Thin Wall Gaps and Floor Corner Holes: How to Fix Both Defects

Why a fixed nozzle leaves voids in a thin wall

Your nozzle has one fixed diameter, so it lays down a bead of roughly constant width. When a wall in your model is only a few beads wide, the slicer has to decide how many passes fit inside it. Simplify3D documentation uses a clear example: a 1.0 mm wall sliced with a 0.4 mm extrusion width. Two perimeters take up 0.8 mm, which leaves a sliver in the middle too narrow for a third bead. Without a dedicated setting, the slicer leaves that sliver empty, and the wall comes out hollow or riddled with holes.

Floor corner gaps have a related but different origin. Simplify3D points out that every layer depends on the foundation laid by the layer below. When the part changes size, say a 20 mm cube printed on top of a 40 mm cube, the side walls of the smaller cube need something solid to sit on. If there are too few perimeters, too thin a ceiling, or too sparse an infill, that foundation lacks material and holes open up right in the corners.

Both defects boil down to the same thing: not enough plastic landing in one specific place to close the surface. That is why this guide covers them together, with fixes that partly overlap.

Telling a thin wall gap from a floor corner gap

Start by looking at where the hole sits. A void that runs the full length of a thin wall, visible from above or when you hold the part up to the light, and that repeats on every layer, points to an extrusion width problem. The perimeters simply never meet in the middle. You will see it most on box dividers, fins, raised lettering and ventilation grilles.

A hole confined to a corner, right where a flat surface meets a vertical wall that starts above it, is a floor corner gap. Simplify3D describes it as the spot where the top layer fails to join the outline of the next layer. The defect clusters wherever the part cross-section shrinks and almost never shows up on straight side walls.

Before you change anything, open the layer-by-layer preview. If the slicer already shows the empty sliver or the uncovered corner on screen, the issue is in the slicing, not the hardware. If the preview looks clean but the printed part has holes, you are more likely dealing with under-extrusion, covered further down. Making this distinction keeps you from piling on perimeters for a defect that does not need them, or from recalibrating your extruder when the file was simply sliced wrong.

Thin walls: adjust extrusion width or let the slicer fill the gap

You have two routes. The first is to let the slicer close the gap for you. According to Simplify3D, the default behavior uses gap fill, a back-and-forth pattern that adapts to whatever space is left between the perimeters. The software also offers a single extrusion option that replaces two perimeters and their gap with one bead whose width adjusts dynamically to fill the wall exactly. Other slicers name these options differently, so look for thin wall or gap fill settings in the advanced tabs of whatever you use.

The second route is to change the extrusion width itself. Simplify3D gives the example of a 1.0 mm wall printed with the extrusion set to 0.5 mm: two beads then fill the wall with no void, faster and stronger. This works best on parts whose walls have a fairly consistent thickness, because a manual width applies to the whole print, not just the thin sections.

If your models regularly include very thin walls, your choice of nozzle matters too. A smaller diameter allows narrower beads at the cost of longer print times. Stay within the width range your slicer accepts for your nozzle, or you will trade one defect for an extrusion problem.

Floor corners: perimeters, top solid layers and infill

Simplify3D lists three settings to check, in order. The first is the number of perimeters. Since the inside of a part is usually partly hollow, wall thickness has an outsized effect on how strong the foundation is. The example is simple: if you were printing with two perimeters, run the same part with four and see whether the gaps disappear.

The second setting is the number of top solid layers. A thin ceiling cannot properly support whatever is printed on top of it. The documentation suggests going from two top solid layers to four. The same logic applies to the gaps in top layers guide: a top surface that never fully closed becomes a bad foundation the moment a wall rises from it.

The third lever is infill percentage. Top solid layers are built on the infill, and if it is too sparse they sag between the cells and leave voids at the corners. Simplify3D proposes comparing 20 percent against 40 percent to see the effect. Before raising density everywhere, read up on infill: a different pattern sometimes helps as much as a higher percentage, with less material and less machine time.

Rule out under-extrusion first

Adding perimeters fixes nothing if your printer is putting down less plastic than the slicer expects. Simplify3D itself lists under-extrusion and inconsistent extrusion as related causes of floor corner gaps. If your perimeters look noticeably thin, do not touch each other, or your infill looks like stretched threads, deal with that before anything else.

The under-extrusion guide walks through the usual checks: a partially clogged nozzle, wet filament, a temperature set too low, or a drive gear slipping on the filament. A flow calibration on a simple test part then confirms that the extrusion width you set matches what actually leaves the nozzle. If the two do not line up, every rule in this guide falls apart, because the slicer is counting on a precise bead width to close the wall.

Print a minimal test: a small box with thin walls and a stepped cube with one clean ledge. If both defects show up at the same time on these simple shapes, under-extrusion is your first suspect. If only one appears, you know which section of this guide to apply. This two-step approach takes a few minutes and saves hours of blind tweaking.

When the model, not the slicer, needs the fix

A slicer can compensate for a lot, but it cannot invent material where the model leaves no room for it. When you design your own parts, size the walls around the bead width of your nozzle: a wall drawn as a multiple of that width slices with no leftover gap. Simplify3D notes that extrusion width adjustments work best on parts with consistent walls, so a clean model spares you much of this tuning.

With downloaded files you do not control the geometry, but you do control orientation. A thin wall laid flat on the bed becomes a solid surface that is easy to close. A troublesome ledge can become a supported surface if you flip the part, as long as the supports you add are not a bigger headache than the original defect.

Keep in mind that a wall left hollow for lack of beads does not behave like a solid one. It cracks more easily and lets light through. If the part has to carry any load, fix the defect at the source rather than hiding it with sanding or filler.

Sources and limits

• Simplify3D — Gaps in Thin Walls
• Simplify3D — Gaps in Floor Corners

We organize manufacturer and community documentation; we have not measured these products ourselves. The manual for your exact model takes precedence over general guidance.