having a certain width w, wherein w < W, and same length L. The inventors have surprisingly observed that the output ultrasonic vibration, when in contact with liquid disposed on a surface, makes liquid cavitate at high frequency, thus removing dirt from the surface 65 of the part 60 on which the liquid is applied. The ultrasonic vibration that is input to the ultrasonic sonotrode 4 are longitudinal waves in a direction perpendicular to the output surface 4e. The sonotrode 4 also has at least one slotted aperture extending through the sonotrode. It preferably has two slotted apertures 43 44. In a particular embodiment, the slotted apertures 43 44 extend from the input-side end section 41 to the output-side end section 42. Two apertures 43 44 are preferably used because they contribute to achieve homogeneous vibration and therefore to achieve homogeneous cleaning. In order for the sonotrode 4 to be capable of cleaning a surface, the design (geometry) of the output surface 4e of the sonotrode 4, together with the vibration amplitude provided by the sonotrode, plays an important role. The output surface 4e is preferably curved (arc-shaped).

In order to clean the surface 65, there must be a liquid interfase (liquid drop) between the surface 65 and the sonotrode tip 4e. It is well-known that ultrasonic vibration reduces liquids' surface stress, which increases the contact surface between the liquid (cleaning solution) and the solid (sontrode 4 and surface 65). If the contact surface increases, the liquid interfase will be able to retain a bigger liquid volume. The more vibration amplitude we have, the more contact surface we will have and so, a higher volume of cleaning solution will be retained. In order to gather as much liquid volume as possible between the sonotrode's tip 4e and the surface 65, it is desired that the sonotrode's tip 4e has as much output surface as possible. Therefore, although the sonotrode's tip may be flat-shaped in a possible embodiment, in a preferred embodiment the sonotrode's tip (output surface 4e) is chosen to be arc-shaped, that is to say, curved, as highlighted in figure 4. In other words, the sonotrode's tip preferably has the shape of half a cylinder having length L and radius r. There are two main reasons for this curved output surface 4. On the one hand, it increases even more the contact surface between the liquid and the sonotrode. Indeed, a semicylinder (half a cylinder) gives the optimum ratio between perimeter and surface. On the other hand, it lets the user tilt the equipment without changing the relative distance between the sonotrode's tip 4e and the surface to be cleaned. Other shapes of the sonotrode's tip are possible, being those shapes having large surface the preferred ones.

For a given frequency, depending on the liquid to be disposed between the sonotrode's tip and the surface to be cleaned, there is a critical threshold value, at which vibration amplitude is so high that the liquid drop gets unstable and it gets atomized. This threshold depends on the physical properties of the liquid. In particular, it depends on the liquid's density, viscosity and superficial stress. For example, in the case of tap water being the cleaning solution, and given a frequency of 20 kHz, this critical threshold is 1 1 μηη. In another example, in the case of acetone (cleaning solution), and given a frequency of 20 kHz, this critical threshold is below 3 μηη. The vibration amplitude above which the liquid drop gets atomized also depends on the frequency applied. In particular, the critical threshold (threshold above which the drop atomizes) decreases with the squared frequency. For example, if the frequency varies from 20 kHz to 40 kHz (that is, x2), the vibration amplitude threshold is divided by 2². In general, if the frequency is multiplied by N (xN), the vibration amplitude threshold is divided by 2^N.

