This question implied that VPP has been evaluated and discarded as a potential zero-G manufacturing method.
That may be true for layer-by-layer resin systems, but what about a two-photon system with a fully enclosed vat? Why wouldn't that work?
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2$\begingroup$ From what I can see about VPP on the web it appears the system relies on gravity to keep the already constructed parts away from the liquid polymer surface so new layers can be created via a laser. In zero G there would be a problem with securing the already constructed parts & keeping it away from the layering interface. There would also be a problem with keeping the surface of the liquid polymer level or horizontal to a plane. $\endgroup$– FredCommented Jul 15, 2021 at 16:23
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$\begingroup$ @Fred the cured resin is more or less the same density as the surrounding liquid, and gravity wouldn't be enough anyway to keep things from moving around during the print. Parts are generally anchored to a build plate, with support structures added if needed. $\endgroup$– Christopher James HuffCommented Jul 16, 2021 at 17:27
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The "surface curing" type of VPP obviously is near-impossible in microgravity, as gravity is needed to confine the uncured resin and level its surface. It might be possible to do the same with surface tension, but this would be far more difficult.
The "window curing" type is certainly doable, though it loses its biggest advantage of not requiring a full vat of resin. It also adds complications such as parts or contamination adhering to the window.
Two-photon photopolymerization should have no problem with microgravity, but is much more limited in terms of size and material. The resin has to be transparent and optically uniform for obvious reasons, and issues of focal length limit the build volume.
However the resin is actually cured, all of the resin-curing approaches have additional problems with the large quantities of liquid resins and cleaning solvents required. It's a messy process here on Earth with solvents cheaply and easily available enough to discard after use, in space you'd have to recover them through distillation for it to be practical, and recovery of unused resin and safe handling of all the fluids and wet prints/printer components would be complicated problems to solve.
With all that complexity, the end result is just cured epoxy. Other approaches are better for producing functional objects like tools or experiment fixtures, being able to use stronger and more durable materials, or even mix multiple materials in a single print. It's not impossible to make vat photopolymerization work in space, but it'd be of relatively limited utility.
answered Jul 15, 2021 at 22:46