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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. value and is dependent on the melt temperature, so it's a value that's not available for everyday processing unless you have exten- sive rheology data to model it. However, it's been proven helpful to simply note the separation between shear-rate/viscosity curves at different temperatures, which is an indication of the effect of temperature on viscosity. The consistency is related to the vertical location of the shear-rate/viscosity curve at a specific temperature. For example, the curve in the graph on p. 34 shows a change of 3000 poise with a change of 20° C, or 150 poise per °C. That would be a moderate degree of temperature sensitivity, and the resin would tend to maintain its viscosity with changing temperature and lend it to some degree of possibly effective barrel cooling of the melt. Of the 11 polymers I checked, the viscosity changes range from 10 poise per °C to 1080 poise per °C at equal shear rates.. Assuming you can arbitrarily pick zone settings to control the melt temperature, like you were using an oven, will sometimes result in no change or even a slight increase in the melt tempera- ture. Use of shear-rate/viscosity curves at different temperatures can assist you in determining what effect barrel cooling may provide. As mentioned, it's surprisingly difficult to change the melt temperature significantly with the barrel-cooling zones working against the overpowering effect of the screw and drive, due to the poor thermal conductivity of the polymer and the continuous shear heating that occurs in the flight clearance. As a result, arbitrary barrel-zone temperature selection and/ or overcooling can lead to problems like unstable operation, insta- bility as the cooling switches on and off, high wear, poor melt homogeneity, and wasted energy. Energy consumption is particu- larly affected, because you are putting energy in with the drive and taking it out with the barrel cooling. It's not unusual to find extruders using 20% more total power (heating and cooling) than necessary because of such conditions. Finding the best barrel settings requires evaluation of each of these negative aspects of overcooling compared with the effects on melt temperature. For example, it's not smart operating practice to use 20% more power in attempting to achieve a few degrees lower melt temperature. Comparing shear-rate/viscosity curves at different temperatures will be a guide in what to do with your barrel zones. For major changes in melt temperatures, the screw design is usually where the solution is found. For polymers with a high viscosity change with temperature, it's likely that override can only be solved with screw design. 525 East Stop 18 Road Greenwood, Indiana 46142 Phone: 317-887-6352 Email: sales@TempTek.com For information online www.Temptek.com Factory Phone 317-887-6352 Outdoor Central Chillers • Complete Outdoor Chilling System with Refrigerant Zones and Pumping System • For Use In Most Outdoor Climates • Central Unit for Plant Wide Cooling for Many Processes • Integral Air-Cooled Condenser Indoor Portable & Central Chillers • Use Inside Plant Facility for Single Process or Plant Wide Cooling • Microprocessor Controller and Display • Small Footprint to save floor space • Environment Friendly Refrigerants • Internal Coolant Reservoir And Cirulation Pump Water Chillers with New Product Warranties, industry recognized components for Maximum Reliability, Economically Affordable capital equipment providing perfect bottom-line returns, and a network of factory trained technicians for Unbeatable Service. • Environment Friendly Refrigerants • Microprocessor Instrument Controls • Non-Ferrous Construction • Small Footprint • Other Products: Temperature Control Units, Pump Tanks, Cooling Towers, Material Dryers, Loaders, Granulators 36 SEPTEMBER 2017 Plastics Technology PTonline.com K now How