Engineering details might seem dull or trivial, but they can make or break us. There won't be any ISRU and bootstrapping on the Moon without metal casting. Three dimensional printers that use lasers or electron beams (which require vacuum, free on Luna) to fuse powdered metals, glass and/or ceramic into small parts that can fit in the hand and medium size parts with dimensions of one or two feet will be used to produce all sorts of parts on the Moon. Barring the creation of giant 3D printers we will need to cast large metal parts on the Moon like vehicle chassis frame members, axles, wheels, struts, etc. When we can we will use rolling mills and/or extruders to make things, but we might want to cast up the rolling mills and extruders with lunar materials rather than ship these massive devices to the Moon! Aluminum and magnesium are often sand-cast but they might also be cast in plaster molds with plaster obtained by sulfuric acid leaching of highland regolith. Plaster, CaSO4, is also a cement setting time retardant and Portland cement contains up to 5% of this compound by weight. Iron, steel and iron alloy (e.g. iron-aluminides, iron-silicon, iron-manganese, iron-nickel) casting is going to require sand molds. Foundry sand, also called green sand, is made of either silica or olivine sand, two substances we have on the Moon, water that can be obtained in various ways and rigorously recycled, bentonite clay and sometimes pulverized coal (forget coal on the Moon). We will have to do casting in pressurized chambers with dehumidifiers to recover water vapor from the drying sand mold and powerful cooling systems because it will get very hot inside when working with molten metals and interior pressure will increase as temperature increases. Casting chambers will be built of metal and concrete, not Kevlar! Here's the detail I am hung up on; there is no clay on the Moon because there was never any water for the hydrological processes that produce clay to occur. Clay is essential to bind the sand mold. We might be able to synthesize clay on the Moon. At: w w w.patentstorm.us/patents/6565643/description.htm l we read that, "U.S. Pat. No. 3,803,026 describes a process for preparing a clay-type material in which an amorphous gel comprising silicon oxide, aluminum oxide, and, e.g., magnesium oxide is subjected to a high-temperature ageing step in an autoclave. C.R. Acad. Sc. Paris 7 292 describes a process for preparing, int. al., clays comprising aluminum, silicon, and, e.g., magnesium by way of a co-precipitation process." Silicon, aluminum, magnesium and their oxides exist on the Moon. I suspect that it will be more practical to synthesize clay on the Moon than upport it from Earth! It is also possible to bind foundry sand with resin. At http://en.wikipedia.org/wiki/Shell_molding we read: ""Shell molding, also known as shell-mold casting, is an expendable mold casting process that uses a resin covered sand to form the mold. As compared to sand casting, this process has better dimensional accuracy, a higher productivity rate, and lower labor requirements. It is used for small to medium parts that require high precision. Examples of shell-molded items include gear housings, cylinder heads and connecting rods. It is also used to make high-precision molding cores" The Wiki article states that this is used for small to medium sized parts so we might still need clay for large part casting. Resin could be made in lunar labs and we would need some way to recycle it possibly by leaching unused resin out of spent molds with an organic solvent and by extracting vaporized resin from air and CO2 from burnt resin. Resin will contain rare lunar elements like H, C and N, and we cannot afford to waste these. If we cast in chambers with inert gas filled interiors the resin won't burn to form CO2 and other gases. If we can synthesize clay on the Moon, and i suspect that we can and must, and do it cheaply, then we could even work clay and fire it with electric furnaces into pottery and other desirable items. That would be a nice added benefit. An unconventional approach to metal casting on the Moon might be the use of microwave sintered regolith molds, but data is lacking. This should be investigated.