Paul D. Fleming III, Paper Engineering, Chemical Engineering and Imaging
Most gravure cylinders have a copper surface covered with a thin layer of chrome. The copper carries the image and the chrome protects it. The basic functions of the copper are engravability, stability and reproducibility. Chrome serves a protective layer against the friction of the doctor blade and the substrate. Unprotected copper would wear more rapidly with time than with the chrome plating.
Since the chrome and copper are plated electrochemically, it is important to review the principles of electrochemistry. A schematic of a typical plating tank is shown below. The anode is usually made from the metal being plated.
The cylinder being plated acts as the cathode. A rectifier is included to convert AC to DC for electroplating. The cathode supplies excess electrons and the anode removes electrons, supplying cations. The cations are attracted to the cathode, recombine with the electrons and plate on the surface. The thickness of metal electroplated is determined from Faraday’s laws. Faraday’s first law says that the amount of material deposited on the cathode in a given time is directly proportional to the current.
Faraday’s second law states that the weight of a given substance deposited is proportional to the equivalent weight. Thus, the integrated current (i.e. the amount of charge) required to deposit an equivalent mass in grams is a universal constant. This constant is 26.8 Ah (Ampere hours) or 96479 coulombs ( a Faraday).
This factor is called the electrochemical equivalent. The electrolyte is circulated constantly to maintain uniform electrolyte concentration.