NEARFIELD ULTRASONIC STRIP CLEANING

WILLIAM C. NOBLE PROCESS CLEANING SPECIALIST

THE PROBLEM A major manufacturer of metal parts, stampings, and bimetals for the electronics industry located in Massachusetts, found TCE (trichlorethylene) in test wells drilled on it's property. A survey of the many manufacturing departments found that within

the Massachusetts facility, there were 54 vapor degreasers using approximately 500 tons of TCE per year. A project was initiated to find a technology to replace vapor degreasing which would not have an adverse effect on production or product quality and would be safe for employees and the environment. VAPOR DEREASING Vapor degreasing is a common method of cleaning many types of parts. Parts are exposed to solvent vapors (usually chlorinated or fluoronated hydrocarbons) which condense on the surface and dissolve oils, lubricants, and some buffing compounds.

The process is well understood, is used in a wide variety of industries, and generally produces acceptable results. Parts processed by this method are defatted and dry. In recent years, worker safety issues and concerns about the environment have prompted manufacturers to seek alternate cleaning methods. CLEANLINESS STANDARD FOR VAPOR -DEGREASER

STAMPING LINES

Cleanliness was measured using the Soxhlet Extraction Method with distilled CycloHexane. An average production day with the degreaser in good operating status would yield a cleanliness level of 20 to 40 micrograms of carbon per square inch still on the work. The processing speed was limited to 20 feet per minute because time was required for the temperature of the strip to rise to the temperature of the vapor so that the strip would dry and not 'mil solvent from the degreaser. While testing was performed for organics, there was no testing for inorganics which cannot be removed by the vapor degreaser. Frequently, production was rejected because of fingerprints, shop soils, chips, and other inorganic soils. Problems also occurred with subsequent operations such as contact welding because of inadequate cleaning. MATERIAL HANDLING PROBLEMS VAPOR DEGREASER

The continuous metallic strip product being cleaned was fed through the vapor degreaser on a series of pulleys. The pulleys were aligned to feed the strip through each section of the vapor degreaser allowing the proper exposure time in each section. It is fair to say that the strip W'aS fa3 over so many pulleys that the inside of the degreaser resembled a motion picture projector. The frequent bending of the strip cause dimensional problems on blanked strip where caster and camber were critical. In addition, bimetallic strip material suffered stress from frequent bnding and had a measurable affect on metallurgy.

Loading of the vapor degreaser was a very dangerous process. It required the operator to place his a m and hands into the vapor zone to thread the strip through the pulleys. His face was in close proximity to the freeboard and although protective clothing was worn, it is obvious that this was not a desirable situation.

OTHER CONSIDERATIONS

VAPOR DEGREASER

With line speed limited to 20 feet per minute, there was little need for exploring the sophistication of progressive dies for blanking presses which had degreasers dedicated to them. The company had the usual problems and costs associated with the storage and handling of a hazardous material and of the storage and handling of the hazardous waste while waiting for the material to be picked up for treatment. In addition to the cost of TCE, the energy requirement to run a vapor degreaser was substantial. The boiling sump for TCE required a temperature of 185 degrees F, and refrigeration was required for the cooling coils. In the Commonwealth of Massachusetts, a permit to install a vapor degreaser is required by law. Adding a degreaser to a production line, or moving an existing degreaser from one building to another required the company to reapply for the permit. Ekyloyee safety, the Right To Know law, environmental concerns, and a general concensus among management that solvent cleaning was not desirable for the long range goals of the company, prompted 'the search for a new technology.

The Solution The use of solvents in industrial degreasing applications can frequently be eliminated by converting to ultrasonically activated aqueous solutions. This report details a new type of ultrasonics and the development of a turnkey system to clean continuous strip products. Benefits include elimination of hazardous solvents in the workplace, higher standard of cleanliness, improved product quality, line speed increase, and total elimination of all hazards from the cleaning process. The challenge which was laid before us was stated simply, "Develop a machine to clean metal strip on ths fly at line speeds up to 80 feet per minute. The strip must be completely dry and be extremely clean. This cleaning method must be totally safe for our employees and the machine must not take up more than six feet of production space." Finding an alternative to solvent cleaning is no easy task in the electronics industry where concerns of corrosion, surface contamination, metallurgy, and conductivity must be addressed. Solving the problem involved the development of a system where machine technology and chemical technology had to work together to provide a turnkey solution. Lewis Corporation of Oxford; Corm., a manufactlxer of industrial ultrasonic cleaning equipment, designed the ultrasonic hardware while Intex Chemical of Greenville, S.C. provided the detergent formulation. NEARFIELD ULTRASONICS This new technical development by Lewis Corporation is made possible because of their experience with magnetostrictive, silver brazed, transducer manufacturing techniques. Nearfield consists of two opposing plates in close proximity to each other which operate at 20 kHz and 16 kHz respectively to produce an extremely intense ultrasonic field.

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When the chamber between the two opposing plates is flooded with the cleaning solution, the alternate compression and rare faction waves cause minute vapor bubbles to form and then collapse. These bubbles transform the relatively low energy density of a sound field into a high energy density by absorbing energy from the sound waves over one or more cycles and expelling it during a very short interval of time.1 It's the action of these microscopic bubbles imploding on the surface of the strip that performs the actual cleaning. The collapsing bubbles cause high speed micro jets of liquid which impinge uiwn the surface and cause surface erosion and localized heating. The action of the bubbles occurs in all directions so that even strip with complex profiles can be effectively cleaned. The field is so intense that only a brief zxposure is required.

