Plastics Technology - Dedicated to improving Plastics Processing.
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PTonline.com E X T R U S I O N K now How 48 FEBRUARY 2018 Plastics Technology polymer breakdown, color shift, loss of additive effectiveness, and plate-out, to name a few issues. For a polymer that melts easily, the melting length should ideally be shorter, as excessively long transitions can actually reduce melting rate. The same is true of pressure develop- ment, as widely used melt pumps greatly reduce the need for long metering sec- tions to handle the discharge pressure. As a result, the tendency to buy longer and longer L/D extruders can actually penalize overall performance. Custom extrusion houses may simply have to live with this reality because they never know what they will be running next year, and a longer extruder has more inherent flexibility than a shorter one. But if you have a dedicated process, there can be self-imposed limits with an extruder that is too long. Just like selecting the extruder size and drive combination, the L/D should be carefully evaluated. Everyone wants the most usable output from their extruder, but if the material comes out too hot or too degraded then the singular focus on rate is actually reducing the extruder's capability. Data such as diffusivity, power-law coef- ficients, melting points, head pressure, viscosity, and crystallinity should be part of the evaluation process. ABOUT THE AUTHOR: Jim Frankland is a mechanical engineer who has been involved in all types of extrusion processing for more than 40 years. He is now president of Frankland Plastics Consulting, LLC. Contact jim.frankland@comcast.net or (724)651-9196. Are there any disadvantages associated with longer extruders? Some polymers melt much easier and faster than others. Also, some processes typically have low head pres- sures, while others have much higher discharge pressures. Inherent viscosity differs a lot between polymers, and some shear-thin significantly while others do not (i.e., are more "Newtonian"). As a result, screw performance is optimized at a variety of L/Ds rather than at any one standard L/D. A screw that is too long for the overall processing situation can actually limit output. The limitation generally shows up as excessively high melt temperature that can cause The tendency to buy longer and longer L/D extruders can actually penalize overall perfor- Complete Blown Film THE X -DIE & High Performance V -RING ALPINE AMERICAN T H E B L O W N F I L M E X P E R T S www.halpine.com T: (508) 655-1123 F: (508) 655-9337