Mechanical Plating Equipment

Mechanical plating is accomplished in mild steel or stainless steel variable-speed tumbling barrels. Because the entire process takes place at low (1 to 2) pH, the barrels must be lined with an inert, abrasion-resistant protective coating such as neoprene, polypropylene, polybutylene, or urethane. The lining is usually applied at a thickness of 19 to 25 mm (0.75 to 1 in.); thicker linings may be used in high wear areas such as the inner base of the barrel.

Typical tumbling barrels have capacities of 0.04 to 1.13 m3 (1.5 to 40 ft3), where capacity is the total available working volume, which is typically about 25 to 35% of the total volume. For example, a 0.57 m3 (20 ft3) plating barrel will hold approximately 910 kg (2000 lb) of 25 mm (1 in.) long steel threaded fasteners or about 680 kg (1500 lb) of 8d common nails. After the parts are loaded in the barrel, approximately 1000 kg (2200 lb) of glass beads are added with sufficient water to form a slurry.

The tumbling of part on part and glass bead on part creates kinetic energy that serves to cold weld the spheroidal metal particles to the substrate. The diameters of the most commonly used glass beads are nominally 5 mm (0.187 in.), 1.5 mm (0.056 in.), 0.7 mm (0.028 in.), and 0.25 mm (0.010 in.). The ratio of glass beads to parts is about 1.5:1 by weight, but varies depending on the part mass and geometry and on the coating thickness required (greater thicknesses sometimes require a higher ratio of beads to parts and the use of larger beads). The barrels rotate at a surface speed of 43 to 75 m/min (140 to 250 ft/min) and at a tilt angle of about 30° from horizontal.

The glass plating beads perform a number of functions. They:

• Assist cleaning and oxide removal through a mildly abrasive scrubbing action

• Facilitate mixing and displacement of the chemicals

• Consolidate the metallic coating

• Protect and separate parts from one another, thereby preventing edge damage and tangling

• Help push the plating metal into corners, recesses, and blind areas

Glass beads are constantly reused and recycled through a glass bead recovery and pumping system, which is part of the standard mechanical plating equipment package.

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