Metal Finishing Guide Book


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the drain zones with normal stage exposure, and three minute stops in both the stages and drain zones. A fourth set was prepared with normal sequence times as a control. Used production TMC solution was utilized for the lab treatment. The time increment of 3 minutes was used as this was the time it took to hand spray a rack of parts and change colors. Adhesion testing and salt spray testing was performed to compare the variations. To my surprise, all of the cross- hatch adhesion (ASTM D3359) results were perfect. This was not expected and was most likely due to the differences between the laboratory and the production washers. The neutral salt spray (ASTM B117/ D1654) results at 336 hours demonstrated better variation. The test data suggests that the line stops in between stages have a greater impact on performance than stopping in the stages that continued to spray solution. Spray impingement prevents the formation of loose deposits that can interfere with paint adhesion and corrosion resistance. Other process-related issues include flash rusting. Figure 1 illustrates a uniform coating on HRPO rims and an appearance that is not uncommon with TMCs. The HRPO substrates were treated in a 5-stage washer with the TMC in stage 4, followed by a reverse osmosis rinse. As the parts continued towards the e-coat tank their color changed. You couldn't wipe off the TMC coating, although it was evident that the reaction continued. About the same time the e-coat ultra-filters were beginning to plug with an iron containing residue. Drippings were collected to determine how much iron was in the residual water entering the e-coat tank. A high number was 10 ppm and values were typically 2–3 ppm. Assuming 10 ppm of iron was constant, it would have taken 52,000 gallons of iron containing water dragged into the 7,000 gallon e-coat tank to match the amount of iron in the e-coat tank. The metal finisher was conducting TMC trials with multiple vendors. It was unknown how much came from the first or second vendor. Based on material balance, there was no way that water on parts alone established the iron levels in the e-coat tank. High iron loading had to be related to line stops, and I suspect that the parts were rusty due to line stops in pretreatment and then dissolved in the cathodic e-coat tank. A power and free conveyor would eradicate this situation. Iron entering an e-coat tank is a concern you need to consider. Not all lines have this issue, but I have heard of at least four e-coaters having this issue. In one instance, the e-coat solution had to be replaced. Iron accumulation in rinse tanks has also been seen. In as much as I've heard of this problem, one would think that it could be remediated with better rinse overflow. This is not always the case and generally leads to the use of "rinse aids" or rust preventatives. High operating cost has also been observed. High water usage was seen on a line where the total RO water usage in the stage before and after the TMC was 52 gpm. This metal finisher also had extreme TMC usage as well. There was high carry-over from the parts processed on this line. High carry is a concern not only for product usage but for quality as well. Phosphate discharge wasn't a concern for this finisher. They were solely interested in energy savings. Once they converted to an ambient iron phosphate they reduced chemical purchases by 58%. This is the crux of this article. I believe in being green. I reduce, reuse, and recycle whenever I can. But I also believe you need to run your line efficiently to survive and prosper. You need to make good decisions so that you aren't the guy at yet another company saying the conversion to TMC "was a mistake." 138

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