Due to the interactions between the fillers, the polymer and the die, the printing of filled polymer composites using the material extrusion process is challenging. Numerical computations have been carried out to investigate the fluid dynamics and heat transfer within the hot end of the material extrusion process. Firstly, a standard nozzle equipping the majority of 3D printers is considered. However, it is not possible to print an object on a commercial printer with a composite made of polylactic acid with 20wt% of carbon black due to clogging issues. This impossibility is supported by the application of the Q-criterion, which is an indicator of flow kinematics. As a result, a novel nozzle design is proposed. The printing of the same object on the same printer equipped with the new nozzle succeeds. The electrical properties of walls printed with both the standard and the new nozzles are evaluated. The conductivity of a wall printed with the new nozzle is 42% higher than that of a wall printed with the standard nozzle. Furthermore, complementary analysis using scanning electron microscopy shows that the porosity of the walls printed with the new nozzle is reduced which is a promising improvement.