Q. We have recently started using some metallic powders on our large sheet metal cabinets. When we spray, we see a variance in color—some areas are darker than others, and the surface looks splotchy. We use manual, corona-charging spray guns. Why is the surface so inconsistent in color and appearance?—B.A.
A. Metallic powders are manufactured with different methods. Many are “bonded,” a process that fuses the base resin and the metallic flake together to create a more homogenous blend, while others are “dry-blended.” In a bonded powder, the metallic flakes are attached (bonded) to conventional powder particles (those combined of resin, pigment, various additives, etc.) to form a mixture in which all particles have similar specific gravity, size, dielectric properties, shape, etc.
Metallic particles have different electrostatic properties when compared to conventional powders that do not contain any metal flake. They charge differently, orient themselves differently in the electric field and often position themselves differently on the surface of the part, depending on the electric field strength. When an operator changes the gun-to-target distance, it changes the resistance between the gun and the part, and the electric field strength changes. For example, if an operator moves the gun too close to the part, those metallic flakes that are deposited on the surface of the part can stand up on edge because they are aligning themselves with the electric field. The metallic appearance of the cured powder depends on the angle at which the light reflects off the metallic particles embedded in the finish. If those particles stand up on their edges, the reflectivity will be lost and the finish will appear dull or dark compared to the rest of the part.
The difficulties described in your question are more often observed with dry-blended metallic powders, especially when applied manually, because of the non-homogeneous nature of the mix. The differences among the components combined in the dry-blended powder mixture may create application challenges due to aerodynamic and electrostatic influences. The metallic flakes have different specific gravity, shape, mass and size than those of conventional powder particles. They will react to the changes in the current level and voltage when the gun is moved in and out or across the part. And they may be concentrated in particular areas depending on the polarity of the coating and the strength of the electric field, or they may be oriented in different positions as described above.
In addition, metallic particles act differently when propelled by the air stream from a spray gun—aerodynamic differences. Depending on the size of the metal flake and the manufacturing process, the metallic particles may segregate from base resin particles within the spray pattern and be more concentrated in certain areas of the spray pattern. Additionally, being of a different mass and shape, they respond differently to the changes in the direction of the spray pattern and to the air turbulences generated by the geometry of the part. For example, the metallic particles can “lag behind” as an operator moves the spray gun along the parts surface. The difference in the way metallic particles respond to the changes in the spray pattern direction and velocity is often the first contributor to the blotchy appearance of the finish when dry-blended metallic powders are applied manually.
Of course, understanding what causes the splotchy appearance does not fix the problem. What can you do to get a more consistent look? First, consider using a bonded metallic powder. They may be somewhat more expensive, but they will be easier to apply due to their more consistent nature, reducing rework and improving quality. Second, try automating your powder coating process as much as possible. Automation eliminates the variables introduced by manual coating and typically yields more consistent and repeatable results. Third, train the operator to minimize changes in the spray gun velocity as it is passed over the part’s surface and in the gun-to-part distance. He should move the gun smoothly and consistently. Fourth, if your equipment has adjustable current limiting capability, set the current limit to a very low number (10-15 µA). This will minimize the fluctuations in the electric field strength during application. If your spray system does not have current limiting capability, lowering gun-tip voltage often yields improvement in finish uniformity when spraying metallic powders. Be sure to keep the gun tip clean, as metallic powder will often collect around the electrode tip and inside the air cap.