Too Much E-Coat
Q. We outsource black e-coating on the automotive seat belt cable assemblies we manufacture. Our customer is complaining there is too much e-coat, which is making our cable assemblies too stiff. Our competitor’s parts clearly have less coating than ours. We only have to pass a 70-hr salt spray test, but I am told ours would pass 1,000 hrs. Our suppliers tell us they cannot put on any less coating. Wouldn’t less time or current yield less coating? —M.T.
A. Your supplier could make some changes to the way it runs your parts and, as a result, obtain lower film thickness. The stiffness comes from the cured e-coat film bridging over the small gaps between wires. This bridging takes away the desired flexibility from the cable assembly. At lower film thicknesses, the bridging and stiffness will decrease significantly.
Critical parameters that your e-coat supplier could change and that have a direct impact on film thickness are voltage, percent solids, deposition time, bath temperature, and current or load density. Some are easy to change for your specific part production runs, while others not only affect your part production runs, but also those of other customers getting their parts coated at the same supplier.
If your supplier is using a black cathodic epoxy e-coat, the film could be adjusted from 10-12 microns or less all the way to 34-38 microns. The e-coat technology is capable of providing a wide range of thicknesses by varying application parameters. That is one of the advantages of e-coat.
For your specific application and salt spray performance, you would only need to apply 14-16 microns, or even less. At this low film, your seatbelt cables will have some bridging but will still remain perfectly flexible after cure. The 70-hr requirement for salt spray performance is not difficult to obtain at those electrocoat films. Keep in mind that typical epoxy electrocoat technologies should be capable of going over 1,500-2,000 hrs salt spray at typical film thicknesses of 20 to 25 microns.
Why does your supplier gives you more film thickness than you really need? It may be that it does not adjust the process parameters for your specific parts. It may have established the parameters based on other high-volume parts and is not adjusting the process to the lower square footage obtained when running your parts. Your supplier not only could apply lower film thickness and improve field quality, but also could reduce material and application costs and convert a problem into a win-win for both of you. You could also give your supplier more volume so it could hone in its process more to your specific application.
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The main task of this work was to study the influence of the different parameters on the electrolytic coloring process for aluminum.
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.