I googled this, and found that many have this problem, but I did not see a solution that worked. I did set the extra shells to 0, and no difference (that's the common "solution" to this) I don't know why the slicer does not put any infill between the walls, there should be enough space, but it just leaves it empty! The wall thickness for my part is 1.5 mm, if I make the part thicker it will putby CVMichael - Skeinforge