Fig. 2: Computational modeling assisted in the screw geometry selection, estimation of torque requirement for the motor, comparison of mixing based on thread cut shape, and estimation of thermal insulation provided by an extrusion needle cap.

In order to limit printhead weight by limiting the screw operating motor weight, modeling was used to select the screw geometry that reduced the operational torque requirement. The geometrical parameters that could be varied included thread cut width and the depth, as well as the pitch of the cuts in the normal and compression zones (A). A set of base parameters was selected and variations of each parameter to a higher and a lower value were tested (B). In addition to the torque requirement determination, the models provided a way to visualize flow speed and shear rate distributions as well as local flow directionality (C) in the various designs tested, as well as comparison with a square thread cut profile. Finally, comparing the torque requirements of the design variations tested (D) enabled the selection of a screw design for manufacture, design 4 indicated by the red rectangle in B. The effect of the screw profile on mixing was tested using a particle tracing model, where transfer of particles starting in one half of the inlet (red) to the region occupied by the other half of the particles (blue) was visualized (E). The overlap of the regions occupied by red and blue particles was used as a quantitative measure of mixing (F), and to compare a triangular thread cut profile (bottom row) with a square cut profile (top row) of roughly the same area. Lastly, heat flow modeling was used to visualize temperature distributions in the various parts of the printhead (G), and compare the utility of extrusion needle insulation against convective and radiative losses (H).