Particulate Friction Measurements

With screw conveying efficiency being dependent on friction, both between pellets and steel, and within beds of pellets, friction data can be useful. Measurement techniques range from inclined planes to annular shear cells [6]. 

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The innovative contribution of the model is the consideration of friction between the powder and the internal die, when determining the axial pressure distribution inside the particulate graphite medium [1, 36]. As a result, model results can be used to predict the temporal profiles, which are in a good agreement with the measured values by using thermocouples at two different positions inside the particulate graphite medium. 

Measurement of the external coefficient of friction of particulate polymers is very difficult because of the very large number of variables that influence the coefficient of friction. Many investigators have made elaborate measurements on the external coefficient of friction [24-32]. The result of this work is that many variables have been identified that affect the frictional behavior however, most measurement techniques do not yield accurate and reproducible results that can be used in the analysis of the extrusion process. The most elaborate measurements and the most meaningful results have probably been obtained at the DKl in Darmstadt, Germany [95]. It is possible to obtain reproducible results by very careful experimental techniques and special surface preparation of the metal wall. However, the frictional coefficients determined in this fashion are hardly representative of the frictional process conditions occurring in an extruder. 

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