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EXTRUSION Zone override is a common concern of almost every extrusion operator, and the typical response is to impose more cooling on the offending barrel zone—even including chilled water. That may or may not work in favor of the process. All zone override is telling you is that the polymer at the inside of the barrel wall is at a higher temperature than the barrel setting. Since the barrel is a very good conductor of heat—and the polymer is a very poor conductor— it's logical that some of the shear heating near the barrel wall would be absorbed into the barrel. If the barrel settings are well below the internal melt temperature, the barrel temperature rises above the setting and overrides. How readily one can change the melt temperature simply by changing the barrel settings seems to be where the misunderstanding occurs. It's generally difficult to overcome the effects of the screw and drive motor by reducing the barrel temperatures, because the drive power on extruders is typically four to 20 times the maximum barrel-cooling capacity, depending on the extruder size and type of cooling. The larger the extruder, the less effective barrel cooling becomes, since the mass of polymer in the screw increases exponentially with screw size compared with the barrel surface area available for cooling. Cooling will extract heat from the barrel but not necessarily from the process because of the low thermal conductivity of the polymer. In fact, overcooling can actually add to the viscous dissipation by cooling the polymer near the barrel wall, thereby requiring more torque. Extruder drive torque is largely the resistance of the screw to rotation in the viscous polymer. The electrical energy used by the drive is converted to rotational force to turn the screw. The rota- The Facts About Barrel- Temperature Override tional force is then converted again to a rise in polymer temperature by shear heating. Energy input from the drive mostly ends up in the polymer. The greater the resistance to rotation, the higher the input torque requirement and the more heat entering the polymer. The viscosity of the melted polymer between the screw and the barrel determines the resistance to rotation and the resulting torque requirement, regardless of the amount of the solids in the screw. That explains why it's harder to melt a 25 MI polymer than a 2 MI of the same polymer on the same screw. It's harder to get energy into the polymer because of the lower torque requirement. To illustrate another way, think of turning a solid cylinder inside a hollow tube with some lubricating oil in between. The torque to turn the cylinder (screw) is largely a function of the lubricating oil. If you have a very low-viscosity lubricant, the torque requirement is low and the energy input is low. Excessively cooling the tube can increase the viscosity of the lubricant and increase the torque requirement. This becomes a "Catch 22" as the more heat extracted in the barrel cooling, the more viscous the film becomes, and the more energy the screw must put into the film to rotate the screw. The overall result depends on the effect of temperature on the viscosity of that polymer and the effectiveness of the barrel cooling. For polymers whose viscosity is less strongly affected by temperature, it is easier to reduce the overall melt temperature to some degree with barrel cooling. The property that reflects the polymers' viscosity response to temperature is called the consistency index. For a power- law fluid, which fits most polymers, the viscosity-thinning behavior is a function of its consistency (m), Power-law coefficient (n), and shear rate. It's described by the Ostwald de Waele equation: Viscosity (µ) = mγ n-1 m = consistency γ = shear rate n = Power-law coefficient The consistency index is determined by the intercept on a linear plot of shear rate and viscosity at 1 sec-1. As a result, it is not a fixed It's generally difficult to overcome the effects of the screw and drive motor by reducing the barrel temperatures. Cooling the barrel more may extract heat from the barrel, but not necessarily from the process. Here's why. By Jim Frankland Determining the effects of temperature on viscosity can be done by measuring the vertical displacement of the shear rate/viscosity curves at two different temperatures. 10,000 1000 100 10 Viscosity, Poise Shear Rate, sec -1 10 1000 10,000 3000 poise 230 C 250 C Estimating Temperature Effects on Viscosity 34 SEPTEMBER 2017 Plastics Technology PTonline.com K now How