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Cleaning Process Techniques

Ultrasonic cleaning compatibility concerns

the substrate - Ultrasonics is most effective with hard substrates, such as metals, glass, and ceramics, and is not as effective in cleaning soft materials.

the cleaning liquid - The Ultrasonic cleaning process can be used with aqueous, semi-aqueous, and solvent-based systems, but both effectiveness and efficiency differ depending on the liquid used.
In principle the technique is compatible with numerous media, including caustics, acids, or solvents like fluorocarbons, acetone, alcohols, ether, and other hydrocarbons, but applications may be strongly reduced by their characteristics. Main factors influencing the performance of an ultrasonic system are:
• The temperature, affecting the other properties of the cleaning fluid
• Viscosity and density affect the effectiveness of the ultrasonic energy.
• Vapor pressure affects the extent of cavitation
• The surface tension affects the wettability of the surface to be washed.

The effectiveness of a cleaning process using ultrasonics will be therefore the result of

a balance among factors of different influence, and a compromise between the higher temperatures and concentrations normally needed for optimal cleaning and the lower temperatures and concentrations necessary for optimal energy transfer.

Common rail - the "common rail" fuel injection systems for diesel engines spray the fuel at a pressure of 1600 bar, therefore requiring an extremely accurate cleaning during the whole manufacturing process. The cleaning process must prevent the risk that small chips may affect the correct functioning of the system. At the pace of one piece every 15-20 seconds, each component is submitted to precision cleaning with tolerance of 10m or less.  The cleaning methods are different depending on the components and the nature of the contaminant to be removed.
The housing is submitted to a high pressure process up to 400 bar followed by a low pressure wash, rinse and drying. Internal components are washed by spray after each manufacturing cycle. The liquid is constantly submitted to precision filtration within the tolerance of cleanliness and the oil is separated from the cleaning liquid. The final cleaning before assembling in the "white room" is carried out by immersion with ultrasonic
Removal of ..........

....... carbon from internal combustion engines

The once difficult operation of removing carbon deposits from pistons and cylinder heads during overhauling is now as simple as a normal degreasing. Thanks to the experiments based on sport car engines where deposits are extremely hard due to the high temperature, PASSAPONTI has developed a simple process removing carbon deposits in few minutes without damaging the substrate, even if very sensitive like the grafite coated pistons. Based on a water base detergent working at medium temperature the process is carried out by immersion using a simple washer properly dimensioned to parts.

........ stains after heath treatment

Oxidations from heat, typical of surface treatment of steel are hard to remove since acids are causing oxygen enbrittlement and intergranular corrosion and therefore cannot be used. The problem has been solved by PASSAPONTI using a special but simple process based on aqueous detergent in immersion with ultrasonic. The system is low cost and offers the best advantage on small and medium parts.

.........Grinding, Honing, and Lapping Compounds
Residues remaining on parts after honing or grinding are usually mixtures of metallic and abrasive particles with oil-based or water-based cutting fluids.

Thus, the methods recommended for the removal of chips and cutting fluids are applicable also for the removal of grinding residues in a majority of instances.

Lapped parts are usually more difficult to clean than honed or ground parts. Lapping residues are composed of extremely fine particles of various abrasives, minute metal particles, semi-solid greases and oils, and some graphite. Even if graphite is not a part of the original lapping compound, it accumulates from wear of cast iron laps.

Allowing compounds to dry increases cleaning difficulty. In many instances, methods used for removing polishing and buffing compounds are applicable also for removing lapping compounds.

However, parts that are precision ground, honed, or lapped present special cleaning problems because:

  • such parts are commonly used in precision machinery, and consequently the degree of cleanliness required is usually higher
  • they are frequently intricate in design
  • they are commonly susceptible to damage and frequently require special handling.

Characteristics of the lapping compound in combination with parts are the main issue in determining the cleaning method and can vary case by case. Anyway, PASSAPONTI has experienced several methods to remove lapping residue, and the most suitable is based on a cold immersion in hydrocarbon solvent with additive or emulsion cleaner followed by a suitable rinse.

Agitation, by mechanical movement of parts, dislodges and floats away the solids resulting from grinding or lapping. The hydrocarbon is constantly filtered and recirculated.

The presence of dissolved oil in the solvent will leave a slight oil film on the parts. This can be removed easily by a rinse or alkaline final washing before assembly.

An extremely high degree of cleanliness without damage is required on some expensive and delicate parts, such as those used in instruments or fuel injection equipment.

Ultrasonic cleaning with alkaline solution, followed by spray with alkaline and immersion/spray rinsing can be ideal and in many applications replaces old technologies which were only partially effective.

Parts which took normally one hour and more to clean using solvent cleaning processes are now effectively cleaned in just a few minutes of alkaline cleaning with ultrasonic energy.

Other inherent advantages of this approach are that is environmentally friendly and safe.

Case Stories

Oil separation -1
The cleaning process is based on three stages (wash - rinse - dry) in a spray tunnel posed after a broaching machine using oil as a coolant. Parts have a mass of 4 kg and a surface of 0,135 sqm. Each part retains 93 g. of oil mostly removed before the cleaning process. Residual oil removed by the process is 18-23 g. Retention of the wash liquid amount to 35 grams per part. Parts are processed at a pace of one piece every 30 seconds during three shifts The wash stage uses an emulsifier creating a stable emulsion.
Verification after processing 5000 pcs. (2 days) revealed a light oil film floating in a milky wash solution. The rinse stage was transparent white. The cleaning result was still good.
Verification after 7500 (3 days) revealed a heavy presence of oil floating on the surface and further contamination of the rinse liquid with stains on parts after drying.

The problem was solved in two separate steps.
The first step faced the contingency by prolonging the duration of the process to five days to enable the maintenance during the weekend. It consisted in the application of the OILNET® to separate the oil when the emulsion was saturated.
The second step involved some tests to select a different type of detergent. Using the proper detergent the life of liquid was extended to four weeks for the wash stage and two weeks for the rinse. The OILNET® removed all the oil during the process. Cost of detergent was reduced by 70%
Inprocess treatment of cleaning liquids