Film-build Control

One of the keys to success with powder coating is good control over the average film build. The parts must be fully covered to provide the appearance and performance required by the market that the product will be coated for but it also should be relatively consistent in thickness from one area of the part to another.

Coaters sometimes have a specification for a minimum and maximum film thickness to make sure the coating performs and to avoid excess thickness that may cause orange peel. The question is how big should the range be between the minimum and maximum and how can it be controlled?

Control of film-build with a manual gun is somewhat challenging. If the spray gun pattern is smooth and consistent and the operator technique is smooth and consistent it should be possible to meet the specification a reasonable range of film thickness control.

Gun-to-target distance is particularly important. If the gun moves in and out a lot or it is “arced” or “fanned” a lot it will be harder to achieve a uniform thickness. Consistent hanging patterns are also important. If the same part is hung in the same pattern at all times it is easier to achieve a consistent stroke pattern that provides uniform thickness. Inconsistent racking makes it much harder to be accurate with the stroke pattern.

There are a couple of other factors that could be affecting your film build control. First of all, there must be good path from the part to earth ground. Poor ground will have a big impact on film build control. Clean hooks are needed to avoid loss of earth ground. Earth ground can be measured with a good ohm meter and the resistance must be below 1 mega-ohm. 

The geometry of the part is also critical. If the surface is irregular it will be more difficult getting consistent thickness. One thing that will help with control over irregular shapes is limiting the current draw. A current draw above 20 micro-amps may have more influence on the film thickness due to areas of resistance on the part surface (Faraday Cage effect). Experimentation is recommended to find the optimum level of current to improve the film build consistency. A typical effective current level for a manual gun over an irregular surface is between 10 and 40 micro amps.

Consistent gun settings, consistent hanging, consistent technique, good ground and current limiting should help a coating line achieve better film build control. Be sure the powder pattern is uniform and the velocity of the powder is low. Make sure the parts are spaced correctly. If the parts are too close it is possible that the application will have trouble building up powder along the edges. If the parts are too far apart the edge of the part may have strong voltage lines and create some picture framing.

Automatic guns can also be dialed in to provide uniform coverage following the same principles. Automatic guns should be controlled to cover the areas of the part that are more easily accessible. If the flow rates are set too high to try to push powder into recesses the coverage may be inconsistent and heavy on the more accessible surfaces. Manual touch-up can be used to fill areas that are hard to cover with automatic spray guns.

What should the film range be? That will vary depending on the shape and size of the part. A film build specification should be narrow enough to provide the needed control but not too tight where control becomes difficult to achieve.

On flatter surfaces a range of 25 to 50 micron may be possible. If the minimum film build required for performance is 50 micron the maximum could be set at 75 micron for tight control. If that target proves to be too difficult to regularly control a range of 50 to 100 micron is still a good range and more doable on a daily basis.

On parts with more geometry and Faraday areas a wider specification may be necessary to allow for the variance in resistance on the part surface. In some cases the range may need to be as much as 75 micron. With a minimum of 50 micron required a range of 50 to 125 micron could be a good range on difficult surfaces.