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Zinc Plating Deposition in Holes

We outsource zinc plating (barrel) and are having a problem with plating being deposited in a 12.0mm hole with size tolerance of 0.08mm at about 0.01 - 0.05mm thick when the specification requires a minimum of 6 micrometer. Obviously this causes the hole to be undersized. While the plater states that this is a current density issue and will always occur, my customer demands that we do better. Is there any reference article or technical information available that illustrates this phenomena?

Jerry March
- Sterling Heights, Michigan
2002

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Sorry, I can't quite interpret your question. I understand that the hole is 12.0 mm dia. I think I understand that the required minimum plating thickness is 6 micrometers.

Are you saying the actual plating thickness in the holes is coming in at .01 mm to .05 mm thick? That is, the plating in the holes is 1-2/3 to 8-1/3 times the minimum required thickness? I don't know what a size tolerance of 0.08 mm means in this context.Are you suggesting that if the plating in the holes could be held below .04 mm everything would be okay?

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2002

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First of two simultaneous responses --

Essentially correct. The allowable hole size is 12.00-12.08mm. It is punched at 12.08mm with a range of ±0.02 to allow for plating buildup. The resultant plating buildup from run to run varies from 0.01-0.05mm thick with a range of ±0.04 periodically resulting in undersize holes. Is this the result of an improper plate or an expected result? Either way, my customer requires documentation to explain the condition.

Jerry March
- Sterling Heights, Michigan
2002

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Second of two simultaneous responses --

Without addressing your specific hole size, here are some general rules that may pull you closer to spec.

1. Alkaline Zinc "throws" better than Acid Zinc. And within that rule higher ratio caustic/zinc throws better than the more efficient standard barrel plating formulation. I have done some work whereby we used alkaline zinc to throw under to get a pretty brighter finish.

2. If #1 is not close enough, then can you afford to individually rack the parts so ,that the hole faces the anode, then we can use high throw chemistry. The defeating mentality is that more profitable efficient chemistry does not throw as well as high-throw chemistry.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2002