Printing an object with a nozzle, which is only a few times smaller than the thickest wall, it might be that the slicer leaves a gap in the middle. For example: the wall is 1.5mm, and you are going to print with an extrusion width of 0.6mm, you need to advance some slicer settings for a solid print without gaps. For this reason, adjust the thin wall behaviour and/or the extrusion width in your slicer.
Adjust thin wall behaviour
Gaps can occur in the slicing process if the diameter of the wall cannot be divided precisely trough the diameter of the nozzle. The place between, would be free. To avoid gaps in thin walls, choose the thin wall settings. If your slicer does not have this option, vary the extrusion width, and the diameter of internal and external perimeters.
Change extrusion width
Gaps can occur in the slicing process if the diameter of the wall cannot be divided trough the diameter of the nozzle. The one line, which does not fit anymore will be missed. Set the extrusion width so it can be divided precisely.