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Technical aspects of ultrasonic cleaning
TECHNICAL
ASPECTS
ULTRASONIC by 0.
OF
CLEANING A. ANTONY*
To a great extent, the cleaning fluid used determines the effectiveness of an ultrasonic cleaner. This article describes experiments for identifying cavitation and determining its distribution so that the workpiece can be put in the right position in the tank. Viscosity, surface tension and vapour pressure of a liquid decide whether it has good cavitating properties, and it is important to choose the right solvent for each cleaning task. Greasy, dustcovered and oxidized surfaces are each cleaned best with a particular range of solvents
U
ltrasonic cleaning is based on cavitation, a phenomenon produced in most liquids when an ultrasonic wave is made to travel through them .lv2 When an ultrasonic wave passes through a liquid, the hydrostatic pressure fluctuates, and if the wave is of sufficient intensity, the pressure can become negative in the rarefaction half of the cycle. This gives rise to the hollows or cavities from which the phenomenon takes the name of cavitation. All liquids contain a certain amount of gas, which exists either as dissolved gas or as microscopic gas bubbles attached to minute solid particles. During the rarefaction the bubbles grow and absorb more gas, which comes out of solution in the water and passes into them. When compression begins, the bubbles may either shrink steadily, if their internal pressure balances out the external pressure, or may implode violently, generating local pressures as high as 1,000 atm as they do so. Such pressure waves, coming from numerous tiny imploding bubbles, penetrate the smallest crevices; their scrubbing action helps the solvent to wet the submerged surface and dissolve dirt from it which would otherwise adhere too firmly to be dissolved off. Therefore the efficiency of a cleaning fluid depends on its properties as a solvent and its ability to support cavitation. In most ultrasonic cleaning tanks the ultrasonic vibrations are transmitted from transducers cemented to the bottom of the tank. A certain amount of energy is lost by reflections at the cement-to-tank and tank-to-liquid interfaces, and from this point of view aluminium, polystyrene or even glass would be more suitable materials than stainless steel to make the tank from. However, stainless steel is usually chosen in spite of its greater reflecting properties because of
*Thorn-A.E.I.
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1963
its ruggedness and lack of rusting; it also resists most of the solvents used for cleaning. Cleaning systems today use frequencies of 20-60 kc/s because cavitation is more efficiently produced in this range than at 1 MC/S, which was used at first. At 1 MC/S a more intense wave was needed to produce cavitation and the ultrasonic field was highly directional. PRODUCTION OF STABLE SONIC WAVES IN A LIQUID
Various types of sonic field can exist in the liquid in a container, depending upon the dimensions of the transducer and the shape and material of the tank. The tank material influences the field because different materials have different reflectivities. A stable standing wave inside the liquid is the best condition for cavitation, as the pressure variations in the medium are then a maximum. Thus, minimum energy is required for cavitation or, vice versa, maximum cavitation occurs for a particular energy. The column of liquid inside the container can be compared to a closed pipe where resonance is produced when the ends are the nodes and the length of the closed pipe is an integral number of half wavelengths. The wavelengths of the sonic wave can be calculated for the particular temperature of operation if the velocity of sound in that liquid is known. Since velocity is substantially constant over a large range of frequencies the same value may be used from 20 kc/s to 60 kc/s. Thus in an ultrasonic tank the theoretical ideal level of liquid would be an integral multiple of h/2 for the particular liquid and frequency. This will not always be strictly true in practice for maximum cavitation, owing to complex cavitation regions produced in small tanks by energy which is reflected from the sides of the tank and interferes with the main beam.
WIRE FRAME,?
0.00047 ’ ALUMINIIJM
1. Pattern of punctures observed on a foil after 2 min exposure to a 40 kc/s ultrasonic field in perchlorethylene at 28°C. Frame was held vertical in the middle of the tank. Half inch (1.28 cm) separations are equivalent to .I/2 for 40kc/s waves travelling at 1027 m/s. Punctures were about a quarter of the size of pinheads Fig.
