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.

Selective plating is exactly what it sounds like.

Specified areas on a part are “masked off” from the plating process.

Typical features that may require masking are:

  • Threaded holes
  • Dowel pin holes
  • Electrical contact points
  • Dissimilar metals on a single substrate
  • Tightly toleranced features

Properly utilized, the masking procedure will achieve the desired results.

Masking materials include:

  • Liquid masking chemicals
  • Plugs
  • Plastic fasteners
  • Tape
  • Die cuts

But there are potential pitfalls, and masking does add time and cost to the plating process.

The masking process is seldom foolproof 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.

If you have questions about masking, the Nimet engineering staff is available to discuss the best solutions. And our masking department has the training and experience to assure the best possible outcome for your selective plating needs.

Adapted from “When masking is a must: Understanding the role of selective plating,” an article inThe Hard Truth, a digest published by Nimet Industries, Inc.

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