  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.
