Extrusion basics: Pumps and lumps, or life between the screw(s) and die: Page 2 of 2

Lumps in your product? The gear pump evens out the axial flow, but doesn’t do any mixing. In fact, if it lowers the pressure inside the extruder, the mixing may actually be worse than without the pump! That’s why pump makers also offer static mixers installed after the pump—after, so the pump pushes it through the mixer, not the extruder. Static mixers are tubes with baffled or otherwise twisted innards, so the flowing melt is split and resplit again and again. They are usually horizontal, and typically one extruder size below the diameter (e.g., 3.5 inches for a 4.5-inch extruder). In retrofitting blown film or any line where neither takeoff nor extruder can be moved easily, they can be inserted vertically, changing height but not horizontal length. 

It is useful but not common to have pressure gauges at both entry and exit. You can also calculate this pressure drop if you have good viscosity data for your material as a function of shear rate and melt temperature.

Lastly, if you have a mixing problem, consider altering materials as well as a mechanical remedy like a static mixer or water inside the screw. Changing a filler type or even just its particle size may help, or adding an internal lubricant, or (for mixtures of melts) getting the viscosities of the components in proportion to the proportions. 

Chemistry illustration from Allan GriffIf you are going to NPE/ANTEC in Orlando on May 7 to 11 and want to talk with me in person about extrusion or plastophobia (or anything else, for that matter), I’ll be on the Neutrex/Purgex stand (S20035) giving half-hour talks on "The Ten (11) Principles of Extrusion" on Monday and Wednesday at 2 PM and Thursday at 10 AM, and on "Plastics Chemistry for NonChemists" (including my cartoons, one of which is pictured here) on Tuesday and Thursday at 2 PM. You can register for the sessions on the Purgex website.


Allan Griff is a veteran extrusion engineer, starting out in tech service for a major resin supplier, and working on his own now for many years as a consultant, expert witness in law cases and especially as an educator via webinars and seminars, both public and in-house. He wrote the first practical extrusion book back in the 1960s as well as the Plastics Extrusion Operating Manual, updated almost every year, and available in Spanish and French as well as English. Find out more on his website, www.griffex.com, or e-mail him at [email protected].

Griff will present a live seminar in Amherst, MA, on May 22. Seminars in your plant are also available. If you can’t attend his live events, he offers a Virtual Seminar, which can be seen at any time, anywhere. E-mail Griff at the address listed above for more information.

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