The L/D ratio (Length-to-Diameter ratio) refers to the effective length of the extruder screw compared with its diameter. For example, if a screw has a diameter of 60 mm and an effective length of 1,500 mm, its L/D ratio is 25:1. This ratio is an important factor in determining how much time the material spends inside the extruder during processing. https://uniqueextrusion.com/twin-screw-vs-single-screw-extruder/