Powder coating is a simple and straight forward process. Follow it blindly and you know you will get great results. Hunt for short cuts, and be ready to fall flat on your face.
1. Cleaning the surface to be coated: Also known as pretreatment, powder coating (as with all other surface finishing methods) requires that the surface to be coated should be cleaned of all contaminants, rust, oils and grease) and properly prepared as required before it is coated. There are two ways to do this -
a. Mechanical: At the industrial level this is done by shot- or snad-blasting the surface to be coated. This creates a fresh virgin surface which should be coated as soon as possible to ensure that fresh corossion does not set in. At the DIY level and start-up level, this is done using sand- or emery-paper to manually remove all contaminants from the surface.
b. Chemical: The parts are dipped in a series of tanks (or are passed through a series of spray-chambers) containing different chemicals or water. These steps remove the surface contaminants and then prepare the surface with a special coat (eg phophate coat on steel or a chormate coat on aluminum). Such special fine coats ( a few microns thick) improve the bond between the metal and the powder particles. Due to the pollution generating potnetial of the chemical process, various authorities in different parts of the world have laid down stringent norms and regulations governing the treatment of these chemicals.
2. Coating the surface with the powder: Virtually ALL powder coating is done by spraying powder on the part being coated. Powder is electrostatically charged as it leaves the tip of the powder gun (either using electronics to create a high electrostatic charge or using friction to create ‘tribostatic' charge) and is attarcted towards the part being coated which is properly grounded (earthed). The charge ensures that the powder particles stick to the surface of the coat. The level of charge will generally define a limiting thickness of the coat. Usually the charge is adequate to ensure that the particles stick to the surface through the process of moving them from the coating booth to the curing oven.
3. Curing the powder coated part: The coated part is introduced into an oven (box type in case of batch production and tunnel type in case of conveyoised and automated production), where at temperatures in the range of 180 - 220oC (356 - 428oF) for 10-20 minutes, the powder first melts and then polymerises and cross-links to create the final finish which is aesthetically good and mechanically tough while being resistant to most types of chemical abuse. The coated part, when cured, is removed from the oven, allowed to cool down and taken away from the paint-shop, ready.