THE F U N D A M E N T A L TECHNOLOGY

FOR U L T R A S O N I C C L E A N I N G

I MISUNDERSTOOD ULTRASONIC CLEANING

1 What Is Ultrasonic Cleaning? (Its mechanism and characteristics)

It is indispensable f o r the solid understanding of CFC free

aqueous cleaning to define the fundamental mechanism of the

ultrasonic cleaning method.

Ultrasonic waves of more than 20kHz (compression waves) produce

cavities in liquid. When they implode, they produce impact force

(micro jet). This impact force is utilized for the ultrasonic

cleaning. Therefore it is essential to understand the fundamental

mechanism of cavitaion when you discuss ultrasonic cleaning since

this method is one of the applications of the impact force of

cavitaion, implosion of cavities produced-by ultrasonic waves. ~

Ultrasonic waves produced in liquid can not function for cleaning

unless they produce cavitation.

Following examples only produce bubbles not cavitation:

* ultrasonic bath

* ultrasonic face cleaner

* bubbles produced by air pump in a fish tank

* aeration of activated sludge tank

* ultrasonic face cleaner

Although ultrasonic waves could be detected with a modern detector

in the above examples, they are not actually utilized for cleaning.

Cleaning results can not be obtained only by compression waves.

Generation of cavitaion is indispensable for the ultrasonic cleaning

method.

1 -

'' 2 Misbelief of MHz

"Sound is compression waves of a .substance and the speed, Lf and

the acceleration, Cr are formulated as:

v = / A w l cos&

F = 1 ~ w t j sinwt , where V i s the frequency.

Therefore the higher frequency causes greater acceleration.

Acceleation is hundreds of thousands of times greater than the force

of gravity. MHz produces great cleaning end results because of its

huge acceleration of molecules."

Are the above sentences correct? The answer is NO.

Compression waves are the statistical energy density difference of

water molecules and do not define the direction of the movement.

Namely water molecules move random and can not be put in the same

direction; propagation of sound through the energy density change of

the molecule does not mean the transport nor the position change of

the molecule and does not produce any acceleration accordingly. So

you can not expect cleaning results from the movement of water

molecule with the huge acceleration.

It should be noticed that the wavelength(h) of the sound of 1 MHz

in water at normal temperature is about 1 . 5 mm, not in ,M m nor 1 units .

3 Core of Cavitation

Cavitaion core is mainly constituted of the gas of the medium.

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.. The core varies according to energy difference between denseness and

non denseness and vapor pressure depending on the temperature of

liquid '(kinetic energy). Generally cavitation occurs in the liquid

with the higher steam pressure.

The cavitaion core in water is mainly constituted of water, but

the generation of core is heavily affected by the gas contained in

water as well as ions and impurities in water.

4 Shape of Cavity and Cavitation in Pure Water

When air is reduced to 0.5ppm in pure water, the shape of cavity

is linar or flat and the cavities implode in a zigzag line. The

core is considered to be constituted of water molecules.

5 Visualization of Cavitation

Cavitation core (cavity) can not be seen because of the high

frequency. However cavitation can be visualized with the aid of

luminescent material or by refraction of monochromatic light.

6 Elimination of Ultrasonic Waves

Air bubbles are the most effective factor to eliminate ultrasonic

waves in liquid. They block and absorb the most of the powerful

compression waves. Visible air bubbles result in restraing the

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’ generation of cavitation. Therefore air bubbles in liquid are

adversary to ultrasonic cleaning.

Some of other adverse factors for the conventional ultrasonic

cleaning(the first generation) are;

* fine wire netting

* plastics, rubber

* turbulent flow

* dirty water, floating particles of dust

7 Ultrasonic Cleaning Using Solvents Including C F C s

Solvents have a good absorbent effect of alr. They dissolve alr

rapidly from the surface when they are condensed or recovered. They

maintain saturationr with air. ~

bubbles

transducer

Air bubbles can be actually seen coming from the surface of a

transducer. They never implode. When a sheet of alminum foil is

put in a tank where a lot of air bubbles are generating, it does not

tear.

