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3D Printing in a Vacuum

Electrostatics plus "vacuum welding" produces sturdy metal copies
  (+5, -2)
(+5, -2)
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In the early days of Space Exploration, it was discovered that metal parts exposed to vacuum sometimes became stuck together so thoroughly they might as well have been welded. Closer inspection revealed that "welding" is quite a good descrption of what had happened to those parts. The behind-the-scenes explanation goes something like this:

In the interior of most solid substances, atoms are usually linked/bonded to other atoms. At the surfaces of those substances, however, there are almost always atoms with "dangling bonds", potential links that have not been fulfilled, because the body of the substance ends at its surface. However, on Earth we tend not to notice this much, because the atmosphere provides plenty of air molecules which can become loosely attached to those dangling bonds. That means when we put two separate pieces of some substance together, the attached air molecules prevent the two pieces from forming bonds directly between them.

There are a few exceptions to that general description. Gold, a fairly non-reactive substance, does not form strong connections with air molecules, and so two pieces of gold leaf, when hammered together, will let escape what little air has attached, and will indeed form direct bonds between the two leaves, quite literally becoming welded together (and at room temperature, too!).

Well, in Outer Space there is no atmosphere, and what do air molecules tend to do in a vacuum? They disperse. This means that when we send something to Space, the air molecules that are loosely attached to its surface can escape in the vacuum, leaving dangling bonds exposed. And so two metal parts can potentially become "vacuum welded" together. Obviously we now tend to send stuff to space that is either made of materials that have very few dangling bonds at their surfaces, or is coated with something that ties down those dangling bonds, to prevent vacuum welding.

Nevertheless, it is widely known than a phenomenon which is troublesome at various times and places can be useful at other times and places. There is a recently-developed and still-being-developed technology which is known as "3D printing" (see link). One simple way to describe it is to say that the "print head" of the device can move in 3 dimensions, depositing particles of the working substance, which are fused together at the point where the print-head is currently positioned. One layer at a time, an overall 3-dimensional object can be constructed from the fused particles.. There are variations on that theme (such as the print head moving in only 2 dimensions and the workpiece being moved in the 3rd), but result is the same.

If 3D printing has a Holy Grail, it is the direct production of metal parts. Currently most 3D printers work with plastics of one sort or another, because particles of plastic are easily fused at modest temperatures. These parts are suitable for making molds for casting metal parts, and a new industry of "rapid prototyping" has been born because of the versatility of 3D printing. But if the technology could directly produce metal objects, that will be the day it reaches the Big Time.

So let us put our 3D printer in a vacuum. Let's design it so it uses electrostatics to manipulate dust particles, because electrostatics can handle a far greater range of materials, and with greater simplicity and efficiency, than can electromagnetics. Depending on the substance we plan on manipulating, we might find it convenient to grind or otherwise blast the dust off a solid block, while in the vacuum, thereby directly preventing any air molecules from attaching to the surfaces of the dust particles. Note that if we simply imported dust into the vacuum, we could have to wait a considerable time for the dust to "de-gas", before it could be used in the 3D printer. And if the imported dust fuses (vacuum welds) before the printer can use it, that could be a problem, too!

Each dust particle is given an appropriate electrostatic charge (the more mass, the greater the charge needed), and then electrostatic fields are used to manipulate the particle, guiding it through the print-head onto the part of the printer where the particles are expected to automatically vacuum-weld themselves together. There are a couple of special issues which now need to be discussed.

First, the part of the printer where the dust arrives (the "initial working surface") needs to be constructed of something that will not interact with the dust. Obviously we don't want the finally-constructed object to be welded to the printer! There do exist various quite-inert substances which can fulfill that need, though.

Second, the electrostatic charge on the dust needs to be removed, as it arrives. (If it wasn't removed and accumulated, then that electrostatic charge would greatly interfere with the arrival of more dust.) For an object being constructed from metal dust, this is a quite simple thing to do. The printer simply needs at least one wire extending from its body to the initial working surface. The first particles of dust that arrive should be aimed at that wire. They vacuum-weld to it, and as more dust arrives, the first layer can be built up, having electrical continuity throughout. The electrostatic charge is removed, and perhaps even reversed, using that wire, of course. (Reversing the charge will accelerate the arriving dust so that as it impacts, the probability that it vacuum-welds goes up a lot.) If the workpiece has a complex shape it may be necessary to begin with more than one of those discharge wires. When the print-job is finished, and the workpiece is ready to be removed, we simply snip the wires as close as possible to the workpiece (perhaps by using laser beams), and the wires can be re-used for the next thing to be printed.

