Twenty years seems a little optimistic. We don't have ground-based automated factories yet -- not fully automated anyway. And the cost to lift a ground-based factory (or even its parts) to space on tonnage alone would be astronomical. Plus, many things are harder in a vacuum. So you're looking at some significant technological progress before what you're suggesting will be possible.
Automated factories are coming from both ends - continuous production, and custom on-demand stuff. Mines / oil rigs / pencil factories are getting almost entirely automated, because they never have to change what they are doing. 3D printers are becoming automated, for the opposite reason. Being squeezed from both sides, non-automated manufacturing is not here to stay.
I'd be more pessimistic on the grounds that there isn't a huge market for space-based factories. Making the best parts requires heavy machinery (whether or not it's automated), which you can't affordably launch into space.
I would imagine that it would be quite difficult to design a 3D printer that works in zero gravity. All current designs that I'm aware of rely on gravity to either keep the base material on a level surface (e.g. SLS, SLM) or to draw the build material down from a nozzle of some sort (e.g. FDM)
Everyone forgets about 3D printing's older brother, 3D milling. Start with a big block of aluminum (or any metal softer than the cutting bit), use what amounts to a CNC dremel tool to carve away everything you don't want. It is vastly simpler than 3D printing and creates objects that are much more sturdy. But it is slower and uses a more expensive feedstock than printing.
Good match for space fabrication, too. Zero-g makes it much easier for an air nozzle to dislodge debris. Feed it big ingots of metal straight from the asteroid smelters. Not sure if you could do the milling in a vacuum, but it would be worth a shot. Downside, greatly limits cooling. Upside, easier recovery/purification/recycling of shavings. A big electrostatic charge on the ingot and tools should make the shavings literally fly off of the work to a collection plate.
Do you mean having a blob of molten metal floating in a chamber? Surface tension means it won't keep a shape. (I guess you could make cheap ball bearings.) If you get around that, any time you touch the blob it would start rippling and ripples won't really damp out. Might make for some pretty sculpture if you can flash freeze it, but nothing of industrial value.
Unless you are trying to obliquely refer to casting, which should be pretty much identical with or without gravity/air. And still needs tooling to make the molds.
(And please don't say that the molten metal could be a feedstock for a printer. That would be a circular discussion.)
I guess you meant something more like blacksmithing. Chamber could add heat by microwave induction. No air to oxidize the surface so you might be able to work trickier metals. We already have power hammers, we already have 6DOF manipulation tables. I'm not entirely convinced that a power hammer could do the work accurately enough, might want to use a milling machine for the final fitting.
Repraps print fine on a vertical surface, or upside down. The inter-layer adhesion is achieved by melting the new layer into the previous one, and the build material is pushed down with positive pressure. It would work fine in zero gravity, though cooling might be a problem if it were working in a vacuum.
Let's target rational exuberance.