These phenomena are illustrated in figures 2(a-c). Figure 2(a) shows a situation in which no ultrasounds (US) are applied by the sonotrode 4' to the layer of liquid (cleaning solution) disposed below it. In this case, conventional liquid drops may adhere to the output surface 4e' of the sonotrode 4'. Figure 2(b) shows a situation in which the sonotrode 4' (and the ultrasonic vibration it produces) has managed to reduce stress on the liquid below without atomizing it. When this happens, the liquid tends to 'stick' to the sonotrode output surface 4e' (in other words, the liquid is attracted towards the output surface 4e'), generating a big linear homogeneous liquid drop 20 (also referred to as linear homogeneous column or static column of liquid 20). The larger the output surface 4e' of the sonotrode 4 is, the more amount of liquid will form the static column of liquid 20. Any physical object that gets contact with this drop 20 will be exposed to a very intensive cavitation and therefore, cleaned. If there is a relative movement (such as a sweeping movement) between the drop 20 and the dirty porous object, the drop 20 will lose part of its content, which will remain inside the pores. To avoid this problem, the porous surface to be cleaned must be previously wet. Finally, figure 2(c) shows a situation in which the ultrasonic vibration produced by the sonotrode 4' has atomized the liquid disposed below the sonotrode output surface 4e'. The dirty surface is therefore not cleaned. In the examples of figures 2(a-c), the liquid is tap water. As already mentioned, the critical threshold of the vibration amplitude applied by the output surface 4e' of the sonotrode 4' at which tap water does not atomize is 1 1 μηη given a frequency of 20 kHz. That's why the desired effect (figure 2(b)) happens when said vibration amplitude is equal or below 1 1 μηη, while the undesired effect (figure 2(c)) happens when said vibration amplitude is above 1 1 μηη.

Referring now to figures 3 and 4, the amplitude of the vibration applied at the output surface 4e of the sonotrode 4 when it is in use, depends on the amplitude of the vibration at the input surface 4c (provided by the converter) and on the ratio between the input cross-section and output cross-section 4f of the sonotrode 4. The input surface 4c is flat, and so, it coincides with the input cross section. The input cross- section 4c follows the formula L x W. Nevertheless, in the preferred embodiment illustrated in figures 3 and 4, the output surface 4e is arc shaped (it follows the shape of a semicylinder (half cylinder), and so, it does not coincide with an output cross-section 4f. The output cross-section 4f follows the formula w x L. The output surface 4e follows the formula π x r x L, wherein "r" is the radius of a cylinder having length L. It is remarked that the sonotrode's tip (output surface 4e) does not comprised the area of the two bases of the semicylinder, but only the lateral area defined by half a cylinder. Figure 4 shows the difference between the output surface 4e and the output cross section 4f. Because the output section 4e is a semicylinder, it's radius is determined by the width w of the cross section: r = w / 2. Therefore, the output cross-section 4f follows the formula 2 x r x L.

As already explained, the inventors have observed that the applied vibration amplitude at the output surface 4e of the sonotrode 4 is very important, since for each cleaning solution that may be used, there is a critical threshold or maximum value of this vibration amplitude, in such a way that if the applied vibration amplitude exceeds the threshold associated to each cleaning solution, the cleaning solution will atomize (figure 2(c)) instead of forming a linear homogeneous liquid drop 20 or static column of liquid 20 (figure 2(b)). The inventors have also observed that, when the vibration amplitude does not exceed said critical threshold, the larger the output surface 4e of the sonotrode 4 is, the more amount of static column of liquid is kept between the output surface 4e and the surface to be cleaned.

In order to achieve the desired effect (figure 2(b)), the sonotrode's tip 4e, 4e' preferably has a semicylindrical (half a cylinder) shape of surface n x r x L = n x w x L / 2, with corresponding output cross-section 4f being w x L = 2 x r x L, such that, at nominal power conditions, and taking into account the area W x L of the input surface 4c, the vibration amplitude produced by the output surface 4e in use of the sonotrode doesn^'t exceed the critical amplitude for the liquid being used (1 1 μηη in the case of tap water at 20 kHz). Each converter 2 (figure 1 ) has a nominal electric power that ensures a given vibration amplitude. The sonotrode 4, 4' amplifies this vibration proportionally to the ratio between its input cross-section 4c and its output cross-section 4f. For example, assuming that the cleaning solution is water, if at nominal power conditions, a converter gives a vibration amplitude of 5.5 microns (5.5 μηη) and the frequency is 20 kHz, the ratio between the sonotrode's output and input cross-sections must not exceed 2, because otherwise, the sonotrode output surface 4e, 4e' will provide a vibration amplitude larger than 1 1 microns (1 1 μηη) and therefore, water will be atomized.

