The Process Of Selective Plating
Selective plating has a long-established technology; it’s only in the last few years that Industry has started to notice its range of applications. The engineering and plating schools have not taught any type of selective plating technology until recently. The process has been growing and unfortunately the process has been given a number of names, making recognition difficult.
In the trade, the technology has been most commonly referred to as brush plating largely because of its origins. The basic process dates back to before the turn of the century, when silverware makers used it to touch up the bowls of spoons, its first industrial application. They could get the silver to adhere to all but the bottoms of the spoon bowls, since they didn’t yet have plating solutions with good “throwing power,” meaning the solutions could not carry electric current far enough to reach the bottoms of the bowls. So they would take an old file, wrap a rag around it, dip it into the plating tank, and touch up the bowls.
The range of industrial applications didn’t expand much until just prior to World War II, when Georges ixci developed organic-based plating solutions in Paris France in 1938. By comparison with the inorganic solutions previously used, like those used in most tank plating today, the new solutions “held” considerably more metal, making them more conductive. This made it possible to pass more current through the solutions, resulting in a faster rate of deposition and better adhesion. What’s more, in many instances the quality of deposit was now far superior to that obtainable from other plating processes.
Most selective plating experts now consider the term brush plating a misrepresentation, since it no longer adequately describes the state of the art of the process. Once deemed practical only for touching up existing plating it has evolved into a highly technical process used for repairing, finishing or improving the surface properties of everything from sophisticated components to axles on trains.
It is also being used as a cost-effective production technique, and thus has become an important factor in the engineering design stage. As a result, “brush plating” has given way to several new names for the process, the most prominent of which are stylus plating, contact plating, electrochemical metallising, selective plating. Among plating experts though, the process is now usually referred to as simply Selective Plating.
The Fundamentals Of Selective Plating
The fundamentals of the process are fairly simple. A fixed or hand-held graphite or platinum anode designed to conform to the shape of the work piece is used to pass current through the plating solution. The solution is flowed between it and the surface to be repaired or modified.
The anode carries a positive charge and the work piece a negative charge via a negative contact, or cathode. When anode and work piece are brought into contact, completing the circuit, the current passed through the system takes the metal ions from the plating solution and deposits them only where the anode touches the surface of the work piece. Thus the size and shape of the anode determines the size and shape of the area built up.
The periphery of the anode is covered with an abrasive material to hold solution and to prevent direct contact with the area to be plated. The anode can be covered with a thin absorbent material such as “dacron felt”, to help hold the solution during the operation, while a special abrasive material affixed to the face of the anode abrades the surface being plated as the anode passes over it.
A DC rectifier manufactured specifically for selective plating is used to ensure accurate control of the working voltage and current density. Some rectifiers have a built-in microprocessor capable of gauging the amount of current actually passed through the system, making it easier for the Operator to compute and monitor the metal buildup. Throughout the process, either the anode or the work piece must be is in constant motion. With cylindrical pieces, usually the part is rotated. For pieces with plane surfaces, usually the anode is placed in motion by using a special designed traversing arm machine, in linear or rotary motion.
Since the design and setup of the tooling is critical to the success of the process, it is usually tailor-made of high-grade pure graphite or platinum clad sheeting for the specific application. The anodes are specially designed for the best solution flow, in some instances it is even cooled to ensure a metallurgically sound deposit. For applications requiring manual operation, the anode is mounted on an insulated handle.
The entire process is capable of being semi or fully automated for volume applications. All these applications have to be assessed in there own merit. Conventional lathes and rotary machines with less expensive computer controls, linear-motion machines, and robotics are available within the industry.
Most selective plating companies, like LDC Selective Plating, maintain facilities for in-house work, capable of handling everything from single items to high-volume production work.