Cleaning parts before coating or painting with ultrasound

Most coating failures are cleaning failures. Paint that peels, powder with craters, plating that blisters and adhesive joints that let go usually have the same root cause: a film of oil, a fingerprint or a residue on the surface that the coating could not wet. The coating line cannot correct that. The surface has to arrive clean, rinsed and dry, and it has to stay that way until it is coated.

What the coating needs

A coating adheres by wetting the surface. Oil, grease, wax, silicone and dried cleaner residue lower the surface energy, so the coating pulls back from those spots instead of spreading. The results are familiar:

Silicone deserves a special warning. A trace from a spray can used near the line is enough to crater a whole batch. Keep silicone-containing products out of the cleaning and coating area altogether.

The cleaning sequence

Two rinses are normal before coating. The first rinse removes the cleaner; the second, in demineralised water, removes the salts that the first leaves behind and that would otherwise dry into spots under the coating. Tap water in the last rinse is a common cause of blistering. See rinsing and drying and multi-tank lines.

Ultrasonic cleaning and pretreatment

Many coating lines add a chemical conversion step after cleaning: iron or zinc phosphating, zirconium or silane pretreatment, chromate-free passivation, or the activation step in a plating line. Ultrasonic cleaning does not replace these steps. It comes in front of them and makes them work, because a conversion coating only forms on a surface that is already free of oil. Where the parts have blind holes, threads or pockets, ultrasonic cleaning removes the oil there too, which a spray pretreatment line often cannot. For simple sheet parts without cavities, a spray line alone is often enough; see spray washer or ultrasonic bath.

Testing the surface

The cleaning is released by a test on the part, not by its appearance:

• Water-break test: rinse the part and hold it up. On a clean surface the water forms a continuous film for at least 30 seconds. Where it breaks into droplets or pulls back, there is still a hydrophobic film. It is fast, free and belongs at the end of every cleaning line.
• Test inks or dyne pens: a set of inks of known surface tension shows the surface energy of the part. The coating or adhesive supplier states the minimum value their product needs.
• Adhesion test on the coated part, such as cross-cut or pull-off, checks the whole chain of cleaning, pretreatment and coating.

From the dryer to the coating

• Coat soon. A clean surface picks up dust and airborne oil. Aim for hours, not days.
• Handle with clean gloves, and never touch the surfaces to be coated.
• Hang or rack parts for the coating line directly after drying, so they are not handled again.
• Keep the bath fresh. A bath loaded with oil puts oil back on the parts. A skimmer and filtration, and a concentration check every day, keep the result constant; see filtration and bath life.

Which machine

For oily parts the chemistry side is covered in removing oil and grease, and for machined parts in removing swarf, chips and coolant.