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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?
hobbyist - Trowbridge, Wiltshire, UK
2005
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
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
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