Any good scientific papers about how 3D printers work?  

Re: Any good scientific papers about how 3D printers work?

1. If you move the motors manually you actually do cause the motors to lose their place in most cases.
2. The plastic is thick enough that it does not flow as easily as water. Also the loaded filament acts as a plug. Think a straw filled with water and you cover one end with a finger before pulling it out of the glass.
3. Diameter of nozzle does not affect layer thickness. Consider using a caulking gun. If you were able to control as precisely as a printer controls a 0.4 nozzle you could still make a 0.05 layer height.
4. The idea is controlled cooling. Some filaments (ABS) don't require a part cooler.

Probably no one has done a scientific manual specific to 3D printing as all of its properties have manuals out the wazoo already. From your questions definitely hydro and thermo dynamic journals abound.

Hope that helps,

Neal

Re: Any good scientific papers about how 3D printers work?

Get a text book on thermodynamics. It's all there.

Re: Any good scientific papers about how 3D printers work?

Any good articles or scientific papers that you could point me at?

The best way I've found (not that there aren't other/better ways) is to search google scholar (not quite the same as google, though you can preface a google search with "scholarly:") for fdm printing or whatever you're interested in. 😉

Some of the questions you ask are handled via firm/software.

Both 3D printers and "Slicers" (software that creates printable gcode from STL source files) have many settings that can adjust most variables involved in FDM printing.

Why does one need a cooling mechanism and not let the plastic get cold on its own due to ambient air?

This is largely dependent on the type/formula of the particular plastic (its physical properties) and the speed at which you print it.