Precision masking is the process by which specific areas of a part are protected or "masked" off within .001". Masking materials include liquid masking chemicals, plugs, plastic fasteners and tape or die cuts.
Typical features that may require masking are threaded holes, dowel pin holes, electrical contact points or areas, dissimilar metals on a single substrate and any tightly toleranced feature. What does it do? When parts are initially designed, they are typically prepared with the machinist in mind. The designs often do not factor in the plating process and allow for the build-up of the plating until the part is completely machined.
If critical dimensions are not adjusted during machining, the part will probably need to be selectively plated. Properly utilized the masking procedure will achieve the desired results. The masking process is seldom fool proof and may come with its own set of potential problems. For example, masking polished surfaces is difficult because the masking material requires a degree of surface roughness for a mechanical bond. A good analogy would be the difficulty of getting paint to adhere consistently to a high gloss finish. Masking on these surfaces has a tendency to lift off at the edges. Plug masking, the most straightforward solution, also can pose challenges as plugs can and do leak, or even fall out during the plating process.
Parts designers should endeavor to avoid the time and expense of masking whenever possible. One way to accomplish this is to be sure to make allowances for coating build-up when the part is machined. Another option is to allow for a secondary machining operation to remove the coating in the selected areas.