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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 jim.frankland@comcast.net or (724)651-9196. this point, there is no opposing pressure opposite the pressurized area over the solid and the screw is forced sideways in the barrel, causing wear on the fights opposite the pressure. I recently inspected an 8-in. barrier screw with this wear pattern. I have seen such pres- sures occur in laboratory-scale extruders with barrels having multiple transducers along their length, but there was no such data available for this screw. However, by approximating the forces necessary to bend an 8-in. steel bar enough to completely wear the fights to the root diameter over a span of about fve diameters, I could estimate the scope of the pressure development (Fig. 2). In this case the screw had a solids-channel depth of approximately 0.100 in. and a melt- channel depth of 0.800 in. with a 7/8-in. fight width. Although this is an indeterminate shape and not a truly fxed beam, I approxi- mated the cross-section at the wear point as 7.375 in. diam. round. Assuming a concen- trated load, the force to bend that shaft 0.100 in. over a length of 45 in. is almost 350,000 lb. While this analysis is far from perfect, it indicates the magnitude of the force generated when a solid plug flls the end of the solids channel. Remembering that a compression section of a screw is essentially a spiral wedge, it can be calculated that a surprisingly small amount of torque goes into creating such enormous pressures. There is a tendency to make the barrier section as long as possible to prevent plugging along its length by exceeding the melting rate—i.e., "wedging." Although that can reduce the wear along most of the length of the barrier, it can also result in relatively catastrophic wear near the end of the barrier by undershooting the melting rate and allowing melt to accumulate at the end of the solids channel. Additional investigation of this type of wear is ongoing and a more quantitative solution is being developed. A tremendous amount of force is generated when a solid plug flls the end of the solids channel in a barrier screw. On this 8-in. barrier screw, the wear pattern indicated a force of about 350,000 lb over 45 in. caused the shaft to bend 0.100 in. Trapped Solids Can Wear Screw Flights in Barrier Designs FIG 2 Radial Wear Measurements .035" .130" .030" 45" SCHNEIDER CAN ALL OF YOUR PLASTIC WWW.SCHNEIDERPACKAGING.COM AUTOMATION EQUIPMENT ROBOT END OF ARM TOOLING STACKING & PACKING EQUIPMENT CASE PACKING Whether it's blow molded, flm based, or injected molded Schneider can handle it. We can keep up to speed with any Blow Molding in the industry and the small footprint and portable features make Schneider the smart, logical answer. We can handle and package your plastic bottles neck up or neck down keeping your operation moving quickly and effciently. Do yourself and your customers a favor. Contact Schneider Packaging today. P.O. BOX 890 5370 GUY YOUNG ROAD BREWERTON, NY 13029 PHONE / 315.676.3035 FAX / 315.676.2875 INFO@SCHNEIDEREQUIP.COM WWW.SCHNEIDERPACKAGING.COM No matter the bottle... NPE 2015 Booth# W8663 30 DECEMBER 2014 Plastics Technology PTonline.com K now How