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In the early 1960s, extruders typically had a length/diameter ratio of 20:1, and a machine with a 24:1 L/D was considered long. Since then, extruders have gotten longer, with 30:1 to 36:1 L/D becoming the industry "standard." Some extruders even exceed 40:1 L/D for special purposes like double venting, compounding, or high- speed processing. What benefits does the additional length provide? Mostly increased output and improved homogenization. Since the feed section stays approximately the same length, regardless of the L/D, the rest of the screw is devoted to melting and pumping. The deeper the screw channels, or the higher the specific output (lb/rpm), the more length you need to complete melting and develop the pressure necessary to push the polymer out the die. As designers reached these limits, extruders were built longer to handle the economic requirements to pump out more and better product. However, there are actual limits on increasing output as L/D is increased. Usually these limits are due to the inability of the feed section to deliver more polymer. On smaller-diameter screws, that limit often is determined by screw strength. On small screws you can only go so deep in screw channels before the screw is overtorqued and fails. On larger extruders, the effi- ciency of feeding decreases as the channels get deeper until there is no further increase in output. Two-stage screws benefit more with increasing L/D because about 4-6 D is consumed in the vent area, which contributes very little to melting or pressurization. For most applications, you'll need a 30:1 two-stage screw to match the output of a 24:1 single- stage screw (see illustrations above). EXTRUSION How Much L/D Do You Really Need? A screw that is too long for the overall processing situation can actually limit output. Just like selecting the extruder size and drive combination, the L/D should be carefully evaluated. By Jim Frankland Relative Output Levels Single-Stage Screws Output 20:1 24:1 30:1 36:1 Two-Stage Screws Output 20:1 24:1 30:1 36:1 0.8 1.0 1.20 1.38 0.5 0.8 1.0 1.15 For most applications, you'll need a longer two-stage screw to match the output of a single-stage screw. L/D L/D Get more insights on Extrusion from our expert authors. short.ptonline.com/extrudeKH Learn more at PTonline.com KNOW HOW EXTRUSION 46 FEBRUARY 2018 Plastics Technology PTonline.com K now How