Surface Defects

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.

• People,Air sampling of enclosed coating systems has shown that as much as 80% of the airborne contamination comes from the clothing, hair and shoes of personnel.

• Nature,?Pollen, road dust and other outdoor dirt are sometimes brought into the plant through doors, windows and unfiltered air supply systems.

• The Building ,Concrete floors may wear and give off dust.

• Manufacturing ,Mechanical operations in the manufacturing process may generate dust, metal shavings, or wood dust.

• Process Equipment ,The conveyor, the oven, the washer or other operations within the coating line can generate or concentrate dirt that will create a defect.

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.