IN
I FOIL
/
REGIONS WHERI PUNCTURES WERE OBSERVED_
Fig. 2. Pattern on foil after 30 set exposure to 40 kc/s ultrasonic field in water. Foil surface is eroded at areas separated by $ in (1.91 cm) which corresponds to A/2 for 40 kc/s ultrasonic waves in water
Fig. 3. Aluminium foil 003047 in thick after 15 set exposed to a 40 kc/s ultrasonic field in water. The theoretical half wavelength is 0.72 in and the ruptures appeared with approximately 0.8 in separation between centres. Individual punctures can be observed near the areas of rupture
ENERGY DISTRIBUTION
Depths at which cavitation is greatest should, theoretically, be separated at half wavelengths for standing waves. Thus for water v = 1484 m/s and for a 40 kc/s transducer system the wavelength should be 1.46 in. The depths of maximum cavitation should thus be spaced out at O-72 in intervals. Jn perchloroethylene v is 1027 m/s at 28°C. This gives us a figure of l-02 in for h, or approximately 4 in separation. The method of plotting the levels of cavitation is to dip into the liquid a OWI47 in thick strip of aluminium foil fixed on a wire frame and to observe the effect produced on the foil when the sonic energy is applied. The unit is switched on for a known time with the foil in the liquid. The foil is then taken out and a series of punctures is observed at approximately 4 in separations. This is shown diagrammatically in Fig. 1. These punctures are due to the minute gas bubbles imploding and releasing sufficient energy to puncture the aluminium foil; values of a few tens of atmospheres have been quoted as common figures for this implosion energy. A measure of the energy is made by counting the number of punctures in each experiment and the number obtained with water, taken as 100. Since the puncture pattern is found to be uniform, the average of three exposures of the foil in the liquid is considered to be sufficient to assess the magnitude of the cavitation energy. This method of energy plotting has been used throughout the investigation for the identification of energy levels. Similar tests done with water as the liquid show that the foil is torn and frittered away at approximately O-8 in separations, as is shown in Fig. 2 and 3. The foil rupture
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1963
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was evident after as little as 15 set exposure. This is probably due to the higher surface tension of water (see later section). Simple reasoning will show that maximum cavitation regions will give maximum cleaning, and this can be verified by two experiments. In the first experiment the 0*00047 in thick aluminium foil was sprayed with a thin layer of calcium carbonate coating and dried. The foil was then fixed onto the frame and immersed in water contained in the ultrasonic tank. The unit was switched on and left for a few seconds. On close examination it was found that coating was removed from the foil in proportion to the number of punches. Regions of greatest removal were separated at half wavelengths of sound waves in water. In the second experiment a nickel wire 1 mm in diameter was taken and straightened. Measurements of thickness were made accurately with the aid of a micrometer. The wire was dipped in an etch consisting of 10% formic acid, 10% hydrogen peroxide and 80% water and placed in the ultrasonic tank with an aluminium foil alongside it. After sonic energy had been applied for a known time, the thickness of the wire was measured at various points. It was found that at the depths corresponding to the maximum energy shown by the aluminium foil, the wire diameter was Spin less than the other parts of the wire. It can thus be assumed that the positions of the punctures on the foil can be taken as a good guide to the cavitation regions. This verifies the theoretical postulate that maximum energy levels are at X/2 intervals. It must, however, be emphasized that in small tanks the field is not always a standing wave. One may get diffuse fields which give rise to complex cavitation regions as well as patches of cavitation. In actual practice the punches on the foil obtained near corners of the tank have shown slightly different patterns to the uniformly spaced h/2 separations.
the same vapour pressure were used in an ultrasonic tank, and the aluminium foil method of energy plotting was carried out. It was found that the amount of pitting had a direct relation to the surface tension of the liquid. This is shown in Fig. 4.
0
20
I
30
I
I
I
I
40
50
60
70
Surface
tension,
dynlcm
Fig. 4. Cavitation plotted against surface tension for different liquids. Units of cavitation on the Y axis are referred to water as 100 Surface tension, dynJcm Amy1 alcohol 22 Toluene 30 Phenol Aniline :‘: Water 12
CAVITATION IDENTIFICATION
Regions of cavitation can be detected by observing a beam of light passed through the liquid being irradiated by sonic energy. Regions of negative pressure have different transparency to light. It is necessary to use a pulsed beam of light, e.g. with a Kerr cell, to observe them. LIQUID CHARACTERISTICS WHICH AFFECT CAVITATION
Three basic characteristics physically affect the cavitation energy available for cleaning, namely, surface tension, viscosity and vapour pressure.
04 Viscosity,
I.0 poise
Fig. 5. Cavitation plotted against viscosity for water and glycerol. Units of cavitation energy are referred to water as 100
Surface tension Inside a bubble of radius R in a liquid the pressure is where P = pressure inside bubble -_ P, = hydrostatic pressure at that depth in the liquid u = surface tension of liquid. The internal pressure of a bubble depends largely upon the surface tension when the bubble is small. When the bubble collapses the potential energy of the bubble surface is concentrated into a very small volume, and thus the higher the surface tension the higher the energy of collapse. This was borne out experimentally in the following experiment : Liquids of different surface tension and approximately
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rrLTRAsoNIcs/October-December
1963
Viscosity The more viscous a liquid, the higher the sonic energy required to produce cavitation. This is borne out by the results from glycerol and from low viscosity materials like water and alcohol. A given sonic energy produces far less cavitation in glycerol. Fig. 5 shows the amount of cavitation energy obtained in glycerol compared with water; water cavitation energy is taken as 100. The straight line drawn between water and glycerol is purely tentative owing to lack of sufficient data. It is known definitely that the higher the viscosity the lower is the cavitation energy, but it is yet to be determined what law it follows with increase of viscosity.
