Investigations on nozzle geometry in fused filament fabrication

Abstract

This investigation focuses on geometric parameters of nozzles used in Fused Filament Fabrication. They are mainly responsible for the extrusion force. Therefore, the influence of the geometric parameters was investigated. Typical nozzles are made of brass and feature a decrease in diameter from an entry channel to a capillary with a conical section in between. At a fixed entry and capillary diameter, variable parameters are the angle of the conical section and the length of the capillary. Commercially available and custom nozzles with various of these parameters were investigated on a test stand using Polylactic Acid (PLA) filament. All nozzles exhibit a common behavior. The extrusion force rises linearly with increasing filament feed velocity. At a certain point the forces fluctuate and increase rapidly. Here, unmolten plastic reaches the nozzle. This characteristic is dependent on extrusion temperature and geometric parameters of the nozzles. A work envelope was defined; the lowest extrusion forces were found for a 56° conical section, but differences in force between the angles from 30° to 118° are low in comparison to the total extrusion force. Different capillary lengths were used to determine the entry pressure loss at different filament feed velocities. The material and coating of the nozzles had no significant influence on extrusion force. A higher thermal mass, two conical sections or two entry channels have a positive effect on extrusion forces and maximum filament feed velocities, thus maximal build rate.