New to FDM; some questions about printing solid walls with Ultimaker 3

[Bruce61 0]

I am new to FDM printing,  and am thinking about getting a Ultimaker 3.   I have a questions about how the FDM printers will print my objects.

The objects I want to print that are cylindrical about 60 mm in diameter and 60 mm tall.  They are about 1.5 mm thick.

In the pics below from Cura the object dia. was 20 mm and I had the .4 nozzel set.

I would like them to be basically solid.  I loaded a model into Cura and had 100% infill set, but I noticed that the model was sill showing an inner and outer wall.  

I am trying to understand what I am seeing in the pic.

How does an FDM printer print a solid layer, since it is extruding a certain thickness line.  What if a cross sectional area of a layer is not an exact multiple of the extruded line with?  Does it by necessity of the way FDM works have to leave voids? and that is what the pick is showing?  

In this picture below, where you see the outer shell and there is extruded lines inside the shell, are the lines on this layer (the outer shell and what you see inside )is this all going to be fused together?

Thank you

[DidierKlein 696]

Hi!

This is a graphical representation of the layers, in order to make the layers visible they are represented like that. When printed they will fuse together