Big printing differences with small modelling differences

Hi there,

I have a strange topic I need to try to understand:

In the attached picture you see a part of a simple print with different cuboids. I used Blender to model the whole thing. For some particular reasons the width of the cuboids are not exactly identical. I gave some examples in the picture "Cuboids.png" attached (please refer to widths of A, B, C, D and E).

What really puzzles me are the differences how the whole thing is printed - the toolpath preview (MANY thanks for that feature by the way) however looks really fine - please refer to "Toolpath.png".

I need to understand some strange things:
a) Why are there so big differences in printing, even when the model difference (width of cuboids) are likely to be neglected.
b) Why are there some gaps, even when the model should be filled, see profiles of B, C and D? Why is A filled completely but not B, even if the difference is just 1 µm in the model? (A vs B )
c) Why is C not filled, even if its width is smaller than A? If it would be a stepper motor problem, then I would assume that the left part of C should be printed with the same width, but it isn't.
d) What can I do to get the profiles closed, without any gaps in between (e.g. at B, right part of C and D)?

I have to say that issues like these often appear in my models and I would be happy to solve these :-)

Background info:
model height: 1.5mm
Slic3r version 1.2 (similar behaviour also seen in earlier versions)
Printer: RepRap TriColor Mendel.
Host: OctoPrint on a RaspberryPi.

Many thanks for you help,
Leo

Well, I cannot actually believe that the differences are as small as pointed out.
From what can be seen from the cuboids.png, you have 2 perimeters, and, assuming a 0.4 mm extrusion width, dimensions vary somewhere between 1.6 and 2 mm.
Firstly, if you wanna control things, you should explicitly set the extrusion width. I assume you have a 0.4 mm nozzle. However, if you do not set the extrusion widths (in Advanced Settings) to explicit 0.4 mm values, then the actual extrusion width will be greater that 0.4 mm (die swell effect).
Secondly, if you really want to avoid some strange situation, set the nozzle diameter a bit less than 0.4 mm, e.g. 0.39 mm. Chances are that Slic3r will try to fill areas that otherwise won't.
Might be that all of your issues will disappear when applying the above.