Gaps in Top Layers

To conserve material, most 3D prints are designed with a solid outer shell and a partially hollow interior. For instance, a print might use a 40% infill, meaning only 40% of the interior is solid, while the rest remains hollow. Despite the hollow interior, the top and bottom layers are intended to be fully solid. This approach saves time and material while maintaining strength.

However, if the settings aren’t quite right, you may find gaps or holes in your top solid layers.

Common Causes and Solutions:

There are not enough top solid layers

The first thing to check is your number of top solid layers. When printing solid layers over a hollow interior, these layers need to span across air pockets created by the infill. This can cause the extruded plastic to droop or sag, leaving gaps. To avoid this, you generally need multiple solid layers to create a smooth surface. A good rule of thumb is to have a top solid layer thickness of >0.5mm. For instance, with a layer height of 0.2mm, you’d need at least three top layers.

If you’re seeing gaps, try increasing the number of top layers. For example, if you initially used two top layers and noticed issues, try increasing them to four. This adjustment won’t affect the external dimensions of your print but will help achieve a solid surface.

The infill percentage is too low

The infill acts as the support structure for your top solid layers. If the infill percentage is too low, there will be large gaps between the infill, leaving the solid layers without enough support. For example, a 15% infill would result in 85% of the interior being hollow, creating substantial air gaps for the top layers to span. If increasing the number of top solid layers didn’t resolve the issue, try increasing the infill percentage. For example, moving from a 20% infill to 60% would provide a much better base for the top layers, reducing the likelihood of gaps.

Under-Extrusion

If increasing the infill percentage and the number of top solid layers doesn’t fix the gaps, the issue may be under-extrusion, meaning the printer isn’t outputting enough filament. Refer to the under-extrusion article for more information on diagnosing and fixing this issue.