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The main purpose of infill is to provide internal support for top layers, which would otherwise have to bridge over empty space. Infill also affects the speed of printing, structural strength, filament consumption, and even the look of the printed object.

Some options will only appear when Expert mode is selected.

Fill density

Most models can be printed with 10-15% infill. If the top of the model closes gradually, it can be printed hollow (0% infill), though we generally do not recommend it. If you need the model to be heavier, want more compression resistance or higher stiffness, you can increase the infill. You'll rarely need infill higher than 30%. Finally, you can print the model with 100% infill and the infill pattern will be forced to rectilinear. Keep in mind that 100% infill can have a negative impact on the look of the printed object.

Increasing model strength
The strength of a model is mostly defined by the number of perimeters (not infill). If you want to have a stronger print, increase the number of perimeters. That said, infill does help with compression resistance.

Fill pattern

PrusaSlicer offers many infill patterns to choose from - check the dedicated Infill pattern documentation page.

Length of the infill anchor

The infill is usually connected to the perimeters with a so-called anchor, which is a short line of another (inner) perimeter that turns into an infill line. This makes the infill sturdier and is required by some materials (namely PETG) to stabilize the extrusion flow at the start of an infill line.

The length of the infill anchor value sets how many millimeters of this anchor will connect the infill and the perimeters. If you set this value to 0, the infill will be printed independently on the perimeters. Set a higher value to print the anchor that will continue as the infill itself.

If expressed as a percentage (15%) it will be calculated over the infill extrusion width.

Maximum length of the infill anchor

This value sets the maximum length of the infill anchor connecting the infill with perimeters.

Set this parameter to 0 to disable anchoring completely.

Top fill pattern

You can pick a custom infill pattern that only affects the top visible layer, not adjacent solid layers below. 

Bottom fill pattern

You can pick a custom infill pattern that only affects the bottom visible layer, not adjacent solid layers above.

Combine infill every X layers

Different layer heights will be used for infill and for perimeters in order to speed up the print without sacrificing the print quality.
Example: Combine infill every 3 layers, layer height 0.1 mm, infill will be printed at 0.3 mm layer height.

The maximum layer height is limited by your nozzle diameter. If you try to combine infill every 1+ layers with 0.4 mm nozzle and 0.3 mm layer height, no change will actually happen because you cannot print layers bigger than (about) 0.32mm (80% of nozzle diameter).
Infill (grey) printed at double the layer height of perimeters (orange)


Solid Infill every X layers

This feature allows you to force a solid layer after every given number of layers. This can be useful to increase the strength of the printed part, however other options, such as increasing the number of perimeters or infill percentage are preferred. Another application would be to create separate internal cells in order to increase thermal insulation.

Set to 0 to disable.

Fill angle

Default angle for infill orientation. Cross-hatching will be applied to some infill patterns.

Bridges will be printed using the best direction PrusaSlicer can detect, so this setting does not affect them.

Solid infill threshold area

Force solid infill for regions having a smaller area than the specified threshold.

Set to 0 to disable.

Solid infill threshold 0 (left), 15 (middle), 100 (right)

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