How Does Powder Coating Work
Powder coating is a type of coating that is applied as a free-flowing, dry powder. Unlike conventional liquid paint which is delivered via an evaporating solvent, powder coating is typically applied electrostatically and then cured under heat or with ultraviolet light. The powder may be a thermoplastic or a thermoset polymer. It is usually used to create a hard finish that is tougher than conventional paint.
Painting metal parts can be a royal pain. It’s time consuming, messy and imprecise with runoff and dripping and uneven coats. Even worse, after you paint it, the coating is not permanent. It has to be touched up time and again, and eventually if the paint gets so thick the whole thing needs to be stripped and redone. Powder coating can be a great alternative that provides a rock hard permanent and even coating with a minimum of fuss. Here’s how the powder coating process works.
Understanding Powder Coating
The powder coating process is an alternate means of coloring metal parts, whether they are automotive parts or pieces of a model. The process uses a specialized pigment that starts off as a powder that is applied to the dry parts (hence the term, “powder coating”).
When exposed to heat and electrical charges, the powder goes through a chemical bonding process and melts, hardening into a solid surface that perfectly contours to the item covered. It is eco-friendly, inexpensive and far more durable than paint.
The first step in applying the powder coat is to clean the part. It must be completely clear of grease, oil or dirt. This includes oil from the skin that may be transferred from handling the piece. Usually, when cleaning the item it is sandblasted or an acid bath used to clear debris then brushed to remove remaining grit and moisture.
Applying the Powder Coat
After being cleaned off, the item is connected to an electrode which gives it a positive electrical charge. The powder is then applied using a corona gun, which is similar to a paint sprayer, but contains the powder coat, a thermoset (or heat curing) polymer which has a negative electrical charge. As the powder is blown over the part, the positive and negative charges are magnetically attracted to one another, causing the powder to cling to the part, like a balloon sticks to a wall from static electricity.
After the item is completely covered in powder, it is placed into an industrial oven. Once inside, the oven bakes the powder onto the piece. The powder does not simply melt over the part rather, some of the powder carbonizes from the heat. A process which releases no noticeable gas or smoke. The remaining powder bonds to the carbonized powder at a molecular level.
This leaves behind a rock-hard shell which is highly resistant to scratches, chipping or rubbing off. After the part cools, it is ready to go, with a coat of vibrant color that forms perfectly to every contour of the item in question.
Curing Stage: The particularities and characteristics of the powder coating process curing stage are mainly determined by the method in which the powder coating is applied, as well as the type of powder coating material employed.
- Curing ESD coated parts: Parts that are powder coated via ESD must be cured in a powder curing oven while the cure schedule—the temperature and time that a powder coating must endure in a curing oven to achieve a full cure—for a powder coated part is mostly dependent on its size, shape, and thickness, generally a curing oven operating between 325 to 450 degrees Fahrenheit will result in cure times that range between ten minutes to over an hour. Accordingly, smaller powder coated parts require less curing time and lower volumes of heated air to cure fully, and larger parts need more. As the ESD coated part reaches the optimum curing temperature within the oven, the powder particles melt and flow together to form a continuous film over the part’s surface.
- Curing fluidized bed coated parts: For parts that are powder coated within a fluidized bed, the parts are heated before the coating application stage in ovens similar to those used to cure ESD coated parts. As the preheated part is immersed in the coating material, the powder particles melt and flow together upon contact with the part’s heated surface. Parts that are coated via electrostatic fluidized bed powder coating can be either preheated before being passed through the powder coating cloud in which case the powder coating formed would be to those produced by the regular fluidized bed method or the part can be heated and cured in a curing oven after it has been coated, like with coatings produced by the ESD coating method.
Powder Coating Material Considerations
As indicated in the previous section and Table 1, below, the powder coating process utilizes two main types of coating materials—thermosets and thermoplastics. Each type can be similarly applied, but undergoes and experiences the curing stage differently, as well as demonstrates distinct physical and mechanical characteristics.
