As a (Mechanical) engineer, I would respond by saying you turn a bolt with a plastic wrench just like any other wrench, providing the moment of torsion provided by the person turning the wrench does not exceed the shear strength of the wrench material.

I'd guess the wrench isn't the problem - rather the nut which is in direct contact with the metal screw's edges and bound to fail very soon.

Is it possible to UV-harden the plastic maybe to give it a harder surface?

Using a socket wrench rather than, say, a crescent wrench means there's more nut in contact with the wrench at any given moment. So the wrench doesn't have to be quite as strong, since the torque is distributed over a larger surface area and thus a larger volume of wrench is available to absorb it.

You've already got some better replies, but I have always wondered about the strength of 3D printed materials.

I think it's just amazing that we're at this point anyways. Regardless of how good the tools are, we can print them in space.

We use some where I work. They are useful in niche applications, such as our one-off research lab equipment. They wear out slowly. They work best where we have 5 different common orifices for fluid that need to be merged into one. When they wear out, we order new ones.

We pay a tech who knows CAD and a fab house rather than a machine shop. A machine shop would probably cost a lot more. Although now that we have the design down pat, we wouldn't waste as much money on prototypes.

Our design includes 2" diameter threaded ends and it holds pretty firmly although the threads do wear out and tear off the tube with vibration and heat changes.

We've used 3D printed plastic parts in surprisingly (to us, anyway) rigorous applications. We've used printed PLA parts as prototype bearing holders on seriously high speed rotating equipment. Strength is usually close to fiber reinforced Delrin or similar plastics.

3D printing let us try and tune successive iterations for best performance far more rapidly than we could have done with classical subtractive machining. OTOH, our 3D printers care about the room temperature and HVAC breeze, whereas a lathe or mill wouldn't notice. Tradeoffs....

How do you turn a wrench with your human hand without breaking your arm? :-)

The same way you fire a plastic gun without it compromising its integrity:

http://www.wired.com/2014/05/3d-printed-guns/