Discrete element method in polymer processing – Method for development of material models

• Calibration of DEM material models is required in polymer processing. • Calibration of coefficients of friction in shear cell is inadequate. • Coefficients of rolling friction should be calibrated in angle of repose test. • Influence of calibrated coefficient of rolling friction only at low normal forces. • Material model development should assess angle of repose, bulk density, shear cell. Solids conveying can be a key aspect for polymer processing equipment and is typically simulated utilising the Discrete Element Method (DEM). As material models for the DEM are limited in the accuracy of particle shape representation, behaviour of bulk material can deviate between simulations and reality. Therefore, a defined approach for developing material models is required in order to improve the accuracy of numeric simulations. This need is intensified, as recycling processes with direct processing of regrind with complex particle shapes become more common. In this paper, an approach commonly referred to as calibration is applied to several polymer pellets as well as regrind. The approach is altered in order to produce accurate models suited to replicate the conditions (flow conditions, pressure and relative movement) occurring during polymer processing in screw machines. A method for material model development is defined, focusing on calibrating the coefficient of rolling friction based on an angle of repose test and subsequently assessing the representation of the coefficients of friction in a shear cell and bulk density in annular-recess-shaped containers. The developed single- and multi-sphere models can be used for further investigations on the behaviour of polymer processing screw machines.