Table 1 – Comparisons between Types of Powder Coating Material
|Capable of withstanding high temperatures||May soften/melt if subjected to high temperatures|
|Cannot be remelted, reformed, and recycled||Can be remelted, reformed, and recycled|
|Higher scratch and mar resistance||Higher impact resistance|
|Susceptible to brittleness and over-hardening (especially in thick coatings)||More flexible in thick coatings|
|Requires a cure cycle to harden||Does not require a cure cycle to harden|
|Undergoes an irreversible chemical reaction||Does not chemically change|
|Applied only via the ESD method (generally)||Applied via both ESD and fluidized bed method|
When first applied to a substrate, thermoset powder coating material has short polymer molecules. However, during the curing process, the powder undergoes an irreversible chemical cross-linking reaction, which bonds together long chains of polymer molecules. This reaction changes the physical properties and chemistry of the material and allows it to harden into a thin, even, hard finish, if the proper cure schedule is followed.
Thermoplastic powder coatings do not require a cure cycle. Instead, the thermoplastic material only requires the time and temperature necessary to melt, flow out, and create the film-like coating. Unlike with thermoset material, which undergoes a chemical reaction during the curing stage, thermoplastic materials do not change their physical or chemical properties when heat is applied. Therefore, they can be remelted, reformed, and recycled for future coating applications.
Some considerations to keep in mind when choosing between thermoset and thermoplastic coating material are the application method and the intended application for the coating. Generally, thermoset powders are only applied via the ESD method. This limitation exists because dipping preheated parts into thermoset powder can cause any excess powder to cross-link due to built-up and residual heat within the fluid bed. As the cross-linking reaction causes permanent changes to the powder material, such occurrences would lead to excessive coating material waste. The curing process enables thermosets to attain harder coatings than thermoplastics, thus allowing them to withstand higher temperatures and demonstrate greater scratch and mar resistance. However, the harder finish can also limit the impact resistance of thermoset coatings, and over-hardening can cause the coating to become brittle, particularly in thicker coatings. Thermoplastic powder can be applied via both the ESD and the fluidized bed coating method, and generally can produce thicker, more flexible and impact resistant coatings than thermoset powder. While the ability to be remelted offers some advantage in regards to material costs, it also makes thermoplastic powder coatings less suitable for high and intense heat applications as the coating material may soften or melt off.
Substrate Material Considerations
Powder coatings are primarily applied to metal substrates, such as steel, stainless steel, and aluminum. However, they can also be applied to non-metal substrates, such as glass, wood, or medium density fiberboard. The range of suitable materials for the powder coating process is limited to materials that can withstand the temperatures required to melt and cure the powder coating material without melting, deforming, or burning itself.
The chosen material also helps determine the coating method employed. Since metals can be electrically grounded, the coating material is generally applied to metal substrates via the electrostatic spray deposition method, but they can also be applied via the fluidized bed method. On the other hand, since non-metals cannot be sufficiently grounded, they require that the powder coatings be applied through the fluidized bed powder coating method.
Powder Coating Finishes and Capabilities
Examples of colored powder coatings on spring parts.
Powder coatings can be applied in a wide range of colors, finishes, textures, and thicknesses that are not readily achievable through conventional liquid coating methods. Capable of being manufactured in virtually any color, powder coating materials can be formulated for both protective and decorative applications. The final finish achieved by the powder material ranges from matte to glossy, and clear to glitter or metallic. Various textures are also available for decorative purposes or hiding surface imperfections.
The powder coating process allows for a wider range of coating thicknesses. Compared to the liquid coating process, powder coating can more readily produce thicker, even coatings, especially when using the fluidized bed coating method. Using the ESD method, it is also possible to achieve thin, even coatings; albeit, not as thin as the coatings achieved via the liquid coating process.
For further enquiries about Powder Coating, contact GZ Industrial Supplies Nigeria.