Metal Finishing Guide Book

2012-2013

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the bath. It also helps maintain a uniform, homogeneous solution throughout the process tanks. Flat-sided barrels tumble parts more effectively. This tumbling is optimized when the flat interior surfaces of the barrel are not smooth. They can be ribbed, grooved, or dimpled. The various types of uneven surfaces also minimize sticking of parts to the panel surfaces, as mentioned previously. Additional tumbling ribs, cross bars, or load breakers of various types are usually needed only for round-plating barrels. They can be added to flat-sided barrels for specific applications. Most oblique-type barrels incorporate uneven, stepped bottoms to attempt to produce these same effects. Perforations The type of work being processed in a barrel must be considered when specifying the perforation shapes and sizes. Barrels are available with round, slotted, tapered, and mesh perforations. Job shops generally use barrels with smaller perforations to accommodate the widest range of potential workpiece sizes. Captive shops often have the luxury of using barrels with larger holes because they can more easily predict their minimum part size. Larger perforations usually exhibit faster drainage, more efficient exchange of metal-depleted solution, and less drag-out (carryover) contamination of adjacent tank solutions. This is because larger perforations minimize the negative effects of liquid surface tension. Many shops maintain extra barrel assemblies that have the smallest perforation sizes that will be needed. In this way, the line can be operated the majority of the time using larger-hole barrels. The smaller-hole barrels are used only when necessary. It is very important that all barrels used in a single production line have the same open-area ratio, regardless of perforation size. The open area ratio is defined as the total number of holes in a barrel panel multiplied by the individual open area of each hole and divided by the total area that contains the included perforations. Open Area Ratio = (Number of Holes x Open Area of Each Hole)/(Total Area of Included Perforations) For example, if you count 133 holes, 3/32" in diameter (0.0069 square inch open area for each hole), in a 4 square inch perforated area, the calculation would be as follows: Open Area = 133 x 0.0069/4 = 0.23 or 23% Interestingly, there is a convenient geometric relationship between hole-size, center-distance from hole-to-hole, and open area. When the distance between centers, of given diameter holes, is twice the diameter of the holes (in a staggered-center pattern that has six holes equi-distant all the way around), the open area ratio is 23%. Consequently, 1/8" diameter holes on 1/4" centers, 3/16" diameter holes on 3/8" centers, and 1/16" diameter holes on 1/8" centers and 1/4" diameter holes on 1/2" centers are all 23% open area ratio patterns. Experience indicates the 23% open area ratio optimizes barrel strength relative to plating performance. Because the open area of any barrel determines the access of the plating current to the work, the plating performance is directly related to the percentage of open area; therefore, barrels with the same open-area ratio can be used in the same plating line regardless of hole size. Because the access of the plating current to the work will be the same, there is no need to adjust rectifier settings or current density. It 426

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