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As long as air bubbles generate (deaeration by ultrasonic waves)

and they eliminate ultrasonic waves most effectively, no cavitation

is achieved. Therefore no desired results is obtained by the

ultrasonic cleaning using chemicals including C F C - 1 1 3 , IPA,

ethylalcohol and chlorinated solvents such as 1 . 1 . 1 .

trichloroethane.

8 Easy Experiment for The Appropriate Ultrasonic Cleaning

water (Hz0)

tank ~

transducer

( 1 ) Run ultrasonic waves until air bubbles are gone.

(Stop running ultrasonic waves d s soon as cavitation makes

water turbid as air bubbles have dissappered.)

( 2 ) Put a sheet of alminum foil there.

( 3 ) Run ultrasonic waves for 60 sec. and watch how the alminum

foil change. You see it beginning to get torn.

To ensure the effective solvent used cleaning, air must be removed

from solvents including C F C s 1 - 1 . l . trichloroethane, alcohol and so forth.

The method of removing air does not use water but double

interfaces produced by temperature difference of liquid. It is

called the double interfacial method.

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I1 B A S I C T E C H N I Q U E FOR U L T R A S O N I C C L E A N I N G

I C a v i t a t i o n C o n t r o l

U l t r a s o n i c c l e a n i n g is a me thod u s i n g a n i m p a c t f o r c e o f

c a v i t a t i o n p r o d u c e d by u l t r a s o n i c w a v e s . T h e r e f o r e p r o p e r c o n t r o l

o f c a v i t a t i o n is e s s e n t i a l f o r e n s u r i n g t h e e f f e c t i v e a n d s t a b l e

c l e a n i n g ; where a n d h o w c a v i t a i o n i s g e n e r a t e d .

C a v i t a i o n c o n t r o l i n v o l v e s t h e c o n t r o l o f l o c a t i o n , d e n s i t y and

i m p a c t f o r c e o f c a v i t a t i o n . T h i s is a n i n d i s p e n s i b l e t e c h n i q u e f o r

u l t r a s o n i c c l e a n i n g t o o b t a i n t h e g r e a t e r e f f e c i e n c y o f t h e

c a v i t a t i o n g e p e r a t i o n a n d a p p l i c a t i o n .

C l e a n i n g s y s t e m wh ich c a n a c h i e v e t h e p r o p e r c a v i . t a t i o n c o n t r o l is

q u a l i f i e d t o be c a l l e d " t h e u l t r a s o n i c c l e a n i n g s y s t e m o f t h e s e c o n d

g e n e r a t i o n " .

2 C o n t r o l . o f C a v i t a t i o n L o c a t i o n

You a r e r u n n i n g a h i g h p r e s s u r e s p r a y e r . B u t i t d o e s n o t h i t a n

o b j e c t t o be c l e a n e d . Can you s a y you a r e a c t u a l l y c l e a n i n g ? Of

c o u r c e , t h e a n s w e r is NO.

h i g h p r e s s u r e n o z z l e

6 - s p r a y

You are running an ultrasonic cleaner properly. But if YOU put a n ”

object o f f the area where cavitaion is produced, you can not get

the desired cleaning results at all.

object -

! I I

transducer

Effective ultrasonic cleaning requires both of the full

understanding of cavitation location and the proper placement of an

o b ~ e c t to be cleaned in an ultrasonic cleaner tank.

3 Factors to Determine Cavitation Location

Followings are listed as factors to determine where cavitaion is

produced.

* frequency of ultrasonic cleaner

* waveform and its distribution

* capacity of ultrasonic cleaner

* depth of liquid

* nature and purity o f liquid

* solvents

* quantity and nature of contained gas

* temperature of liquid

* distance between transducer and water level

* location and placement of transducer

* way of mounting in a tank

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* shape of tank

* design conditions o f reflection surface

* temperature distribution of liquid

* turbulent flow or laminar flow boundary of liquid

* objects to be cleaned

* tools (wi.re netting , mesh belt, etc)