Added Nov 15, 2007
It occurs to me that the very first layer might simply be a flat piece of metal that has had its shape stamped out of large flat piece. Then we don't have to worrry about interactions between the dust and the body of the printer, because we can immediately start adding to the metal of that first layer.

Vernon, Nov 12 2007

More info on 3D printing http://fabathome.or...php?title=Main_Page
As mentioned in the main text [Vernon, Nov 12 2007]

Explosive forming of metal http://www.fsb.hr/d...ng%20of%20Metal.htm
As mentioned in an annotation [Vernon, Nov 12 2007]

Rapid prototyping http://en.wikipedia...i/Rapid_prototyping
sintering etc. [bhumphrys, Nov 14 2007]


       [bigsleep], electrostatics has NO problem manipulating dust particles in a gravity field. I did not mean to imply that this printer needed to be in Outer Space; I only specified a vacuum. It needs to be a quite-good vacuum, but we CAN do that here on Earth.
Vernon, Nov 12 2007

       The problem is the small contact area between the "dust" particles. Regardless of how large they are, the proportion of their surfaces which touch eachother will presumably be very small. If the particles are rigid and don't yield on contact, the contact areas will be a very tiny proportion of the total area. I don't think there will be much "yield" in the system you describe. So, what you are going to wind up with is effectively a very weakly-sintered material. Even if the particle-to-particle bond is as strong (at the point of contact) as the within- particle bond, it is going to be over such a small area as to be very weak.   

       3D printers of the type you describe work by extruding molten plastic (or sometimes other materials) so that there's a reasonable contact area between one blob and the next.
MaxwellBuchanan, Nov 12 2007

       No, it's not. Aside from the fact that "While cold welding is real, an unqualified claim that "in space metals stick" should be treated as an urban legend.", there is a requirement to compress the structure to get a decent contact area. This won't work.
MaxwellBuchanan, Nov 12 2007

       I think the idea as posted was to use the vacuum 3D printing to create finished articles out of metal dust, which won't work.   

       When you say "making a cast first", do you mean making a template from which replicas may be made by casting? If so, there's no point making it in metal - you want it in wax or plastic.
MaxwellBuchanan, Nov 12 2007

       This is the phenomenon that was advanced as the explanation for gage blocks "wringing" together. Air molecules do not appear to inhibit this phenomena, if that is truly the explanation.   

       edit: spelling corrected, thanks [MaxwellBuchanan]
normzone, Nov 12 2007

       Sp: phenomenon.
MaxwellBuchanan, Nov 12 2007

       [MaxwellBuchanan], if what you say causes a problem (which should be lessened by what I wrote about reverse-charging the workpiece to increase the impact speed of dust particles), there is an additional trick that could be invoked, after the workpiece is finished. This trick is a variant of something called "explosive forming of metal" (see link).   

       In this application, the trick would require the workpiece to be made a little larger than its final intended size (the exact percentage would be something that experimentation would quickly reveal).   

       Basically, you place the otherwise-finished workpiece in a liquid in a mostly-spherical container, and then simultaneously detonate a number of small explosive charges inside the surface of the sphere. The shock wave compresses the whole workpiece equally from all directions, squeezing out a lot of that empty space that you find worrisome. Re-exposure to vacuum would allow additional vacuum welding to occur, of course.   

       I suspect though, if the impact speed of the dust was simply high enough, the problem you describe would not exist in the first place. I suppose we might call it a "warm weld" that occurs during the impact, in a vacuum, in that situation.   

