Darnell and Mol Model

Darnell and Mol neglected the width of the flight (e = 0) so the formulation of was reduced in their model as follows  

Because Darnell and Mol neglected the width of the flight and allowed the dynamic coefficient of frictions to be set equal for part of the derivation, the model is rarely used. The model adaptation of Tadmor and Klein is used instead. This model will be described next. 

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Perhaps the most severe assumption in the Darnell and Mol model is the isotropic stress distribution. Recalling the discussion on compaction in Section 4.5, the stress distribution in the screw channel is expected to be complex. The first attempt to account for the nonisotropic nature of the stress distribution was made by Schneider (36). By assuming a certain ratio between compressive stresses in perpendicular directions and accounting for the solid plug geometry, he obtained a more realistic stress distribution, where the pressure exerted by the solids on the flights, the root of the screw, and the barrel surface are all different and less than the down-channel pressure. The ratio between the former and the latter is of the order of 0.3-0.4. 

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