This paper studies granular flow and compaction behavior of high-density polyethylene by discrete particle modeling in order to gain greater understanding of the stress distribution within the solids-conveying zone of a single-screw extruder. The contact force-displacement model used in the simulations was first validated by simulating uniaxial compression in a batch compaction cell. Subsequently, the discrete particle approach was used to model in 3D the movement of particles within the solids-inflow and solids-conveying zone of a 32-mm single-screw extruder. Results of the simulations showed that axial pressure development did not increase in an exponential manner, as suggested by continuum models, largely due to the compressibility of the solids. The nature by which pressure developed was shown to be further complicated by the retarding frictional forces of the granular bed, indicating Archimedean transport phenomena close to the feed opening when the head pressure was low and inadequate stress transmission occurred along the screw. In the cross-channel direction, the anisotropic stress field predicted found that the highest pressure in the screw channel was located at the screw root, while the lowest pressure corresponded to the retreating flight. The results were subsequently discussed in comparison to available continuum models.