In-process monitoring of part warpage in fused filament fabrication through the analysis of the repulsive force acting on the extruder

A solution for in-process monitoring of part warpage in fused filament fabrication is presented, based on real-time measurement of the upwards-oriented repulsive force acting on the extruder during deposition. The force signal is processed with the help of simulation software implementing a mechanical model which incorporates the stiffness of the machine and the compliance of the part as it is fabricated. The model acts as a digital twin, where compliance is constantly updated while part geometry evolves through the creation of new layers. The incorporation of stiffness/compliance simulation helps the monitoring system in the interpretation of the force signal, leading to a better isolation of warpage events. The proposed monitoring solution is validated through the implementation of a sensorised hardware prototype and the execution of experiments involving the fabrication of test parts. The results indicate warpage detection performance with 92.9% accuracy, 91.5% specificity and 95.7% sensitivity, and demonstrate that it is possible to use a relatively affordable, quick and non-destructive sensing solution (that is, measuring the repulsive force acting on the extruder) as an effective means to detect part warpage. However, the developed solution highlights the challenge of developing accurate digital twins for the prediction of part compliance, essential for the correct interpretation of the force signal.