Q. We are facing a problem of bad marks on our final e-coated automotive parts, which are most probably caused by dripping ultra filtrate. Please suggest a solution. Thank you. H.S.
A. Electrocoat marks or stains are a very common defect many e-coaters face. It is very difficult to give you a full solution to your problem without seeing pictures of the “marks” on the parts and the racking you use, and additional details of the process. However, your question indicates that the most probable root cause of the marks are the ultrafiltrate drippings. If you are right and this is the case, the solution could involve one or several of the following:
1) Modifying the racking so that the drips from parts above do not fall on the parts below. The difference in pH or conductivity of the falling permeate drips with the deposited electrocoat can leave a mark or stain that could be visible once the permeate is evaporated and the electrocoat is cured. I know that in some cases this is a solution that can’t be implemented for manufacturers like you, for production reasons, but it’s easy for electrocoat job coaters where they custom coat parts.
2) Lowering the last permeate rinse tank conductivity and suspended electrocoat paint. This is accomplished by increasing the ultrafilter flux rate and increasing the amount of UF entering your final tank increasing the cascade/return effect of the UF closed-loop system.
Increased UF flow minimizes paint carryover to the last ultrafiltrate rinse tank. The lower conductivity you maintain in the final rinse tank, the better visual parts you will get from the system.
3) Modifying your ultrafiltrate pH (higher or lower depending if your system is cathodic or anodic), so that the pH of the UF is near close to the paint pH. This prevents permeate drips from dissolving or removing any electrocoated paint on the parts below and leaving permeate marks. This modification to the anolyte pH is accomplished by controlling the anolyte dump rate, dumping more or less.
4) Substituting your last ultrafiltrate or permeate rinse tank for one that includes DI or RO water instead. This ensures that your last and most important rinse is made with a liquid of the lowest conductivity possible to provide the best rinsing and visual for the electrocoated parts. This solution involves re-piping your clean ultrafiltrate main line so that it flows not to the last tank but to the one before last. This last DI or RO tank will be maintained by conductivity and will be isolated from the electrocoat closed loop UF tanks. The overflow from this tank will be directed to your wastewater system.
This is the typical system used by automotive companies electrocoating class A flat surfaces where the production volumes and visual requirements are high.
I hope the above gives you direction as to what to do next. Regards and good luck!
E-coat can produce uniform finishes with excellent coverage and outstanding corrosion resistance.
PPG launched the first use of waterborne compact paint technology in a U.S. automotive manufacturing plant at the BMW assembly plant in Spartanburg, S.C. This painting process has turned out to be a 2012 award winner and has opened up a new way for auto manufacturers to go leaner and become more efficient in their operations.
I am responding to the article in the January 2001 issue regarding the comparison between powder coat and electrocoat performance.