What exactly is brush-plating?
Brush-plating, also known as selective plating or spot-plating is a technique which makes it possible to deposit metals and/or alloy's on conducting materials. Actually the process exists as long as bath-plating and formerly was used to repair parts that were not completely covered, electrolyte from the bath was used then. In the sixty's they started to develop the process to a complete surface-technique and this development is still going on.
At first brush-plating looks more like welding then plating. The part to be plated is connected by a flexible wire to the negative output of a Power Pack, this is a DC-transformer with rather some electronics added, and acts in this way as a cathode. As an anode mostly a piece of carbon is used. This anode is connected to an anode handle and wrapped with an absorbing material, such as polypropylene wool or something like that. This wrapping is needed to absorb the electrolyte, which contains a high concentration of the metal to be deposit. The anode handle now is connected by a flexible wire to the positive output of the Power Pack. When the anode is placed on the part, the current circuit is closed and the metal from the electrolyte will deposit on the surface. (see figure) As long as the anode moves over the part to be plated and the anode is supplied with electrolyte this process will go on continuously. The electrolyte can be supplied by dipping or pumping through. In this way it is possible to apply metal on a selective area, for instance to repair a scratch in a printing cylinder.
Pros and cons
Advantage is the equipment is mobile, the work can be done locally in the customers workshop mostly without disassembling the part to be repaired, therefore repairs can be carried out much faster then applying any other method. Mostly there is no need for machining after depositing the metal especially with thin layers.
Disadvantage is limitation of the layer thickness of about 0.7 mm. Technically a thicker layer is no problem, but financially it can be a problem. Also when a part has to be plated overall, bath-plating often added will be more economical.