* cleaner's design conditions to cope with variety of liquids

and cleaning methods

* cleaner's design conditions to control dirt. stain ,

floating particles

* inaterial and surface condition of objects to be cleaned

4 Basic Distribution

Understanding the basic distribution patterns of cavitation is

vital. Followings are the examples;

Vertical d i se jh t inn

lior-izontal distribution

Basic d i s t r ibu t ion for water

Grid d i s t r i b u t i o n

Basic dist r lbut ion Tar water - 8

Uniform d i s t r i h t i o r t7

I I

Basic distribution for water

Radiation ( d i r e c t )

Basic d i s t r i b u t i o o far water Basic d i s t r i b u t i o n for water

Column (cav i t a t ion ball)

V

Basic d i s t r i b u t i o n for water

( l a u l t y temperature s e t t i n g ) 1--+=[

k s i c d i s t r i b u t i o n for vatex

Cavi ta t ion ball in - deaera ted water

Basic d i s t r i b u t i o n for mter

"low &sic distribution tor water

Plar baske t des ign Object of c leaning h w efficiency

- -----.---I - ,\\ \ I / / /-

Basic d i r t r i b u t i a o for water Basic d i s t r i b u t i o n far water

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I I Basic distribution far water

Oasic d i s t r i b u t i o n for water

5 Cavitation Density Control

( 1 ) Proper cavitation density is obtained by followings:

* concentration and dispersion of cavitation generation

* optimization of sound pressure

* contrivance of ultrasonic waveforms

* selection of ultrasonic frequency

* contrivance of tank shape

* temperature and nature of liquid

( 2 ) Note

As cavitation generates heat, its excessive concentratlon

without cooling could affect adversely, eliminating its impact

force .

1 0 -

6 Cavitation Impact Force Control

It 1s lmportant to optlmlze the impact force dependlng on an

oblect of cleaning.

( 1 ) General formula for Impact force

[liquid pressure] X [surface tenslon] X [other factors]

Impract force

[vapor pressure] X [frequency] X [contained g a s amouncl X [ o t h e r s ] 11

(liquid temperature)

( 2 ) Note

If you set a cleaner to obtain the maximum impact force with the

same energy, efficiency of cavitation becomes lower. Impact force

must be controlled to produce cavitation efficiently according to

an object of cleaning.

(3) Efficiency improvement of cavitation generation

Air contained in liquid is a key to obtain the efficient

utilization of ultrasonic waves. A s for water, it contains 6 - 8

ppm of oxgen at the normal temperature. Deaeration can not be

achieved only by running ultrasonic waves. On the contrary

running ultrasonic waves helps aeratlon from the surface and

results in being air saturated.

It is vital for the efficient ultrasonic cleaning to eliminate

or control the contents in solution. For this purpose we must

develop a method of blocking air coming in from the surface and

/or a deaeration system which enables deaeration faster than

dlssolution. A new system which achieves proper cavitaion

control, meeting the requirement mentioned above is called "the

reinforced aqueous cavitation system or the ultrasonic cleaning

method of the third generation".

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7 Basic Technique Regarding Waveforms

( 1 ) What is the most suitable form of ultrasonic waves for the

ultrasonic cleaning ?

O f cousre it is the one that creates the most desired cavitaion

for the specific cleaning. What we must realize here is that the

requied cavitation must be discussed with the relation to the type

of the cleaning object, the stains, the way of adhesion and the

system of cleaning. In a nut shell, it is useless to seek the

most suitable waveform for the multiple-use regardless of a

cleaning system and a cleaning object.

It is important to select the best ultrasonic cleaning system by

focusing on the characteristics of waveform, generation type and

so on.

(2) Waveforms of typical ultrasonic generators

* Multipurpose ultrasonic generator(c0ncurrent multi-waves)

Feature: Uniform cavitation in a certain area if handled

properly. For cleaning small objects in a tank of less

than 20-30 1

Basic frequency: 40kHz. S O O W ( 3 0 0 W ) , commercial frequency

modulation

Correct usage - vibration +

-

m /

less than 200m/m

measuring point

R

alminum sheet / bonding

transducer 4 0 kHz bolt fixed

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Frequency of c o r r e c t u s a g e

general I

m u l t i p l e

Frequency of f a u l t y u s a g e

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Faulty Usage(Mu1ti-type is used for immersible application.) * Bulkhead application gives the same results if the liquid is

deep.

less than

Conclusion

Although this type of generator is interesting, it loses its

strong point and far less effective than a single-wave generator

of 40kHz when applied for immersion. It is affected by a tank

shape and is unlikely to control cavitaion. It is only usable

for a small object in a shallow tank.

