Metal finishing Q&As since 1989

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Taper Locks of mold cracked after chrome plating

May 22, 2008

I am a plastic injection mold maker. On our mold, there are male and female taper locks (58 diameter). The taper locks are 52 HRc. Over the years, the tapers has worned and scored. We have cleaned the female side by grinding off until rust and distortion is removed. Then we did the same on the male side of the taper and hard chromed to build it back up. Before putting back in to the mold, we inspected to ensure the preload is as per the original mold specification. Few days later,the tapers has cracked (on the chrome side) from the chrome side and through the steel. Questions:
* What are the potential reasons to cause this failure.
* Does hard chrome not ideal for build ups where there is constant loading
* Does the hard chrome initiates higher steel brittleness during the chroming process
* What is the maximum thickness of hard chrome can be applied on each process.
* Is there minimal chrome composition required during the hard chroming process

Thank you

Kenneth Kang
Mold Maker - Korea

YOU CANNOT ASSUME A PROPER FIT BY CHROME PLATING. REPLACE THE LOCKS WHEN THEY ARE WORN OR REGRIND THE LOCKS AND SHIM THEM UNDER LOAD.

JACK LEMKIN
- COLUMBUS, Ohio, USA
May 29, 2008

The most probable reason your parts cracked is hydrogen embritlement. Also, please note that thick chrome is always cracked due to its own high internal stress and low ductility. These cracks act as stress risers and tend to propagate to the base metal.

Guillermo Marrufo
Monterrey, NL, Mexico
June 1, 2008

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The cause is almost certainly hydrogen embrittlement from the plating process. Your description doesn't mention after-plating treatment to reduce that risk, and with a hardness as high as 52 HRC the steel is extremely susceptible.

Bill Reynolds [deceased]
consultant metallurgist - Ballarat, Victoria, Australia
We sadly relate the news that Bill passed away on Jan. 29, 2010.
June 2, 2008

With the cost of plating and grinding , Why don't you just replace the locks , If you want good registration between Cavity and Core sides new is the only way to go on this one Plating isn't made to repair everything,Although it can at times be the only way to repair certain things, but not here.

Henry Wallman
Toolmaker - Eagle River, Wisconsin, US
July 4, 2008