Robbers & Current distribution - better plating

**woodjames** · 09-28-2009 09:52 PM

You'll find that during plating, the deposit is not very even, due to the throwing power of the bath. When a piece is set with the current too high, these areas are prone to burning as the majority of the current is accepted at high points, edges, points, areas closest to the anodes, etc... You can use robbers to absorb some of the deposit, so that overall the plate on the piece is more even, and help prevent burning. This isn't readily apparent until one tries to sand or buff a copper plate, and the low current density (lcd) areas are quickly buffed/sanded through, whereas quite a bit of time can be spent on the high points, because the deposit built up thicker there. This also becomes unacceptable in respect to longer plating times, often encountered when a piece has pits/scratches in it, such as when more time is spent in the acid copper bath to build up more thickness to make the repair. When left in too long, too much builds up in high current density areas (HCD's), and coral shaped deposits begin to grow. Robbers take away some of that deposit, instead of it all being focused on HCD areas.

Here's a step by step pic of correcting this problem. every situation is different, this piece in particular seemed to provide a good example of HCD areas.

Usually I polish before placing an object in the bath, however in this case, being a relatively simple shape, without any deep recesses, it'd be obviously easier to buff copper than steel (the base metal). It's been ground and sanded to remove and pitting/metal stretch marks, then sand blasted. I've already put the item in Flash Copper for 30 minutes. Flash Copper has excellent throwing power, and it's purpose is just a strike coat, so current distribution/uneven build isn't of great importance.

It may not seem too extreme in the photo (nickel for scale), but all the areas highlighted in red will attract most of the deposit. I can't give you a percentage, but it'll be enough to disturb how even the thickness of the deposit is, which also affects corrosion protection.

The piece is now ready for acid copper. The plating time will be longer, to build up enough thickness to be able to buff away the texture from sandblasting. It's difficult to set up robbers without a rigid method of securing the item in the tank. In this case I used some scrap copper pipe, tapped a 1/4 x 20tpi hole in it, and bolted the piece right to it. The piece eventually gets a bolt covering this hole when it's installed on the car. It's okay in this case to fasten this way, as normally, this method will leave some "evidence" and be noticeable after the brighter subsequent baths. Directly behind the work piece, I bent a 'U" shape in the robber wire, and when I tightened the piece down to the bar, it secured the robber wire at the same time. The object is to conform the robber wire near to the areas that will normally accept more deposit, so that it will limit the amount, in this case copper; that is deposited on the workpiece in that area. You can see how it's secured and bent to shape in the photo. There are other ways to secure the wire, and other shapes can be made, but this is what works best for me.

The last photo is the piece secured in the tank. Often times I flatten and bend the end over (that is coming out of the tank) so it mates and rests on the pipe running across the tank. However, it can be very difficult to rinse the plating solution out of the tube without two open ends. A solid bar would be a better option, but more expensive.

Understand more.
-Jimmy