Weak infill — a loose, hole-ridden structure inside the print

Symptom: after cutting a print open, or looking into an unfinished layer, you can see there is almost nothing inside: the infill lines are thin and broken, they do not touch where they cross and they do not join the walls. The model looks fine from the outside, but under pressure it collapses or cracks along the internal void. Sometimes it sounds hollow.

Infill has two jobs: to support the top layers and to carry load between the walls. Both require the lines actually to touch one another and to have time to fuse before they cool. Inside the model, however, the conditions are quite different from those at the surface: the slicer prints there faster, the paths are shorter and broken up, and each of them hangs over a void. That is why infill is the first thing to reveal every shortcoming in extrusion that is not yet visible on the smooth outer walls. Before you start raising the density, check whether the problem is the number of lines or their quality.

Most common causes and fixes

1. Infill density and pattern do not suit the application

The default 15 per cent is enough for figurines and decorative models, but not for a bracket meant to carry load. The pattern matters too: lightning supports only the top layers and deliberately leaves the interior empty, gyroid gives uniform stiffness in all directions, grid and cubic work well in structural parts, and concentric carries practically no transverse load.

How to check: look at the layer preview in the slicer before printing. If the cross-section shows a sparse mesh with large openings, the print will be exactly that.

What to do: for functional parts set infill around 30–40 per cent and a gyroid or cubic pattern. Above 50 per cent the gain in strength is small relative to the material and time it costs.

2. Infill speed is too high

Infill is the most common place where users claw back time by setting speeds twice as high as for the walls. The extruder then cannot build pressure over short distances, the start of every line comes out lean, and the flow breaks up over the void. The symptom is very characteristic: the infill lines are noticeably thinner than the walls and have gaps just after a change of direction.

How to check: stop the print on an infill layer and compare the width of the internal lines with the perimeters. They should be similar.

What to do: bring the infill speed down to something close to the speed of the inner walls. With TPU, also bear in mind that the manufacturer classifies this material's printing speed as low and recommends a direct drive extruder.

3. Infill overlap with the walls is too small

Even dense infill adds no strength if it is not connected to anything. The parameter responsible for the contact is called Infill overlap in Cura and Infill/perimeter overlap in PrusaSlicer and OrcaSlicer. With too low a value a gap visible to the naked eye is left between the mesh and the wall, and the part cracks around its perimeter, separating the shell from the interior.

What to do: increase the overlap gradually, by a few percentage points, until the infill lines start to merge visibly into the inner wall. Too high a value has the opposite effect: the infill pattern begins to show through the outer walls.

4. There is not enough material inside the part

Insufficient extrusion shows up first in the infill, because there are no neighbouring lines there to mask the shortfall. There are several reasons: a partially clogged nozzle, too low a flow, a slipping drive gear, and very often wet filament, from which steam escapes as gaps in the flow.

How to check: extrude 100 mm of material from the hot nozzle while stationary and measure how much actually came out. Listen for crackling as well.

What to do: start by drying the spool, because that step is reversible and free. Then check the flow calibration and that the nozzle is clear. Only at the very end should you touch the extrusion multiplier.

5. Cooling is working against the material

Infill hangs in the air, so it reacts to the fan more strongly than any other part of the model. With shrink-prone materials — ABS, ASA and PC/ABS — too strong a fan cools the lines before they fuse, and the infill comes away from the walls. With PLA the problem is the reverse: too little cooling means the lines do not set in time and sag into the void instead of forming a mesh.

What to do: stick to the manufacturer's ranges rather than a single setting for all materials.

Porima material	Recommended fan	Effect on infill
PLA	100%	Full airflow, the lines set immediately
Tough PLA	100% (required)	Without cooling the mesh sags between the walls
PLA/CF	60–100%	The fibre stiffens it, cooling can be reduced
PETG	100%	Good fusion, but infill strings easily
HT PETG	0–10%	Almost no airflow, otherwise the layers come apart
ABS and Eco ABS	0–40%	Excess cooling tears the infill away from the walls
ASA	0–30%	Requires a closed chamber, approx. 50 °C
PC/ABS	0–30%	The most sensitive to draughts in the chamber
PA (nylon)	30–100%, first 5 layers with no airflow	A wide range, matched to the model's geometry
HIPS	100% (can be reduced)	Tolerant, copes well with dense infill
TPU Flex 98A	40–70%	Requires slow printing, infill sparse as a matter of course

The fan recommendations come from the official FDM technical data table from the manufacturer, Porima. The comment in the third column describes workshop practice. The full parameter listing: 3D printing temperature table.

6. Infill lines are too wide or printed every few layers

Settings that speed up printing can throw the infill geometry out. A line width much greater than the nozzle diameter means the material does not fit where it was planned to go and the mesh comes out irregular. The option to print infill every second or third layer works the same way: it saves time, but every such line has to bridge double the height and breaks up more often.

What to do: set the infill line width close to the nozzle diameter — with a standard 0.4 mm nozzle, around 0.4–0.45 mm. Turn off printing infill with layers skipped in parts that are meant to carry load. Remember that PLA Wood and PLA Star require a 0.6 mm nozzle according to the manufacturer, so for those you recalculate all line widths from scratch.

Walls give more strength than infill

This is the most important conclusion on this page, and at the same time the most often overlooked. In bending, the greatest stresses occur at the surface of the part, which is exactly where the walls run. Infill lies in the neutral zone and carries far less. In practice this means that a model with three walls and 25 per cent infill will usually be stiffer and more break-resistant than the same model with two walls and 50 per cent infill, and it will print faster and use less material as well.

The tuning order that works: first increase the number of perimeters to three or four, then add top and bottom layers, and raise the infill density only at the end, and only if the part is going to work in compression or has to support a large flat surface. The exceptions are models with thin walls, where there is simply no room for another perimeter, and solid parts, in which the infill approaches a hundred per cent anyway.

A final note about the material. No setting will make up for an unstable filament diameter, because variation in diameter translates directly into the amount of material delivered in each line, and in infill that shows up immediately as paths that are sometimes thicker and sometimes broken. Porima filaments are produced to a ±0.03 mm tolerance.

Related problems

• Under-extrusion — shortfalls of material show up in the infill earlier than on the walls.
• Pillowing / holes in the top layer — the direct consequence of infill that failed to support the model's ceiling.
• Layer separation — the same weak-fusion mechanism, only visible from the outside.
• Wet filament — steam in the flow breaks the thin infill lines first.
• Poor bridging — every infill line is in essence a short bridge over a void.
• Gaps in thin walls — a related problem of path contact, only at the surface.
• Clogged nozzle — a partial clog spoils the infill first and the rest of the print afterwards.