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Fusable alloys

What is the best Fusable alloy for copper plating? The object is to form the alloy then plate the part. After plating you melt the alloy away leaving the copper shell. I am seeking the best fusible alloy.

Roy Bentley
R.B. Mfg - BC, Canada
2005

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It sounds as if you are trying to do electroforming. Fusible metal mandrels are well known and were quite common some years ago. However, in lots of applications they have been superceded. You have adequately described how to use fusible alloys, but perhaps you may like to consider using waxes, plastics or low melting point metals. If you use waxes, you will need to coat them with powdered graphite as their surface is extremely hydrophobic and hence difficult to wet. (Graphite isn't exactly hydrophilic!). There is an electrically conducting wax available in Germany, but I haven't any idea about the supplier; perhaps someone else knows. You can also use metals such as aluminium, tin, and zinc; all of them can be melted out of the electroform and are also soluble in alkali. One down side is that if they are melted, they can form intermetallic alloys with copper and impart unwanted properties into the copper (Note: I am not sure about an aluminium-copper intermetallic). Zinc alloys are also of interest as ones are available that have superplastic properties and can be blow moulded at about 250C. Other low temperature metals include lead, antimony, bismuth, cadmium and indium, but you have to be careful that any electrocoating process does not get near to their melting points! You can also use babbits and solders, with the same caveat. If you use zinc alloys, you will need to give it a flash of Rochelle or cyanide copper as most copoper electroforming is done with acid copper and this is not suitable for zinc surfaces. Finally, in the fusible metals vein, you can use cerro alloys; these are low temperature alloys based on mixtures of bismuth, lead and tin, with other additions such as cadmium and indium. Probably the most well known one is Woods Metal (Bi 50%/Pb 25%/ Sn 12.5%/ Cd 12.5%); this melts at about 70C. There are others that melt at higher or lower temperatures, depending on what your operating criteria are. The major down side of the cerro alloys is that if they are melted out, the bismuth can cause grain boundary embrittlement with both copper and nickel forms.
As an alternative, why don't you consider using something like MIP (medium impact polystyrene)- this can be dissolved out of the mandrel and recovered by distilling the solvent off and reusing the polymer. You can also use ABS and other easily metallised plastics. I would discourage using polypropylene as this is less dense than water and can be quite difficult to remove from the mandrel.

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

Ed. note: Shhh. No hints for David letterman. Will it flo-o-oat, will it float :-)