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Colour change in powder-coated parts

2007

We build products for Telecommunication Industry and the body of the product is machined aluminum (6000 series). After machining, they are powder-coated externally through a supplier with thermosetting polyester.

The problem is that after coating, they need to be put in an oven at 65 C to 85 C degrees for 8 to 14 hrs as per our internal process and after getting out the oven, only 10-15 % of them change their colour to yellowish appearance. When the issue was raised to the supplier, their response was "those colour-changed ones were re-worked bodies (stripped and re-coated: means they went through the same chemical cycle twice!) therefore, when they subject to prolonged heat, they would react differently comparing to the ones processed only once".

Anyone has a comment(s) on the Supplier's response? If it is the case, what would be the mechanism for the colour-change (change of surface properties or retaining chemicals or other ?)? What would be the practical advice to the supplier to prevent this colour change on re-worked powder-coated parts?

Your quick reply would be appreciated.

Regards,
Alex Kultur
Australia

Alex Kultur
Buyer - QLD, Australia

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You're aging the parts after paint? I'm surprised the paint isn't just falling off afterwards. Check to see if the discolored parts are positioned right in front of the oven baffles, in other words, is hot air blowing directly onto these (the discolored) parts causing a hot spot? Another thing that you can do is put a load of parts in your oven that have only been through the paint cycle once(no reworks) and see what you come up with. You could also ask your painter to mark his reworks so that you can be sure that they are the parts coming coming out discolored. I think you'll find that the painting, stripping and repainting cycle isn't causing this particular problem.

Sheldon Taylor
supply chain electronics
Wake Forest, North Carolina
2007

I often repaint powder coated parts for customers and the only pretreatment that is done is the surfaces are rubbed done using scotch cloth which dulls the finish and roughens the surface.there have never been problems with the finished appearance but it can often be tricky to get the paint to adhear to the corners.but patience and often spraying the powder from the reverse side of the piece works instead of spraying it directly at the work piece.

Ian Purcell
powder coat operator - Ireland
January 31, 2008

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February 5, 2008

Dear Alex,
I think the other responses are excellent and once you have tried their suggestions please inform us all of the results. This information would add to our knowledge base for future reference.
It sounds like your supplier is involved and I would imagine that they have already tried to prevent the discolouration by supplying the polyester resin based powder. Nevertheless, there are different grades within the technology of the polyester powders available to the applicator. I would suggest that a "Super Durable Polyester" is worth a try ø the resin plus additives, is cooked a little longer to remove certain chemical groups from the chain of the polyester resin. The resultant polyester powder has superior weathering properties and can resist heat far better than your standard industrial polyester powder.

Terry Hickling
Birmingham, United Kingdom

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Dear Alex,
If your supplier (or anyone)would like to know a little more re "Super Durable Polyesters" you can inform them that the polyester powder is formulated using the "Aliphatic" powder route of chemistry and not the cheaper "Aromatic" route used for the manufacture of industrial polyester powders. However, I think your problem may be, as already suggested by others, connected with the re-coating of rejected work. Please let us know the outcome.

Terry Hickling
Birmingham, United Kingdom
February 6, 2008