That is to say, given a certain cleaning solution (having specific physical properties) and given the width W of the input cross-section 4c of the sonotrode and the width w of the output cross-section 4f of the sonotrode (which, in the particular case in which the sonotrode's tip is arc-shaped, its radius r follows the expression r = 2 x w), there is a vibration amplitude threshold above which the ultrasonic vibration produced by the sonotrode 4, 4' at its output produces the atomization of the liquid disposed below the sonotrode output surface 4e, 4e'. As a consequence, the dirty surface is not cleaned. On the other hand, it is advisable to work with a vibration amplitude close to (but below) said threshold, because if the vibration amplitude is much lower than the threshold, cleaning capacity will be reduced. In sum, depending on the output power of the transducer 2 cleaning solution and width W of the input cross-section, there is an optimum sonotrode's radius that retains the maximum volume of cleaning solution without atomizing it.

The surface 65 to be cleaned may be a substantially single curvature surface (linear, cylindrical, parabolical...) so that it can be cleaned by a sweeping movement. The surface 65 must be solid. This surface may have geometrical irregularities, such as cracks, cavities, pores or any other irregularity, such as an ornamental pattern. In order to be cleaned properly, the height or depth of the irregularity is preferably smaller than the height of the retained liquid volume (column 20), as shown for example in figure 2(b).

The use of the device 10' for ultrasonic cleaning a part 60 is as follows: The device 10' is swept along the surface 65 of the part 60 to be cleaned, as illustrated in figure 5. This operation can be performed either manually or by means of automatic devices such as robotic manipulators. In order to avoid damage (such as scratch) on the sonotrode's tip and/or on the surface to be cleaned, physical contact between the surface 65 and the sonotrode tip 4e must be preferably avoided. This problem concerns especially to manual use. Therefore, the device 10' preferably includes mechanical means, such as wheels, for maintaining a certain gap between the tip 40e of the sonotrode 40 and the cleaning solution disposed on the surface 65 to be cleaned. Such mechanical means are specially recommended during manual sweeping movement. Figure 6, which will be described in detail next, shows the sweeping distance D between the sonotrode's tip 50e and the surface 650 to be cleaned when sweeping the device 100. The maximum distance (D_max) from the surface 650 at which the sonotrode 50 can be swept is determined by the largest obtainable drop or column of cleaning solution. The maximum distance (D_max) is therefore determined by the maximum volume (amount) of liquid that can be retained between the sonotrode's tip 50e and the surface. In other words, it depends on the size (length) of the static column of liquid. If the distance D is above the maximum value D_max), there is no more contact between the drop contained between the sonotrode tip 50e and the wet surface 650 and therefore the static column breaks. This maximum volume of retained liquid depends on the type of cleaning solution (having specific physical properties), on the material of the sonotrode and on the output surface 4e of the sonotrode (in the particular embodiment in which the output surface 4e is given by half a cylinder, the maximum volume of retained liquid depends on the radius r an on the length L). In a particular example, in which a sonotrode made of aluminium is used, and the sonotrode has a semi-cylindrical output surface 4e having radius = 5mm and length L = 100 mm, it has been observed that if the distance D exceeds 5 mm, which is indeed the largest obtainable water drop or water column (see column 20 in figure 2(b)) under those circumstances, there is no more contact between the drop contained between the sonotrode tip 50e and the wet surface 650; no cleaning will occur. In other words, the static column breaks. In a particular example, the distance D is selected to be 3 mm in order not to work at limit conditions.

With reference to either figure 5 or 6, between the surface 65, 650 of part 60, 600 and the sonotrode 40, 50, a layer of liquid or film of liquid (cleaning solution) is applied (not visible in figure 5). The ultrasonic vibration applied by the sonotrode 40, 50 makes the cleaning solution cavitate, releasing strong shock waves and water jets on surface 65, 650. Both shock waves and water jets remove dirt particles (sand, soil, dust, moos, mud...) and accelerate dissolutions (paint, oil, grease...). Cavitation can penetrate into pores, cracks, cavities or any other pattern present on the surface 65, 650 to be cleaned. In other words, ultrasonic cavitation power is concentrated on a thin layer of liquid (cleaning solution) when the sonotrode 40, 50 applies ultrasonic vibration to the cleaning solution. Non-limiting examples of cleaning solutions that may be used are water (such as tap water) and aqueous solutions comprising chemical agents, such as soap, acetone and alcohol. The distance D between the output surface 40e 50e of the output-side end section of the sonotrode 40, 50, closest to the surface 65, 650 to be cleaned, is preferably larger than the thickness of the layer of cleaning solution. That is to say, the output surface 40e, 50e of the sonotrode 40, 50 is preferably not in constant contact with the cleaning solution. This distance D, illustrated in figure 6, between the output surface 50e of the sonotrode 50 and the surface 650 to be cleaned is preferably constantly maintained along the sweeping process.

