Q: We have a problem when we apply metallic silver powder coating over black e-coated truck wheels. During tire assembly, an iron rod is used to assemble the tire. When the rod is withdrawn from the parts, we can see a heavy scratch mark at the edge of the part.
Is this due to compatibility over e-coat? How can we avoid this? Is there any need to improve the powder system?
My scratch resistance is 3,000 gms, and impact resistance is 250 kg-cm.
A: This sounds like a tough one. First of all, the surface properties of the powder topcoat do not appear to be a function of the compatibility with the e-coat primer. Incompatibility with the e-coat might cause inter-coat adhesion issues, but not surface slip problems.
The scratch resistance and surface slip of the powder coating can be improved. Your powder supplier can increase the crosslink density and thus the hardness of the powder by using a more functional resin (containing more chemically reactive groups). The supplier can also increase the surface slip of the coating by incorporating a PTFE (polytetrafluoroethylene)/wax blend into the formula.
Both measures will help, but I do not think that this will completely eliminate scratching caused by an iron bar. You may not have the ability to influence the manner in which the tires are installed, but using a more forgiving tool may be the best answer. If the tire installer can use a bar that has a softer surface (like nylon or PTFE rich), the scratching could be eliminated.