In FDM, what does “retraction” refer to?

Retraction in Fused Deposition Modeling (FDM) refers to the process of pulling filament back into the nozzle during travel moves. This technique is crucial for preventing unwanted stringing and oozing of filament while the print head moves between different areas of the print. When the nozzle is not actively depositing material, retraction retracts a small amount of filament to relieve pressure within the nozzle. This helps maintain the quality of the print by ensuring that only the desired amount of filament is extruded at any given time, thereby enhancing the precision of the printed object.

In contrast, the other choices do not accurately capture the concept of retraction. The speed at which the nozzle moves without depositing filament pertains to travel speed but does not involve the pulling back of filament. Cleaning the nozzle involves different maintenance procedures and does not relate to filament management during printing. The cooling down stage is a part of the printing process that occurs after printing has finished, not during the movement of the nozzle.