Filament extrusion is a crucial manufacturing method to fabricate filament for fused deposition modelling (FDM) 3D printing. Despite its importance, filament extrusion remains an underexplored area in FDM-related research. This study investigates the effect of nozzle diameter on manufacturability of Polyamide 12 (PA 12) and high-density polyethylene (HDPE) filament. Prior to filament fabrication process, the polymers’ thermal stabilities were initially assessed using thermogravimetric analyser. The effect of varying nozzle diameters (1.2, 1.4 and 1.6 mm) on filaments dimensional accuracy and process capability indices were assessed post filament fabrication process. Besides, regression models were also established to quantify the relationship between nozzle diameter and filament thickness. In general, PA 12 and HDPE demonstrated high thermal stabilities with initial degradation temperatures of more than 400 °C. Increasing the nozzle diameters affects the thicknesses of both PA 12 and HDPE filament. The process capability indices for both PA 12 and HDPE remains below than 1, indicating significant variability. Meanwhile, the established regression model could be used to predict appropriate nozzle diameter to produce filament with ideal thickness. The findings contribute to advancing filament fabrication techniques while addressing scalability and precision challenges in additive manufacturing.