Q:

Do 3D printers in space stations mean a significant cost reduction?

I just watched a VSauce2 video, at the end of which certain NASA animations were shown about some of their recent innovations. One of them included use of printers in the ISS.
Do they actually mean a noticeable decrease in cost? I mean, I'm aware that bringing equipment up to these places has huge costs, and I assume 3D printers may decrease the amount of said equipment, as they can create it up there.
What are they precisely used for up there?

A:

A 3D printer on station isn't likely to lead to direct cost savings; the range of items it can replace are limited, and it has to be supplied with feedstock mass in any case; it won't allow for significantly fewer supply launches. 
The primary benefit is that it can allow for the repair of a system the astronauts would otherwise have to do without until the next scheduled supply - whether a nonessential but convenient item, or a scientific experiment, or, conceivably, even a mission-critical or safety-critical part. This would have a definite impact in terms of return-on-investment, but that's not the same as cost savings.

A:

The 3-D printer on the ISS is more for testing purposes than anything. The idea is that for really long duration missions, a 3-D printer will allow them to make something in case something breaks, or they realize they need something that didn't come initially with the ship. So far I can only find a few things that have been made, including a ratchet, several test items, and a wrench. It seems the later was actually used as needed, the rest are just test artifacts. There is a lot of promise, a 3-d printer can potentially save a significant amount of launch weight, not to mention reducing the spares required on a ship, but overall, they are still trying to figure out exactly how to best use them.
As has been mentioned, the real benefit will be for those things that break, aren't life critical, but are needed sooner than a few months that can be taken to get the part from the ground. To my knowledge the 3-D printer has been used once for such, to print a tool.

A:

I disagree vastly with the majority of the answers here - there are a large number of recent NASA SBIR/other grants that show the agency firmly believes otherwise!
Currently, the 3D printer on the space station is naught but a research one and is a poor one at that. The company that made it is run by a... not so nice fellow that didn't do a good job of building it. It has served limited use due to this. 
The primary cost savings could arrive through recycling of the materials onboard. This would allow for reusable utensils, reusable tools (tool broke? if you still have material, it can be reprocessed), purpose-made parts for performing experiments (recycling makes this not at all wasteful), and so on. Utensils especially are useful - if they can be recycled, they will be sanitized when the material is melted at its usual temps! Currently, astronauts must wash all of their utensils with wet wipes since running water isn't the best idea on the ISS. This generates a (hilariously silly) amount of waste in the form of nothing but wet wipes. Which also must be sent up as supplies. And the astronauts say that the utensils can get fairly gnarly after a while... so being able to print single-use recyclable utensils lowers waste generation, lowers supply requirements, and increases quality-of-life (in some small measure) for the crew. All large cost savings! 
The issue isn't with the technical readiness of 3D printers for space station employment. Even the most basic of printers is capable of printing in zero-g in its stock form. The issue that requires some engineering is bed adhesion, and removal of the byproducts generated by melting ABS/PLA/Ultem-1010. These fumes can be hazardous and must be removed from the build volume with air cycling (or must be contained while the machine is running, then purged).
ERASMUS is the utensil recycling idea I mentioned - can see that it was funded here: http://www.parabolicarc.com/2016/05/06/nasa-selects-tethers-unlimited-sbir-awards/
Note that the same company/division is currently building this as well - probably not a coincidence, once you figure one of these out the rest are natural evolutions of the starter concept. Neat stuff! -
http://www.geekwire.com/2016/nasa-firmamentum-3-d-printer-recycler-space/

