Plastics Technology

SEP 2018

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EXTRUSION Head pressure never seems to be a big concern of extrusion pro- cessors, while melt temperature is something that always worries them. But they are closely connected. The melt temperature at open discharge is a function of the extruder alone; it is controlled by the screw design, screw speed, L/D ratio, polymer properties, condition of screw and barrel, and effi- ciency of barrel heating/cooling. That becomes a baseline temperature that can be altered only with changes in one or more of those extruder variables. When head pressure is applied to the end of the extruder, the melt temperature rises non-linearly from the baseline with increasing pressure. That's due to a cascading effect of the pressure flow—output from the screw is reduced as the head pressure increases. As the screw continues to rotate with reduced output, the energy via shear stress going into the polymer is increasing. The increasing energy input results in an increase in the melt temperature and a decrease in the viscosity of the polymer, further increasing the pressure flow and further decreasing the output. So, melt temperature is connected to head pressure and results in reduced output, greater power requirement, more downstream cooling, and maybe even degradation of the polymer properties. This results in greater manufacturing cost How to Estimate and Control Head Pressure and needlessly compromises the performance of whole systems when it is easily diagnosed and corrected. A melt pump can correct much of this effect by allowing a head pressure that is usually much lower than the full head pressure. However, many processes cannot tolerate use of a melt pump due to the incorporation of fillers and the possibilities of polymer degradation and contamination. In those applications, the downstream tooling design is important to the performance and profitability of the line. Unfortunately, there is often no consider- ation of the effects of head pressure in the selection of the down- stream components and its effect on the overall process. Head pressure can be accurately estimated and controlled by proper design. This involves mostly simple things, such as limiting the length of adapters and flow pipes, proper sizing of the screen changer, die designs specific to the polymer proper- ties, proper sizing of all the flow channels for the expected output, and proper heating of the downstream components. The simplest forms of flow channels are the circle, slit and annulus. There are simple approximations for calculating each of these basic shapes. By using basic Newtonian formulas for each shape, you can illustrate the principles and get a good estimate of the head pressure (see accom- panying table and Fig. 1). Using Newtonian equations requires determinating the viscosity from shear-rate/viscosity curves for that polymer at the appropriate temperature. Newtonian analysis neglects some viscoelastic effects—like viscous heating at the wall and entrance effects as the flow shapes change. The shear rate must be calculated and then applied to the shear-rate/viscosity curves at the processing temperature to obtain the correct Head pressure further increases the melt temperature from the extruder baseline. You rightfully worry about melt temperature, but don't overlook head pressure, because the two are closely linked and will influence line performance. By Jim Frankland For a round orifice, the pressure is increased by eight times the passage length but decreases by the fourth power of the radius. For a slit orifice and an annular passage, the pressure is increased by 12 times the length and reduced by the first power of the width and the third power of the height or difference in the radii. R H Ro Ri W Round Slit Annulus FIG 1 40 SEPTEMBER 2018 Plastics Technology PTonline.com K now How

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