Table 1. CHARACTERISTICS OF TWO VERY DIFFERENT SOLVENTS Viscosity, poise
Surfa;;n~msion, --
-_ Water
72
Glycerol
63.4
0.01
13.4
Vapour pressure, mmHg ---17.0 at 20°C
at
20°C
From the above table it is clear that there is not a great deal of difference in surface tension between water and glycerol, but a thousandfold difference in the viscosity. Allowing 10-15 % decrease in cavitation energy for the lower surface tension of glycerol, we must attribute the rest of the difference in the cavitation energy to the greater viscosity.
100 0
6
n y
.z
1
90
ao-
5 705 5 60k6 50 sy 40._ j; 305 8 20 1 j 10 8 ” ” 8 ” “1 300 240 ZOO170 IS0 I20 100 72 60 44 30 20 16 IO
Size
of
mesh
Fig. 6. Transmission of cavitation energy through a stainless steel mesh
Vapour pressure . It has been found experimentally that liquids of higher vapour pressure give lower cavitation energies available for cleaning if other quantities like surface tension and viscosity are approximately the same. This may be explained as follows. At a particular frequency, but for liquids of different vapour pressure, the amount of vapour released by the liquid during the negative half of the cycle depends upon the vapour pressure of the liquid: the pressure inside the liquid is less for liquids of lower vapour pressure. The implosion energy of a bubble of lower internal pressure would be more than that of a bubble of higher internal pressure. This probably accounts for the falling off of the cavitation energy for liquids of higher vapour pressures, but of the same surface tension and viscosity. Thus the three principal characteristics of a liquid which contribute to good cavitation are high surface tension, low vapour pressure and low viscosity.
moisture and form acidic solutions. The other obvious factor is to choose a solvent which does not attack the workpiece chemically. GAS DISSOLVED IN THE LIQUID
Ultrasonic cleaning would be impossible if the liquid contained no gases at all and if the vapour pressure was also very low. In practice most liquids under normal conditions contain gases dissolved in them and this factor is not of undue concern in the choice of solvent. SCATTERING BY GAUZE; POSITION OF WORKPIECE;
CONTAINERS
Sonic energy may be reflected or scattered or transmitted by a rigid body placed in the liquid. Scattering of the sonic waves by gauze was studied by the pitting method of energy plotting to give an indication of the amount of energy scattered by the gauze. Pieces of stainless steel gauze of different mesh were immersed in the liquid and the cavitation energy transmitted was plotted for each mesh. The distribution is shown in Fig. 6. A large part of sonic energy seems to be scattered by gauze between 10 and 170 mesh. The most satisfactory position for cleaning of the workpiece is to suspend it in the liquid without any containers. This is not always possible, especially if very small components are being cleaned, and the use of trays or containers becomes essential. From the experiment conducted above, it is seen that gauze of mesh finer than 240 and coarser than 10 is suitable for transmission of sonic energy. Trays may thus be made out of 240 or 300 mesh gauze. The material best suited for this is stainless steel because of its resistance to most solvents. Almost complete transmission of sonic energy may be obtained with trays made out of a solid material which offers low impedance to the sonic waves: such materials are glass, aluminium and polystyrene. Since it is not always practicable to make trays out of these materials, stainless steel sheets 0.003 in thick may be used, as with thin stainless steel only a small fraction of the energy is reflected. Suitable glass beakers may be used with the same solvent in them as in the tank; or a small quantity of the solvent may be used in the beaker and water used in the tank to act as the coupling agent. TIME OF CLEANING
Though it is difficult to estimate the time of cleaning, it has been found that in a period of 2-4 min the bulk of the contaminants has been removed. Bell Laboratories Monograph No. 3143 reports that 99% of the contaminant is removed in 120 set with the most suitable solvent for that particular contaminant. It may be said that in general 2-4 min is the optimum period for cleaning. If sufficient cleaning is not achieved by this time it may be necessary to change the solvent or the other characteristics of the system, e.g. sonic output power or filtering of the solvent,
OTHER LIQUID CHARACTERISTICS FOR CHOICE AS SOLVENT
Other liquid characteristics to be looked for in good cleaning liquid are efficient wetting, good solvent properties for the particular type of contaminant to be removed, stability in ultrasonic fields and safety to the component and the tank material. Stability in ultrasonic fields is important as it is known that certain solvents, e.g. trichlorethylene, tend to be unstable and break down in the presence of
FLOW RATE OF SOLVENT IN THE TANK The flow rate of water in a tank with recirculating arrangements, which are sometimes used to filter away contaminants removed from the workpiece, affects the cavitation energy produced. The higher the flow rate, the less the cavitation. This is due to the turbulence caused in the
C
ULTRASONICS/October-December
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197
cavitation regions which obstructs the formation of negative pressures. Bell Laboratories report in their monograph that for flow rates of up to 1% of the volume of liquid in the tank per minute no effect is produced on the cavitation energy. The present author has found that cavitation energies are not affected even up to 10% flow rate for his particular tank. No general figure can be recommended as this is purely a feature of the tank dimensions and flow conditions. DISCUSSION ON THE GENERAL CHARACTERISTICS OF SOME SOLVENTS
The uses of some solvents and their characteristics are given in Tables 2 and 3.
