How to reduce Blistering for plating on Zinc Die Cast?
A discussion started in 2001 but continuing through 20182001
Q. 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
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.
Anoplex - Dallas, Texas USA
May 18, 2010
A. 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 :-)
Ted Mooney, P.E.
Pine Beach, New Jersey
"Finishing and Electroplating Die Cast and Wrought Zinc"
by Safranek & Brooman
from Abe Books
Q. The problem is delayed blistering of zinc die cast parts that are 1st plated with Cu and then Ni. A copper strike solution is utilized 1st and small concentrations of zinc are co-deposited with the copper.
My questions are: what levels of zinc will be harmful (if any) to the copper deposit? How much does a grain refiner like (rochelle salts) actually help to prevent blistering? No organic additives are presently being used.
Would appreciate any feedback.[name removed at request]
- East Berlin, Pennsylvania
A. Skip the strike copper as I suspect you are getting a powdery deposit here causing the delayed blistering. Ensure proper degreasing, pickling and cyanide copper plating directly.
Bangalore, Karnataka, India
A. Have seen this problem myself and is usually associated with oxide films on the zinc castings that are plated over. Better cleaning and complete removal of the oxide layer from the zinc before a high cyanide low copper strike should solve the problem. If parts have been vibrator finished this is an ongoing problem we have had when the customer allows the surface to become ingrained with material from the vibrating process.
We have never found a complete solution to the problem sorry.
Q. Hi there
our company does jobs for Zinc Die Cast Parts and found Blistering to be one of the prominent problems.
Could anyone please suggest a solution to prevent it?
plating shop employee - Singapore
A. Hello, YC, welcome to the finishing.com community. Blistering on diecastings has been a perennial problem (see threads 4846, 29746, 33784, 47424 for a sampling) and is due to a number of factors including poor surface condition of the diecast "skin", outgassing from porosity, entrapment of solutions due to porosity, and improper pretreatment.
Probably the first thing that needs to be done is the institution of QA measures regarding the quality of the castings. A general rule is that a lot of castings that are suitable for painting or organic coatings are simply not good enough for plating; for example, a part with a cold shut is not useable. A second thing that may be considered is impregnation of the castings to fill the remaining porosity. Third, your chemical process supplier can recommend a proper mechanical and chemical pretreatment cycle, and work with you to implement it.
Conventional wisdom is that you can't "break the skin" because underneath the skin the casting will be porous. However, there are shops that disagree with this idea. New Brunswick Plating [a finishing.com supporting advertiser] has for decades now championed the idea of what they call "chemical milling" of diecastings, and have produced millions of highly reliable parts with it. I do not know whether they would license their process to a company in Singapore, but it's worth an e-mail. Good luck!
Ted Mooney, P.E.
Pine Beach, New Jersey
November 16, 2018 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread
Q. Hey everybody thanks for doing what you do!!
Having some issues w zinc die cast racked parts; we have a process where the piece is milled on the inside for a latch to click in. I already had a cross section done and know that there are pores and cold shuts. Based on that I would think milling the piece would "open a can of worms" in that area.
Is there anything I can do to mask blistering? What should I be looking at specifically? Have pulled parts out of copper cyanide rinse tank before bright nickel and have not seen evidence of blisters. We have a 2 Cu plate tanks before bright Ni. Degrease, Electroclean, Acid Etch pretreatment.
What typically causes blistering in a Cu cyanide of bright Ni tank?
Plating Supervisor - Chicago, Illinois, USA
November 21, 2018
A. Blistering on ZINC DIE CAST IS MAINLY DUE to two things, one is porosity & second one is Precleaning.
This blistering mainly occurs in the oven while polymerizing the E-coat. This can be avoided by using low bake lacquers & good precleaning cycle using 1-3% sulphuric Acid as an activator, followed by Alkaline Copper of 10 microns, then followed by your required plating.
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