Co-rotating twin-screw extrusion is an energy consuming process that is generally not fully optimized to specific polymer. From the point of view of the efficiency of the extrusion process, the starting material should be characterized by small grain size in comparison to the screw channel area, small surface area to volume ratio and small internal friction between the pellets. In order to develop a model describing the effect of PLA grain size on the extrusion efficiency, a series of experiments with a twin-screw extruder were carried out during which the energy consumption, torque on shafts and temperature of the melt on the extruder die were monitored. As feed material, both the neat PLA with different grain sizes as well as the PLA with expandable graphite filler and phosphorous based flame retardant were used. Morphology and dispersion quality of the composites were examined by scanning electron microscopy (SEM), flammability, smoke production, mass loss and heat release rates was tested using cone calorimetry and melt flow rate was determine using plastometer. Moreover, the thermal properties of the obtained composites were determined by differential scanning calorimetry DSC. The results show that the choice of the starting material affects both the efficiency of the extrusion process and the flame retardancy properties of the composite materials.