Application of a cleaning solution to the surface 65, 650 to be cleaned may be done in different ways. In a particular embodiment, prior to moving or sweeping the sonotrode 40, 50 over the surface 65, 650 to be cleaned, a layer of cleaning solution is applied on that surface, in such a way that substantially the whole surface 65, 650 to be cleaned is covered with a layer of liquid. To work in this way, an initial impulse (a single liquid drop) is necessary. If ultrasonic vibration is activated, as the drop touches both the liquid layer and the sonotrode's tip, a static liquid column (a big linear drop, as the one illustrated in figure 2(b)) will be generated. It does not matter if there is relative movement between the sonotrode and the liquid layer because the surface stress of the liquid column is so low that it tends to 'stick' to both elements. In other words, once the column is formed, it will follow the sonotrode's sweeping movement. Another way of generating this liquid column is by moving down the sonotrode until it gets in contact with the liquid layer.

As already mentioned, the sonotrode 4, 40, 50 (in fact, the whole device 10, 10', 100) is moved over the surface 65, 650, sweeping the surface from a first end 65a to the opposite end 65b, along the length of the surface 65, 650 to be cleaned, as represented in figure 5. If the surface to be cleaned is wider than the width of the sonotrode, it will be necessary to sweep the device 10, 10', 100 along the surface 65, 650 as many times as required for the sonotrode 40, 50 to apply ultrasonic vibration on the water disposed on the whole surface to be cleaned. In another particular embodiment, a layer of cleaning solution covering the whole surface to be cleaned is not applied prior to sweeping the sonotrode 4, 40, 50 over the surface 65, 650 to be cleaned. Instead, the mentioned liquid column is constantly regenerated. In this embodiment, a cleaning solution may be provided externally to the sonotrode 4, 40, 50 for example by means of a syringe or a spraying nozzle. Alternatively, the cleaning solution may be provided internally to the sonotrode 4, 40, 50 for example along a channel disposed within the sonotrode, designed to take water to the output-side end section 4b of the sonotrode 4, 40, 50 and more preferably, to the output surface 4e, 40e of the sonotrode 4, 40, 50.

Figure 6 illustrates the operation of a device 100 for ultrasonic cleaning according to the invention. The ultrasonic wave generator and the booster are not shown in figure 6. The part 600 to be cleaned is also shown. In figure 6, arrow M represents the sweeping movement of the device 100. The surface 650 of the part 600 to be cleaned is cleaned while the device 100 moves forward. Dirt 70 on the surface 650 not yet cleaned is shown. Figure 6 refers to an embodiment in which no layer of cleaning solution is disposed on the surface 650 to be cleaned prior to the movement of the sonotrode 50. Instead, a cleaning solution is provided to the surface 650 as the sonotrode 50 moves forward. Arrow A1 represents the direction of droplets 80 of cleaning solution being supplied while the device 100 moves. When the cleaning solution droplets 80 become in contact with the output surface 50e of the sonotrode 50, ultrasonic vibration V occurs, as a consequence of which dirt is removed from the surface 650 of the part 600. Arrow A2 represents the direction of dirty droplets 71 being sucked up by an external vacuum device while the sonotrode 50 moves forward. In a particular embodiment, in which the cleaning solution is water, dirty water 71 may be filtered in the device 100, which continuously provides filtered water 80, thus reusing water. The same system might be used for any kind of cleaning solution. In this case, arrow A3 schematically represents the direction of dirty water (or cleaning solution, in general), which is filtered in order to be reused. In a particular embodiment, dirty water 71 is feedback and filtered by means of a hydraulic system that includes particle filtration (not shown). In other words, in this embodiment, the water layer is constantly regenerated thanks to filtering means. In this way, the only water consumed by the device 100 is the water that remains on the clean surface, for example on the pores, if any, disposed on the surface 650 of the part 600. Water consumption is very low (just the necessary amount to wet the surface 650 to be cleaned).

