Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 654 of 707

Fig. 2. Hook shapes. ing 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 relation- ship 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 alu- minum 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 noncrit- ical 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 common- ly used in zinc, electroless nickel, cadmium, or silver baths. A spring tension tip is used in baths, which require greater throwing power and positive contact such as chromium plating or anodizing. They are also needed whenever mechanical or air agitation is used. Spring-Type Tension Tip Some principles of plating must be remembered in designing a plating tip. Fig. 3. Four basic types of plating rack construction; A — single spine; B — T type; C — box type; D — multiple spine. 653

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