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Ask the Expert Question-and-Answer Archive

by Ron Joseph

October, 2006

Cratering, (Fish Eyes) in Powder Coating

Q: I am Senthil working as a Quality Executive in Akzo Nobel coatings India in Powder BU. Today for the first time, we received a complaint of cratering defects by one of our customer. They have used our pure polyester powder product. The finish is a structured one. They are not using any silicone for masking. Please advice us about the probable root cause, corrective and preventive measures to avoid such problems in future.

A: The fact that only one customer seems to have the complaint would indicate that it is unlikely that there is an inherent problem with the batch of powder. This statement would not be true if you made the batch solely for this customer, in which case the batch might also be suspect.

I have seen cratering in both liquid and powder finishes and very often the cause is not at all obvious. Assuming that the cratering is due to the customer's environment rather than due to the batch of powder, you will need to perform many different tests to try to identify where in the factory the contamination might be initiating.

You might need to go as far as performing tests on each individual person not only in the powder coating facility but in the entire manufacturing facility. For instance, I solved one problem by identifying that a single person was using a coconut oil skin conditioner to soften the skin on his hands. When we asked him to stop the cratering went away. Others, who ate potato chips with their bare hands did not cause cratering. As I said, the cause is not obvious.

You might need to perform tests to determine if the contaminant is entering the plant from the outside air. Have you looked at the pretreatment (phosphating) equipment? What about the rinse water?

You can analyze the painted surface to determine what the contaminant is, but even if you get an answer you will still need to identify the source.

This can be a large task but it might take a Sherlock Holmes to find the culprit.

Best wishes,

Ron Joseph

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