Powder coating is a type of coating that 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 the 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 is 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 household appliances, aluminium extrusions, and automobile and bicycle parts. Newer technologies allow other materials such as MDF (medium-density fibreboard), to be powder coated using different methods.
Powder coatings emit near zero volatile organic compounds (VOC)
Powder coatings can produce much thicker coatings than conventional liquid coatings without running or sagging.
Powder coating overspray can be recycled and thus it is possible to achieve nearly 100% use of the coating
Powder coated items generally have fewer appearance differences between horizontally coated surfaces and vertically coated surfaces than liquid coated items.
The Powder Coating Process
The powder coating process involves three basic steps:
- Part preparation or pre-treatment.
- The powder application.
Part preparation or pre-treatment
Removal of oil, soil, lubrication greases, metal oxides, welding scales etc is essential prior to the powder coating process. It is 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 soil to be removed and the performance requirement of the finished product.
Chemical pre-treatment involves the use of non- chromates in submersion application. These often occur in multiple stages and consist of degreasing, etching, de-smutting, various rinses and the final chromating of the substrate. The pre-treatment process both cleans and improves bonding of the powder to the metal.
Powder application process
The powder coating is applied to the metal in a spray from an electrostatic gun, or corona gun. The gun imparts a positive electric charge to the powder, which is then sprayed towards the grounded material by compressed air spraying and then accelerated toward the material by the powerful electrostatic charge.
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. The 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 powder cure at 200deg C(390degF) for 10 minutes. The curing schedule varies according to the material being coated. We use a convection cure oven to accomplish the above which reduces the curing time.