Small spots of colour contamination; may be difficult to see
Insufficient housekeeping when colour changing
Totally clean plant, gun and recycling equipment and recharge with new powder
Contamination from general area cleaning
Avoid brushing in the spray area; vacuuming is recommended.
Cross contamination from dust and particles in oven.
Do not cure other colours at same time. Reduce air exchange rate.
Deposits of dust and powder on cured film, often these can be felt on the surface as slightly raised bumps.
Dust free cooling zones, avoid drafts.
Cross contamination at manufacture stage
Larger patches of colour difference. Powder particles cannot align into large off-colour effects; when patches happen the root cause is always contamination on the substrate and/or curing conditions.
Reaction of the coating with contaminants on the substrate surface. For hand-degreased systems, look for wipe marks - cleaning with a highly volatile solvent often results in oil smearing after the solvent has evaporated. Look also for straight lines caused by tooling, for instance during the automatic welding process. For pre-treatment systems, look for drip marks – pre-treatment chemicals not washed off, or trapped may react with pigments during the cure process.
Review pretreatment steps and the use of various oils. Orient parts for maximum coverage. Clean and adjust spray nozzles for maximum coverage. Maintain high rinse tank purity. Allow sufficient time for adequate drainage but not dry off between stages. Reduce temperatures. Utilize fog nozzles and adjust nozzles for maximum coverage.
Heat stability of the pigments in the coating is not sufficient to cope with variations in temperature, especially noticeable in high chroma colours.
Check that there is no significant difference in thickness of the substrates. If this is unavoidable consider curing the different sizes under different curing conditions. Check oven for hot and cold spots. Contact us.
Continuous or suddenly appearing changes in colour or effect compared to original sample part or compared to first parts coated. For metallic effects, the colour is less metallic than expected. See also 'Striping'.
Oven air is not exchanged enough to eliminate by-products of gas combustion
Adjust the air exchange rate to reduce the concentration of by-products. With light colours the oven volume must be exchanged several times more often than with dark colours to minimize discolouration and haze. Consider powder stabilised for gas by-products in oven.
The coating formulation may not have sufficient pigment opacity
Increase coating thickness to provide coverage. Contact us for coverage requirements.
Heat stability of powder coating resin system is not sufficient to cope with variations in temperature. Particularly noticeable in paler colours, where the discoloration is towards yellow/brown.
Check that the coating is not being subjected to prolonged exposure to temperatures above the recommended curing temperatures.
For light shade sandpaper textured effects, variation in film thickness can cause the degree of texture to change. An increase in the peak size of the coating can cast a shadow and be observed as the colour moving darker.
Takes steps to ensure a uniform dry film thickness.
Differences between the expected metallic effect, either too bright, or not enough metallic appearance, and the actual work. These issues are usually found in blended metallics rather than bonded metallic effects. 'Bonding' is the process of fusing the metallic effect flakes to the powder coating base, which renders the product more suitable for reclaim, and also overcomes many undesirable electrostatic phenomena.
The kV is too low.
This is especially noticeable in blended metallic effects (non-bonded). The metallic flakes do not pick up a charge as readily as the powder coating. Lowering the kV reduces the charge on the flake to the point that it is not attracted to the workpiece, making the effect less metallic than expected, or even virtually non-existent.
Increase the kV to achieve the desired effect; reducing the gun-part distance may also help. The situation needs to be carefully monitored to avoid ‘orange peel’ caused by high film thickness or back ionisation.
The gun is too far away from the work.
Metallic pigments (particularly gold-bronze) are often heavier than powder coating particles and will naturally fall out of the cloud first. If the gun is too far away from the part this results in a comparatively lower metallic look than the expected result.
Moving the guns closer should be the first alternative, with the KV being increased as a second step. The situation needs to be carefully monitored to avoid ‘orange peel’ caused by high film thickness or back ionisation
The air flow through the gun is too high.
If the velocity of the transport air through the gun is too high, it will reduce the residence time of the coating in the gun. Whilst this may have no effect on the powder coating base, the metallic effect flakes will not have enough charge. This will lead to the effect being lower than expected, if not totally non-existent.
Reduce the transport air flow. Recall that a powder cloud around the workpiece is required.