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-----:Zinc die castings get black spots from vibratory finishing
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Diecastings usually require mechanical finishing, such as processing in a vibratory finisher to remove sprue lines. But diecastings have a hard skin and a porous interior, so overdoing the mechanical finishing process can expose that porosity, leading to defective plating. Read on ...
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Q. When we use ceramic media with zinc parts after 2 hours I see on zinc some spots and blackish color forming? What is the problem behind this please guide me !!
Er. Mohammad Adil- Aligarh, U.p India
January 8, 2026
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A. Hi Mohammad.
Tony Kenton suggests that it is a chemical reaction. I am not an expert on media selection for vibratory finishing of diecastings, but I suspect that you are using the wrong media, or are lacking abrasives, because what you are describing sounds to me like "fretting corrosion", i.e., you are annoying the metal on the surface, but not removing it.
If you <send pics for posting> it may be possible to say more or differently.
Hopefully an experienced vibratory finisher of diecastings will chime in as well 🙂
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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⇩ Similar, related, Q&As -- oldest first ⇩
Q. Dear Friends,
We are a Zinc Diecasting company which uses standard Zamak 3 alloy. We wish to electroplate our products with Chromium & brass plating, so before plating we wish to make the surface of product very smooth & scratchless WITHOUT USING ANY MANUAL PROCESS. Please advice how the surface can be smoothed w/o including any manual process.
Best regards,
- India
2003
A. It is possible to vibratory finish small parts or drag finish large parts, Asheesh
Good luck.
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Need quick confidential answers? $25
Need project assistance? $100/hr.
Q. Dear Mr. Ted Mooney,
Thanks for your reply. Please advice me about Drag Finish ... Procedure, machines, chemicals ?
Best regards,
- India
A. Drag finishing uses similar parts and solutions to what is used for vibratory finishing--it's just used for bigger, individual parts which vibratory finishing can't accommodate.
Each part is racked individually, hanging down from a spindle and into the container of media. A chain or similar power transmission device drags the spindles and the parts around and around through a race track style loop; while at the same time the part is also rotating around the spindle. Similar to the way the earth revolves around the sun but also rotates around its axis. By adjusting the orbital speed and the rotational speed (as well as the media), different finishes can be obtained.
Luck and regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Need quick confidential answers? $25
Need project assistance? $100/hr.
Black Spots after Vibro-finishing
Q. We have zinc diecast parts. After vibro finishing the parts are dried in oven, and we we facing black spots.
Using corn cob was having 100% clogging problem; due to that we have adopted this process.
Kindly suggest which way to avoid the black spot problem?
Regards,
Managing Director - Bangalore, Karnataka, India
2007
Ed. note: You haven't told us what process you are using in place of the corncob. Please get back to us.
A. Sounds like a chemical compound reaction problem during your deburring cycle. Two things. Check the chemical compatibility of the product to zinc. Second, try adding a 5 to 15 minute straight water rinse cycle to deburring before trying to dry. About drying. You might want to clean parts in hot water or use a hot air blower on parts before the oven to reduce dwell time
AF Kenton
retired business owner - Hatboro, Pennsylvania
Q. We are facing black spots problem in zinc die casting parts after shot blasting in few days in rainy season. these black spots increase as the time passes. Please suggest, How we can solve this problem. ⇦ Answer?
VIKAS SinghGROZ - Gurgaon, haryana India
August 16, 2019
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