Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 331 of 707

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 pre- dict their minimum part size. Larger perforations usually exhibit faster drainage, more efficient exchange of metal-depleted solution, and less drag-out (carry- over) 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 perfo- ration 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, cen- ter-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 pat- tern 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" -in. diameter holes on 1/8"-in. 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 and 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 plat- ing 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 is very apparent from this that for operational convenience and optimization of operations most barrels utilized should be manufactured with a 23% open area. As mentioned above, there are other types of barrel perforations available to the plater. These include herringbone, screen, fine mesh, and slots. To produce her- ringbone perforations, the barrel panels are drilled halfway through each panel at a 45° angle relative to the inside and outside panel faces (see Fig. 8). In this way, the holes intersect at the middle of the panel in a 90° angle. Small-diameter, straight workpieces, such as nails, pins, etc., cannot pass through the perforations because the holes are not straight. Plating solution and current can pass through the perforations, although at a reduced rate. Barrels with fine-mesh panels with very small openings are generally made of polypropylene and are used to plate very small or delicate work. Larger workpieces will tear, gouge, or wear through the mesh in an unusually short period of time. Some barrels are manufactured with thinner panels in perforated areas to aid 330

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