Victor – ...With 3D printing on the moon first step will be to separate fractions of a lunar dust. For such porpoise can be used low gravity, vacuum conditions and old vacuum tubes technology. Cathode's material under the heat (as it was in tubes) emits electrons. Anode on some distance from cathode, over regolith (I believe long time ago was an idea to use cathode-anode as a solar battery and theoretical efficiency was pretty high, and heavy) accelerates electrons and part of electrons beamed to a surface of regolith, after some period of charging, charging process stopped, and another plate (charges negatively) placed over regolith, acting as a capacitor’s plate (all of this is from 10 grade physics’ practice book, I believe). Depend of a voltage on a plate, smaller fractions of duct will levitates from a surface, and distributes by weight vertically. Last step - another positive charge applying by tore shaped collector will discharge particles of regolith. Lowing or elevating collector (or by changing voltage of the “capacitor” plate - preferably) will select different fractions. Actually it does not matter how to harvest fraction - static charges, and known formulas can apply. If some fraction will be suitable to make practical object (glass will be first to try) the same principle can apply to a way of delivery dust particle to a surface of printing layers (it will be less mechanical parts in 3D printer).
Surprisingly some of old “school’s” physics bring modern interests – for example charged, same sign, 0.1-100 micron particles, in some conditions can attract each other instead of commonly believed action. http://journals.ioffe.ru/jtf/2010/05/p75-79.pdf
Compact version of useful formulas are in: http://genphys1.phys.spbu.ru/People/Karasev/docs/meth2.pdf
Effects on charged dust particles on Moon - http://nuclphys.sinp.msu.ru/school/s10/10_01.pdf
Effects on charged dust particles on equipment: http://220.127.116.11/ft/7938/739975/13859267.pdf
Plasma with injected 5-60 mkm particles: http://www.ebiblioteka.lt/resursai/Uzsienio%20leidiniai/ioffe/ztf/2004/11/ztf_t74v11_24.pdf
Despite of your “contra” about complication of metallurgy process on the moon – and you correct point that this is not our priority today - it is known that aluminum is like “accumulator”, it accumulates energy. Circle – (a) day time producing Al from Al2O3 and (b) night time producing from Fe2O3 +2Al -> heat + 2Fe0 + Al2O3 solves a problem - technological process can work all time without nuclear source.
Surpluses oxygen with aluminum by itself is perfect solid (Al) + liquid (O2) rocket’s fuel+oxidizer.
Can you answer me how that aluminothermic reaction will work not on a tank’s plates welding but on a kainda micro-level – in vacuum, 25 -50 micron size Al and similar size Fe2O3 intersect each other at specific point where controlled heat can apply? It will be nice to make such experiment, today on earth, in big vacuum chamber – but no time, – can you answer what it will be at least theoretically?
And with 3d printing - look what did Markus Kauser : http://www.thisiscolossal.com/2011/06/markus-kayser-builds-a-solar-powered-3d-printer-that-prints-glass-from-sand-and-a-sun-powered-laser-cutter/