5.2.1 Single and Twin Screw Extruders. Screw extruders are divided into single screw and multiscrew extruders. Single screw extruders are most popular for their low cost, simple designs, ruggedness, and reliability. Typical sizes vary from 1 to 6 inch (25 to 150 mm) and L / D ratio from 20 to 30 with 24 being most common.