____-Removing
Solvent
__-
light coatings
of grease
--
Perchlorethylene Tk;;kJ;eth ylene 1 : 1 : 1 Trichlorethane Carbon tetrachloride
Removing adhering non-greasy contaminants on small components (e.g. carbon, dust pressed on to metal)
Water/alcohol mixture; water 92-88 “/,, alcohol 8-12 “/,
Components
Formic acid/distilled water mixture plus hydrogen peroxide
slightly oxidized
Table 3. CHARACTERISTICSOF SOME SOLVENTS
-Surface tension, dyn/cm
Trichlorethylene
32
32
1 :l :l Carbon Trichlor. tetraethane chloride
0+)09
Acetone
__
28
25
23.7
----
--------____
Viscosity, poise -
Perchlorethylene ---_--~
0+)055
0.0061
0409
17 at 20°C 100 at 60°C
CONCLUSIONS 60 at 20°C 319 at 60°C
100 at 20°C 500 at 60°C
91 at 20°C 450 at 60°C
184 at 20°C 1.14 atm at 60°C
DEGREASING SOLVENTS
Trichlorethylene, perchlorethylene, acetone and 1 : 1 : 1 trichlorethane can all be used as general degreasing agents quite satisfactorily. The solvents dissolve the grease into solution. The cavitating properties of the chlorinated solvents are better than those of acetone, which has comparatively high vapour pressure. Perchlorethylene appears to have the best cavitating properties and is claimed to be more stable than trichlorethylene in the presence of moisture. Chemically there seems to be little to choose between these solvents as far as their cleaning goes. Their cavitation energies are far less than that of water owing to their lower surface tensions. It can be said that the solvent most suitable for ultrasonic degreasing is perchlorethylene. Trichlorethane is probably more suited to degreasing in large industrial installations.
198
Various combinations of hydrogen peroxide, formic acid and distilled water have been recommended by Koontz in Bell Laboratories’ Monograph No. 3143 for slight descaling. For oxidized nickel wire the appropriate solution gives extremely good cleaning in an ultrasonic field. The procedure was to immerse the wires in the ultrasonically agitated bath for about 2 min and again in distilled water to remove residues from the wires. They were later washed in running de-ionized water until the conductivity fell to a low level. 0.5 x 10m6mho can be taken as a “low level.” It is to be noted that the de-ionized water should initially have this conductivity so that it will fall back to this value after the wires have been washed in it. When this is done with enough care it is possible to get a finish where no interference films due to the slight oxidation are visible on examination under a microscope. The surface was found to be lightly pitted on giving this treatment. In the light of the success obtained for nickel, the author assumes that the proportions recommended by Koontz for other metals will give equally satisfactory results for these metals under ultrasonic conditions. It may be added that, after one month’s storage in open air, these wires had kept their shiny appearance.
00031
---__________
Vapour pressure, mmHg
Where degreasing is not necessary, distilled water may be used as a solvent for removing dust adhering to and dirt pressed onto the component and because of the high surface tension of water, good cavitation is available. A mixture of 8-12 % alcohol and 92-88 ‘A water has proved very satisfactory for the same purpose. The small’ percentage of alcohol probably adds to the cleaning effect, as its wetting properties are better than those of water and it brings down the surface tension. Components cleaned in this mixture were found to be much cleaner than those cleaned in plain water. OXIDIZED COMPONENTS
Table 2. USESOF SOLVENTS Use
SOLVENTS FOR CLEANING OFF DUST ADHERING TO COMPONENTS
uLTRAsoNICS/October-December
1963
The solvent most suitable for degreasing is perchlorethylene, even though a liquid with similar chemical properties but with a high surface tension would be preferable. For general cleaning of dust-contaminated components a mixture of 8-12% alcohol and. 88-92% water has been found very satisfactory. For removing oxides and slight descaling of special metals like nickel, molybdenum, tungsten, copper and stainless steel, various combinations of a solution of hydrogen peroxide, formic acid and distilled water in an ultrasonic field are probably the best method known, apart from heat treatment in a hydrogen atmosphere. If containers are used, the most suitable material to use is 300 mesh stainless steel gauze made up in the form of a tray.
REFERENCES 1. HUETER and BOLT, “Sonics,” Wiley (1955). Wiley (1938). 2. BERGMANN, “Ultrasonics,” 3. Bell Laboratories Monograph No. 3143.
