Sealer Elimination

Question:

I work as a paint line manufacturing engineer in London. The product we are supplying to the automotive industry is strictly limited to medium to heavy truck wheels. We make the rims and discs, weld, then put into a wheel and than e-coat them. I am using a normal pretreatment wash line before painting with acrylic e-coat. Salt spay test is specified to 336 hours.

I am working at this time on a project changing from DI-water to RO water. Could you explain to me if it is true when using RO water, a sealer before the final rinse before painting can be eliminated? I have worked before for other tier 1 suppliers to the Japanese automotive industry and sealers are usually not being used there either.

I would appreciate it if there is a chance to address this question at your earliest time since I would love to move ahead with this project and could use this info for capital cost justification. S.E.

Answer:

As I understand your question, you are asking if you can eliminate the final seal in the pretreatment system by changing to RO (reverse osmosis) water instead of using DI (deionized) water between the seal and the e-coat stage. I am not sure of the source of your information, but I do not see why the switch from DI to RO water would have a significant impact on your need for the sealer.

Reverse osmosis and deionization are just two different ways of creating very pure water. The deionization process takes tap water and flows it through ion exchange resin beds. Resin beds are divided into anionic and cationic type to describe the ions they remove. A mixed bed describes the use of both types of resin systems for the removal of both anions and cations. The dissolved solids (generally hard water ions such as calcium and magnesium and potentially other contaminants such as chlorides and sulfates) are picked up by the ion exchange resin bed. The resulting water has no dissolved solids, and therefore, no conductivity. While this is very effective for removal of dissolved solids, it does not remove any organic species that could be present in the water.

The reverse osmosis process pumps water at fairly high pressures through membranes (filters) of very fine porosity, generally in the range of 0.0002 to 0.001 microns (about 0.000005 to 0.000025 inches!). This level of filtration is effective at removal of both organic and inorganic species.

The seal following a phosphate coating stage is intended to “fill in” voids and imperfections in the original coating. Some seals are reactive while others are not. If not dried before painting (as is the case before e-coating), the seal is rinsed to eliminate dragover into the e-coat tank.

Based on the above, you can see that regardless of the type of rinsewater used, the function of the seal is the same. That said, however, it may still be possible to eliminate the phosphate seal stage of your process considering that a 336-hour salt spray requirement is not very difficult to meet with an e-coat. I would encourage you to do trials with your current system to measure the effectiveness of the final seal step. This would easily be done by processing some parts as normal, then processing another set with the pumps of the seal stage off (assuming a spray system). Then run those parts through the salt spray testing to see how they compare and if you can still maintain the 336-hour requirement without the seal.