Force Controlled Printing for Material Extrusion Additive Manufacturing

Abstract

In material extrusion additive manufacturing, the extrusion process is commonly controlled in a feed-forward fashion. The amount of material to be extruded at each printing location is pre-computed by a planning software. This approach is inherently unable to adapt the extrusion to external and unexpected disturbances, and the quality of the results strongly depends on a number of modeling and tuning parameters. To overcome these limitations, we propose the first framework for Force Controlled Printing for material extrusion additive manufacturing. We utilize a custom-built extruder to measure the extrusion force in real time, and use this quantity as feedback to continuously control the material flow in closed-loop. We demonstrate the existence of a strong correlation between extrusion force and line width, which we exploit to deposit lines of desired width in a width range of 33 % up to 233 % of the nozzle diameter. We also show how Force Controlled Printing outperforms conventional feed-forward extrusion in print quality and disturbance rejection, while requiring little tuning and automatically adapting to changes in the hardware settings. With no adaptation, Force Controlled Printing can deposit lines of desired width under severe disturbances in bed leveling, such as at layer heights ranging between 20 % and 200 % of the nominal height.

Figures (14)

• Figure 1: Schematic representation of the sensing extruder. The torque sensor is used to characterize the extrusion process.
• Figure 2: Block diagram of the closed-loop extrusion control scheme utilized in Force Controlled Printing
• Figure 3: Custom-built FFF printer used for Force Controlled Printing
• Figure 4: Snake trajectory, used as a benchmark in this work. The trajectory is a continuous raster composed of 40 equally spaced 40mm long lines (oriented in the $X$ direction) connected by a 1mm long segment (in the $Y$ direction). Printing begins at $X=-20mm, Y=-20mm$, and finishes at $X=-20mm, Y=19mm$.
• Figure 5: Comparison between the measured reaction forces during conventional printing and Force Controlled Printing. Both experiments were conducted on the same trajectory (shown in Fig. \ref{['fig:snake']}) at a feed rate of 100mm/s, a temperature of 300℃, a layer height of 50µm, and with a nozzle diameter of 0.15mm. Note that the force reference is not used by the open-loop print.