Powder bed 3d printing

Kragen Javier Sitaker, 2015-09-11 (updated 2019-05-28) (10 minutes)

I just wrote this long thing in Flux deposition for 3-D printing in glass and metals about a powder-bed 3-D printing technique that deposits a binder that’s completely inert at room temperature, but upon firing the print in a kiln, becomes active.

I think there are a variety of other possibilities in powder-bed 3-D printing that have not yet been fully explored.

Magnesium oxychloride Sorel cement

Sorel cement is a combination of highly water-soluble magnesium chloride (nigari) with highly water-insoluble magnesium oxide (milk of magnesia); it’s a cement similar to Portland cement, but more refractory, less water-resistant (and won’t harden underwater), and nearly twice as strong.

So, although I’d have to investigate more, I think you could use an aqueous solution of magnesium chloride to moisten a powder bed of sand and dry magnesium oxide to form a very strong mortar.

Selective hammering

Instead of squirting binders onto a powder bed like an inkjet printer, you could bang the shit out of it with hammers like a dot-matrix printer, ideally under vacuum so that you don’t generate explosive gas expulsions. The impact will stick together the particles in the vicinity, affecting a total mass of powder material similar to the total mass of the hammer. (This suggests that low-mass hammers are in some sense optimal.)

Selective electrical sintering

For beds of metal particles, instead of squirting binders, you could touch the surface of the powder with an electrode and drive a large current into it, sintering the nearby particles together through joule heating of their contact points, like an old-fashioned coherer.

The electrode would probably have to be a carbon rod, since any other plausible material is likely to stop working due to surface oxidation.

This probably won’t produce a strongly bonded part, but might be enough to produce a solid part that can then be solidified further by other means.

Cross-linking with calcium or other polyvalent cations for cement precipitation

A number of anions, such as phosphate, carbonate, and alginate, form water-soluble compounds with monovalent cations like those of the alkali metals (sodium, potassium) and ammonium, while forming water-insoluble compounds with divalent cations like those of the alkaline earth metals (calcium, magnesium). Calcium and magnesium also have highly water-soluble salts, such as their nontoxic chlorides. Phosphate is also water-soluble in the form of phosphoric acid.

This means that by mixing two liquids you can precipitate a solid through a double ion replacement reaction. This is used in molecular gastronomy spherification of foods, forming a flexible calcium alginate membrane around a liquid center with sodium alginate dissolved in it.

(I’m pretty sure this is because these anions are polyvalent and are strongly enough bonded to their cations that they are solvated together with them, rather than separately, so that once the cations are also polyvalent, the individual anions floating around with their individual cation harems are replaced by endless chains in which each cation links together different anions. But I’m no chemist.)

Other polyvalent cations, like Cu₂₊, Zn₂₊, Fe₃₊, and Fe₂₊, should also work for this. Most of these also have relatively innocuous water-soluble salts; ZnCl₂, Fe(NO₃)₃, Cu(NO₃)₂, FeCl₃, as well as blue, white, and green vitriol, of course, which last are innocuous enough to use as nutritional supplements, but are subject to onerous reporting paperwork in places nowadays; acetates of calcium, magnesium, copper, zinc, and ferrous iron (II) are also all soluble, though zinc only a bit, and ferric iron (III) not at all. Ferrous citrate is also soluble.

So the plan is that you precipitate a solid cement in the interstices of an aggregate or filler, such as quartz, carbon black, fumed silica, chopped carbon fiber, chopped basalt fiber, powdered graphite, powdered copper, powdered silver, hollow glass spheres, hollow steel spheres, chopped cellulose fiber (such as sawdust), clay, or a mixture. Different possible resulting cements include the following; I’m including Mohs hardnesses as an imprecise but readily available and roughly accurate guide to strength:

So you should be able to get relatively high strength, almost as high as portland cement (whose strength comes mainly from belite, which is known as larnite in nature, Mohs 6), by precipitating calcium phosphate crystals from a water-soluble calcium salt such as calcium chloride and a water-soluble phosphate salt such as monoammonium phosphate; you may be able to get a highly refractory bond by calcining the phosphate or carbonate of magnesium into magnesia; you can get an instant nontoxic aqueous elastomeric gel with calcium alginate; you can get biocompatibility (and guaranteed working recipes) from zinc and magnesium oxides with buffered aqueous phosphoric acid; and there are thirteen other combinations that will probably work as well.

Further alternative polycations might include nickel, mercury, and vanadium ions, but these have some disadvantages (carcinogenicity, higher toxicity) and not much in the way of available information. Further alternative polyanions might include sulfate (which does have some insoluble salts, notably calcium sulfate (gypsum) and barium sulfate), oxalate, silicate (see below), sulfide (soluble with lithium, sodium, and ammonium, but should precipitate transition metals) and perhaps some carrageenans.

Iron sulfide in particular — fool’s gold — is 6–6.5 on the Mohs scale, harder than apatite. It has the disadvantage of gradually oxidizing in air, though, with corrosive results, and of course the soluble sulfides are toxic.

It may be desirable to prevent homogeneous nucleation in order for the cement particles to be big enough to bridge the gaps between grains of filler. For of these most cements, if the temperature is kept high enough, cement particles will only nucleate on the surfaces of grains of filler; this may help to produce a solid mask. (More speculatively, pressure control is another possible lever to control nucleation, but this would probably require a liquid-filled chamber.) It may also be possible to solve this problem by making the precipitation mass-transport-limited.

Filler particles with more extreme aspect ratios — clays such as bentonite being the champion here, though a less expansive clay may be more practical for this use — should lower the critical percolation threshold needed to form a solid mass, thus placing less stringent demands on the nucleation process.

Densification may be needed after the initial precipitation, since when the cement precipitates from solution, the water and other solute remain. (For example, if reacting aqueous dipotassium phosphate (which dissolves 150 g per 100 mℓ of water) with calcium chloride to produce hydroxyapatite, you have potassium chloride and water taking up space in the result.) Densification can be carried out by passing a supersaturated solution of the same cement, or a compatible cement, over the printed object once it is removed from the powder bed; or it can be carried out by infusing the pores with a different material, perhaps a melt, again after powder removal.

Bicarbonate as a hydroxyl donor

Cyanoacrylates polymerize in the presence of hydroxyl ions; dripping cyanoacrylate onto NaHCO₃, stealing hydroxyl ions and converting it to sodium carbonate, is a well-known manual additive manufacturing technique which can probably be improved by adding filler to the NaHCO₃.

Bicarbonate as a CO₂ donor

Waterglass (sodium or potassium silicate) forms a silica gel rapidly upon exposure to CO₂; maybe you can use NaHCO₃ as a CO₂ donor for this purpose. Certainly you can harden it with acids instead.

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