Missing top layers in Cura
Photo Credit: Courtesy of 3DStockPhoto ( image)

What is happening here is that Cura is struggling with the quantised extrusion width. This behaviour can be improved in the slicer, they have a issue tracked here https://github.com/Ultimaker/Cura/issues/1303 and 2.4 beta has improved it.

There are some pictures in this answer which show how Cura 2.3 slices a rectangle of increasing width. I got nothing at the nozzle width, then a pair of overlapping lines up to twice the width (which I assume will be over-extruded due to the overlap). For anything wider than simply two sides, the result depends on the number of walls requested.

  • One wall results in 100% infill for small regions (regardless of the infill setting). This is good, but maybe 1 wall is not apropriate for the whole model.
  • Multiple walls seem to prevent the infill untill there is space for pairs of walls. So width of 3x nozzle has a gap. Small tweeks to the nozzle size can maybe push this quantisation point about, if 1 wall is not a good workaround.

For rectangular geometry, it's worth trying to quantize small parts in 0.8mm increments. For curves, setting 1 wall shell might be better.