ULTRASONIC CLEANING by F. John Fuchs

Blackstone Ultrasonics, Inc., Jamestown, NY

The ultrasonic cleaning process is beneficial in that when properly applied, it can save time, money and increase cleanliness to a level that can not be achieved by any other means. Workpiece sizes range from screws to cast heavy equipment transmission housings weighing in excess of 300 Ibs each.

For purposes of cleaning, ultrasonic vibrations are transmitted into liquids consist- ing of either water-based or solvent-type chemicals, which in turn contact the surfaces to be cleaned.

When ultrasonic waves of sufficient amplitude are introduced into a liquid, the re- sult is cavitation of the liquid at the "rarefaction" or low pressure points of the sound waves. As waves pass by a given point in the liquid, low pressure areas are replaced by high pressure or "compression" areas. Cavitation bubbles produced under rarefaction implode during compression, resulting in the production of extremely small but highly intense shock waves radiating from the point of implosion. It is these high intensity shock waves that do most of the work we associate with ultrasonic cleaning.

Since cavitation and implosion take place wherever an activated liquid penetrates, ultrasonic cleaning can be accomplished on even the most complex and intricate assem- bly literally from the inside out - right down to the surface porosity. Many factors go to make up a successful ultrasonic cleaning operation.

MAXIMIZING THE ULTRASONIC CLEANING PROCESS The effectiveness of any ultrasonic cleaning process is dependent on the cavitation

intensity achieved in the cleaning liquid, as well as other factors. I n turn, cavitation in- tensity is dependent on several parameters and properties of the cleaning liquid.

Te m para tu re: Temperature is the parameter which has the most dramatic effect on the ultrasonic

cleaning process. The general rule is that increasing temperature will result in higher cavitation intensity and better cleaning, as long as the boiling point of the chemical is not too closely approached. Near the boiling point, the liquid will boil in the negative pressure areas of the sound waves, resulting in no effective cavitation.

Maintenance of the proper operating temperature is important because of its sig- nificant effect on the speed and effectiveness of most chemical reactions. Water is known to cavitate most effectively at approximately 160°F. A caustic/water solution, however, cleans most effectively at a temperature of 180°F because of the increased chemical effect at a slightly higher temperature. The increase in chemical effect over- powers the decrease in cavitation effect.

Additional temperature increase results in diminished cleaning effect because of continued rapid reduction in cavitation intensity. Some chemicals, on the other hand, are designed especially for use at lower temperatures. Operation of one of these chemi- cals at an inappropriately high temperature can cause chemical breakdown and resul- tant ineffective cleaning. Solvents should be used at temperatures at least I O " below their boiling points. Dissolved Gas in the Liquid:

Dissolved gas results in low cavitation intensity. This is because the gas diffuses into cavitation bubbles formed during thenegative pressure portion of the sound wave, and then acts as a cushion to the implosion of the bubble during the positive portionof the wave. As a rcsult, there is no violent implosion, hence no useful effect produccd.

138

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&solved gas can be eliminated from liquids in several ways in preparation for

I . Apply ultrasonic energy intermittently. Gas bubbles will form as the energy is

2. Heat the liquid. As temperature is increased, liquids are able to hold less dis-

3. In case of aqueous chemicals, add a wetting agent. Surface tension reduction

effective ultrasonic cleaning:

applied and then float to the surface when it is turned off.

solved gas.

will speed gas removal when ultrasonic energy is applied. Surface.Tension:

In general, liquids with higher surface tensions exhibit higher cavitation intensities. This is thought to be because the higher surface tension results in greater energy being released as cavitation bubbles implode. Viscosity:

More viscous liquids require more energy to cavitate. As viscosity is increased (per- haps to that of motor oil) ultrasonic cavitation is no longer possible using normal tech- niques. Ultrasonic Power:

Ultrasonic power must be matched to the task. Excess power can be just as detri- mental as insufficient power in most applications. In some cases where especially deli- cate parts arc involved, an ultrasonic powcr control may bc used 10 rcducc powcr and the likelihood of part damage caused by vibration or erosion.

Ultrasonic Frequency: As ultrasonic frequency is increased, more power must be applied to maintain the

same cavitation intensity. This is because at higher frequencies, fewer sites are present which can become nuclei for cavitation bubbles. The higher the frequency, the smaller must be the nucleus for cavitation. Fewer cavitation bubbles of a smaller average size re- sult in less cavitation intensity overall. Most ultrasonic cleaning equipment today oper- ates at frequencies between 21 and 45 KHz.

Ultrasonic frequency may occasionally be considered as a variable in achieving max- imum cleaning. Cases where i t may be important are those where very small areas must be penetrated.

