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Zinc Diecast skin is blistering

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We are brass plating Zinc Diecast #3 & #5. We etch in Sulfuric and have success with most parts, on the other hand some parts seem to have excess skin, which blisters in powder coating at 350 F. My question is if there is a normal thickness or range for this skin or at what point should it be rejected back to the diecaster as a casting problem.

Thanks,

Max T. Faeth
plating shop - Los Angeles, California

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Max,

Even a part that hasn't been buffed shouldn't have a skin like you described. Your diecaster should check his process for even heat distribution in the die as well as his pressure...if it's a pressure casting. As far as plating, I always use a 4 oz/gal Fluoride based acid salt. I also keep the electrocleaner mild and time less than 30 seconds since a smut will form quickly and the copper strike will not adhere.

Jim Conner
- Salisbury, Maryland

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May 18, 2010

Hi. I'm having a little bit of a hard time picturing this "excess skin". I would think that the skin is nothing more than the surface which is contacting the die.

But I have seen plenty of "cold shuts" where there is a crack in the skin due to two flows being squeezed together after the zinc is too cold to fuse together. I suppose if the zinc were colder still, it might be possible for one skin surface to sort of flow over the other, leaving an excess thickness and a laminarity to the skin.

Then again, my actual experience is in plating; and my experience in diecasting is pretty much limited to watching -- so diecasters may be laughing at this idea :-)

Regards,

Ted Mooney, P.E. finishing.com Brick, New Jersey