ASPECTS
ULTRASONIC by 0.
OF
CLEANING A. ANTONY*
To a great extent, the cleaning fluid used determines the effectiveness of an ultrasonic cleaner. This article describes experiments for identifying cavitation and determining its distribution so that the workpiece can be put in the right position in the tank. Viscosity, surface tension and vapour pressure of a liquid decide whether it has good cavitating properties, and it is important to choose the right solvent for each cleaning task. Greasy, dustcovered and oxidized surfaces are each cleaned best with a particular range of solvents
U
ltrasonic cleaning is based on cavitation, a phenomenon produced in most liquids when an ultrasonic wave is made to travel through them .lv2 When an ultrasonic wave passes through a liquid, the hydrostatic pressure fluctuates, and if the wave is of sufficient intensity, the pressure can become negative in the rarefaction half of the cycle. This gives rise to the hollows or cavities from which the phenomenon takes the name of cavitation. All liquids contain a certain amount of gas, which exists either as dissolved gas or as microscopic gas bubbles attached to minute solid particles. During the rarefaction the bubbles grow and absorb more gas, which comes out of solution in the water and passes into them. When compression begins, the bubbles may either shrink steadily, if their internal pressure balances out the external pressure, or may implode violently, generating local pressures as high as 1,000 atm as they do so. Such pressure waves, coming from numerous tiny imploding bubbles, penetrate the smallest crevices; their scrubbing action helps the solvent to wet the submerged surface and dissolve dirt from it which would otherwise adhere too firmly to be dissolved off. Therefore the efficiency of a cleaning fluid depends on its properties as a solvent and its ability to support cavitation. In most ultrasonic cleaning tanks the ultrasonic vibrations are transmitted from transducers cemented to the bottom of the tank. A certain amount of energy is lost by reflections at the cement-to-tank and tank-to-liquid interfaces, and from this point of view aluminium, polystyrene or even glass would be more suitable materials than stainless steel to make the tank from. However, stainless steel is usually chosen in spite of its greater reflecting properties because of
*Thorn-A.E.I.
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u~~AsoN~cs/October-December
1963
its ruggedness and lack of rusting; it also resists most of the solvents used for cleaning. Cleaning systems today use frequencies of 20-60 kc/s because cavitation is more efficiently produced in this range than at 1 MC/S, which was used at first. At 1 MC/S a more intense wave was needed to produce cavitation and the ultrasonic field was highly directional. PRODUCTION OF STABLE SONIC WAVES IN A LIQUID
Various types of sonic field can exist in the liquid in a container, depending upon the dimensions of the transducer and the shape and material of the tank. The tank material influences the field because different materials have different reflectivities. A stable standing wave inside the liquid is the best condition for cavitation, as the pressure variations in the medium are then a maximum. Thus, minimum energy is required for cavitation or, vice versa, maximum cavitation occurs for a particular energy. The column of liquid inside the container can be compared to a closed pipe where resonance is produced when the ends are the nodes and the length of the closed pipe is an integral number of half wavelengths. The wavelengths of the sonic wave can be calculated for the particular temperature of operation if the velocity of sound in that liquid is known. Since velocity is substantially constant over a large range of frequencies the same value may be used from 20 kc/s to 60 kc/s. Thus in an ultrasonic tank the theoretical ideal level of liquid would be an integral multiple of h/2 for the particular liquid and frequency. This will not always be strictly true in practice for maximum cavitation, owing to complex cavitation regions produced in small tanks by energy which is reflected from the sides of the tank and interferes with the main beam.
WIRE FRAME,?
0.00047 ’ ALUMINIIJM
1. Pattern of punctures observed on a foil after 2 min exposure to a 40 kc/s ultrasonic field in perchlorethylene at 28°C. Frame was held vertical in the middle of the tank. Half inch (1.28 cm) separations are equivalent to .I/2 for 40kc/s waves travelling at 1027 m/s. Punctures were about a quarter of the size of pinheads Fig.