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* Alternate multi-frequency system

Feature; Aiming to el.iminate standing waves by running a few

different kinds of ultrasonic waves of different

frequencies altenately.

Baic frequency; 28, 40, 60 kHz

Conclusion;.The system is possible in terms of idea. However it

is impossible to ensure the maximum amplitude for the different

frequencies, ),l, x2, and A 3 respectively while fixing the

temperature of liquid and transducer.

Correct application (if possible)

Note; Temperature of a transducer and liquid must be maintained

at the fixed temperature(norma1 temperature?).

Run 1 1 , A 2 and A3 simultaneously from the same transducer

* Concurrent 3-waves "VARIOUS"

Feature; Combining the basic frequency and 2-3 kinds of high

frequencies makes it possible to generate the plural

kinds of cavitation simultaneously.

28kHz 40kHz 1 OOkHz

synergistic frequency 1 OOkHz 1 OOkHz 200kHz

200kHz 200kHz 500kHz

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Waveform output

ZBkHz changes according to synergistic frequencies of 100, 2 0 0 k ~ ~

28X 100. 200kHz

2 Cavitation generation

basic frequency frequency

Cavitation generates at the maximum change

- of sound pressure.

1 6 -

Conclusion

Effective and powerful cleaning results can be obtained 'easily

even in a deep tank by combining the high frequencies of l O O k H z

and ZOOkHz and the powerful frequency of 28kHz or 40kHz which

enables the easy cavitation control. This system is not

susceptible to temperature change and gives the desired end

results even when applied for immersion. Therefore it does not

have such problems as other systems have.

Note ~

Although this system gives a superior performance, it would be

ineffective if the ultrasonic waves dissappear on the surface of

a transducer or cavitation is not well controlled because of

operator's ignorance about how and where cavltaion is generated.

output (sound p r e s s u r e )

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General principle for the powerful aqueous cleaning

To utilize ultrasonic waves -efficiently, cavitation should be

controlled by maintaing dissolved oxygen vaulue under 2 ppm.

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I11 REINFORCED AQUEOUS CLEANING SYSTEM - BASIC TECHNIQUE

1 New Viewpoint of The Ultrasonic Cleaning Technology

When running ultrasonic waves in liquid, what are the requirements

for the best liquid to propagate ultrasonic waves and create the

desired cavitation most efficiently?

2 Dependence on The Quali.ty of The Liquid

Cleaning performance depends on cavitation. The nature of: the

generated cavitation varies according to the purity of the liquid

and the amount of the dissolved air. Furthermore the optimum depends

on an object of cleaning and the quality of oil. A s for aqueous

system, it would be reasonable to classify water based on 5 levels

of purity and 5 levels o f dissolved air as follows, which gives 25

kinds of water.

(Variety of purity)

* water containing detergent

* tap water

* p u r e water l e s s than 2 M&/cm

* pure water 2 - 5 ~ a / c m

* p u r e water more than 5Ma/cm

(Variety of dissolved oxygen)

* water of DO value 6-8ppm

4-6ppm "

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.- * water of DO value Z - ~ P P ~

f 0.5-2ppm

" less than O.5ppm

a ,

I

The ultrasonic cleaning of the third generation or reinforced

cavitaion system utilizes ultrasonic waves most effectively by

focusing on the quality of liquid, more specifically purity and

dissolved oxygen of liquid. Conventional ultrasonic cleaning

systems utilize partially the capacity of ultrasonic waves.

3 Example of Controlling Dissolved Oxgen in Tap Water(referred to as

DO below)

cooler

Note; Reinforced module is for deaeration.. The design varies

according to liquid such as water, alcohol, pure water, light oil,

etc.

