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WHEN IS IT TIME TO REPLACE YOUR SCREW? After you have established your baseline data and tracked your extruder perfor- mance over time, it is important to set a target range for optimal performance. When the system wear causes you to fall outside your optimal performance metric, it is time to consider replacing the extruder screw. The optimal performance metric could be a balance of extruder output/ energy usage. If you can still maintain your target production rate, but because of screw wear you are operating at too high a screw speed and using more energy, you may find it valuable to replace your screw to increase your extruder efficiency. The next step would be to pull the screw and measure the tolerances to determine the amount of wear. As a rule of thumb, it is consid- ered time to replace the screw when the flight clearance increases by 4 times the original clearance. For example, the typical flight clearance is the diameter divided by 1000. Thus, a 6-in. screw would have a clearance of 0.006 in. Based on the process meeting our defined performance metric, if this screw were pulled and measured at a clearance of 0.024 in., it would be recommended to replace the screw. Being in tune with your extrusion equipment will allow you to identify issues before they impact production. Creating a baseline set of data from a standardized formulation and standardized process conditions will be critical when trying to track extruder performance over time to identify potential issues. Aside from generating data, film-quality fade over time should be monitored, as this could also be a key indicator of something wrong in your extrusion system. Finally, establish some target metric to determine when to replace the screw, then replace it as soon as this tolerance is exceeded, to improve efficiency and reduce cost. ABOUT THE AUTHORS: Wes Hobson is a senior engineer in The Dow Chemical Co.'s Performance Plastics Technical Service and Development group in Freeport, Texas. He has 22 years of experience focusing on cast and blown film and sheet extrusion. Hobson in currently application development leader in Dow's Industrial and Consumer Packaging group, focusing on stretch film. He has a Master's Degree in Packaging Science from Michigan State University and is a member of ISTA and the Society of Plastics Engineers. Contact: 979-238-1610; jhobson@ dow.com; dow.com. Mike Rutkowske is a senior technologist with The Dow Chemical Co. and has 13 years of expe- rience in polymer research and technical development. He has spent the majority of his time operating both small and large-scale packaging equipment (blown film, twin-screw extru- sion, extrusion coating, etc.). He is an author of one patent and has significant experience in optimizing fabrication processes and equipment. Rutkowske is currently in the process of completing his BS in Mechanical Engineering Technology from the University of Houston and is a member of TAPPI. Contact: mrutkowske@dow.com. As you establish your baseline data you should look at the heater output for each of your extruder zones and catalog this. Make Perfect Parts and Save Money! • Preload cores for perfect parts. • Save $$$$ with a smaller cylinder. • Simplify mold operation and design. • Smaller mold base = low cost parts. • Fully Preloads at only 1,500 psi. • Stays locked without hydraulics. • 400˚ F systems available. • Rush 0-2 day shipping available. pfa-inc.com N118 W18251 Bunsen Drive Germantown, WI 53022 (262) 250-4410 Fax (262) 250-4409 KOR-LOK ® , Hydra-Latch ™ and Hydra-Jaws ™ are trademarks of PFA, Inc. ©2016 PFA, Inc. All Rights Reserved. Mold OPEN Core Out Mold CLOSED Core OUT Mold CLOSED Core IN Core PRE-LOADED Against FULL Injection Pressure CAN'T DO IT! CAN'T DO IT! KOR-LOK ® TRADITIONAL SIDE-ACTION VS. SIDE-ACTION SYSTEMS SYSTEMS YES! YES! YES! YES! YES! YES! SIDE-ACTION SYSTEMS Designed & Made in USA HYDRA-JAWS ™ Every Mold, Every Machine, Every Time! Adjusts to Fit Every Mold Quick and Secure Consistent Clamping Force @plastechmag 69 Plastics Technology E X T R U D E R W E A R