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Release treatment of stainless electroforming mandrels

adv.  

I am trying to electroform copper and wire reinforced nickel/cobalt/tungsten parts for small model rocket engines. I have made some 304/316 stainless mandrels, and mostly there is suitable relief, but some portions have to be cylindrical - so I need the electroformed part to release especially easily. Any tips on technique?

I have read that bichromate passivation is done sometimes, anything else?

Peter Fairbrother
hobbyist - Trowbridge, Wiltshire, UK
2005

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Dichromate passivation is good. You can also add a very, very thin layer of oil by applying oil dissolved a large excess of evaporating solvent, say 1% oil, 99% solvent. Even with that, it is unlikely you will be able to remove the electroform without some draft.

Jeffrey Holmes, CEF
Spartanburg, South Carolina
2005

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You can often get away with no release layer with stainless steel because the stainless produces its own. However, if you need to enhance it, you can try a dip in sodium metabisulphite solution (say 150-250 g/l) for about 10 minutes. You can also use a dichromate dip for about 3 minutes or an electrolytic (cathodic) dip for about 15 seconds. Similarly you can use permanganate solutions or even sulfides (but this stinks). However, to give easiest release, you need a relief angle of 1-2 degrees, then you can just tap out the mandrel with a hammer or press tool. To ensure good electroforming, you must have a good clean mandrel, so pay particular attention to the pretreatment stages before electroforming starts.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
2005