3D Printing in a Vacuum

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.

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Added Nov 15, 2007
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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.