IN
I FOIL
/
REGIONS WHERI PUNCTURES WERE OBSERVED_
Fig. 2. Pattern on foil after 30 set exposure to 40 kc/s ultrasonic field in water. Foil surface is eroded at areas separated by $ in (1.91 cm) which corresponds to A/2 for 40 kc/s ultrasonic waves in water
Fig. 3. Aluminium foil 003047 in thick after 15 set exposed to a 40 kc/s ultrasonic field in water. The theoretical half wavelength is 0.72 in and the ruptures appeared with approximately 0.8 in separation between centres. Individual punctures can be observed near the areas of rupture
ENERGY DISTRIBUTION
Depths at which cavitation is greatest should, theoretically, be separated at half wavelengths for standing waves. Thus for water v = 1484 m/s and for a 40 kc/s transducer system the wavelength should be 1.46 in. The depths of maximum cavitation should thus be spaced out at O-72 in intervals. Jn perchloroethylene v is 1027 m/s at 28°C. This gives us a figure of l-02 in for h, or approximately 4 in separation. The method of plotting the levels of cavitation is to dip into the liquid a OWI47 in thick strip of aluminium foil fixed on a wire frame and to observe the effect produced on the foil when the sonic energy is applied. The unit is switched on for a known time with the foil in the liquid. The foil is then taken out and a series of punctures is observed at approximately 4 in separations. This is shown diagrammatically in Fig. 1. These punctures are due to the minute gas bubbles imploding and releasing sufficient energy to puncture the aluminium foil; values of a few tens of atmospheres have been quoted as common figures for this implosion energy. A measure of the energy is made by counting the number of punctures in each experiment and the number obtained with water, taken as 100. Since the puncture pattern is found to be uniform, the average of three exposures of the foil in the liquid is considered to be sufficient to assess the magnitude of the cavitation energy. This method of energy plotting has been used throughout the investigation for the identification of energy levels. Similar tests done with water as the liquid show that the foil is torn and frittered away at approximately O-8 in separations, as is shown in Fig. 2 and 3. The foil rupture
unR.ksoNIcs/&tober-December
1963
195
was evident after as little as 15 set exposure. This is probably due to the higher surface tension of water (see later section). Simple reasoning will show that maximum cavitation regions will give maximum cleaning, and this can be verified by two experiments. In the first experiment the 0*00047 in thick aluminium foil was sprayed with a thin layer of calcium carbonate coating and dried. The foil was then fixed onto the frame and immersed in water contained in the ultrasonic tank. The unit was switched on and left for a few seconds. On close examination it was found that coating was removed from the foil in proportion to the number of punches. Regions of greatest removal were separated at half wavelengths of sound waves in water. In the second experiment a nickel wire 1 mm in diameter was taken and straightened. Measurements of thickness were made accurately with the aid of a micrometer. The wire was dipped in an etch consisting of 10% formic acid, 10% hydrogen peroxide and 80% water and placed in the ultrasonic tank with an aluminium foil alongside it. After sonic energy had been applied for a known time, the thickness of the wire was measured at various points. It was found that at the depths corresponding to the maximum energy shown by the aluminium foil, the wire diameter was Spin less than the other parts of the wire. It can thus be assumed that the positions of the punctures on the foil can be taken as a good guide to the cavitation regions. This verifies the theoretical postulate that maximum energy levels are at X/2 intervals. It must, however, be emphasized that in small tanks the field is not always a standing wave. One may get diffuse fields which give rise to complex cavitation regions as well as patches of cavitation. In actual practice the punches on the foil obtained near corners of the tank have shown slightly different patterns to the uniformly spaced h/2 separations.
the same vapour pressure were used in an ultrasonic tank, and the aluminium foil method of energy plotting was carried out. It was found that the amount of pitting had a direct relation to the surface tension of the liquid. This is shown in Fig. 4.
0
20
I
30
I
I
I
I
40
50
60
70
Surface
tension,
dynlcm
Fig. 4. Cavitation plotted against surface tension for different liquids. Units of cavitation on the Y axis are referred to water as 100 Surface tension, dynJcm Amy1 alcohol 22 Toluene 30 Phenol Aniline :‘: Water 12
CAVITATION IDENTIFICATION
Regions of cavitation can be detected by observing a beam of light passed through the liquid being irradiated by sonic energy. Regions of negative pressure have different transparency to light. It is necessary to use a pulsed beam of light, e.g. with a Kerr cell, to observe them. LIQUID CHARACTERISTICS WHICH AFFECT CAVITATION
Three basic characteristics physically affect the cavitation energy available for cleaning, namely, surface tension, viscosity and vapour pressure.
04 Viscosity,
I.0 poise
Fig. 5. Cavitation plotted against viscosity for water and glycerol. Units of cavitation energy are referred to water as 100
Surface tension Inside a bubble of radius R in a liquid the pressure is where P = pressure inside bubble -_ P, = hydrostatic pressure at that depth in the liquid u = surface tension of liquid. The internal pressure of a bubble depends largely upon the surface tension when the bubble is small. When the bubble collapses the potential energy of the bubble surface is concentrated into a very small volume, and thus the higher the surface tension the higher the energy of collapse. This was borne out experimentally in the following experiment : Liquids of different surface tension and approximately
196
rrLTRAsoNIcs/October-December
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Viscosity The more viscous a liquid, the higher the sonic energy required to produce cavitation. This is borne out by the results from glycerol and from low viscosity materials like water and alcohol. A given sonic energy produces far less cavitation in glycerol. Fig. 5 shows the amount of cavitation energy obtained in glycerol compared with water; water cavitation energy is taken as 100. The straight line drawn between water and glycerol is purely tentative owing to lack of sufficient data. It is known definitely that the higher the viscosity the lower is the cavitation energy, but it is yet to be determined what law it follows with increase of viscosity.
