Metal Finishing Guide Book

2013

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Fig. 2. Hook shapes. support all the parts, plus be wear resistant, especially for use on an automatic machine. The plating bath in which the rack will be used has a known current density rating in A/ft2. Multiply this factor by square feet of parts on the rack to determine how much current the rack spine and hook must carry. (See Table I for plating solutions—cathode current densities.) Generally, most single spines are fabricated with 1/4 × 3/4 in. or 1/4 × 1 in. copper, which will carry 200-250 A. (See Table II for a chart of relative conductivity.) Copper is the most commonly used material, as it has the highest conductivity in relationship to price. Sometimes cathode hooks fabricated of copper and spines fabricated of steel, stainless steel, brass, or aluminum can be used if the connection is below solution level. Again, the main factor is conductivity. Steel, stainless steel, brass, and aluminum have lower conductivity than copper. The most common practice is to use steel for supporting members and not where conductivity is needed. DESIGN OF PLATING RACK TIPS Some practical objectives in the design of the tip are easy racking and unracking; adequate current flow (contact) to the part; tip designed to hold part in noncritical area; type of tip—gravity or spring type; and material. Gravity Tip A gravity tip is one that is styled for easy racking and unracking. The part to be plated usually has a hole for the tip to fit through. This style is most commonly used in zinc, electroless nickel, cadmium, or silver baths. Spring-Type Tension Tip A spring tension tip is used in baths, which require greater throwing power and Fig. 3. Four basic types of plating rack construction; A — single spine; B — T type; C — box type; D — multiple spine. 710

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