Infill Fundamentals

What infill is

An FDM part is not solid plastic by default. The slicer prints a shell, made of walls (perimeters) around the sides and solid top and bottom layers, and fills the space inside with a sparse internal pattern: the infill. Prusa gives infill's main job as providing internal support for the top layers, and notes it also affects print speed, strength, filament use and appearance.

Infill density is a percentage: 0 percent is hollow, 100 percent is solid. The pattern is the shape of the internal structure. Both are set in the slicer, not in the model.

Walls matter more than infill

Prusa states that the strength of a model is mostly defined by the number of perimeters, not by infill, though infill still helps a part resist being crushed. So when a part needs to be stronger, the first change is usually more walls, not more infill. Prusa's design guide gives approximate wall thicknesses for a 0.4 mm nozzle with its default line width Rule of thumb:

Perimeters	Approximate wall thickness
1	0.45 mm
2	0.9 mm
3	1.35 mm
4	1.8 mm

Walls are covered in Walls and Perimeters, and top and bottom shells in Top and Bottom Layers.

How much infill

• Most models: Prusa says most models can be printed with 10 to 15 percent infill Rule of thumb.
• Rarely more than 30 percent: Prusa says you will rarely need infill higher than 30 percent Rule of thumb.
• Hollow (0 percent): possible for models whose tops close in gradually, but Prusa generally does not recommend it.

Higher density costs time and filament, and since walls carry most of the strength, extra infill adds less than extra walls. Because infill's main job is holding up the top layers, too little of it tends to show on top surfaces. If that happens, see Gaps in Top Surfaces.

Patterns in plain terms

There are many patterns. A few cover most needs. The notes below are from Prusa's description of the patterns in its slicer; other slicers have most of the same ones.

Pattern	What it is good for
Gyroid	Prusa's favorite: Prusa says it gives great support in every direction, prints fairly quickly, and its paths do not cross within a layer.
Cubic	A 3D pattern whose paths cross within each layer.
Adaptive cubic	Gets denser near the walls and sparser in the middle; Prusa says it uses about a quarter less material than rectilinear Rule of thumb.
Support cubic and lightning	Made to hold up the top layers with as little material as possible. Prusa says support cubic does not improve mechanical properties; lightning saves even more material.
Rectilinear	Fast and simple, and the only pattern Prusa recommends for 100 percent infill.
Grid	Simple and solid, but material builds up where lines cross, which can cause noise or a failed print.
Honeycomb	Strong, with no crossings, but Prusa says it uses about 25 percent more material Rule of thumb and can take up to twice as long.
Concentric	Follows the outline inward; useful for transparent or flexible parts, with long print times.

Pattern names, angles and the remaining settings go deeper in Infill Patterns and Density.

Choosing by job

• Display models and prototypes: low density in a fast or material-saving pattern; the infill only has to hold up the top.
• Everyday functional parts: more walls first, then moderate density in a pattern Prusa describes as strong in every direction, such as gyroid.
• Flexible filaments: Prusa notes that some patterns, such as concentric and Archimedean chords, can help flexibility.
• Parts you will fill with resin or epoxy: Prusa notes gyroid and the Hilbert curve make that easier.

Strength also depends on how the part is oriented on the bed, because FDM layers bond less strongly than the lines within a layer. See Print Orientation.

A useful time saver

PrusaSlicer can print infill at a thicker layer height than the walls, called combine infill. Prusa's worked example of the setting: combining every 3 layers at 0.1 mm layer height prints the infill at 0.3 mm Formula. Prusa notes the combined layer cannot exceed about 80 percent of the nozzle diameter. The walls keep their fine layers, and the hidden infill prints faster. Other slicers have similar options under their own names.