Next, an example is disclosed.

An ultrasonic converter works at 20 kHz and provides vibration having amplitude of 20 microns (20 μηη) at its nominal power (1200 W). This output amplitude is too high for the cleaning application. Therefore, it has been reduced to 20% of the nominal power (4 μηη of output vibration). A sonotrode having an input cross-section of 30x100 mm² (W x L) is used. The sonotrode has a curved output surface, having a semicylindrical shape defined by a radius r = 5 mm and length L = 100 mm. The output cross-section is therefore 2 x r x L = 10 X 100 mm². If we divide both values, the ratio is 3, which means that this sonotrode will increase the input amplitude (4 μηη) by 3, obtaining in this way 12 μηη. This value is 1 μηη higher than the maximum vibration amplitude for tap water. However, due to mechanical damping, it becomes slightly smaller, therefore meeting perfectly the optimum value of 1 1 μηη without atomization. If vibration is activated and water is poured on the tip of the sonotrode, a linear water drop is generated. The device is swept over the surface to be cleaned at a substantially constant distance D= 3 mm. When this water drop gets in contact with a dirty surface (a brick in this case, figure 7) it cleans it deeply, but only on the surface below the sonotrode's tip (100 mm x 10 mm). In order to clean the whole brick surface, the system is swept, in this case, at 10 m/min speed.

In conclusion, the proposed method and device permit to clean parts of any size using ultrasonic technics, without immersing the part into a tank filled with water. The method and device are indicated for ultrasonic cleaning of both parts having substantially flat or single curved surfaces, having geometrical irregularities, such as cracks, cavities, pores or patterns. Non-limiting examples of applications of the invention are cleaning of walls having graffiti and cleaning of bricks or tiles. Among the advantages of the invention, no physical contact between the sonotrode and the part to be cleaned is required. Besides, the used layer of cleaning solution can be constantly filtered and reused. In fact, over impermeable surfaces, such as glass or metal, when a static liquid layer is generated, liquid consumption is nearly zero because it can be constantly filtered and reused. Ultrasound waves penetrate into small pore, cracks or cavities of the surface to be cleaned. It can be used either with only water or with an aqueous solution comprising chemical agents. In this last case, ultrasonic cavitation drastically accelerates chemical reactions, dissolving the dirt adhered to the surface to be cleaned. The method and device can be applied both for industrial and for home cleaning tasks.

On the other hand, the invention is obviously not limited to the specific embodiment(s) described herein, but also encompasses any variations that may be considered by any person skilled in the art (for example, as regards the choice of materials, dimensions, components, configuration, etc.), within the general scope of the invention as defined in the claims.

Claims

1.- A method for ultrasonic cleaning of a part (60, 600), the method comprising:

-applying a cleaning solution on a surface (65, 650) of a part (60, 600) to be cleaned;

-moving (M) a device (10, 10', 100) comprising a sonotrode (4, 4', 40, 50) along said surface (65, 650), maintaining a distance (D) between said surface (65, 650) and the output surface (4e, 4e', 40e, 50e) of the sonotrode (4, 4', 40, 50) closest to said surface (65, 650);

-applying ultrasonic vibration (V) by the output surface (4e, 4e', 40e, 50e) of the sonotrode (4, 4', 40, 50), wherein the applied ultrasonic vibration (V) has a certain vibration amplitude;

the method being characterized in that it comprises:

- generating a linear homogeneous column (20) of cleaning solution between said output surface (4e, 4e', 40e, 50e) of the sonotrode (4, 4', 40, 50), said output surface (4e, 40e) of the sonotrode (4, 4', 40, 50) being curved, and said surface (65, 650) by either applying a drop of cleaning solution to both the cleaning solution on said surface (65, 650) and to the sonotrode's tip or moving down the sonotrode (4, 4', 40, 50) until it gets in contact with the cleaning solution on said surface (65, 650), and exposing said surface (65, 650) in contact with said linear homogeneous column (20) of cleaning solution to cavitation, thus removing dirt (70) from said surface (65, 650).

