The powder coating process is a dry painting process that is mainly used for coating metal. There are several methods for applying powder coating. The most commonly used method is spraying electrostatically charged particles of pigment and resin onto an electrically grounded piece. Heat is used to melt the powder, causing it to flow, form a thin film and eventually dry to a hard, scratch-resistant shell.
As with any painting job, the most important step in the powder coating process is preparing the surface. Before the powder coat is applied, metal surfaces are thoroughly cleaned to remove oils, dirt and grease. After the metal has been cleaned, it is generally rinsed and given an acid bath to etch the surface. An etched or slightly rough surface helps to ensure proper adhesion and an even distribution during the powder coating process.
The next step in the process is the actual powder application. The application method varies depending on the type of powder being used. The two main types of media used in the powder coating process are thermoplastic powder and thermoset polymer. Both types of powder rely on heat curing, but they do so in different ways.
Thermoplastic powder is usually applied to a heated work piece. The hot metal causes the powder to melt, flow and then harden upon cooling. Thermoplastic powder does not undergo any chemical change as it hardens. That means that the finished work can often be reheated to even out any thin or uneven spots in the finish.
Thermoset polymer is applied to the work at room temperature, then heat-cured in an oven. The part is heated and held for a set period of time at temperatures of 320-410 degrees Fahrenheit (about 160-210 degrees Celsius). During that time, the polymer undergoes a chemical change known as "cross-linking." The curing of thermoset polymer happens in four phases, which include melting, flowing, forming a gel and, finally, curing to a hard shell.
For powder coating equipment, there are several types of guns that can be used in the powder coating process. They generally all achieve the same result in different ways. These powder coating tools are designed to fluidize the paint media and electrostatically charge the particles as they leave the gun. The main difference from one style of gun to the next is the method used to introduce the charge.
Some powder coat professionals also use a piece of equipment called a fluidized bed during the powder coating process. A basic fluidized bed system includes a tank to hold the powder, a means for introducing the charge and a low pressure air supply. As air enters the tank, the charged powder is fluidized, or suspended in the air column, and it often resembles a boiling liquid. The part is then dipped into the suspended powder and then cured. The fluidized bed method lets the powder coater apply a thicker coat in just one application.
The powder coating process adds more thickness to the piece than traditional painting methods do. This is especially important for items that are meant to fit together after powder coating. If the work piece is disassembled for powder coating, it might require some sanding or grinding at reassembly.