Q. What causes some parts to only receive a thin film thickness while parts on the same rack are fully coated? How can this problem be corrected? T.G.
A. Theoretically, parts will coat if they are grounded and will not coat if ground is not established. In reality, some parts only receive a partial coating and are referred to as "light coats," "grays," etc. Parts that only partially coat are usually the result of the part moving during the coating process and losing ground after it starts coating or not having ground when entering the tank and establishing ground during the coating process. Either way, the normal coating time is cut short and therefore a thin film is deposited.
Lightweight parts are most likely to produce "light coats" since they will move more easily than heavier parts. Another candidate for "light coats" are parts that trap air while entering the tank resulting in a floating condition even if the trapped air escapes out of holes or by tilting the part. A third cause of "light coats" comes on systems where some or all e-coated parts get a topcoat (powder or liquid) while on the same rack. The extra coating will create grounding problems when more raw parts are loaded for e-coating again.
As you probably noted, the cause is usually the result of tooling (racks, hooks, hangers, etc.) and/or cleaning the tooling. Lightweight parts are best handled several ways: (1) clean racks more frequently; (2) use two hooks/part; (3) hang a heavier part from the lightweight part as an "anchor;" (4) have special racks built with spring steel or titanium clips to "grab onto" the part; (5) use of more wedge shaped pins or square-stock hooks.
Systems (monorail or square transfer with power-and-free) that have topcoat after e-coat as a primer adds coating to racks that create grounding problems. I have found that the part density for E-Coat is so much greater than part density for topcoat presentation that a "part transfer" between E-Coat and topcoat is justified. This eliminates the added coating on the E-Coat racks. If part transfer is not practical, the only solution to "light coats" is more frequent cleaning of racks.
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.
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