To save plastic, most 3D printed parts are created to have a solid shell that surrounds a porous, partially hollow interior. For example, the interior of the part may use a 20% infill percentage, which means that only 20% of the interior is solid plastic, while the rest is air. o save print material and to speed printing, the interior of a model is not printed 100% solid. Instead, some kind of infill is printed, typically only 30% of the material, the rest of the model remains hollow. Only the top layers (and the walls and the bottom) are printed 100% solid. The gaps in the top layers can have these causes:

  • When There Are Too Few Solid Top Layers: 

               Increase the number of solid top layers in the slicing software. There should be at least 0.5mm of solid layers (how many layers that means depends on the layer height). The additional solid top layers do not add height to your printed model. When you increase the number from 3 to 5, for example, the last 5 layers are printed solid (instead of 3). In Cura, use the Shell Thickness setting on the Basic tab to increase the solid top layers.


  • When The Infill Percentage (for the interior) Is Too Low: 
               When the hollow gaps in the model are too wide, because the infill percentage is only – say – 10 or 20% – the solid layers may sag. If adding more solid top layers does not correct the problem, increase the infill percentage in the slicing software. This will increase the filament consumption. In Cura, you’ll find the Infill Density setting on the Infill tab.


  • Underextrusion: 
           Refer Not Extruding Enough Plastic Notes