Why Not Barrel E-Coating
For E-coating, the parts have to be individually placed on racks. It was explained that E-coating is a lot like electroplating. If that is the case, why can’t they be coated in a barrel?
Q: I work at a stamping company and many of our small brackets have been zinc electroplated in the past. I understand that the process consisted of putting the parts in a barrel--type container and rotating them in the plating solution.
Now many of these parts are being E-coated. For E-coating, the parts have to be individually placed on racks. It was explained that E-coating is a lot like electroplating. If that is the case, why can’t they be coated in a barrel? Y.B.
A. Well, they can, but the conditions have to be right. Some small parts do lend themselves to barrel E-coating. And, in some cases a “basket” E-coating procedure is used in lieu of racking. A lot of work has been done to find ways to utilize bulk processing techniques for E-coat. As we noted from the letter above about “Slick” E-coat, there have been some breakthroughs in recent years.
With electroplating you get a hard surface right away as the plating is deposited on the substrate. That finishing technology can withstand limited contacts with other parts without causing damage to the finish. It is very different with E-coat.
I have had some personal experience with projects where we set up bulk application systems. We were applying standard cathodic epoxy E-coat materials. Those projects encountered several challenges. For example, all of the parts must be in electrical contact in order to have the paint electrodeposited on them. The contact points would leave bare spots. Also, where the parts were in contact they would stick to each other when the coating was cured. The parts were tumbled in a rotating barrel to prevent them from sticking together. As a result of the tumbling action the coating film was damaged by abrasive contact with other parts before the coating was cured. If the quality requirements for the part will allow some of these deviations from the norm, then a bulk method could be worth looking into.
Some castings with rough surfaces and somewhat complex configurations can be successfully E-coated in bulk by placing them in basket carriers on the conveyor. One technique I have witnessed utilized a tilting mechanism to tilt the basket carriers back and forth. The tilting action was to cause the parts to move slightly so they will not always have the same contact points. This reduces areas of coating voids. In one of the applications I observed, the surface was course enough to obtain an acceptable film consistency with virtually no visible bare spots.
Bracket type parts as you mentioned normally require complete coverage and higher coating quality than you would expect from any of the barrel coating processes with which I am familiar.
However, this is a good question. A lot of work is still being done to find ways to have E-coat be more of a bulk coating process. Papers have been presented on this subject at The Electrocoating Conferences. If any of our readers have some experiences to add to what I have covered, please drop me an e-mail.
How do you measure the surface area of a threaded fastener? How much coating would you put on it? How thick of a coating? What about non-threaded fasteners? The U.S. Department of Agriculture’s Forest Service, of all people, may have come up with the solution for those pondering how to coat sometimes-difficult small pieces using computer imaging and software to compute the area.
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This paper is a peer-reviewed and edited version of a presentation delivered at NASF SUR/FIN 2012 in Las Vegas, Nev., on June 12, 2012.