Cob-Webbing in the Booth

Question:

I am putting in a re-bar powder coating unit in India. We intend to use fusion bond epoxy (FBE) powder. In this re-bar system the part is preheated to 220C and then coated with powder using electrostatic guns positioned on top and bottom. Curing time is less than 2 min. Previously, I used to work with a company that did the same thing. During my time with this company I observed the powder “cob-webbing” within the booth. This is not only wasting powder but causes maintenance problems. We tried many things to solve the problem by working closely with the powder and gun suppliers but without success. Can you please inform us of causes and remedies for this problem? B.D.

Answer:

The heat coming off the part melting the powder overspray causes the “cob-webbing” within the powder booth. This is inherent to the FBE process, where preheated parts are coated with powder that fuses, bonds and cures upon contact with a preheated part. The part is normally water quenched upon exiting the powder coating booth to cool the surface to prevent the roller conveyor from disturbing the coating. Often the curing process continues after the part’s surface has been cooled, because the inside of the part is still quite warm. Nevertheless, the part is fully cured quickly after it has exited the booth and proceeds toward packaging (usually banding into bundles).

This “cob-webbing” is inherent to the FBE process but can be minimized with better control of the preheating process. The preheating process normally uses an induction coil to rapidly heat the re-bar(s) to 220C or hotter. Controlling this coil to provide the exact amount of energy needed to heat the re-bars to the desired temperature is required to reduce “cob-webbing”. For instance, the larger the re-bar diameter (mass), the more energy is required to heat it. If you do not adjust the coil settings between runs of different re-bar sizes then the lower mass parts will be overheated, adding to the heat within the powder booth and causing more “cob-webbing.” This is the only suggestion that I can make to help you. Good luck.