Powder Coating

What is Powder Coating?? Source
Powder coating is a type of dry coating, which is applied as a free-flowing, dry powder. The main difference between a conventional liquid paint and a powder coating is that the powder coating does not require a solvent to keep the binder and filler parts in a liquid suspension form. The coating is typically applied electrostatically and is then cured under heat to allow it to flow and form a "skin." 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. Powder coating is mainly used for coating of metals, such as "whiteware", aluminium extrusions, and automobile and motorcycle parts. Newer technologies allow other materials, such as MDF (medium-density fibreboard), to be powder coated using different methods.

Properties:
Thickness: 50 to 80 micron
Hardness: 6H compared to SP which is 3H
Labor Cost: Similar to Spray paint
Paint/Powder cost: ~10% higher than spray painting
Maintenance cost: Lower than Spray painting as SP needs thinner to clean while PC only needs to blow dry.

Advantages and Disadvantages of Powder Coating?? Source






















VOC = volatile organic compounds

Types of Powder Coating?? Source
There are two main types of powder available to the surface finisher:

1. Thermoplastic powders that will remelt when heated, and
2. Thermosetting powders that will not remelt upon reheating.

During the curing process (in the oven) a chemical cross-linking reaction is triggered at the curing temperature and it is this chemical reaction which gives the powder coating many of its desirable properties.

The most common polymers used are polyester, polyester-epoxy (known as hybrid), straight epoxy (Fusion bonded epoxy) and acrylics.

How to Powder Coat?? Source

1. Determine the type of material you are going to powdercoat and then select a suitable powder for the finish. Powdercoating is done with thermoplastic or thermoset polymer powder, and these materials are formulated for bonding with different base metals to give the best results.
2. Clean the base metal thoroughly. Using bead or abrasive blasting on hard metal, such as cast iron or steel, will remove mill and rust scale, dirt and foreign materials. Chemical solvent cleaning will remove any grease, oil, or paint, and light sanding can be done to finish preparing the surface. Aluminum, magnesium, and other soft alloy metals can be solvent cleaned and wire brushed, or sanded if needed.
3. Apply the powder to the object to be powder coated. This is done using a "gun" or compressed air sprayer which electrostatically charges the powder material so that it sticks to the grounded base metal object receiving the coating. These guns are available from various suppliers, and cost as little as $100. For experimental purposes, you can apply the powder to a flat metal surface by dusting it directly on, and spreading it to a thin, even layer.
4. Cure the metal at a temperature appropriate for the powder material you use. A conventional oven is suitable for this purpose if the metal is small enough to fit, otherwise, an infrared heat lamp or other flame less heat source needs to be used. Normally, the object is heated to 350 to 375 degrees F. for about 10 to 15 minutes, and allowed to cool.

Some Tips

1. Powdercoating is an enviromentally friendly, corrosion and UV light resistant finish. Although it is best done with precision industrial equipment, it is possible to experiment with it in a home workshop.
2. Clean and rinse all metal surfaces thoroughly, as with all paint coatings.
3. Use in a well-ventilated, clean area.
4. Collect all excess powder for reuse in future applications.
5. Remember, you will have to heat the coated object to cure the powdercoat, so you will have to have an oven which will hold the piece, or be willing to apply heat directly with an infrared lamp for a sufficient time to cure it.
6. There are a number of sources for powdercoating powder, guns, and other equipment online.
7. Always Pre-heat the part in the oven prior to coating. This will outgas any grease or oil still embedded on the surface. If the part is not pre heated, any grease or oil still left after cleaning will outgas during curing and bubble the finish during curing.

2 Common Coating method Source

1. Using an electrostatic gun, or Corona gun
The powder is applied with an electrostatic spray gun to a part that is at earth (or ground) potential.

Before the powder is sent to the gun it is fluidised:

1. to separate the individual grains of powder and so improve the electrostatic charge that can be applied to the powder and
2. so that the powder flows more easily to the gun.

Because the powder particles are electrostatically charged, the powder wraps around to the back of the part as it passes by towards the air offtake system. By collecting the powder, which passes by the job, and filtering it, the efficiency of the process can be increased to 95% material usage.

The powder will remain attached to the part as long as some of the electrostatic charge remains on the powder. To obtain the final solid, tough, abrasion resistant coating the powder coated items are placed in an oven and heated to temperatures that range from 160 to 210 degrees C (depending on the powder).

Under the influence of heat a thermosetting powder goes through 4 stages to full cure.
MELT, FLOW, GEL, CURE

The final coating is continuous and will vary from high gloss to flat matt depending on the design of the powder by the supplier.

2. Fluidized Bed method

Heating the substrate and then dipping it into an aerated, powder-filled bed. The powder sticks and melts to the hot object. Further heating is usually required to finish curing the coating. This method is generally used when the desired thickness of coating is to exceed 300 micrometres. This is how most dishwasher racks are coated.

Electrostatic Fluidized Bed Coating
Electrostatic magnetic Brush (EMB) Coating: