Wood's metal is an alloy with a low melting point, about 60 degrees C. A lot of information on Woods metal can just be found on WikiPedia:
Wood's metal contains cadmium and lead, and therefore is poisonous. However, there are a variety of alloys with similar low melting point that do not contain cadmium.
A few things that I found so far (September 2014)
- non-toxic are field's metal (62 C) and cerrolow 136 (58 C)
- cerrosafe (74 C) is said to be specifically suitable for machining: this is probably because on solidification is first shrinks a little and then slowly expands again. after 1 hour it has almost exactly the same size as in the molten state
- cerrobend (basically Wood's metal) expands on solidification by about a factor 1.006
- solid field's metal has a thermal expansion coefficient that is very close to Aluminum (22 x 10^-6)
Where to buy
Wood's metal is sometimes sold under the name 'Cerrobend'. Mari form VDL pointed us to these companies:
For Field's metal I found this company:
In the end we have ordered from Lowden Limited 1kg of Fields metal and 1 kg of Cerrolow 136. These are less toxic.
How to decide on what to use?
Cerrolow 136 (58 °C, 8.5 kg/l, 1.7kEuro/liter) is about half the price of Field's metal (62 °C, 7.88 kg/l, 3.4kEuro/liter).
What do we need to look at to decide which one we use?
- shrinkage expansion after solidification: Clow136 skrinks 2x10-4. For a thickness of 2mm, that is less than 1 micron, so probably irrelvant. For Field's metal, we don't know.
- expansion coefficient? Both should be very close to Al (22-23x10^-6/°C).
- melting temperature/conductivity? (how warm could foil become while machining?)
- fluidity: how easily does it flow? does it have sufficient capillary action to make a good film between foil and mold?
- Al affinity: does it stick well to the Al during milling in its solid phase? can we easily get it off again in its liquid phase?
- recoverability: stuff is rather expensive (count 5 liter for final box?). how well can we recover it?
- toxity: Field's metal does not contain lead and less bismuth