An extensive experimental and theoretical study has been performed on screw filling in starve fed/flood fed single screw extruders. The effect of screw speed, flow rate (level of starvation) and die geometry has been investigated. A “screw pulling-out” technique was used in the study. The solid conveying, melting positions, the extent of starvation, and the fully filled regions were observed. It was seen that filling of the screw increases with an increase of the flow rate at a fixed screw speed, and decreases with an increase of the screw speed at a fixed flow rate. The screw filling is strongly dependent on the die geometry and increases with an increase of the resistance of the die. The pressure was also measured, and it was observed that it considerably decreases with starvation. Mechanism of screw filling has been discussed. The simulations and experiments were performed for low density polyethylene (LDPE) and high density polyethylene (HDPE) at various operating conditions, both for starve fed and flood fed extrusion. Fill factor, pressure, temperature and melting profiles have been computed and validated experimentally. All the experimental observations were consistent with the simulations.