Selective Plating

When plating must be applied to only certain areas of parts, the areas not to be plated must be stopped off or masked, which means they must be covered with materials that will not conduct current, such as waxes, lacquers, or rubber tape.

Waxes. Ordinarily, a petroleum-derived wax is used for stopping off. The wax must not contain any oil or other organic materials that will dissolve in the plating solution and contaminate it. It must also be capable of adhering tightly to the part, to prevent the plating solution from coming in contact with the stopped-off area.

Before being applied, the wax is heated in a pot to about 27 °C (80 °F) above its melting point, so that it does not solidify too rapidly and will adhere more readily. Still-better adhesion is obtained if parts are warmed on a hot plate before the wax is applied.

Parts must be positioned so that only the area to be coated is placed in the molten wax. This means that, normally, only end areas or protrusions can be stopped off with wax. The wax can be applied with camel's hair brushes, but this is time-consuming if many parts are to be treated. For a large number of similar parts, a fixture can be used that will dip each part to the proper depth.

A sharp, uniform demarcation between plated and nonplated areas can be obtained by the use of pressure-sensitive tape and wax, following either of two procedures:

• Apply the tape to the part so that the trailing edge of the tape follows the demarcation line; dip that portion of the part to be left unplated in molten wax so as to overlap the trailing edge of the tape slightly; and then remove the wax when it has solidified.

• Apply the tape to the part so that the leading edge follows the demarcation; dip that portion of the part to be left unplated in molten wax so as to overlap partly the trailing edge of the tape; and then, when the wax has solidified, plate the part without removing the tape.

Waxing must be done carefully, so that areas that are to be plated have no wax on them. If wax does get on areas to be plated, it must be thoroughly removed. After plating and postplating treatments, the wax is removed from parts by placing them in hot water.

Lacquers may be used instead of wax as stop-off coatings, but their use is generally limited to instances in which the plating bath is operated at a temperature at which the wax would melt. Lacquer is applied by dipping or painting the areas to be stopped off. Normally, two to four coats of lacquer must be applied. One disadvantage of lacquer is that it is difficult and time-consuming to get all of it off. Heavier coatings prevent leakage and make stripping easier.

Plastic Tape. For stopping off irregular areas of heavy parts that cannot be dipped or that are too large to be painted (e.g., splines, large shafts, or bearing shoulders), a plastic tape is used. The tape is wound tightly and stretched over the irregular areas. To prevent leakage, each turn should overlap the preceding one at least half-way. At the edge of the stop-

off area, a pressure-sensitive tape is used to form a sharp line and prevent the leakage of plating solution under the plastic tape.

Plastic tape is expensive to use. When many similar parts are to be selectively plated, rubber sheet, held in place by pressure-sensitive tape, may be used for stopping off areas not to be plated. Rubber stoppers, plastic plugs, or corks, sealed with wax, are used for stopping off internal areas of cylindrical parts. Rubber or plastic tubing can be used to stop off areas of small cylindrical parts.

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