Plating Q&A: Dog Bones and Plating
How to evenly coat parts.
Q. Our facility plates bright nickel on studs and nuts using racks, but we have an ongoing problem: the studs have a heavier deposit on the ends compared with the middle of the studs. The required plate thickness is 20 microns, but we end up with 60 microns on the end of the studs. This is a classic “dog bone,” and it’s a major problem because we cannot assemble the studs and nuts properly. The pieces on the bottom of the rack are more likely to have a heavy deposit and the anode to cathode ratio is approximately 2-1. I have attached a drawing showing the arrangement of the anode and plating rack in our tanks (note that the rack with the parts, the cathode, is shorter than the anodes on either side of the rack).
We have experimented with shields and current robbers, but have not had very good results. Do you have any suggestions on how to resolve this ongoing problem?
A. You did not mention whether you have looked at the relationship of the anodes to the cathode in your plating tank. A typical part being plated has areas of higher and lower current density. Areas with higher current density will have a heavier plated deposit. In this case, I would try shortening the anodes so they are a few inches shorter than the plating rack. This will help reduce the current density on the bottoms of the racks.
You mentioned experimenting with shields and robbers; have you experimented with anode placement and reducing the anode to cathode ratio?
Also, consider the amount of agitation of your plating bath. You did not mention how large the plating tanks are, but if they are fairly large, oftentimes the amount of mixing in various areas of the tank will vary and will, in turn, affect the thickness of the deposit. This may help with the variation of thickness across the rack.
Originally published in the October 2015 issue.
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