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Root Causes of Defects

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Q. We currently have a customer who purchases aluminum forged products from us. Some product has the surface prepared by polishing and some product that is not surface polished. What I’m experiencing now is a vast number of returns for base metal defects found on parts after powder coating, however the defects that are claimed to be in the base material are not possible since all product is visually inspected 100% of the time in two separate occasions. In addition, the defects are obviously covered by the paint but what is being shown as the defect is so large of an area that it is humanly not possible to be created by forging. Do you have a visual chart that would show the potential painting defect and the root cause of it to help us better identify if the defect is caused by us or by the painting application? Also, what is the best method for preparing these surfaces to allow an even and consistent coating of paint? The defects that I’m looking at appear to be like the material is pooling up and when cooled will either raise a quarter to half dollar coin sized section or recess this same size area but the paint is darker in color. P. T.

 

A. I have no idea what the defect can be on your parts without seeing a photo or actual part showing the defect. Your description of the defect does not help me much either. However, there are defects in castings and forgings that can wreak havoc in a powder coating system that are not visible to the naked eye before coating. These only become apparent when the part is heated to cure the powder coating and/or the coating is on the part to provide a contrasting surface. One of these hidden defects is entrapped gas that can out-gas through the coating. This does not sound like your problem, since you are not describing pinholes as the defect.

I know of no photographic document that describes paint related defects. There is however, a great verbal discussion of powder coating defects in the Powder Coating Handbook, published by the Powder Coating Institute. In this text, under Quality, several defects are described in great detail and causes are noted. I would obtain a copy if I were you. 

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