2. -The method of claim 1 , wherein said vibration amplitude applied at the output surface (4e, 4e', 40e, 50e) of the sonotrode (4, 4', 40, 50) depends on the vibration amplitude at the input surface (4c) of said sonotrode (4, 4', 40, 50) and on the ratio between the input cross-section (4c) of the sonotrode (4, 4', 40, 50) and the output cross-section (4f) of the sonotrode (4, 4', 40, 50) .

3. -The method of claim 2, wherein the curved output surface (4e, 40e) of the sonotrode (4, 4', 40, 50) is defined by a radius r, wherein r = w / 2, w being the width of the output cross-section (4f) of the sonotrode (4, 4', 40, 50).

4. -The method of any preceding claims, wherein said vibration amplitude applied at the output surface (4e, 4e', 40e, 50e) of the sonotrode (4, 4', 40, 50) does not exceed a threshold above which said cleaning solution atomizes, said threshold being dependent on the type of cleaning solution and on the working frequency.

5. -The method of any preceding claim, wherein said device (10, 100) is moved along said surface (65, 650) maintaining a substantially constant distance (D) between said surface (65, 650) and the output surface (4e, 40e) of the sonotrode (4, 40) closest to said surface (65, 650).

6. -The method of any preceding claims, wherein said cleaning solution is applied on the surface (65, 650) to be cleaned prior to starting operation of the sonotrode (4, 4', 40, 50), in such a way that a layer of cleaning solution is disposed on said surface (65, 650).

7. -The method of any preceding claims from 1 to 5, wherein said cleaning solution is applied on the surface (65, 650) to be cleaned as the sonotrode (4, 4', 40, 50) moves along said surface (65, 650) to be cleaned, in such a way that said linear homogeneous column (20) of cleaning solution is disposed between said surface (65, 650) and the output surface (4e, 4e', 40e, 50e) of the sonotrode (4, 4', 40, 50).

8. -The method of claim 7, wherein said cleaning solution is supplied externally to the sonotrode (4,4', 40, 50) by means of a syringe or a spraying nozzle.

9. -The method of claim 7, wherein said cleaning solution is supplied internally to the sonotrode (4, 4', 40, 50) along a channel disposed within the sonotrode, (4, 4', 40, 50), said channel being designed to take cleaning solution to the output surface (4e, 4e', 40e, 50e) of the sonotrode (4, 4', 40, 50) closest to the surface (65, 650) to be cleaned.

10. -The method of any preceding claims from 7 to 9, wherein said cleaning solution is reused by the sonotrode (50) sucking up (A2) dirty droplets (71 ), filtering (A3) said dirty droplets and supplying (A1 ) filtered droplets (80) while the sonotrode (50) moves forward over said surface (65, 650) to be cleaned.

1 1. -The method of any preceding claims, wherein said cleaning solution is selected from the following group: water and aqueous solutions comprising at least one chemical agent.

12. - A device (10, 10', 100) for ultrasonic cleaning a part (60, 600), with the aid of a cleaning solution to be disposed on a surface (65, 650) of said part (60, 600), said device (10, 10', 100) comprising: -an ultrasonic wave oscillator (1 ) configured to convert a standard electric signal working at 50-60 Hz into electrical energy working at a frequency comprised within the ultrasonic frequency;

-an ultrasonic converter (2, 20) for converting said electrical energy provided by the ultrasonic wave oscillator (1 ) into mechanical vibration;

-an ultrasonic sonotrode (4, 4', 40, 50) coupled to said ultrasonic converter (2, 20) and configured to generate at its output surface (4e, 4e', 40e, 50e) ultrasonic vibration (V) having certain vibration amplitude, wherein the output surface (4e, 40e) of the sonotrode (4, 4', 40, 50) is curved;

wherein in use of the device (10, 10', 100), said sonotrode (4, 4', 40, 50) is configured to generate a linear homogeneous column of cleaning solution between said output surface (4e, 4e', 40e, 50e) of the sonotrode (4, 4', 40, 50) and said surface (65, 650) by either applying a drop of cleaning solution to both the cleaning solution on said surface (65, 650) and to the sonotrode's tip or moving down the sonotrode (4, 4', 40, 50) until it gets in contact with the cleaning solution on said surface (65, 650), and to expose said surface (65, 650) in contact with said linear homogeneous film of cleaning solution to cavitation, thus removing dirt (70) from said surface (65, 650).