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E L ECTRO C OA TC L IN IC RectifieR Ripple effect Q. How does a high rectifier ripple affect the operation or part quality of an electrocoat tank? C.S. A. AC/DC rectifers convert sinusoidal AC power voltage waves to square DC power waves. Because AC to DC rectifers are not capable of converting 100 percent of the sinusoidal AC waves to square DC waves, they always leave a small sinusoidal variation in the DC voltage output. This unpredictable variation can create unwanted flm deposition and flm quality. The amount of ripple in an AC/DC rectifer quantifes the percentage of unwanted variation of output from the rectifer. Typical electrocoat rectifers are either 1-percent ripple for precise electrocoating operations or 5-percent ripple for typical installations. As long as the percentage of ripple does not exceed 8-10 percent, you will not notice any operational issues or any visual changes on the flm itself. Typical electrocoat flms deposited with high-percentage ripple rectifers are rough with wavy patterns. electRocoAt oveR Wood Q. We are a furniture fabricator and have been told that there is an electrocoat paint that coats over wood panels. How can electrocoat deposit over a wooden surface if the substrate is not conductive? We are a little confused, as electrocoating theory says that electrocoating can only be used on conductive metals. Can you clarify for us? R.Y. A. The electrocoat theory you mention is absolutely right. Electrocoat needs a conductive path to work. The conductivity path starts in pfonline.com/experts the rectifer, and must go all the way to the anodes and through the paint bath, and then complete the electrical loop back to the rectifer through the part, the rack and the ground bar. Electrocoat will not deposit if this path is not continuous or if it is broken in some link of the electrical chain. If the part itself is not conductive (as in this case with the wood panels) then electrocoat cannot deposit flm, as there is not a continuous electrical path from the rectifer back to rectifer. However, if prior to electrocoating the wood panels are sprayed with a layer of conductive wet paint or powder, then electrocoat could be applied to the "chemically modifed" wood surface. The key word here is "conductive." Although not widely extended, this technique has been used in some limited applications. In summary, it is not the electrocoat technology that is capable of applying flm over non-conductive surfaces, but the chemical modifcation of the substrate that rendered the surface of the wood conductive enough for the electrocoat to work. Using this technique, you could coat over any non-conductive surfaces. This theory is also the basis of some "e-coat over ecoat" applications over conductive surfaces in which a UV-stable electrocoat is applied over another epoxy primer electrocoat that has been made chemically conductive by adding carbon black as conductivity agent. RAck StRipping StAndARdS Q. What is the industry best practice standard for e-coat rack stripping? T.F. A. I don't think there is a formal standard for rack maintenance, although there are some guidelines, recommendations and internal standards of practice used by some electrocoaters. The electrocoating industry, representing so many OEMs and other industrial users, offers an infnite array of designs for tooling and racking fxtures to satisfy the many different types of parts and applications, and specifc line designs and conditions. Some racking fxtures are fairly fxed structures that can't easily be removed; others are modular and detachable, allowing for rapid cleaning and change. Part density, whether part loading is manual or automatic, and whether the e-coat line is followed by a topcoat line also determine the design of the racking fxtures and may limit or condition the proper fxture maintenance schedule. Typically, monorail electrocoat systems need simpler racking fxtures than square transfer systems and JOSE A. TIRADO Consultant, Ti6 Consulting International, electrocoat@pfonline.com 40 JUNE 2013