Rejects from coating lines can come from a substrate defect, a surface contaminant or a coating material defect. Process controls should be implemented upstream of the coating line to prevent a defective substrate from entering the line.
If a substrate is defective and has entered the coating line, it is important to recognize the source of the blemish and not waste time trying to fix the coating process to correct a pre-existing substrate condition.
Dirt on the surface of a coated part is a major concern for all coaters.
Dirt may come from the air around the spray environment, from the
application equipment or it may be in the coating material.
If a defect is from contaminated coating material or manufacturing byproducts that have not been removed by the cleaning process it will be evenly distributed and, if it is an external source, the defect will more likely be random.
Dirt in a powder coating can be screened out. Rotary sieves or vibratory sieves can be used to filter power fed from the collector module.
Integrated screeners can be built into collector modules. Even an all manual operation can screen powder by putting it through a stainless
steel screen before loading it into the feed hopper. Powder should
always be screened.
Reclaim needs to screened to eliminate dust that has entered the booth. Virgin material should be screened to make sure that there are no chips or contaminants in the box.
Dirt from ineffective pretreatment can usually be corrected by changes
in the process or control of the pretreatment system. Soils that are onthe part from the manufacturing process that cannot be removed by a spray washer can usually be tracked to the source so a change can be made to eliminate them.
Air borne contaminants are the hardest to track and eliminate because they can come from so many different sources. Keeping a clean environment around the system is one of the best ways to reduce air borne dirt.
Electrostatically applied powder can entrain airborne dirt and deposit
it on the parts.
The list below names some of the common sources of airborne contamination.
With all of these potential sources, the job of coating a part with no
defects is a challenge to say the least. .
The level of dirt prevention that an operation adopts is directly related to their quality standards. In many general industrial applications it is not necessary to reach the highest level of cleanliness.
Case Study: Powder Coating Defects (pdf format)
March 23, 2014