Q: My company uses an outside vendor to E-coat fasteners used in commercial roofing. The fasteners are coated gray. Sometimes we receive dark- colored fasteners in the same box, bucket or tub with light-colored fasteners. This color difference has been explained to me as the darker fasteners being over-baked or cured longer—the fasteners in the top of the basket, fully exposed, compared with those in the bottom of the basket, with 200 lb of fasteners on top of them. The explanation went on to say that the darker/more cured parts have a stronger/better coating because of this.
I’ve also heard the objection to this by saying that the over-baked parts have a harder coating and when the product bounces around in a bucket or box the harder coating chips off easier causing areas of the fastener to not be coated and subject to corrosion.
What is the duration or level of over-baking that will cause the coating to chip off of the fastener? Will this happen to product that is run through the process in the same basket? How big of a concern should this be for us? J.S.M.
A: I would say that the explanation you received is mostly correct. It also sounds like you are pushing the envelope with 200 lb of fasteners in one basket. A fastener by nature is not a massive part. Those fasteners in the top layers will be exposed directly to the oven heat and be cured faster. In order to get enough heat penetration into a mass of 200 lb of fasteners the process would require either a higher temperature or a fairly long heat curve. So, an over-cure of the fasteners on the top layers is to be expected with such a basket load.
Several years ago, I had some experience with the process of E-coating fasteners in baskets. As I recall, we also had issues with the fasteners sticking together after curing. As they were removed from the baskets, they often needed to be broken loose as they were packaged. Yes, there were some minor bare spots, but those were acceptable. However, I don’t believe we loaded over 50 lb in a basket.
If you are over-curing fasteners with a gray E-coat, it would not surprise me if the coating on the external layers would darken. A certain amount of over-curing of the coating can actually enhance some of the coating properties and there are cases where this is done on purpose. I have found E-coat to be more tolerant of over-curing than to under-curing. However, there is a point where the coating starts to harden and can become quite brittle, and that could cause the problems you mentioned.
How big of a concern should this be? It will depend on how you expect the fasteners to be used, what the expectations of the end-user are, and what you want the coating to do. This is the reason why specifications are developed and tests are designed to assure that the coating will perform as expected by the end user of items such as fasteners. If you have not developed any specifications or designed any tests then perhaps you should assign someone on your staff or hire a consultant to put those documents together for you.
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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