Switching to Powder

Q. We are a custom powder coater and a customer of ours wants to switch from e-coat to powder coat on his zinc parts. We have submitted samples of all chemical formulations of powder and have added fluoride to our three-stage wash system in order to get better adhesion. None of these things have given us consistent adhesion. We have discussed this problem with our chemical supplier and the powder manufactures but no one seems to have the answer. Is there something that either we, or our customer, can do to assure these zinc parts will have adequate and consistent adhesion? We coat aluminum and steel all day long and do not have these adhesion problems. B. R.

A. After I contacted you via telephone, we discussed several additional facts:

These are zinc die-cast parts.
You have a three-stage pretreatment system.
You have tried wiping these parts using MEK before pretreatment and had successful adhesion.

These additional facts bring your problems into much clearer focus. Without this information, it would have been very difficult to provide you with an accurate solution to your problem.

First of all, the zinc die-casting process frequently uses mold-release agents to allow the parts to separate from the mold easily and quickly. Secondly, these mold release agents are normally organic in nature (i.e. waxes, oils, petrochemicals, etc.). Finally, the fluoride additive will have little, if any, impact on removing these organic soils since it is designed to remove inorganic soils (i.e. zinc oxide in this case). Although removing zinc oxide is important for better adhesion and corrosion resistance, it has little effect on the removal of the organic soils. As you know, if the part is not perfectly clean (i.e. no organic and inorganic soils), you cannot obtain reliable adhesion.

Therefore, your solution lies in improving the organic soil cleaning in your first stage. Working with your chemical supplier, he may recommend you increase the concentration of the chemical, increase the temperature of the bath or increase the pump pressure at the nozzle. He may even recommend you do all three to obtain the results you need. Finally, the pretreatment chemical supplier should perform a soils analysis on the new part and determine the best overall solution, which may even include changing the first stage chemistry to a completely different formula. Be sure to verify the results by testing the adhesion properties on the finished part after all changes and improvements have been made to your process.

After you have corrected your pretreatment problems, look at your curing system next to ensure that you do not have other problems. For instance, there is a chance that these zinc die-cast parts may have significantly higher mass than the other parts you currently run. If this is so, check your curing process by profiling your oven using this part. If the resultant cure profile curve has a bring-up time to temperature slope that is less steep than your other parts, the powder coating may not have sufficient adhesion. Powder coatings must melt quickly to provide sufficient surface wet-out to achieve maximum adhesion. If your ramp-up time is very long, the time the powder coating stays liquid is reduced and the surface wet-out is reduced, as well. This leads to poor adhesion of the cured film on the part. If your profile curve shows this problem, increase your oven temperature to increase the rate of rise of the metal temperature. Verify these changes and the results by noticing a steeper curve on this verification profile.