THE DETERGENT' The selection of the proper chemical cleaning solution was so critical that research chemists at Intex Chemical worked to formulate a product specifically to meet the requirements of the Nearfield Strip Cleaning system. The detergent, Intex 8125, is a near neutral pH ( 7 . 5 ) , is non-silicated, and contains no hazardous ingredients. It is noncorrosive to metals and a combination of three surfactants enable it to quickly wet-out the surface of the strip so that the soil can be loosened and r2moved by the ultrasonics. The detergent has superior rinsing properties and is capable of removing most stamping and forming lubricants and other non-organic shop soils. When the soil being removed is an oil, a reaction between the surfactants and the emulsion cause the emulsion to break and split the oil to Yne surface where it can be skimmed. This reaction only takes place when the detergent is in a quiescent state. This reaction will not occur with lubricants which are emulsifiable, or are synthetics which contain Triethanolamine (TEA), a fatty acid. The latter type is usually biodegradable and splitting would be of little value. For reasons which are not fully understood, Intex 8125 greatly promotes ultrasonic activity in water.

THE SYSTEM PROCESS The system accomplishes ultrasonic wash, rinse, and dry on a continuous basis. The strip enters the ultrasonic chamber where contact with the detergent takes place and soils are removed. The detergent solution is continuously filtered and recirculated with the cleanest solution in contact with the moving strip u p n it's exit from t'ne cleaning chamber. The strip then enters the rinse section where the detergent solution is thoroughly rinsed off wit.h a direct spray of tap water. The strip is then dried by means of air knives, nozzle-like devices that focus filtered, compressed air on the strip and physically remove all traces of water. The strip emerges from the system without any water droplets that could cause spotting (see fig. 1).

No guides or rollers are employed and the strip does not have to be depressed into a tank for cleaning. The entire cleaning pocess is essentially non-contact and since the process path is straight through, the system can also be used to clean rigid materials such as pipe and bar stock. 2 CLEANLINESS STANDARD FOR NEARFIELD STRIP CLEANER Using the Soxhlet Extraction Method with distilled Cyclo-hexane, the cleanliness

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standard for the Xearfield Ultrasonic Strip Cleaner was 0 to 6 micrograms of carbon per square inch still on the work, cleaner by an order of magnitude over t k vapor degreaser. Nhat is more imprtant, this level of cleanliness was achieved at 80 feet E minute line speed, or four times the speed pssible with the degreaser.

In addition, inorganic soils such as fingerprints, chips, particulates, and other shop soils were removed. One thousand power electron microscope scans of the cleaned surface revealed only trace amounts of debris remained. MTERIAI, HANDLIX

=-FIELD

ULTRASONIC STRIP CLEANER

kith a straight through pass line and a non-contact cleaning process, material handling problems associated with the vapor degreaser all but disapgeared. Caster and camker dimensions on blanked strip were unchanged, and no stress occurred on bimetallic strip. Metallurgy tests revealed that extended contact with bimetallic strip used in thermostats to the intense ultrasonic field, resulted in a temperature change in the bimetal response. Testing showed this temperature change at one degree F for each minute of expsure to the Nearfield ultrasonic cavitation and when the line sgeed vas considered, this vas not a factor because the ultrasonics are wired to shut off when the strip stops. The Nearfield Ultrasonic Strip Cleaner is easy to load and the entire process is-nonhazardous, even if the operator comes in contact with the cleaning chemical.

OTHER CONSIDERATIONS NEARFIELD ULTRASONIC STRIP CLEANER The increase in line speed from 20 feet per minute with'the va,pr degreaser to 80 feet per minute with the Nearfield Ultrasonic Strip Cleaner has enabled the customer to change to progressive dies in the stamping machines. The company no longer has a storage and handling problem with hazardous materials or hazardous waste. The spent solution of Intex 8125 is very easy to baste treat. The cost of Intex 8125 is far less than TCE because it is diluted with water and is used at between 5 and 10 percent solution. While TCE boils at 185 degrees F, 8125 runs at between 140 and 150 degrees F and the chemical tank heater is only used to bring the temperature from ambient to 150 at start up. The solution is maintained at 150 by the heat generated by the ultrasonics. A three inch wide Nearfield Ultrasonic Strip Cleaner (Model SC-3, fig. 2 ) has 3000 watts of ultrasonic power. The only moving part in a Nearfield Ultrasonic Strip Cleaner is the pump impeller. Maintenarxe cost is lower. While the line s p e d at this company was increased to 80 feet per minute, the speed is dictated by the complexity of the profile of the strip as it affects drying. Strip which has little or no profile has been run at speeds up to 200 feet per minute. Production Sroblems with subs,quent operations which were associated with a lack of cleanliness from the vapor degreaser have been eliminated.

Nearfield Ultrasonic Strip Cleaning has exceeded the customer's cleanliness specifications

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by an order of magnitude in addition to a substantial line speed increase. The aqueous cleaning method removed many of the inorganic soils normally left behind by vapor degreasing, and air knife drying insured that all traces of minerals normally present in potable water are removed. A l l design criteria stated in the challenge made to us were met in all cases and

exceeded in most. The customer has eliminated vapor degreasing as a method of cleaning in stamping departments. The Nearfield Ultrasonic Strip Cleaner is currently Patent Pending.

ULTRASONIC WASH

I

SPRAY RINSE

WATER

%-!

BLOW DRY

AIR

FILTER/HEATER

RECIRCULATION PUMP Fig. 1. This block diagram of an aqueous based strip cleaner shows the direction of strip through the cleaner and the three functional sections.

FIG. 2

Nearfiela Ultrasonic Strip Cleaner Model SC-3

REFERENCES

1.

Verall, R.E. and Schgal, C.M.

2.

Lawson,

C.

and Noble, W.C.

Ultrasonics (1987) 25 Metal Finishing

(1)

29

(1987) 85 (5) 51