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.. 4 General Principle of Reinforced Aqeous Cleaning System

To make best u s e of the energy of ultrasonic waves f o r the

generation of cavitation It is vital to control DO value under 2

ppm .

( 1 ) How to know the DO value from the surface condition

( S It C ’ s cleaner: 600W. 28kHz X 100 200kHz. depth; 200”

water; RT)

Observation - DO ( p p m ) .~ ~

6-8 Small, smooth and round swells appear sometimes on the

water surface. Tiny bubbles like white smoke come out

of the surface of a transducer.

BAD - - _ _ - - _

4 - 6 Small and round swells keep coming out for a long

period. Tiny waves appear on some part of the surface.

BAD - _ _ _ _ _ -

2-3 Tiny waves appear on the entlre surface. A

interference pattern of lOOkIiz and 200kHz is observed

Round swells come out occassionaly on a part of the

s u r f ace.

GOOD - - - - - - -

0.5-2 A fine interference pattern is observed on the entire

surface. No round swells appear. Occasional fine

spray is observed.

VERY GOOD - - - - -

- 2 1 -

I .

Note: Recent study shows the close llnk between DO value and

emulsification. A s for emulsifacatlon independantly.

than 1 5 ppm of supersaturation gives the good results.

value control must be studied in the wide range; under

’ . more

DO

G.Sppm, several ppm, and up to 20 ppm of supersaturation

Yet the value under 2 ppm is ideal for the reinforced

cavitation .

( 2 ) Visible evidences for bad cleaning by reinforced cavitation

system

* Visible tiny bubbles in liquid Especially bubbles to be seen at

the moment of turning off the machine

* Round swell on the water surface caused by visible tiny bubbles

* Smoke like tiny bubbles from the surface of a transducer

(Before this condition was considered-to be an evidence for gdod

cleaning.)

* Screechy sound of low frequency (mis-control of cavitation)

* No waveform to be seen (Ultrasonic waves do not reach the surface

even if there are no objects of cleaning in the tank. Peculiar

waveform of the transducer maker should be observed on the

surf ace.

( 3 ) Visible evidences for the defective design of the cleaner

A waveform seen on the surface varies according to the maker of a

transducer. Therfore it is the simple and easy measure for the

judgement of the cleaner’s function to see whether the maker’s

peculiar waveform appears on the entire surface without putting a

cleaning object. If the waveform is not observed, it means

ultrasonic waves dissapear before reaching the surface. A multi-

wave generator creates a interference pattern of 40-60(9G)kHz.

2 2-

, % Alternate multi-waves type creates a wave pattern which changes

continuously according to the frequencies, 2 8 , 4 0 and 60 kHz.

( 4 ) Visible evidences of the powerful effects when reinforced

cavitation system is working properly.

Tiny waves on the entire surface. Occasional spray ( as high a s

5- 3 0 cm ) Visible clearly with strong light exposure

* When using 28 kHz X 1 0 0 , 2 0 0 kHz, a plrex beaker breaks

immediately.

* An alminum f o i l of 30 u is torn up to shreds instantly.

* An alminum bowl is holed easily on the bottom.

* A thin sheet can not be taken of€ from a transducer.

invisible

US

stainless disk with 200-2000tiny holes ( 2 0 - 1 0 0 u )

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.. CFC Free Aqueous Cleaning Technology Incorporated with Reinforced

Cavitation System

Water has cleaned the earth and everything on it for four bil.lion

and some hundred million years after its birth. The life was born

in water. Since then water has been utilized as the harmless and

special liquid for cleaning. To take great advantage of water as

the better cleaning agent, studies must be carried out by focusing

on the nature of the various kinds of liquid in terms of their

application to the new ultrasonic technology. The reinforced

cavitation system opened the new era of the ultrasonic cleaning

technology of the third generation.

Manufacturer S & C CO., LTD

liead office 4087-1, Aikawa-clio, Nakatsu, Aikoh-gun, Kanagawa 243 Phone 0462-86-2779(Rep) Facsimile 0424-86-5709

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..