2 JC Norris Brush Plating Part I Metal Finishing July 1988 p 4448 Key Process Elements

Anode-Cathode Motion. Controlling continuous movement between the anode and the workpiece, or cathode, is a key element in obtaining high-quality brush-plated deposits. However, quality also depends on plating within a specific current density range, so both variables affect ultimate deposit quality. This relationship is illustrated in Fig. 2. Solution suppliers routinely recommend ranges of anode-cathode speeds and current density values for each solution; a representative list is given in Table 5.

Table 5 Anode-cathode motion and current density for selective plating solutions

Table 5 Anode-cathode motion and current density for selective plating solutions

Selective plating solution

Anode-cathode motion

Current density

m/s

ft/min

A/dm2

A/ft2

Cadmium (acid)

0.26-0.561

50-110

86.4

864

Cadmium LHE (low hydrogen embrittlement formula)

0.20-0.41

40-80

86.4

864

Chromium

0.02-0.03

4-6

86.4

864

Cobalt (machinable)

0.13-0.26

25-50

115.2

1152

Copper (high speed, acid)

0.20-0.51

40-100

144.0

1440

Gold

0.15-0.31

30-60

28.8

288

Lead

0.15-0.26

30-50

86.4

864

Nickel (acid)

0.10-0.26

20-50

86.4

864

Nickel (high speed)

0.20-0.41

40-80

144.0

1440

Nickel-tungsten alloy

0.10-0.15

20-30

72.0

720

Rhodium

0.03-0.05

5-10

43.2

432

Silver (heavy build)

0.10-0.31

20-60

72.0

720

Tin (alkaline)

0.10-0.41

20-80

86.4

864

Zinc (alkaline)

0.15-0.612

30-120

115.2

1152

Data courtesy of SIFCO Selective Plating

Data courtesy of SIFCO Selective Plating

Fig. 2 Relationship between current density and anode-to-cathode speed. Source: Ref 3

The visual appearance of the electroplate is also an indicator of quality. A dark gray or black color usually corresponds to a burnt deposit, which results from too high a current density or insufficient movement. In contrast, inadequate current density or too much movement produces a generally shiny surface.

Anode-to-cathode movement may be achieved manually or mechanically, such as by using turning equipment to provide a constant rotational speed for cylindrical parts (Fig. 3) or by using specially designed tilting turntables to rotate large parts at controlled speeds. Another option is the rotostylus (Fig. 4), which rotates the anode instead of the workpiece.

Fig. 3 Turning head. Courtesy of SIFCO Selective Plating
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