Control of Plate Thickness

This section discusses the thicknesses of zinc specified for service in various indoor and outdoor atmospheres. Many combinations of variables must be considered in attempting to plate to a given thickness. To hold each variable at a steady value is virtually impossible under production conditions, so as one variable changes spontaneously, others must be adjusted to maintain uniformity of plate thickness. In automatic plating this is impractical, so the process is set up to give a certain minimum thickness under a great variety of conditions. This accounts for much of the thickness variation normally encountered in automatic plating of a run of identical pieces.

The shape and size of parts that may be plated all over, with or without the use of conforming anodes to attain uniformity of plate thickness, are essentially the same in zinc plating as in cadmium plating (see the article "Cadmium Plating" in this Volume).

Normal Variations. Preferred thicknesses in automatic zinc plating are usually minimum specified thicknesses, and there is little concern regarding the maximum thicknesses obtained. Thickness variations encountered should therefore be over the established minimum thickness.

For example, as shown in Fig. 6, tests were made on 75 samples, over a one-week period, of parts 100 mm (4 in.) long and 39 g (1.375 oz) that were automatically plated to a minimum specified thickness of 3.8 pm (0.15 mil). Although actual plate thicknesses ranged from 2.5 to 7.5 pm (0.1 to 0.3 mil), over 80% of the parts examined exceeded the target minimum.

Thickness of plate, mi I

Thickness of plate, mi I

Thickness of plate, jjfn

Thickness of plate, jjfn

Fig. 6 Variation in thickness of zinc plate obtained in automatic plating in cyanide zinc bath, 75 tests

Thickness variations obtained in barrel plating are markedly affected by the tumbling characteristics of the part and by the density of the load in the plating barrel. Parts that can be tumbled readily are more likely to develop a uniform coating. As shown in Fig. 7, a minimum plate thickness of 12.5 pm (0.5 mil) was the target in barrel plating a 0.12 kg (0.26 lb) S-shape part made of 3 mm (0.125 in.) flat stock. Of 75 parts examined, all were found to be plated to thicknesses that exceeded the target minimum, and a few had thicknesses in excess of 34 pm (0.9 mil).

TtiicfcnEss of plaie, mil

TtiicfcnEss of plaie, mil

Thickness ot platE, (im

Fig. 7 Variation in thickness of zinc plate obtained in barrel plating a 3.2 mm (- in.) thick part in a cyanide

zinc

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