As reported by Plastics Technology in their article “How to Estimate and Control Head Pressure,” head pressure in extrusion processes is often overlooked by processors, who tend to focus more on melt temperature. However, these two factors are closely linked. The melt temperature at open discharge is determined by the extruder’s screw design, speed, L/D ratio, polymer properties, and the condition of the screw and barrel. This baseline temperature can only be altered by modifying one or more of these variables.
When head pressure is applied to the extruder, the melt temperature rises non-linearly from the baseline as the pressure increases. This is due to the cascading effect of pressure flow—reduced output from the screw as head pressure increases, leading to increased shear stress and energy input into the polymer. This raises the melt temperature and decreases the polymer’s viscosity, further increasing pressure flow and decreasing output.
These challenges are addressed by implementing effective head pressure control techniques. Proper design of downstream tooling and the use of melt pumps can significantly reduce head pressure, thereby improving the overall efficiency and performance of extrusion processes. Although some processes cannot tolerate melt pumps due to fillers and potential polymer degradation, careful downstream tooling design remains crucial.
Accurately estimating and controlling head pressure involves simple yet effective strategies, such as limiting the length of adapters and flow pipes, properly sizing screen changers, designing dies specific to polymer properties, and ensuring appropriate heating of downstream components. Understanding flow channel design, such as circles, slits, and annuli, and using Newtonian equations to approximate head pressure, allows processors to make informed decisions without the need for complex computer analyses.
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Article with all rights reserved, courtesy of ptonline.com
