Powder coating is a method of coating a substrate in a polymer powder via directional spray equipment. In this method, the powder is carried out of the spray equipment with low pressure air and the Spray gun applies an electrostatic charge to the powder particles, which are then attracted to the grounded part.
After application of the powder coating, Coated parts are directed to a curing oven where coating chemicals react to produce long chains molecules in the presence of heat to form high cross-link density which is very stable.
Temperature profile in the Curing Oven is the most essential factor when powder curing process is concerned. Most of powder contains relatively high molecular weight solid resins of between 30 and 50 um in particle size and curing agent, catalyst which forms or assists in the formation of crosslink between the polymer chain to none reversible polymer film under the correct temperature. So variation of the temperature from it desire value in the Curing Oven can cause to many properties of above chemicals which finally affects to the characteristic of powder layer, such as Gloss level, toughness, adhesion, chemical / UV resistant, corrosion resistant and flexibility etc.
Furthermore, possible defects of low and high temperature in the Curing Oven can be listed as follows.
|Signs of Under Cure||Signs of Over Cure|
|Brittle, parts chip easily||Gloss too low|
|Poor solvent resistance||Color out of specification|
|Poor adhesion||Texture pattern changes|
|Gloss too high – out of specification||Yellowing of powder|
|Abnormal Orange peel of smooth finish||Poor intercoat adhesion on recoat item|
Hence quality assurance process in oven temperature is very important where SPC ( Statistical Process Control) method can be used. Also it allows decisions to be based upon data rather than on subjective opinions. SPC involves measuring temperature variations in the Curing Oven by using statistical methods and improving it to reduce variation at desired process levels. SPC can also help determine the difference between typical variation inherent in the process and special causes of variation that can be detected and eliminated which drives to the consistently improvement of the quality of the product.