In-line measurement of extrusion force and use for nozzle comparison in filament based additive manufacturing

In fused filament fabrication (FFF), it is important to control the extrusion process for different materials and extruder hardware precisely. The effective extrusion force on the nozzle allows the characterization of the flow behaviour of the molten polymer. Therefore, integrating compact force sensors into the extrusion unit allows the identification of suitable process windows for different materials and the evaluation of hardware. However, actual setups are not compact and restriction-free. This paper presents a new compact and concentric setup, which allows an accurate measurement of the forces in the extruder avoiding parasitic effects. Furthermore, the test rig and the calibration process including influencing factors such as filament contact force and extrusion height is described. On this basis, a varying parameter study using different nozzles, feeding velocities, and temperatures is conducted. The recorded values of the extrusion force are compared with the literature values and the flow behaviour is assigned to the corresponding nozzle geometry. It has been shown that the calibrated test rig is able to measure the extrusion force with sufficient accuracy. These measurements correspond to comparable literature values. The boundary conditions of feeding velocity for processing a specific material and nozzle are determined. In this way, requirements such as the fastest or a uniform extrusion can be specifically served by varying the corresponding parameters. An evaluation of the effects in the nozzles results in a nozzle performance overview.