You're dreaming of today? Because that's what it is. You can walk into just about any machine shop and get your part as long as you have drawings and material spec. You probably won't be happy with the cost, though.

The point is that the spare parts you need can be made with traditional manufacturing processes because that's how the original was done. Just because you only need one, doesn't necessarily make it easier to produce by a 3D printing machine.

The Rapid Prototyping Center at MSOE has several patents around creating mold casts with 3D printers. IIRC, one of their techniques allows building a tetra lattice coolant path around the part cavity, allowing more optimal cooling than traditional methods (increasing water turbulence, getting closer to the cavity face). They weren't printing the tools directly, but the tools could be made from the parts they generated.

That was about a decade ago, so I'm sure they've moved onto more exciting things.

production: noun. 1 the action of making or manufacturing from components or raw materials, or the process of being so manufactured.

I'd say if you make a factory that takes raw materials and creates a series of one-off items, that you are engaging in production.

It seems like what you're really saying is, "this is not how production has been done in the past". With 3-D printing and related automation technologies, this will start to change very quickly. (It's already started, in fact).