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Film quality should be monitored and cataloged as a key indicator of extrusion performance and can be used to determine extruder wear. It was mentioned earlier that an increase in specific rate may not be a bad thing. That is not always true. A problem that may appear with higher specific rate could be degradation via higher shear heating. An increase in background gels—gels that constantly appear in the film—may result from degradation due to shear heating. An increase in unmelt gels in a system could result from a loss of efficiency in the mixing element. Another symptom of mixing issues would be high melt-temperature variation and/or mixing issues that manifest themselves as film abnormalities. A good way to measure the melt-temper- ature variation would be to use a variable- depth thermocouple. With your transfer-line pipe temperature setpoint near the extrudate temperature, start with the thermocouple retracted as far out of the melt as possible. Take a reading, then insert the thermo- couple into the melt one complete turn and stop. Allow the measurement to come to a steady state, then take a reading and repeat. Continue this procedure until you have reached maximum extension. Now take the data and look at the amount of tempera- ture variation across the melt. If you see less than 10° F variation, you have good homogenization of the melt. If your data shows more than 20° F variation, you have poor melt homogenization. Time FIG 2 Screw Wear Over Time vs. Heater Output Heater Output, % 70 60 50 40 30 20 10 0 16 15 14 13 12 11 10 9 8 7 6 Specific Rate, lb/[hr rpm] As the screw wears, shear is decreased, causing the heater to add more energy to maintain setpoint. At Athena, we understand your unique hot runner system needs. That's why we engineer products like our new RMX modular hot runner controller with a touchscreen display and superior functionality and communications. Athena still off ers our popular IMP, RMA and RMB modular controllers and you can mix and match in a standard mainframe to suit your unique needs. And Athena still off ers world-class non-modular controllers. Get all our products from your local distributor or online. Find your hot runner controllers at www.AthenaControls.com Find your hot runner controllers at www.AthenaControls.com At Athena, we understand your from your local distributor or online. BOOTH W5455 Film quality should be monitored and cataloged as a key indicator of extrusion performance and can be used to determine extruder wear. 68 FEBRUARY 2018 Plastics Technology PTonline.com T ips & Technique s