The Powder Coating Process
The powder coating process involves three basic steps:
- Part preparation or the pre-treatment
- The powder application& Equipment
- Curing process
Phosphate pre-treatment process
1a. Part Preparation Process (Chemical) Phosphate and Chromate
Removal of oil, dirt, lubrication greases, metal oxides, welding scale etc. is essential prior to the powder coating process. It can be done by a variety of chemical and mechanical methods. The selection of the method depends on the size and the material of the part to be powder coated, the type of impurities to be removed and the performance requirement of the finished product.
Chromate pre-treatment process
Chemical pre-treatments involve the use of Phosphates or Chromates in submersion or spray application. These often occur in multiple stages and consist of degreasing, etching, de-smutting, various rinses and the final Phosphating or Chromate Conversion of the substrate. The pre-treatment process both cleans and improves bonding of the powder to the metal and offers excellent performance against corrosion and adhesion of the powder.
In many high end applications, the pretreatment process, has been particularly useful in automotive and other applications requiring high end performance characteristics.
Blasted then powder coated (before and after)
1b. Part Preparation Process (Mechanical)
Another method of preparing the surface prior to coating is known as Abrasive Blasting or Sandblasting and Shot Blasting.
Different powder coating applications can require alternative methods of preparation such as abrasive blasting prior to coating. Cast steel shot or steel grit is used to clean and prepare the surface before coating. This method of preparation is highly efficient on steel parts such as I-beams, Angles, Pipes, Tubes and Large Fabricated Construction pieces.
Example of powder coating spray guns used
2a. Powder Application Processes & Equipment
Aluminum extrusions being powder coated.
The most common way of applying the powder coating to metal objects is to spray the powder using an electrostatic gun. The gun imparts a positive electric charge to the powder, which is then sprayed towards the grounded object by mechanical or compressed air spraying and then accelerated toward the workpiece by the powerful electrostatic charge. There are a wide variety of spray nozzles available for use in Electrostatic coating. The type of nozzle used will depend on the shape of the workpiece to be painted and the consistency of the paint.
Another method of applying powder coating, called the fluidized bed method, is by 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 micrometrers.
3a. Curing Process
When a thermoset powder is exposed to elevated temperature, it begins to melt, flows out, and then chemically reacts to form a higher molecular weight Polymer in a network-like structure. This cure process, called crosslinking, requires a certain temperature for a certain length of time in order to reach full cure and establish the full film properties for which the material was designed. Normally the powders cure at 200 °C for 10 to 15 minutes. The curing schedule could vary according to the manufacturer's specifications. The application of energy to the product to be cured is accomplished by Convection cure ovens.
Properties of Powder Coating
Most powder coatings have a melting temperature around 150 °C, and are cured at around 200 °C, for minimum 10 to 15 minutes (Also its depend on thickness of specimen or part) with film build-ups of greater than 50 to 90 μ (Microns) may be required to obtain an acceptably smooth film. The surface texture which is considered desirable or acceptable depends on the end product. Many manufacturers and fabricators actually prefer to have a certain degree of orange peel since it helps to hide metal defects that have occurred during manufacture, and the resulting coating is less prone to showing fingerprints.