Through the experimental investigation of solid granule compaction, it is demonstrated that the combined influence of pressure and temperature on the bulk density of solid polymer granules is very significant for all materials, although the effect is different for different materials, especially amorphous polymer material, such as polystyrene. A distinction is made between the effect of pressure and temperature on the bulk density for various materials and this cannot be neglected in polymer processing operations. The rheological property of solid polymer particles must be considered in the design of the plasticating extruder screw, which forms the basis of many polymer processing methods. Also, it is one of the probable reasons why extrusion using a grooved feed section in the extruder barrel can enhance the throughput of material compared with that of conventional extrusion.