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Infill is the internal structure printed inside a 3D-printed part’s outer walls. It can support the solid layers that form the top, help a part resist compression, and affect print time, material use, weight, and appearance. It is not necessary for every print: many models work with sparse infill, and some shapes can be printed hollow.
What infill does inside a 3D print
In FDM printing, the outer walls—or perimeters—form the part’s shell. Infill occupies some of the space inside that shell, usually as a repeating pattern rather than a solid block. Its key job is to give the top solid layers something to rest on instead of making them bridge a wide, empty gap. Infill also contributes to resistance when a part is compressed and changes how much plastic and printing time a part requires. Prusa Research’s infill documentation describes supporting top layers as infill’s main purpose.
Is infill necessary?
No—not for every model. A part may print successfully with 0% infill if its shape closes gradually and does not leave unsupported top surfaces. That is a shape-specific option, not a general recommendation to print everything hollow. For a typical starting point, Prusa Research says most models can use 10–15% infill; its guidance also says densities above 30% are rarely needed. These are general slicer recommendations, not guarantees for a particular part or load. Prusa’s infill guidance explains the qualifications.
Start with the printer or slicer profile, then inspect the sliced preview: look for broad top surfaces with little infill beneath them. The appropriate setting depends on the model’s shape, material, top skin, and intended use.
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Choose strength settings by the force the part must handle
More infill is not automatically the best way to make a part stronger. Prusa says strength is mostly defined by the number of perimeters, and recommends increasing perimeters when strength is the goal. Infill still helps support top layers and can improve resistance to compression, but it cannot reliably compensate for a weak shell, poor layer bonding, or an orientation that puts force across vulnerable layer lines.
- For a stronger ordinary part: consider adding perimeters and orienting the part so the printed layers suit the expected force before raising infill density.
- For compression or internal support: infill can contribute, but the pattern and density need to suit the part and load.
- For safety-critical or load-bearing parts: generic slicer percentages are not an engineering specification. Use part-specific engineering and testing.
There is no universal strength curve for infill density. Higher density generally takes more material and time, but the strength gained varies with pattern, material, geometry, and loading. UltiMaker cautions that density and strength do not have a linear relationship. UltiMaker’s infill-pattern overview discusses those trade-offs.
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Pick an infill pattern for the job
Patterns differ in printing speed, material use, support beneath top layers, appearance, flexibility, and how they respond to forces from different directions. The descriptions below are specific to the cited slicer guidance; a pattern name alone does not guarantee a particular part’s performance.
| Pattern | When it may fit | What to know |
|---|---|---|
| Gyroid | General-purpose parts that may see forces from multiple directions | Prusa describes it as a three-dimensional pattern with support in all directions, relatively quick printing, and a good strength-to-weight ratio. Actual part strength depends on the complete design and print. Prusa Research |
| Rectilinear | Speed-conscious everyday prints | Prusa calls it one of its fastest patterns; its lines alternate direction. PrusaSlicer also switches to rectilinear at 100% infill. Prusa Research |
| Adaptive cubic | Parts where a slicer-specific variable-density pattern may reduce material use | Prusa describes larger cavities in the center and denser support near walls and top or bottom regions. Its documentation reports about one-quarter less material than rectilinear for the described implementation; that comparison is specific to that implementation. Prusa Research |
| Support cubic | Parts needing more support near top layers while conserving material elsewhere | Prusa says it becomes denser near top layers. Its stated purpose is supporting those layers while saving material, not improving mechanical qualities. Prusa Research |
| Lightning | Models and rapid prototypes where internal strength is less important | UltiMaker describes it as a way to save time and material; it is not a suitable choice for parts that must carry meaningful loads. UltiMaker |
Prevent sagging on top surfaces
The top skin has to bridge between the infill lines beneath it. With sparse infill, that unsupported span is longer, so the first top lines can sag. Prusa suggests using at least three top layers to counter this. In the slicer, also assess whether adding top solid layers or increasing infill locally near the top is appropriate for the model. Prusa’s top solid layers guidance
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How to set infill for a new model
- Begin with the slicer’s profile. Avoid assuming that every object needs a high density. Prusa’s 10–15% figure is a general starting point for most models, not a requirement.
- Check the preview. Inspect broad top surfaces and confirm that the infill and top solid layers provide suitable support.
- Match settings to the part’s job. Consider perimeters and orientation for strength; choose infill density and pattern based on top support, compression, force direction, appearance, and material efficiency.
- Adjust cautiously for non-critical parts. If the result has a poor top surface or does not meet its intended use, change relevant settings and print a sample before relying on the part.
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