Table 1. CHARACTERISTICS OF TWO VERY DIFFERENT SOLVENTS Viscosity, poise
Surfa;;n~msion, --
-_ Water
72
Glycerol
63.4
0.01
13.4
Vapour pressure, mmHg ---17.0 at 20°C
at
20°C
From the above table it is clear that there is not a great deal of difference in surface tension between water and glycerol, but a thousandfold difference in the viscosity. Allowing 10-15 % decrease in cavitation energy for the lower surface tension of glycerol, we must attribute the rest of the difference in the cavitation energy to the greater viscosity.
100 0
6
n y
.z
1
90
ao-
5 705 5 60k6 50 sy 40._ j; 305 8 20 1 j 10 8 ” ” 8 ” “1 300 240 ZOO170 IS0 I20 100 72 60 44 30 20 16 IO
Size
of
mesh
Fig. 6. Transmission of cavitation energy through a stainless steel mesh
Vapour pressure . It has been found experimentally that liquids of higher vapour pressure give lower cavitation energies available for cleaning if other quantities like surface tension and viscosity are approximately the same. This may be explained as follows. At a particular frequency, but for liquids of different vapour pressure, the amount of vapour released by the liquid during the negative half of the cycle depends upon the vapour pressure of the liquid: the pressure inside the liquid is less for liquids of lower vapour pressure. The implosion energy of a bubble of lower internal pressure would be more than that of a bubble of higher internal pressure. This probably accounts for the falling off of the cavitation energy for liquids of higher vapour pressures, but of the same surface tension and viscosity. Thus the three principal characteristics of a liquid which contribute to good cavitation are high surface tension, low vapour pressure and low viscosity.
moisture and form acidic solutions. The other obvious factor is to choose a solvent which does not attack the workpiece chemically. GAS DISSOLVED IN THE LIQUID
Ultrasonic cleaning would be impossible if the liquid contained no gases at all and if the vapour pressure was also very low. In practice most liquids under normal conditions contain gases dissolved in them and this factor is not of undue concern in the choice of solvent. SCATTERING BY GAUZE; POSITION OF WORKPIECE;
CONTAINERS
Sonic energy may be reflected or scattered or transmitted by a rigid body placed in the liquid. Scattering of the sonic waves by gauze was studied by the pitting method of energy plotting to give an indication of the amount of energy scattered by the gauze. Pieces of stainless steel gauze of different mesh were immersed in the liquid and the cavitation energy transmitted was plotted for each mesh. The distribution is shown in Fig. 6. A large part of sonic energy seems to be scattered by gauze between 10 and 170 mesh. The most satisfactory position for cleaning of the workpiece is to suspend it in the liquid without any containers. This is not always possible, especially if very small components are being cleaned, and the use of trays or containers becomes essential. From the experiment conducted above, it is seen that gauze of mesh finer than 240 and coarser than 10 is suitable for transmission of sonic energy. Trays may thus be made out of 240 or 300 mesh gauze. The material best suited for this is stainless steel because of its resistance to most solvents. Almost complete transmission of sonic energy may be obtained with trays made out of a solid material which offers low impedance to the sonic waves: such materials are glass, aluminium and polystyrene. Since it is not always practicable to make trays out of these materials, stainless steel sheets 0.003 in thick may be used, as with thin stainless steel only a small fraction of the energy is reflected. Suitable glass beakers may be used with the same solvent in them as in the tank; or a small quantity of the solvent may be used in the beaker and water used in the tank to act as the coupling agent. TIME OF CLEANING
Though it is difficult to estimate the time of cleaning, it has been found that in a period of 2-4 min the bulk of the contaminants has been removed. Bell Laboratories Monograph No. 3143 reports that 99% of the contaminant is removed in 120 set with the most suitable solvent for that particular contaminant. It may be said that in general 2-4 min is the optimum period for cleaning. If sufficient cleaning is not achieved by this time it may be necessary to change the solvent or the other characteristics of the system, e.g. sonic output power or filtering of the solvent,
OTHER LIQUID CHARACTERISTICS FOR CHOICE AS SOLVENT
Other liquid characteristics to be looked for in good cleaning liquid are efficient wetting, good solvent properties for the particular type of contaminant to be removed, stability in ultrasonic fields and safety to the component and the tank material. Stability in ultrasonic fields is important as it is known that certain solvents, e.g. trichlorethylene, tend to be unstable and break down in the presence of
FLOW RATE OF SOLVENT IN THE TANK The flow rate of water in a tank with recirculating arrangements, which are sometimes used to filter away contaminants removed from the workpiece, affects the cavitation energy produced. The higher the flow rate, the less the cavitation. This is due to the turbulence caused in the
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ULTRASONICS/October-December
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197
cavitation regions which obstructs the formation of negative pressures. Bell Laboratories report in their monograph that for flow rates of up to 1% of the volume of liquid in the tank per minute no effect is produced on the cavitation energy. The present author has found that cavitation energies are not affected even up to 10% flow rate for his particular tank. No general figure can be recommended as this is purely a feature of the tank dimensions and flow conditions. DISCUSSION ON THE GENERAL CHARACTERISTICS OF SOME SOLVENTS
The uses of some solvents and their characteristics are given in Tables 2 and 3.
