Gaps between infill and walls: how to fix overlap and extrusion width

Why infill and perimeters fail to bond

Every layer is built from two different toolpaths. The perimeters trace the outline and give the part a strong, accurate exterior. The infill fills in the rest, usually with a fast back-and-forth pattern. Because those paths run in different directions, they have to merge at the boundary to form a solid bond. According to Simplify3D's troubleshooting guide, small gaps along the edge of the infill mean that merge isn't happening cleanly.

That documentation points to two main causes. The first is geometry: the infill doesn't overlap the innermost perimeter by enough. The second is speed: infill usually runs much faster than the outlines, and if it moves too quickly, it doesn't have time to bond. Extrusion width matters too, since Simplify3D calculates overlap as a percentage of it.

This isn't purely cosmetic. A weak joint leaves the walls and the infill less connected within each layer. If you want the bigger picture on density and patterns, start with our guide to infill. This page has a narrower goal: close the seam between those two zones without reworking your whole profile.

Make sure it's really a seam problem

Start by looking at where the gaps sit. If they follow the inside edge of the walls while the infill lines and perimeters themselves look consistent, the joint is your prime suspect. A solid surface often makes it obvious, with a thin line separating the fill from the outline. Your slicer's toolpath preview can also show whether the infill lines actually reach the inner perimeter.

If lines look skinny everywhere, or the perimeters have gaps of their own, the problem is bigger than the seam. Simplify3D also lists gaps between perimeters and infill as a symptom of under-extrusion, where the printer simply isn't putting down enough plastic. In that case, fix the under-extrusion first, because more overlap won't touch the root cause.

Two neighboring defects are easy to confuse with this one. Holes in the top surface usually point to a top layer issue. Gaps tucked into the corners, where a top layer doesn't join the outline, belong with floor corner gaps. Once you've narrowed it down, pick a small, simple part and keep it as your reference for comparing each change.

Raise the overlap in small steps

Overlap controls how far the infill reaches into the outline to tie the two together. Simplify3D calls the setting Outline overlap, and you'll find it under Edit Process Settings on the Infill tab. It's a percentage of your extrusion width, so it scales with different nozzle sizes. At 20%, the infill overlaps 20% of the innermost perimeter.

The company's own example is simple: if you're at 20%, try 30% and see whether the gaps disappear. Change only that one setting, reprint your reference part, and check the same edge. If the gaps shrink but don't close, go a little higher rather than jumping straight to an aggressive number.

Other slicers may name or place this setting differently. Search for overlap in the infill settings and check whether the value is a percentage or a length. Also watch for the opposite problem. Too much overlap can push excess plastic against the wall and risk marks you can see from the outside. If that starts happening, back down to your previous step and leave it there. A small overlap bump is quick to try, which is why it belongs first on your list.

Check extrusion width and infill anchors

In Simplify3D, extrusion width is the base for the overlap calculation. That means the same percentage covers a different distance whenever the width changes. After any width change, recheck the seam instead of assuming it still holds up.

In PrusaSlicer, Prusa's documentation says infill line thickness lives under Print settings, Advanced, Extrusion width, Infill. It's worth a look if you're running an imported or heavily edited profile. Make sure the value still makes sense for the nozzle you have installed, especially after a swap. Think of it as a sanity check, not a magic fix.

The same documentation covers another useful setting: the infill anchor. That's a short stretch of inner perimeter that turns into an infill line. Prusa says it makes infill sturdier, and that some materials, PETG in particular, need it to stabilize flow at the start of an infill line. Set it to 0 and the infill prints independently of the perimeters. Enter a percentage such as 15% and the length is calculated from the infill extrusion width. If your gaps showed up right after someone changed this value, check it first.

Slow down when overlap isn't enough

If more overlap doesn't close the gaps, look at speed next. Infill generally prints much faster than outlines, and Simplify3D warns that too much speed leaves it without enough time to bond. In that software, the Default Printing Speed on the Speeds tab controls any move where the extruder is actively laying down plastic.

Simplify3D's method is blunt but useful. If you were printing at 3600 mm/min (60 mm/s), cut that value by 50% and reprint. If the gaps vanish, raise the speed gradually until you find the best setting for your printer. The big drop gives you a clear answer quickly, so you're not chasing tiny differences between prints. From there, you can decide how much print time you're willing to trade for quality.

If nothing changes at reduced speed with more overlap, the seam theory gets weaker. Go back to actual flow and the overall condition of the machine, using our 3D printer calibration guide. Once you land on settings that work, save them in a profile for that filament. That way you won't have to start over the next time you load a new spool.

Sources and limits

• Simplify3D — Gaps Between Infill and Outline
• Prusa Knowledge Base — Infill

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