Letter 18011

Flash copper undercoat prior to hard chroming [India]  

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Hi! I would like to know whether an undercoat of copper plating from a rochelle salt bath(15-25 microns) on plastic inj. molds and dies(working surfaces) with subsequent diamond lapping and finally hard chroming the molds would tolerate injection molding conditions. The reason I'm trying this is to "fill up" surface defects but at the same time have to adhere to mold profile parameters.

Thanks!

Solomon Mordecai
- INDIA

Personally I don't think it's a good idea. Good adhesion onto copper is possible, but it is more problematic than adhesion on steel. Plus, depending on the chrome thickness you might have an eggshell effect.

Ted Mooney, P.E.  finishing.com Brick, NJ

Thanks Ted!! Just wanted to know .. whats an "eggshell effect"? as in peeling due to the shearing forces? I planned this out for faster turnarounds for molds and dies reducing process timings.

Solomon Mordecai
- INDIA

Picture the shell on an egg; it is rather hard. But its hardness is of limited value because the egg is soft. Attempts to put the shell's hardness to use--using an egg as a hammer, for example--tend to be unsuccessful. A thin plating of chrome on a soft substrate may be of similarly limited value.

Ted Mooney, P.E. 
finishing.com
Brick, NJ

Molds and dies are usually made of alloyed Cr-Ni steel so the surface has to be activated to get good adhesion of any plating. This is better assured using acidic nickel formulas like Wood´s strike. Then we recomend to plate w/ more nickel (from sulfamate or all sulfate w/conforming anodes)instead of Cu since it will also polish very fast and is harder so it avoids what Ted calls eggshell. Copper will only be good for small very deep pitts or scratches when a large area of the chrome will not be backed by it. I personally think of it as a sheet of glass on a bed. Can't lay on it.

Guillermo Marrufo
Monterrey, NL, Mexico

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