Plastics Technology

OCT 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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Page 34 of 75

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 or (724)651-9196. With a melt pump, simply lowering and raising the suction pressure will give you an indication if a partially filled metering section is the problem, if the frequency and magnitude of the surge are affected as noted above. It takes the "fill length" calculation to estimate the actual degree of fill. Without a melt pump, you'll need to change some part of the downstream polymer flow appa- ratus to create a change in head pressure. If a screen changer is used, changing the screens can usually do the trick. You'll often notice a screw pulse with clean screens that will slowly disappear as the screens plug and the head pressure rises. That can be seen even with a melt pump, as the extruder screw speed chases the backpressure fluctuation. The output to the die is steady because of the melt pump, but the screw speed is changing constantly. Although this may not seem to be a problem, changes in screw speed also change melt temperature, which may affect the final product, particularly complex profiles and very tiny extrusions such as filaments. The cyclic surge is very common in two-stage screws where the design was not specific to the head pressure. That's often the case for a new application where the head pressure cannot be estimated. But rules of thumb—like sizing the second metering section 1.5 times the depth of the first metering section—can result in this type of problem. In single-stage screws, the cause of a partially filled metering section is usually inadequate feeding, either by design, or because the characteristics of the polymer have reduced the output such that the metering section is not filled. Very recently I reviewed a smooth- bore extruder running HMWPE that had a steady cyclic surge. The slippery nature of HMWPE often produces a less-than-antic- ipated feed rate, resulting in a partially filled metering section. The surge was eliminated by raising the head pressure by just a few hundred pounds with the screen changer. A word to the wise: Be sure to stay well under the pump discharge pressure when trying to eliminate the cyclic surge with a melt pump by increasing the suction pressure. That will keep polymer flowing through the melt pump bearings for lubrication and prevent damage to the device. If the output has a rhythmic surge, the cause is almost always a partially filled metering section. @plastechmag 33 Plastics Technology E X T R U S I O N

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