Powder coating is a coating that is applied to objects much like your common household paint, but unlike paint, powder coating does not contain a liquid filler that needs time to evaporate. Instead, after a powder coating is applied, it is heat cured until it forms a “skin”. The powder is made up of either a thermoplastic or thermoset polymer. Using the powder coating method results in a much tougher surface than conventional painting does. And much like paint, a powder coating may be applied to a variety of surfaces, such as metals, automobile and bicycle parts, drum hardware, and household appliances.
There are advantages in using the powder coating method as compared to painting. Since there is no liquid carrier in powder coating to evaporate, powder coating produces much fewer volatile organic compounds than painting does, and with the absence of a liquid carrier, several coats of powder coating can be applied to a surface without running. Another advantage when using powder coating is that many different colors may be applied before being heat cured, which results in eye catching color blending and bleeding. Powder coating is also surprisingly cost effective because the cost of a powder coat gun, booth and oven is comparable to the cost of a typical spray gun system. However, unlike spray paint systems, any over-spray from the powder coating process can be recycled.
Powder coating contains different polymers, including polyester, epoxy, polyester-epoxy, polyurethane and acrylics. To produce the powder in powder coating requires a three step process. In a mixer, granules of polymer, pigments, hardeners and other powder type ingredients are mixed well. The resulting mixture is then well heated in an extruder. Then the mixture is rolled out flat to allow it to cool. When cooled, the mixture is then broken up into chips. Finally, the chips are run through a grinder, resulting in a very fine powder.
The powder coating process consists of preparation, powder application and curing. Before the coating can be applied, dirt, oil, grease, welding scale and metal oxides must be removed. This can be done by mechanical or chemical methods, depending on what material the part consists of and the size of the part to be coated. The chemical process entails immersing the part into a chemical bath containing phosphates and chromates. Another method to prepare the surface is to use an abrasive process known as sandblasting.
Applying the coating is commonly accomplished by spraying, using an electrostatic gun. The job of the gun is to produce a positive electric charge to the powder, which in turn will easily stick to the object being coated. An alternative method is called the fluidized bed method, which is done by heating the object and then dipping it into a powder filled bed, thus allowing the powder to melt and stick to the heated object.
The curing process uses heat, which in turn, causes the powder to melt and causes a chemical reaction resulting in a durable polymer bond.