The extrusion process of 3D printer filament from recycled plastics has various challenges in the effort to produce quality filaments. One of the important factors affecting the filament quality is the diameter tolerance of the extruded filament that should be within the range of ±0,05mm. To produce those filaments, good consistency of the extrusion process is required. The most influential component of the extrusion machine is the screw. In this study, the screw was modified to overcome fluctuations in filament diameter. The polymer material was polypropylene (PP) that comes from recycled plastics. Firstly, the temperature, electric current of the motor, and screw rotation speed were observed to determine systemic fluctuations due to the surging phenomenon. Thereafter, the filament diameter was observed by direct measurement with a vernier caliper and macro-observation to seek defects. Furthermore, to overcome filament fluctuations due to imperfection in the mixing process, the screw was modified by adding a Maddock mixer that its undercut depth had been modified. The undercut depth variations are 0.1 mm (0.04D), 0.5 mm (0.02D), and 1.2 mm (0.47D).  This research showed that the modification of undercut depth on the Maddock mixer did not improve the quality of filament diameter produced. Meanwhile, the undercut depth variations on the Maddock mixer affected the capacity of extrusion.