From an electroplater’s perspective, all parts are not created equal. The shape, or geometry, of the part directly impacts everything in the plating process, and careful considerations and decisions must be made before plating begins. Among the variables an electroplater must consider are:

  • Part Length
  • Diameter
  • Presence of through holes and blind holes
  • Threads
  • Material hardness and weight

Changing any one of these can impact the choice of plating method, lot size, cleaning process, equipment and above all, the plating quality. The interaction of these variables must also be taken into consideration. Ignoring any of these factors could create many challenges from uneven plating to finishes that are prone to premature failure.

Let’s Look at the Science

Plated Parts with Various GeometriesThe shape of a part affects the flow of electrical current and dictates how evenly the metal coating can be distributed. Electricity inherently travels to the extreme point of a part causing that area to have thicker plating due to its higher current density. The middle of a long part, or the bottom of a blind hole, will have a lower current density and less plating thickness. Small diameters of parts will also attract electrical current at a different rate than the larger diameter of the same part. Therefore, specifying a point of measurement for plating thickness is essential.

Dealing with Blind Holes

In the case of complex parts with blind holes, preparation is critical. Any chemical used must be able to flow into the blind holes and still be exchanged with fresh solution. This is necessary so that the intended cleaning or activation is effective. Flushing out those chemicals that you worked so hard to get in the holes, is equally as important. No chemical residue can be left behind, especially if the chemical isn’t compatible with the electroplating solution. As a general rule, blind holes with a depth to diameter ratio of one-to-one will rinse properly and have good plating coverage. When the depth of the hole is greater than the diameter, the risks of residue build-up and poor coverage increase. Good news though; these issues can be avoided by adding a weep hole near the bottom of the ID.

Handling Long Parts with Through Holes

The same rule applies to the through hole of long parts. The middle of the ID will have the least amount of solution exchange and the greatest risk of poor quality. As mentioned previously, this will also be the lowest current density area of the part and will have the least amount of thickness. Electroless plating processes will have the best results for deep IDs of through holes, like a deep drawn part.

The Effects of Material Hardness

Material hardness is another key consideration. For instance, large brass parts with OD threads present a challenge since the material is soft, the threads are fragile and the parts tend to be heavy. Tumbling them in a standard plating barrel can round off the threads and make them not fit an after-plate gage. Rack plating can overcome this issue but is expensive.

Conversely, lighter components have a tendency to float, which is less of a challenge, since parts typically aren’t damaged when this happens. The bottom line here is that the least amount of handling is best for the most consistent quality.

An electroplater can overcome these obstacles by studying geometry-related variables carefully in advance, then choosing the optimal electroplating method. Among them are vibratory plating for fragile long parts, barrel plating for parts that can withstand tumbling and spouted bed electrode for very small parts.