Infill and Top/Bottom when slicing dual extrusion print

[Warbowhunter 5]

Doing quite the interesting print right now. A round shell with a translucent part and a black part. It needs a certain thickness  but shouldn't be 100% infill or I assume it won't be transparent enough. So I sliced with triple layer outer perimeter (default I believe) and no infill. Yet on the bottom and top of the top parts it still shows infill as that is part of the top/bottom of the tiny part. Black part can be 100% infill.

So I want to get rid of the yellow bottom/top infill of the middle parts but I don't want the full piece to have no top/bottom infill or it will be open. How would I fix this?

[GregValiant 1,411]

You are going to have to fool Cura.  How you do it can be tricky.

 

The slices Cura creates are horizontal (parallel to the build plate).  Since that rib (or whatever) is only "near horizontal" the wall thickness is being compensated for.  It's a trig function of the angle to the build plate.

This is a 6 x 6 x 75 rectangular solid tipped up 10° with 1.2mm wall thickness (3 walls / loops).

 

Here is the math.  The top view is at 10° tip and the bottom view is at a 5° tip from horizontal.  The infill is generated to maintain the 1.2mm wall thickness "normal" to the outer surfaces of the model.