From: REMOVE_THISdwilkins@means.net (Don Wilkins)
Newsgroups: sci.chem,sci.engr.chem,sci.chem.electrochem
Subject: Re: Electroforming process
Date: Wed, 04 Feb 1998 11:45:37 GMT

On Tue, 03 Feb 1998 21:39:01 +0800, Chan Lai San
<chanlaisan@pacific.net.sg> wrote:

>Is there a way to control the current distribution for a
>electroplating/electroforming process ?
>
>My experience is that there is a variance is weight ( between different
>parts within the same batch )of the products due to uneven current
>distribution.
>
>I understand this uneven current distribution is quite normal for
>electrolytic process. But is there is way to minimise and control it ?

The plural is more appropriate. There are ways and you should do some
research.

Uneven current distribution is quite normal. For example with two
parallel plates as electrodes there is very little plating on the back
side of the cathode. How would you get around this? Why not use two
plates for the anode one on each side of that cathode? Yes that works
so that is one solution. Electrode placement is important and is used
extensively. The shape and placement of these anodes and the number of
anodes is important particularly for complex shaped cathodes.

With the two plate anode suggested above you still have a problem at
the edges. you will have current flow from both anodes to the edge
whereas there is current flow predominately from only one electrode in
the middle of a flat plate. You could curve the anode so the distance
from the anode to the cathode was greater at the edges, you could
perhaps make the anode plates a bit smaller, or you could place an
inert (not connected) electrode in the current path. (called a thief
in electroplating lingo). You might place a dummy cathode near a place
where there is high current density or a dummy anode where there is a
low current density. With complex shapes the placement of electrodes
is critical for uniform deposition.

You can complex the cation to be deposited so that there is a high
concentration  of metal available near the electrode surface but a low
free ion concentration near that surface. This is why many plating
baths use  e.g. cyanide to complex metal ions. Stirring the solution
so as to replenish the deposited species is also important. All sorts
of things happen when the metal to be deposited is depleted in one
area and not in another. All of those things that happen usually do
not contribute to a good deposit.

You can use pulsating and/or current reversal techniques to even out
the plating. Just as the points tend to plate the heaviest deposits
during deposition they also go back into solution faster during
current reversal. This tends to smooth the surface.

You will find much more in the electroplating literature. It ain't a
subject to discuss in detail in a newsgroup note.