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Mechanical plating is a method that can be used to coat the surface of an object with various metals. Unlike electroplating, mechanical plating uses a physical peening action to achieve the effect. The workpieces are typically placed into a drum that has been filled with a plating medium, and the drum is then agitated. As the particles in the plating medium strike the objects that have been placed in the drum, a thin layer of less than 0.001 inches (0.025 mm) will be peened into place. The similar mechanical galvanizing process uses the same method to create thicker layers.
One of the reasons that certain fasteners are plated with other metals is to guard against corrosion. Plating can protect a component by preventing harmful elements from reaching it, or by acting as a sacrificial metal. Electroplating can introduce hydrogen into certain metal components though, which tends to lead to a phenomenon called hydrogen embrittlement. Mechanical plating does not introduce hydrogen in this manner, and it is typically less costly to perform as well. The main limitation of the method is the size of the workpieces it can be used with, as excessively large or small items may be damaged during tumbling.
The entire mechanical plating process can take place within a tumbler coated with some type of protective material such as plastic or rubber. Each workpiece is typically cleaned before mechanical plating takes place, so it is often tumbled with a degreasing or descaling agent first. In some cases, the workpieces will then be conditioned prior to the actual plating phase, which may result in an initial coating of copper. These steps may be left out if the starter contained in the final plating media contains cleaning or conditioning agents.
After the workpieces have been prepared, the final plating media can be added to the tumbler. This is typically a mixture of glass or ceramic beads, water and fine copper, zinc or tin dust. When the tumbler is activated, the glass or ceramic beads are repeatedly struck against the workpieces. Each of these hits can cause some of the metallic dust to be peened onto the workpiece that was struck. Over the course of about an hour, a layer that is typically less than 0.001 inches (0.025 mm) can be cold welded onto the surface of each workpiece, while mechanical galvanizing can use more media and time to achieve thicker coatings.