Q. I am powder coating using RAL 7035 in polyester as well as in hybrid. I cannot get the required shade per the shade card. What are the assignable causes? We are curing the polyester at 220°C and the hybrid at 180°C. R.L.
A. First, find out if your supplier is matching the color to a RAL master panel. There may be slight variation due to the fact that the original colors were generated with inks and the pigments were not as durable as powder coatings, but the color should be an acceptable match. If the color was not matched to an original master you will have more pronounced variation.
You may also have some trouble with your cure oven or cure cycle time. 220°C (430°F) is a somewhat high temperature for curing powder. A polyester powder will stand some over-bake, but the RAL 7035 is a lighter gray that could oxidize if left in the oven too long at that temperature. Try using more time and less temperature.
Make sure you have enough oven exhaust. If you oven is not exhausting enough air you will build up gases in the oven and cause discoloration of the coating. You can learn about proper exhaust volumes from the Systems Design manual published by Chemical Coaters Association International at ccaiweb.com.