Metal Finishing Guide Book


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amount of each depends completely on the total surface area of the parts to be plated and, therefore, it is important to calculate this number prior to each cycle. The variable-speed plating barrels rotate at a surface speed of 43 to 75 m/min (140-250 ft/min), depending on part type and at a tilt angle of about 30�� from horizontal. Except for precleaning heavy oils or scale, all of the steps are performed in the same tumbling barrel, normally without rinsFig. 1. Specially lined ing or stopping the rotation. A typical variable-speed tumbling/plating barrel. process cycle includes a series of surface preparation chemical additions, designed to mildly acid clean and activate the substrate and then to apply a copper strike. The preparation chemicals normally contain sulfuric acid, surfactants, inhibitors, dispersing agents, and copper in solution. This step results in a clean, galvanically receptive part surface. The next step is the addition of a ���promoter��� or ���accelerator��� chemical, which acts as a catalyst as well as an agent that controls the rate of deposition and subsequent uniform bonding of the plating metals. A defoamer is used to control foaming caused by the surfactant additives, so loss of plating 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 number of equal additions totaling an amount proportional to the plating thickness desired. Table I represents a typical sequence. The process is conducted at room temperature between 15 and 32��C (60 and 90��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 metal 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 plating 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 dryer oven with belt or vibratory transport. Table I. Typical Process Sequence for Mechanical Plating Process Stop Alkaline or acid preclean (if necessary) Time, min 5 Rinse Surface preparation 5 Copper strike or ���flash��� 5 Accelerator/promoter Plating metal additions (series of small equal adds) Water polish 448 3 15-20 5

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