Plating Equipment

Considerations specific to the operation of cadmium cyanide baths in conventional plating equipment are discussed here, with attention to the materials of construction used.

Still Tanks. Usually, unlined steel tanks are used for alkaline cadmium plating; however, steel tanks with plastic linings are useful in preventing stray tank currents. Another advantage is that lined tanks may satisfy the diking requirements of the Occupational Safety and Health Administration (OSHA). Double tanks and containment berms must be considered with the current regulations. Rubber and plastics used for tank linings should be tested for compatibility with the plating bath, to prevent contamination from constituents of the lining. Vinyl plastisols are compatible, commercially available, and require no further testing.

Filters and cooling coils may also be made of steel. Equipment for fume control should be used; such equipment in some cases is required by local ordinances. A typical tank arrangement is shown in Fig. 1. Equipment for baths other than the cyanide must be made acid-resistant.

Barrels may be made of hard rubber, polypropylene, acrylic resins, phenol-formaldehyde or melamine-formaldehyde laminates, or expanded or perforated sheet steel coated with vinyl plastisol. The plastisol coating is about 3.2 mm (- in.)

thick and is resistant to the standard barrel plating solutions and temperatures. Usually, doors and wall ends are of the same material.

Perforated cylinders for oblique barrels also have perforated bottoms and are made of the same materials used for perforated cylinders of horizontal barrels.

Anodes used for barrel plating may be bar- or ball-shape. For maximum current density, the anodes are curved to shorten the path of the current. Curved solid anodes are placed on insulated supports, whereas anode balls are placed in curved holders tied together at the lower ends.

Figure 2 illustrates schematically the use of barrel equipment for cadmium plating. Although not shown in the illustration, barrel installations are equipped with plate coils to remove the excess heat caused by the high current used in the plating bath.

Fig. 2 Schematic showing cadmium plating installation that incorporates the barrel method

Automatic plating machines may be of either the straight-line or the return type. In straight-line plating machines, the work is loaded at one end, carried through the various phases of the cleaning and finishing cycles, and unloaded at the opposite end. Such a machine is considered a heavy-duty unit, because it can be designed for large racks and heavy loads.

Loading and unloading of the return machine is performed in the same area; the work follows an elliptical path, as indicated by the schematic layout of Fig. 3. This unit can be designed for either light or heavy loads.

Exhauil

Fig. 3 Schematic showing process sequence for automatic cadmium plating installation

Exhauil

Fig. 3 Schematic showing process sequence for automatic cadmium plating installation

Both types of automatic machines may be continuous, with the work load in constant motion, or intermittent, in which case the motion of the carriers stops for a predetermined time after the work is immersed in each solution.

Power for cadmium plating is provided by rectifiers, which can be of silicon-controlled-rectifier (SCR) thyristor or tapswitch type. If the SCR type is used, attention should be paid to reducing ripple through a filter choke. This is necessary because bright cadmium is sensitive to a high-ripple current, which produces a dull finish on coatings plated in the medium- and high-current density range.

The rectifier elements are silicon. SCR rectifiers offer the advantage of remote control, finer control, computer interface, and constant current-voltage availability. Tap switch rectifiers offer the advantage of lower cost and eliminate the need for ripple filter chokes.

Rinse Tanks. Although longer tank life will be obtained if rinse tanks are lined or coated with polyvinyl chloride or rubber, all rinsing, with the exception of the rinse following hydrochloric acid pickling, may be done in unlined steel tanks. The use of unlined steel tanks for rinsing following pickling or acid plating is not recommended.

Racking of parts for cadmium plating is subject to the same considerations as in the electrodeposition of other metals. Information on design and use of plating racks is available in the article "Industrial (Hard) Chromium Plating" in this Volume.

Maintenance. Table 4 is a typical schedule of maintenance for plating and auxiliary equipment. Table 4 Recommended maintenance schedule for plating and auxiliary equipment

Daily

Check anodes; replenish when necessary. Check all contacts, anode and cathode. Check solution levels. Check bath temperatures and controls.

Check bath composition, if possible, using chemical analysis and plating cell test., Probe tank bottom for lost parts.

Check motors for signs of overheating, arcing, or failure. Check amperage and voltage to work.

Check lubrication on automatic equipment.

Weekly

Probe tank bottom for lost parts, if not checked daily.

Check rubber tank linings for damage.

Filter plating bath, unless constant filtration is used.

Check bath analysis, chemically and with plating cell, and make additions and corrections, if these functions are not performed more frequently. Oil equipment. Clean all contacts.

Check for preventive-maintenance items that cannot be repaired during the week. Dump and replenish cleaning lines where necessary

Monthly

Pump plating solution to purification tank; treat for impurities, if necessary. Inspect tank linings while plating tanks are empty; repair if necessary. Inspect and clean heat exchangers or plate coils if accumulation or buildup exists. Blow out and check rectifier stacks for condition and power delivery. Check for arcing or scored armatures on generators. Blow out coils. Perform general preventive maintenance examination of all equipment.

Semiannually

Clean out exhaust systems.

Repair exhaust fans.

Check all motors.

Repaint where necessary.

Inspect and clean out all floor drains.

Check for leaks and cross connections between cyanide and acid drains.

Check all items usually covered on annual or semiannual overhaul, such as solenoid valves, limit switches, relays, and automatic electrical equipment.

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