Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Fig. 5. Various types of replaceable tips. cleanings. Graphically, it indicates why platers may experience roughness at varying intervals in the plating filtration cycle. The amount of solids increas- es in the tank as the flow rate decreases to a level that may cause rejects. After the filter is serviced, the increased flow rate agitates any settled solids. Therefore, it is advisable to delay plating of parts until the contaminant lev- el is again reduced by filtration to within tolerable limits. This phenomenon generally occurs in a still tank, since the dirt has more chance to settle. For this reason, when the solution is pumped into a treatment tank, sludge may be found on the bottom of the plating tank. Dirt in an air-agitated tank can settle any time after the air is shut off. If car- bon and/or a filter aid is used in the filter during the continuous filtration cycle, it should be borne in mind that, as these solids are collected on the media, the pressure increases appreciably, reducing the initial flow rate by almost 25% and the overall volume pumped through the filter by as much as 50% before ser- vicing is necessary (Fig. 3). Frequent laboratory checks will verify the amount of insolubles in the plating tank, which will tell whether a uniform degree of clar- ity is being maintained or whether it is increasing slowly toward the reject level. More frequent servicing of the existing filtration equipment will increase the total volume pumped and, in turn, maintain the lowest possible level of contamina- tion and minimize the need for batch treatment. It is, therefore, necessary for the plater to determine the particle size to be removed and then select the media that provides the most solids-holding capacity. Then, knowing the efficiency of the media, multiply it by flow rate so that all of the solution passes through the filter in a certain period of time, such as 1 hr or 1 min. Note the small amount of solution that is filtered in 5 min if a rate of one turnover per hour is used (Fig. 4) as compared with the amount that would pass through at a rate of ten turnovers per hour (assume a 100-gallon solution): At one turnover per hour, 1 100 gal/60min = 1.6 gpm 5 min = 8 galfiltered At ten turnovers per hour, 10 100 gal/60 min = 16.6 gpm 5 min = 83 filtered The point here is that if nearly the entire solution is turned over every 5 min, then the plating bath will exhibit a high degree of clarity and purity. The net result should be fewer rejects caused by occlusion of particulate matter in the deposit. 599

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