Plastics Technology

DEC 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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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. Get more insights on Extrusion from our expert authors. Learn more at KNOW HOW EXTRUSION metal as well as a tearing away or galling of the two materials. Since the screw is contained in the closed environment of the barrel, it's usually not obvious which or how many of these wear mechanisms is taking place. But I have found over the years that there are telltale signs of the cause if you examine the worn screw flights. If you have a burr on the trailing side of the flight, wedging is most likely the cause. That occurs from a combination of the high compressive force and the angular motion of the flight relative to the barrel wall during rotation. Essentially, the flight surface is torn off the back of the flight. This will occur almost exclusively in the compression or barrier section of the screw. Mineral fillers often contain hard particles. However even relatively soft materials can cause a degree of screw wear through an extended combination of particle fracture, removal of lubrication, and impact. The same holds true of virtu- ally any type of recycled material. Wear caused by abrasion will tend to reveal itself by a polishing or rounding of the flight corners on the pushing side of the flight surface, eroding the base screw material Misalignment creates a combination of forces: One is similar to the wedging, but there is also an enormous pressure on the flight as the screw is forced to bend to follow the barrel bore. This results in a tearing and a crushing action that produces a burr on both sides of the flight. This can happen at any area on the screw, While mineral fillers contain hard particles, even relatively soft materials can cause a degree of screw wear. 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 T: (508) 655-1123 F: (508) 655-9337 depending on the barrel support(s) and alignment with the reducer. Determination of the cause of the wear is the first step to eliminating it. Once the cause is determined, corrective actions are pretty clear. Occasionally there can be two simultaneous causes—such as wedging and alignment or abrasive wear that masks the other two—but that is pretty rare and does not negate this analysis. @plastechmag 31 Plastics Technology E X T R U S I O N

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