Estimating Fully Developed Melt Temperatures

It is very useful to be able to predict melt temperature in extrusion, particularly in the extrusion of temperature sensitive polymers. Examples are extrusion of crosslinkable polymers, foamed polymers, and polymers that are susceptible to degradation. Unfortunately, the proper calculation of melt temperature is rather involved and requires the use of numerical techniques, the most popular being finite element analysis. 

In this section we will describe a method to predict the fully developed melt temperature in screw extruders based on simple analytical expressions. The method is easy to use and leads to quantitative results with a minimum of time expenditure. 

The fully developed melt temperature is reached when the viscous heat generation is balanced by the heat flux away from the polymer melt. The viscous dissipation in the extruder will cause an increase in melt temperature resulting in reduced viscosity, which will result in a reduction in viscous dissipation. The melt temperature will reach a steady state value when the viscous dissipation has reduced to the point that it equals the heat flux from the polymer melt. 

The viscous dissipation is determined by the product of shear stress and shear rate. The average shear rate in the screw channel can be approximated by the Couette shear rate  

If the melt viscosity is described with a power law equation, the viscous dissipation 

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C. Rauwendaal, Estimating Fully Developed Melt Temperature in Extrusion, Conference Proceedings, 581 SPE ANTEC, Orlando, FL, 307-311 (2000)... 

This analysis provides a simple and fast method to estimate the fully developed melt temperature in screw extruders. The effect of material properties, processing conditions, and machine design parameters can be determined quantitatively. As a result, the analysis can be used to predict how melt temperature will change when another... 

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