Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Fig. 5. Barrel assembly equipped for use in a rack plating line. Barrels are used to plate chrome where ample current and continuous-contact are available (when gentle abrasion of the part surface is not a problem). One can infer from the previous example that a barrel's value and versatility depend on its capability to (1) plate a particular finish and (2) function prop- erly in system solutions and tempera- tures. This capability is determined by the materials, construction, and detail features incorporated into the barrel unit. Some barrel equipment lines have the capability to produce more than one plat- ed metal or finish type; however, most plating lines are dedicated to one finish type. Elimination of "drag-out" in a plating line that produces more than one finish type is a primary concern. Drag-out, or cross-contamination, of the different plated met- als in stations used for: rinsing, sealing, chromating and cleaning can be minimized by incorporating an "up-rotation" sequence in the barrel operation. Up-rotation is discussed in the section "Hoist Systems, Tanks, and Ancillary Equipment." WORKLOAD The barrel plater needs to evaluate each of the following items to decide if the desired finish on a particular part can be barrel plated: finish function (relative to use of the part), part configuration, part size, part weight, calculated part sur- face area, and total workload volume and square foot surface area. The workload capacity is usually 40 to 60% of the total interior barrel volume. The maximum workload volume is usually determined based on total square foot surface area of the load and the capacity of the bath chemistry and electrical equip- ment to plate. Other factors are the weight of the individual workpieces and their propensity to damage the finish or serviceability of other parts in the load. Damage of this type is usually the result of the weight, configuration, or edge char- acteristics of the parts as they tumble against each other in the barrel. As designated in the section about the uses of barrel plating, plated-finish functions are of three basic types: corrosion protection to increase the useful ser- vice life beyond performance of the un-plated base material; decoration for appearance, which also enhances the value of the base material; and engineering applications to attain (add material) or maintain a dimensional requirement and/or as a bearing surface. There are requirements for plated finishes that need to perform more than one of the previously mentioned three basic functions. Barrel plating is most com- monly used to finish parts for corro- sion protection. Decorative finishes are successfully barrel plated when surface effects from part contact are controlled to an acceptable level. Engineering fin- ishes are not usually applied by barrel plating. Configuration of the workpieces affects the ability of work to be suc- Fig. 6. Special-length barrel assembly for plating elongated parts or for use in a rack plating line. 327

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