Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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is used to control foaming caused by the surfactant additives, so loss of plat- ing solution is avoided and operator visual monitoring is maintained. A series of plating metal (usually zinc) additions added as a powder or water slurry is introduced in a num- ber of equal additions totaling an amount proportional to the plating thickness desired. Table I represents a typical sequence. F) and at a pH range of 1 to 2 to ensure proper adhesion and high metal efficiency. The low pH acts to maintain and oxide-free condition at all times on the surface of the parts as well as the plating met- al particles. The process has an efficiency of about 93%, meaning that approximately 93% of the plating metal added is actually plated on the parts. The mechanical plat- ing cycle usually takes between 30 and 45 minutes. At the end of the cycle, the slurry of glass beads, plated parts, and plating water discharges onto a vibrating "surge hopper" and is then directed to the rinsing and glass bead separation section. This section is a water-sprayed vibrating screen area or magnetic belt, which removes the glass beads for recycling and rinses the parts. Separated parts are then dried by a heated centrifuge or a continuous dry- er oven with belt or vibratory transport. The process is conducted at room temperature between 15 and 32° and 90° variable-speed tumbling/plating barrel. APPLICABLE PARTS Various part types for which coating opportunities were limited to electro- plating, hot-dip galvanizing, painting, or organic finishing are now successfully being mechanically plated or galvanized. Parts now universally accepted for con- sideration include regular and self-tapping screws, bolts (including A 325), nuts, washers, and stampings; nails; chain and wire forms of all types; pole line and tower hardware for telecommunications; electrical connectors; and automotive, aircraft, and marine fasteners. The suitability of parts considered for mechanical plating or galvanizing is determined by its size, shape, and base metal. Part types that would not with- stand the tumbling action of the process are usually not suitable. Parts heav- ier than 1 to 2 kg (2.2-4.4 lb) or longer than about 300 mm (12 in.) are not nor- mally coated in this manner. Parts that have deep recesses or blind holes may Table I. Typical Process Sequence for Mechanical Plating Process Stop Alkaline or acid preclean (if necessary) Rinse Surface preparation Copper strike or "flash" Accelerator/promoter Plating metal additions (series of small equal adds) Water polish Time, min 5 5 5 3 15-20 5 353 C (60 Fig. 1. Specially lined

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