There are usually three main types of powder coating system, namely electrostatic coating with a spray gun, electrostatic fluidized bed coating, and electromagnetic brush coating. Powder coating is a surface finishing technique similar to painting but with the coating material applied in powder form rather than suspended in a fluid. The powder is electrically charged so that it adheres to the item to which it is applied — anything from furniture or personal accessories to walls or large appliances — until it is cured. Choosing the right type for any given project is often a matter of availability as well as project scope. All three types work in roughly the same way, and all follow about the same application process. First is pretreatment of the part to be coated, followed by application of the coating material, and finally curing of the coated article. Getting a good finish is often as much about the type of application as it is about the skill and patience of the person in charge.
Powder Coating Basics
The basic idea behind powder coating is to cover a surface with an even layer of some form of synthetic covering. Most are thermoplastics or other tough coatings that are intended to form something of a barrier. Some are decorative, and may resemble paints; others have more functionality. A lot depends on the specific circumstance.
One of the advantages to a powder coating system is a relatively even application. Unlike painting with a brush, which can lead to lines and drips, applying powder particles can lead to a smooth, uniform finish. Powder is typically applied either by spraying it on with a spray gun, dipping it into a fluidized bed of powder, or applying it with electromagnetic brushes. Electrostatic coating with a spray gun tends to be the most common method, and requires a special device that electrically charges the particles of powder as it shoots them out. The item being coated is electrically grounded when the powder is sprayed onto it. The difference in potential between the item and the powder particles allows the powder to electrically adhere to the item until it is cured.
There are two main types of spray guns. A corona gun charges powder particles by bombarding them with ions as the powder exits the gun, and an electrical field between an electrode near the front of the gun and the article being sprayed helps transport the charged powder from the gun to its target. In what’s known as a tribo gun, however, powder particles are charged inside when they rub against the walls of the inner chamber. Aerodynamic forces are primarily responsible for transporting the powder from a tribo gun to the article being sprayed, though the people running the devices usually need to have at least some strength and precision when it comes to directing the flow.
Bed Powder Systems
Alternatively, an article may be coated in an electrostatic fluidized bed. In this sort of powder coating system, the item is first heated and then lowered into a bed of powder through which air is pumped. Particles of powder become suspended in the stream of air and take on the appearance and some qualities of a fluid. Powder particles then fuse to the article being coated when they contact the hot item.
Electromagnetic brush coating tends to be the least common method. In this sort of system, powder is applied to flat articles with the use of an electromagnetic brush. For large articles, this system often provides a more even coat with faster application than can be achieved with the use of a spray gun.
Priming and Application
Deficiencies in finish are most frequently the result of improper pretreatment of the article to be coated. Pretreatment of an article depends to some degree on the material of which it is made. Generally, the process involves cleaning the item with a solvent to remove oil or grease and etching it to remove any surface corrosion. A thin film called a “conversion coating” is then chemically applied to the article. This conversion coating improves the surface for better adhesion of the powder coating. Areas that are not to be coated are usually masked with tape or other temporary coverings before the application.
The Curing Process
Once the powder has been applied, the coated article undergoes a curing process in a convection or infrared oven. The resulting finish from this curing process depends on the type of powder that’s been used. Thermoset polymer powders undergo chemical reactions, called cross-linking, during heat curing. Once such a chemical reaction has taken place, the coating cannot usually be melted again. Thermoplastic powder is an exception; it usually melts during the cure process but does not undergo chemical changes so in most cases it can actually be melted again after the initial cure is complete.