Retraction Retract Speed
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This is the speed at which the material is pulled out of the nozzle chamber when making a retraction.

In general you'll want to set this as high as possible without grinding the material at the feeder. Moving the filament at greater speed makes the nozzle stand still for a shorter amount of time, which reduces the size of the blob in the location where it retracts and slightly reduces printing time.

If the speed is set too high however, the force excerted by the feeder will be so high that the feeder wheel will start to grind into the filament. This reduces the reliability of the print.