Metal Finishing Guide Book


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load cells, and meters to operate in the exact required sequence. A manual override panel is part of the system, which allows adjustments to be made if needed or to take over in the rare case of computer malfunction. Use of this advanced automated process provides welcomed enhancements to an established manual technology. It provides improved quality and reliability of coatings; increased process speed, productivity, and ease of use; operator safety—reduced liability from chemical handling and exposure; environmental compatibility and minimization of waste products; historical tracking, record keeping, and documentation; and overall cost effectiveness. In an automated system, all chemicals are in liquid form including the plating metal. The powdered plating metal is transformed into a liquid slurry in a two-part metal slurry mixing system consisting of a mixing module and a delivery module. The mixing module combines a measured amount of water and metal dust under constant agitation and then delivers it to the delivery module for the plating process. Metering pumps in this module transfer continuously mixed slurry directly to the plating barrels via permanently fixed flexible tubing. Automation system costs vary widely according to the requirements and degree of automatic control. A range approximately between $18,000 and $100,000 will estimate costs associated with most systems from the most simple to highly sophisticated. POSTTREATMENTS Posttreatments for mechanical plating are similar to those used in electroplating. The coating is more receptive to postfinishing immediately after plating, before drying. A mild acid dip (1% nitric acid) will reactivate parts that have already been dried. Conversion coatings or passivates, such as clear or blue, yellow, olive drab, or black, can be applied. Special trivalent passivates are now available to meet new industry requirements regarding hexavalent chromium. Mechanically plated parts can also accept proprietary topcoats, paint, and other special postfinishes. The color, luster, and iridescence of postfinishes on mechanical plating are somewhat different than those obtained on electroplated surfaces but are well within the normal range of acceptable appearance and performance. Corrosion resistance is demonstrated for a variety of finishes and postfinishes (Fig. 5). With excellent corrosion protection, no hydrogen embrittlement, low energy cost, automation, and consistent coating thickness and uniformity across the wide range of deposits, mechanical plating and galvanizing remains a viable option for today's metal finisher. REFERENCE 1. Standard Specification for Coatings of Zinc Mechanically Deposited on Iron and Steel, ASTM B 695 399

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