Thin-wall gaps in 3D printing — 5 causes of voids and skipped walls

Symptom: long gaps appear in the thin parts of the model, or the wall is not printed at all. The letters of an inscription are hollow inside, a rib has a hole running its whole length, and holding the part up to the light you can see straight through it. The thicker sections of the same print come out faultless.

An FDM printer does not lay down arbitrarily thin lines. The extrusion width follows from the nozzle diameter and stays within a narrow band around it. When the slicer meets a wall narrower than two such lines but wider than one, it has a problem with no ideal answer: either it squeezes two perimeters in there and overfills the wall, or it lays the two separately and leaves a void between them, or it skips the section entirely. By default most slicers choose the option that is safest for the dimension, which means leaving a gap. That is why the fix almost always consists of changing the way the slicer calculates walls, rather than tweaking the printer.

Most common causes and fixes

1. The wall in the model is thinner than the nozzle diameter

If the designer gave the wall 0.3 mm and there is a 0.4 mm nozzle in the printer, there is nothing to print. The slicer will simply skip such a section, because it cannot lay a narrower line. It looks like a printing fault, though it is an ordinary mismatch between model and tool.

How to check: measure the wall in your CAD program or in an STL viewer. You can also open the layer preview in the slicer and check whether the problem section has a toolpath generated at all — no toolpath points unambiguously to the cause.

What to do: thicken the wall in the model to at least the nozzle diameter, and ideally to a multiple of it. With a 0.4 mm nozzle, walls of about 0.8 mm and 1.2 mm behave well, because exactly two and three lines fit into them. If the model is someone else's and cannot be edited, what remains is a smaller nozzle or scaling the whole thing up.

2. The number of perimeters does not fit the wall thickness

The more common variant of the same phenomenon. The wall is 1.0 mm, the nozzle 0.4 mm, and the slicer has been told to use two perimeters — so it lays two lines of 0.4 mm and is left with 0.2 mm of void exactly in the middle of the wall. The result is a gap running the whole height of the element, often mistaken for under-extrusion.

How to check: in the layer preview click on the layer at the height of the problem and see whether an empty strip shows between the two perimeters. If so, the cause is here.

What to do: enable closing of such gaps in the slicer — in PrusaSlicer and OrcaSlicer this is Gap fill, in Cura Fill Gaps Between Walls set to Everywhere. You can also reduce the number of perimeters to one and let the infill make up the rest, or experiment with the perimeter extrusion width: increasing it slightly makes two lines fill 1.0 mm with no void.

3. Thin wall detection is switched off

Cura has a separate Print Thin Walls switch, inactive by default. As long as it is off, anything below the width of a single line drops out of the print without any warning. PrusaSlicer and OrcaSlicer solve this differently, through variable extrusion width on perimeters, but there too the settings can be turned around in an imported profile.

What to do: in Cura tick Print Thin Walls and check Minimum Feature Size and Minimum Wall Line Width — those decide how narrow a feature the printer will still try to reproduce. In PrusaSlicer and OrcaSlicer make sure the perimeter generator is running in Arachne mode, which adapts the line width to the real wall thickness. After any change always look through the layer preview, because the effect is visible straight away.

4. The nozzle is too large for this detail

A 0.6 mm nozzle shortens print time and copes beautifully with large solids, but it takes away the ability to reproduce fine features. The same model printed with a 0.4 mm nozzle comes out with no gaps, and changing only the profile will make no difference. It is worth remembering that some materials require a larger nozzle because of their filler: the manufacturer specifies 0.6 mm for PLA Wood and for the Granite version of the PLA Premium series, while carbon-fibre-filled materials reproduce detail only from 0.4 mm upwards, and require a hardened nozzle at that.

What to do: for models with thin walls, lettering and fine detail stick to a 0.4 mm nozzle. If you are printing PLA Wood or a fibre-filled material, design the walls thicker — with a 0.6 mm nozzle a sensible minimum is 1.2 mm.

Porima material	Recommended nozzle	Smallest detail reproduced	Nozzle wear
PLA	0.4 mm	0.2 mm and above	none
Tough PLA	0.4 mm	0.2 mm and above	none
Hyper PLA+	0.4 mm	0.2 mm and above	none
Silk PLA	0.4 mm	0.2 mm and above	none
PLA Premium	0.4 mm (Granite 0.6 mm)	0.2 mm and above	none
PLA Wood	0.6 mm	0.2 mm and above	moderate
PETG and PETG Transparent	0.4 mm	0.2 mm and above	none
HT PETG	0.4 mm	0.2 mm and above	none
ABS / ASA	0.4 mm	0.2 mm and above	none
PA (nylon)	0.4 mm	0.2 mm and above	moderate
TPU Flex 98A	0.4 mm	0.2 mm and above	none
PLA/CF	0.4 mm	0.4 mm and above	very high — hardened nozzle

Source: the official FDM technical data table from the manufacturer, Porima. Full set of parameters: print temperature table.

5. Too little material flow

A thin wall is the place most sensitive to a shortfall in extrusion, because it consists of one or two lines and has no neighbouring material to mask the deficit. The same printer that behaves correctly on infill and thick walls starts leaving gaps on a single perimeter. The cause may be a flow multiplier set too low, excessive resistance in the feeder, a partially clogged nozzle, or wet filament foaming the strand.

How to check: print a test cube in single-wall mode (vase mode) and measure the wall with callipers. A result clearly below the extrusion width you set confirms a flow problem rather than one with the model geometry.

What to do: check in turn the filament diameter entered in the slicer, the flow multiplier and whether the nozzle is clear. If the material has been lying open for several weeks, dry it — PETG, TPU and nylon absorb moisture fastest, and you will find detailed times in the filament drying guide.

When the model needs fixing, not the slicer

Slicer settings have their limit and with genuinely thin walls you hit it quickly. If a part is meant to be watertight or to carry load, the only honest solution is to design it for printing: wall thickness equal to a multiple of the extrusion width, avoiding wedges that taper to zero, and replacing sharp, thin edges with chamfers. A separate matter is models downloaded from the internet, which are sometimes geometrically faulty — inverted normals, non-manifold surfaces and intersecting solids can give exactly the same symptom. Before you start changing the profile, run such a file through a mesh repair. If the gaps disappear after the repair, the case is closed and there is no point looking for a fault in the printer.

Related problems

• Under-extrusion — the most common cause of missing material, mistaken for gaps in walls.
• Weak infill — the same mechanism inside the model.
• Clogged nozzle — a partial blockage shows up first on thin features.
• Dimensional accuracy and shrinkage — extrusion width affects the dimensions of the finished part.
• Over-extrusion — the opposite problem, after pushing the flow up too far.
• Wet filament — a foamed strand does not fill a single line completely.
• Filament grinding — the feeder loses material, and the shortfall shows on thin walls.