Fig. 1A. Blind hole is not filled with liquid in this orientation. No cleaning will occur.

140

Fig. IB. Blind hole will fill with liquid in this orientation and can be cleaned.

Blackstone Immersible Transducers Transform your Tanks

Part Exposure:

Before a surface can be cleaned i t must some in contact with ultrasonically acti- vated liquid. Unfortunately, this fact is frequently overlooked in the use of ultrasonic cleaners and the result is failure. The most common mistakes fall into the following categories:

1 ) lmpropcr part orientation resulting in air pockets. A blind hold i n a piece 01' metal inverted into a liquid with the opening of the hole facing downward will not f i l l with liquid (Fig. 1). As a result, cleaning cannot be expected in the blind hole. This situation can be corrected by inserting thc part in such a way (hat there is no air trapped. This may, in some cases, require rotating the part after i t is submerged.

2) Overloading of baskets with small parts can sometimes result in ultrasonic ener- gy being absorbed by the first several layers of parts. This happens most fre- quently with small hardware items such as washers, nuts, bolts and screws. In general, large volumes of small parts can be cleaned more quickly a few at a time with relatively short cycles than they can be in large groups with longer times.

3) Baskets or fixtures for holding parts must be constructed in such a way that transmission of the ultrasonic energy will be attenuated as little as possible. An open racking method is best whenever possible.

Contaminants considered for ultrasonic removal generally fall into one of the fol-

I ) Soluble contaminants. 2) Non-soluble contaminant held by a soluble binder. 3 ) Non-soluble contaminant held in place by mechanical attachment or ionic

In the first two cases, cleaning or contamination removal is dependent on chemical

lowing classifications:

'

bonds.

action. In the third case, theeffect is primarily mechanical.

Soluble Contaminants: Soluble contaminants are materials which can be broken up or dissolved complete-

ly by either aqueous or solvent-based chemicals. Removal of these involves supplying a sufficient quantity of the solvent to completely dissolve or break up the soluble material which is then removed along with the solvent.

1JItrawnic energy provides agilalion a1 Ihc solvcnt/conraminan~ interfacc which helps speed the dissolving process. This effect is extremely beneficial whcrc thcre arc rough or irregular surfaces to be cleaned. Agitation provided by mechanical stirring, bubbled air or conventional vibration cannot penetrate into blind hole areas and aid dis- solution as can ultrasonic cavitation.

Non-Soluble Contaminant Held by a Soluble Binder: 'rhk typc of conr:iminnnt ir Ihc snmc a \ that ,iiist dixmscd. cxccpl that Ihc soluhlc

material in this case is acting as a binder which holds non-soluble matcrial to thc ~uII';icc as wNell. An example is machining chips held on a surface by oil and grease. Ultrasonic energy speeds the dissolving process as well as helping to dislodge and remove loosened non-soluble particles.

Non-Soluble Contaminant: In this case, non-soluble materials are held in place by mechanical or ionic forces.

Implosions of cavitation bubbles i n proximity to the attachment points cause nicchan- ical shock waves which break the holding bonds. Once the bonds are broken, the freed partncles can be flushed away. The action is dependent on the chemical media solely where ionic bonds are involved. A polar medium should be chosen which has a greater

\

-. atraceion for the removed particles than the surface they were removed from to prevent redistribution. Scale removal is accomplished when cavitation lifts and breaks off pieces by the process of mechanical fatigue.

Cleaning Chemical Selection: Selection and use of the proper chemical cleaning media is of primary importance

in any ultrasonic cleaning operation. Continued use of a previously satisfactory chemi- cal is most desirable wherever possible. Chemical compatibility with the workpieces has been established and it is usually possible to accomplish a smooth transition into ultra- sonics. Factors that would indicate selection of a new chemical might be flammability, economic considerations, or knowledge of the fact that the chemical does not perform well in conjunction with ultrasonics. Certain wetting agents, for example, are known to diminish the effect of ultrasonic cavitation.

Equipment Selection: The two general types of equipment available for ultrasonic cleaning are ultrason-

ically activated tanks and ultrasonic vapor degreasers. Ultrasonically activated tanks range in size from less than a pint of liquid capacity up to several hundred gallons and are used with water-based and other non-volatile cleaning media.

Ultrasonic vapor degreasers are available in standard sizes up to approximately 30 gallons and are designed especially for use with chlorinated hydrocarbons or fluorocar- bon solvents. in addition to these two general types of equipment, immersible ultrasonic transducers are available which can be added to nearly any dip-type cleaning system 10 give ultrasonic capabilitv.

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