Metal finishing Q&As since 1989

-----

Poor plating coverage in blind holes

Mr. Derek Skaro,

I have to rack plate 6061 aluminum blocks containing several blind threaded holes with .0002-.0003 of silver, but I can't seem to get a good throw into the holes, and the parts stain during the drying process because of debris and fluids leaching and tarnishes my silver coating.

Sincerely,

Jeff Huehn
- Elk River, Minnesota, USA
November 6, 2010

---

Read any good plating book and you will find that this is true. Typically, a hole will plate a depth equal to the diameter of the hole. The main reason for this is that you plate all of the metal ions out of the solution in the holes and since there is no fluid circulation, there is no more plating. Add to this the fact that nearly every hole will retain some of the previous chemicals and or rinse water which further dilutes the amount of metal ions. In bad cases, it reacts with them or changes the pH to the point that it does not plate.
A tiny improvement can be had by jet rinsing each hole before plate. The staining can be greatly reduced by the same jet rinsing of each hole. As you can see, this is labor intensive.

The real question is: does your customer really need the holes plated or they just want it.

It can be done with some very precise exotic plating jigs with constant solution pumped into the holes, but it is a slow and thus very expensive process.

James Watts
- Navarre, Florida
November 9, 2010