       [normzone], it could be that one of the material properties of those gage blocks is similar to what I described for gold, one of the exceptions to the general rule regarding air at the surfaces of most substances. There is also a possible "smoothness" factor, because two bumpy surfaces in contact will have fewer points where they can weld together than two smooth surfaces --and gold leaf is EXTREMELY smooth (I've read that a leaf can be something like 4 atoms thick, not much room for bumps there!). So also are gage blocks smooth, no?
Vernon, Nov 12 2007

       //I suspect though, if the impact speed of the dust was simply high enough, the problem you describe would not exist in the first place.// I really don't think it would. You need to get enough kinetic energy into the collision to cause significant deformation, and the mass of each particle is very low.
MaxwellBuchanan, Nov 12 2007

       [MaxwellBuchanan], electrostatic forces are ten million billion trillion trillion times stronger than the force of gravity, and those forces have been used to accelerate atoms to nearly light-speed. I have no doubt we could use those forces to accelerate dust particles to an adequate impact-weld speed. That might make the notion of "vacuum welding" moot, but it will certainly allow the direct 3D printing of metal parts!
Vernon, Nov 12 2007

       //electrostatic forces are ten million billion trillion trillion times stronger than the force of gravity// That is a bit misleading. The gravitational attraction between two electrons is indeed that much weaker than the electrostatic force between them, but you're unlikely to get a piece of metal dust made out of pure electrons (or with no electrons). However, granted that you can electrostatically drive particles to high speeds.   

       //That might make the notion of "vacuum welding" moot// Well, fair enough - but then the idea should have been "impact sintering". I think the vacuum welding concept is a bit of a red goose.
MaxwellBuchanan, Nov 12 2007

       [MaxwellBuchanan], if I removed the word "billion" from the phrase "ten million billion trillion trillion", then the resulting comparison between gravity and electrostatics would be misleading the other way (two singly ionized U238 atoms have greater mutual electrostatic force than gravitational force, than "ten million trillion trillion times"). Possibly both of the "trillions" could be removed, and two dust particles with a single extra electron each likely would electrostatically interact more strongly than "ten million billion" times than the gravitation between those dust particles. Remember that current vacuum technologies such as Chemical Vapor Deposition is evidence that we can make arbitrarily small dust particles. I therefore submit YOU are being misleading, in trying to specify difficulty in accelerating dust to deformation-at-impact speed, when you have no data to support that.   

       Next, a vacuum is still needed if one wishes to accelerate very tiny dust particles to high speeds. So while the original definition of "vacuum welding" may not be fully applicable, this Idea still involves "welding in a vacuum". Furthermore, at the moment a fast dust particle makes contact with the workpiece being printed, to whatever degree ordinary vacuum welding takes place then, we may be able to imagine this holding the particle in place while the rest of it deforms into final position. Which means that the original definition could still be partly applicable, after all.
Vernon, Nov 13 2007

       Sure, no problem.
MaxwellBuchanan, Nov 13 2007

       "The name's Bond. Dangling Bond."   

       Vern, your sheer eclecticism never ceases to impress me. I have no idea if this is feasible and/or plausible, but once again you've taught me something I didn't know. [+]
lostdog, Nov 13 2007

       Yes, but at high speeds every atom (or particle) knocks off three others, unless you are going to weld after every single placement. Extremely time consuming.   

       For that mater, direct printing of metal parts is currently possible using laser sintering. This uses a layer of metal particulate and a laser to fuse the desired areas in that layer. The fused layer is then lowered and a new layer of particulate is smoothed over top of it. At the end you clean off the loose particulate and you have a finished, sintered part. This is reasonably fast and energy efficient, although higher speeds (and larger particulate) of course produce lower resolution.
MechE, Nov 14 2007

       [MechE], the goal here should ultimately be a beam of metal dust particles, each accelerated just fast enough to deform on impact without splattering or making a crater in the workpiece. I suspect that experimentation with particle sizes and speeds should be able to find something that will work nicely. The not-knowing of that in advance, of course, is what makes this notion half-baked. And of course, the "heavier" the beam particles or beam-density, the faster the print-head can move to print the object.
Vernon, Nov 15 2007

       Vacuum vapor deposition. Vaporize the charged metal (anode) with a laser (or sunlight concentrated with a parabolic mirror) in vacuum, have the target mold (cathode) reasonably close, with a mask in between to control where the vaporized metal is deposited.   

       I suspect it will be one of the standard means of fabrication in lunar factories.
whlanteigne, Feb 03 2013

       "So also are gage blocks smooth, no?"   

normzone, Feb 03 2013


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