The Importance of Melt Pressure

The diehead pressure in the extruder determines the output from the extruder. It is the pressure necessary to overcome the resistance of the die. When the diehead pressure changes with time, the extruder output correspondingly changes and so do the dimensions of the extruded product see Fig. 4.1. As a result, when we monitor how the pressure varies with time, we can see exactly how stable or unstable the extrusion process is. 

It is best, therefore, to plot pressure with a chart recorder or better, to monitor the variation of pressure with a computer data acquisition system. A simple analog or digital display of pressure is much less useful. 

It is also critically important to measure pressure in the extruder to prevent serious accidents that can happen when excessively high pressures are generated. The very high pressures generated in the extruder can cause an explosion. The barrel can crack 

When monitoring pressure, it is a good idea to use an automatic shutoff when the pressure reaches a critical value. In pressure measurements, it is important to determine the absolute level of pressure, but it is equally important, if not more important, to determine changes in pressure with time. In most cases, pressure variation correlates closely with variation in the extruded product. Since high frequency (cycle time less than one second) pressure fluctuations are quite common in extrusion, a fast response measurement is important. 

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