The answer to your question could be found by yourself just by thinking about what you have written (always a good thing).
FDM is not about stretching filament, it's about squeezing filament thru a tight nozzle with some pressure. Otherwise it would not flow. Pressure is an adjustment to the thickness to some degree. If the nozzle is lifted by 90 microns you could guess (or try) what happens if you squeeze filament thru the 0.4 nozzle.
Hint: The horizontal space between the nozzle and the bottom surface or bed is 90 microns. Where does the filament go from that 400 micron output tube and how thin could that be for this direction (wich is vertical [hint#2])?
To clarify, of course I know that the filament is feed through the nozzle. I was not arguing about the actual width of the string, just that a 0.4mm nozzle doesn't mean that you can't get a thinner string than the nozzle diameter.
My point is, if the filament feedrate f (mm/sec) and the distance a (mm) from nozzle tip to perfboard is fixed, then a change in extruderhead velocity (in X & Y directions) v (mm/s) will change the cross-sectional area, and hence, the width and height of the printed filament string.
When the filament is extruded (or pressed if you like) through the nozzle, a part of the string is welded to the perfboard, and thus unelastic, and a part of it is still in molten state and elastic. The molten filament string will be in the "air" between the perfboard and the nozzle. If you now speed up extruderhead velocity, you will stretch that "airborne" string, and decrease its cross sectional area.
So to conclude, the filament is extruded (pressed,squeezed) through the nozzle and then stretched when leaving the nozzle, due to the relative movement between the nozzle and perfboard, thus decreasing cross section area.
Where does the filament go from that 400 micron output tube and how thin could that be for this direction (wich is vertical [hint#2])?
If I understand you correctly, this calculation will give an answer to your hint. I assume in the calculation that you mean that the area of the filament through the nozzle is equal to the area of string on the perfboard. I base that on your comment that "FDM is not about stretching filament".
Cross sectional area of nozzle An (and filament): An=Pi*d^2/4= 3,14*0,4^2/4= 0,126mm^2
To simplify we assume that the printed string is now a rectangle with height h=0.09mm and width w.
Area of Rectangle Ar=h*w (mm^2)
We assume Ar=An
According to http://reprap.org/wiki/Filament: ABS filament through a 0.5mm nozzle will stretch to 0.3 mm diameter, which means that the cross section area will be reduced to about 36% of its solid state area.
So if we apply that to the example above then:
Stretched extruded filament area As=An*0.36=0,126*0,36=0.045mm^2
In other words, due to stretching, the minimum width (or wall thickness) of a 0.09mm layer will be 0.5mm. (At least in theory!!)
So, if I understand you correctly, you say that a layerheight of 0,09mm would make a string 1.4mm wide. If I got you all wrong please excuse me!