Pretty much the same way that we owe the rapid evolution of the internet to porn.
The thing with chemicals is that for some, a very small amount can go a very long way for good or ill, in which case, you aren’t necessarily concerned with feedstocks. That’s only an issue if you want the best yields. If you don’t care about yields and only care about a usable end product, that’s a very different game.
So the only advantage it has over a computerized 8-axis milling machine is that it’s cheaper.
What I’d like to see is a chemputer that works in reverse – that, is, breaks down complex chemical substances – i.e., most of the solid and liquid and gaseous waste-stream – into component elements.
It’s called “the digestive system”.
Great, now we just need one that can handle forms of toxic industrial waste not suitable for processing into snack foods.
Is there really enough left over to worry about? 
There are several reasons why that won’t work:
Trivial alterations to the design specs (as simple as embossing a half-millimeter-deep logo on one part) would change the digital signature of the file without altering the functionality of the output.
Trivial spoofing of the “banned design” download (i.e. connect the 3D printer to a computer that tells the printer “no updates so far” or even “the banlist has been updated to [junk file]”) would circumvent the system altogether.
Any “banned design” file that is sufficiently comprehensive to cover basic firearm building blocks (in separate job lots, perhaps, as yet another way around the naive idea of “banned designs”) would generate a plethora of false positives for anything that used a hollow tube, a spring-loaded feed, etc.
Distinguishing gun parts from the aforementioned hollow tubes, spring-loaded feeds, et cetera used for other purposes would require AI on a level that would render this issue, and all other issues, moot – the computers would be in charge and (if we’re lucky) keeping their original creators around for sentimental reasons.
Someone else has already mentioned the use of saltpeter. Sulfur has some mild anti-fungal properties, which may be why drugstores stocked it once (garden centers still sometimes carry it for use as an organic anti-fungal crop dust).
It often is. This guy installed secret compartments in cars, so the DEA put him in jail. If you are knowingly doing something that aides and abets criminal activity, you can bet they will find a way lock you up.
If that were the case, few luxury brands could exist. You can by near exact knockoffs of most things now. Does a Picasso reproduction make the original worth less? People still, rightly or wrongly, differentiate between a genuine article and a reproduction. Yes, there will be some that will be okay with a knockoff, but this won’t become the problem you foresee it being. Especially if acquiring said reproduction takes the money and expertise involved with 3-d printing.
This. Guns are mainly a problem in the hands of a few small groups: gang members, criminals, violent fuckups, crazy people, and terrorists. All those groups are either too stupid and lazy to bother printing their own guns, or not price sensitive enough to be wondering how to get access to (eventually) cheaper guns. It’s not like it’s hard to get a gun now. There are roughly as many guns as people in this country. You can get a gun on the street for $100-$200. Printing your own gun would need to come down A LOT in terms of price, ease, and reliability before it would become a main source of weapons for criminals.
The genie is out of the bottle, and they really think they can effectively stop it?
I rather think they think they have to appear to be doing something.
Someone should merge these threads.