Powder coating involves applying an electrically charged powder to a surface, which adheres until it hardens. There are three main types of powder coating systems: electrostatic coating with a spray gun, electrostatic fluid bed coating, and electromagnetic brush coating. The pre-treatment of the part, application of the coating material, and curing of the coated item are important steps in achieving a good finish. The finish resulting from the curing process depends on the type of powder used.
There are usually three main types of powder coating systems, namely electrostatic coating with a spray gun, electrostatic fluid bed coating and electromagnetic brush coating. Powder coating is a surface finishing technique similar to painting but with the coating material applied as a powder rather than suspended in a fluid. The powder is electrically charged so that it adheres to the object it’s applied to—anything from furniture or personal accessories to walls or large appliances—until it hardens. Choosing the right type for a given project is often a matter of availability and scope of the project. All three types work in much the same way and all follow the same application process. The first is the pre-treatment of the part to be coated, followed by the application of the coating material and finally the curing of the coated item. Getting a good finish often depends as much on the type of application as on the skill and patience of the person responsible.
Powder Coating Basics
The idea behind powder coating is to cover a surface with an even layer of some form of synthetic coating. Most are thermoplastics or other tough coatings that are meant to form some sort of barrier. Some are decorative and may look like paint; others have more features. Much depends on the specific circumstance.
One of the benefits of 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, even finish. The powder is typically applied by spraying it with a spray gun, dipping it into a fluid 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 charges the powder particles electrically as it shoots them. The article to be coated is electrically grounded when the powder is sprayed on it. The potential difference between the object and the powder particles allows the powder to adhere electrically to the object until it is cured.
Spray guns
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 electric field between an electrode near the front of the gun and the item being sprayed helps carry the charged powder from the gun to its target. In what is known as a tribo gun, however, powder particles are charged into the interior as they rub against the walls of the inner chamber. Aerodynamic forces are primarily responsible for carrying the powder from a tribo gun to the object being sprayed, although the people operating the devices usually need to have at least some force and precision when it comes to directing the stream.
Powdered litter systems
Alternatively, an article may be coated in an electrostatic fluidized bed. In this type of powder coating plant, the item is first heated and then lowered into a bed of powder through which air is pumped. The dust particles remain suspended in the air stream and take on the appearance and some qualities of a fluid. The powder particles then fuse to the item to be coated when they come in contact with the hot element.
Coating brush
Electromagnetic brush coating tends to be the least common method. In this type of system, the powder is applied to flat items with the use of an electromagnetic brush. For large items, this system often provides a more uniform coat with faster application than can be achieved with the use of a spray gun.
Priming and application
Deficiencies in the finish are most frequently the result of improper pretreatment of the item to be coated. The pre-treatment of an item depends to some extent on the material it is made from. 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 item. This conversion coating improves the surface for better powder coating adhesion. Areas that are not to be coated are usually masked with masking tape or other temporary coatings before application.
The polymerization process
Once the powder has been applied, the coated item undergoes a curing process in a convection or infrared oven. The finish resulting from this curing process depends on the type of powder used. Thermosetting polymer powders undergo chemical reactions, called cross-linking, during heat curing. Once such a chemical reaction has occurred, the investment usually cannot be remelted. Thermoplastic powder is an exception; it usually melts during the curing process but undergoes no chemical changes, so in most cases it can actually be re-melted after the initial cure is complete.
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