As soon as the melted plastic leaves the nozzle, it's no longer under the precise control of the extrusion system. There are two problems to consider:

Printing must always be done on an already solidified layer, otherwise, the previously extruded plastic will be moved around and distorted by the new layer
When printing overhangs or bridges, it's important to freeze and solidify the plastic in place as soon as possible. Otherwise, the filament will sag down.

Most 3D printers come with a dedicated print fan. By blasting ambient air at the extruded filament, the cooling can be dramatically improved. However, with some materials, this could make things actually worse and cause warping or layer separation. PrusaSlicer lets you adjust the cooling settings for each filament.

Keep fan always on

The print fan will always maintain at least a minimum speed. This setting is typically on for most materials with the exception of ABS, ASA, PC, and some flexibles.

Enable auto cooling

When enabled, the cooling fan and print speed will change during the print based on the settings below. A text description below will update with every change of any cooling setting and will explain in detail the resulting cooling behavior.

When disabled, the print fan will run at a fixed speed the whole time defined by Fan speed - Min with the exception of bridges, which can still be defined by Bridges fan speed.

Fan settings

Fan speed

You can set Min and Max fan speed as a percentage of the fan's max RPM.

The Max value will be used when the layer print time is below the Slow down if layer print time is below value.

The Min value will be used when the layer print time is longer than Enable fan if layer print time is below value.

If the layer print time is between these two values, the fan speed will run proportionally at a speed between the Min and Max value.

This may seem a bit confusing, but you can use the live-updating text description above Fan settings, which will hopefully make everything clear. Try changing the values and see how the description changes.

Bridges fan speed

Overrides the fan speed with set value when printing bridges and overhangs. Typically you'll set this to a higher value compared to normal printing.

Disable fan for the first X layers

Cooling the very first layer is usually not necessary, as it's being laid on a flat surface. With many filaments, you may even want to skip cooling for a few more first layers (typically between 1-5) to prevent warping and detaching the print from the print bed, which would inevitably lead to a ruined print.

Dynamic fan speeds

The setting enables and controls dynamic, variable fan speed in function of the overlap percentage.

The overlap percentage represents the overlap of extrusion with the previous layer. A 100% overlap is a full overlap (no overhang), while a 0% overlap represents a full overhang (floating extrusion, bridge).

The input fan speed values represent the percentage of the fan's max RPM.

Fan speeds for overhang sizes in between are calculated via linear interpolation.

Cooling thresholds

Enable fan if layer print time is below

If the layer print time is estimated below this number of seconds, the print fan will be enabled and its speed will be calculated by interpolating between the Min and Max speed.

Slow down if layer print time is below

If the layer print time is estimated below this number of seconds, the print move speed will be slowed down to extend the duration to this value. The slowing is done by scaling the print speeds, so some moves will still be faster than others.

Min print speed

The minimum speed that PrusaSlicer will scale the speeds down to. Printing at extremely low speeds for extended periods of time could lead to problems such as heat creep.

Keep in mind that by raising this value too high, you may partially invalidate the minimum layer time set by Slow down if layer print time is below. Because even when printing at this minimum speed, the layer time might be shorter than the set value.

Filament settings

Cooling