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topic 61236

Choosing a plastic for nickel plating installation

April 16, 2021

Q. Small shop is looking to experiment with electro plated diamond abrasive tooling. Why is PP the choice material for tanks? Would HDPE be acceptable for small scale plating tank? Glued or threaded PVC pipes are OK for pump/filter and circulation?

Mitchell Reep
- Minnetonka Minnesota
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April 2021

A. Hi Mitchell. Your specific questions are easily dispatched, but because this will be read by many other readers, let's cover some basics before follow-up questions start ... Several plastics are highly chemically compatible with general plating shop applications, and widely used, including polyethylene, UHMWPE, polypropylene, PVC, teflon, and vinyl ester fiberglass. But most plastics severely soften with temperature, and that often plays a role ... and some fabrication methods are better suited to some types of plastics.

Both of these issues are highly relevant to your situation. PVC and polyethylene both soften pretty substantially in the 140-150 °F range whereas polypropylene is generally considered good for at least 180 °F, and fiberglass and PVDC (CPVC) even hotter. Polyethylene is economically blow molded for small tanks, but for larger tanks polypropylene is more easily fabricated as well as offering greater temperature rigidity. Polyethylene & polypropylene are highly flammable, whereas PVC is not, and use of electric heaters in PE & PP tanks has without question been the largest cause of plating shop fires. Here's the scoop on polyethylene tanks: they are sold as a competitively priced commodity so they are usually manufactured "just rigid enough". So for round tanks, usually no problem. But for rectangular open-top tanks other than rinse tanks, the higher specific gravity of the chemical solutions, plus their temperature, plus mounting things like anode rods, filters, and heaters on the tank rims make those blow-molded tanks "iffy". Further they're usually thin and cheap and tend to bow out -- the bowing may not mean there is imminent danger of them bursting, but when the contents are cold water nobody worries, whereas when it's hot dangerous chemicals they do :-)

In conclusion, if the tank is bigger than a breadbox (love those old time sayings) I personally would have a plating equipment manufacturer fabricate it from polypropylene rather than use a polyethylene tank, but for the rinses I might accept PE. PVC pipe whether glued or threaded should be okay for 140 °F nickel plating solution, but not for hotter tanks like cleaning tanks -- there you'd want PVDC.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Aloha -- an idea worth spreading
finishing.com - Pine Beach, New Jersey
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