____-Removing
Solvent
__-
light coatings
of grease
--
Perchlorethylene Tk;;kJ;eth ylene 1 : 1 : 1 Trichlorethane Carbon tetrachloride
Removing adhering non-greasy contaminants on small components (e.g. carbon, dust pressed on to metal)
Water/alcohol mixture; water 92-88 “/,, alcohol 8-12 “/,
Components
Formic acid/distilled water mixture plus hydrogen peroxide
slightly oxidized
Table 3. CHARACTERISTICSOF SOME SOLVENTS
-Surface tension, dyn/cm
Trichlorethylene
32
32
1 :l :l Carbon Trichlor. tetraethane chloride
0+)09
Acetone
__
28
25
23.7
----
--------____
Viscosity, poise -
Perchlorethylene ---_--~
0+)055
0.0061
0409
17 at 20°C 100 at 60°C
CONCLUSIONS 60 at 20°C 319 at 60°C
100 at 20°C 500 at 60°C
91 at 20°C 450 at 60°C
184 at 20°C 1.14 atm at 60°C
DEGREASING SOLVENTS
Trichlorethylene, perchlorethylene, acetone and 1 : 1 : 1 trichlorethane can all be used as general degreasing agents quite satisfactorily. The solvents dissolve the grease into solution. The cavitating properties of the chlorinated solvents are better than those of acetone, which has comparatively high vapour pressure. Perchlorethylene appears to have the best cavitating properties and is claimed to be more stable than trichlorethylene in the presence of moisture. Chemically there seems to be little to choose between these solvents as far as their cleaning goes. Their cavitation energies are far less than that of water owing to their lower surface tensions. It can be said that the solvent most suitable for ultrasonic degreasing is perchlorethylene. Trichlorethane is probably more suited to degreasing in large industrial installations.
198
Various combinations of hydrogen peroxide, formic acid and distilled water have been recommended by Koontz in Bell Laboratories’ Monograph No. 3143 for slight descaling. For oxidized nickel wire the appropriate solution gives extremely good cleaning in an ultrasonic field. The procedure was to immerse the wires in the ultrasonically agitated bath for about 2 min and again in distilled water to remove residues from the wires. They were later washed in running de-ionized water until the conductivity fell to a low level. 0.5 x 10m6mho can be taken as a “low level.” It is to be noted that the de-ionized water should initially have this conductivity so that it will fall back to this value after the wires have been washed in it. When this is done with enough care it is possible to get a finish where no interference films due to the slight oxidation are visible on examination under a microscope. The surface was found to be lightly pitted on giving this treatment. In the light of the success obtained for nickel, the author assumes that the proportions recommended by Koontz for other metals will give equally satisfactory results for these metals under ultrasonic conditions. It may be added that, after one month’s storage in open air, these wires had kept their shiny appearance.
00031
---__________
Vapour pressure, mmHg
Where degreasing is not necessary, distilled water may be used as a solvent for removing dust adhering to and dirt pressed onto the component and because of the high surface tension of water, good cavitation is available. A mixture of 8-12 % alcohol and 92-88 ‘A water has proved very satisfactory for the same purpose. The small’ percentage of alcohol probably adds to the cleaning effect, as its wetting properties are better than those of water and it brings down the surface tension. Components cleaned in this mixture were found to be much cleaner than those cleaned in plain water. OXIDIZED COMPONENTS
Table 2. USESOF SOLVENTS Use
SOLVENTS FOR CLEANING OFF DUST ADHERING TO COMPONENTS
uLTRAsoNICS/October-December
1963
The solvent most suitable for degreasing is perchlorethylene, even though a liquid with similar chemical properties but with a high surface tension would be preferable. For general cleaning of dust-contaminated components a mixture of 8-12% alcohol and. 88-92% water has been found very satisfactory. For removing oxides and slight descaling of special metals like nickel, molybdenum, tungsten, copper and stainless steel, various combinations of a solution of hydrogen peroxide, formic acid and distilled water in an ultrasonic field are probably the best method known, apart from heat treatment in a hydrogen atmosphere. If containers are used, the most suitable material to use is 300 mesh stainless steel gauze made up in the form of a tray.
REFERENCES 1. HUETER and BOLT, “Sonics,” Wiley (1955). Wiley (1938). 2. BERGMANN, “Ultrasonics,” 3. Bell Laboratories Monograph No. 3143.