Residual stress control in large-format additive manufacturing of polylactic acid via a digital twin and in-operando imaging

Polymer-based Large Format Additive Manufacturing (LFAM) is an extrusion-based technology that deposits large-diameter polymer beads using a robotic arm-mounted nozzle. However, slow cooling rates and heat accumulation generate technical challenges, including significant deformation that requires nozzle path adjustments and the buildup of residual stresses from thermo-chemical shrinkage that may cause debonding. This study integrates two fast modeling approaches, ScanFast (thermal) and QuadWire (mechanical), to reduce the number of degrees of freedom compared to conventional methods while maintaining accuracy. A computationally efficient digital twin of the process is developed and validated experimentally on a thin wall printed with polylactic acid. Anisotropic material properties are characterized, and in-operando temperature and displacement fields are measured using infrared thermography and backward Digital Image Correlation. The results show correlation coefficients greater than 0.80 between experimental and numerical data. The validated digital twin is then applied to assess the influence of process parameters on three key aspects: (i) the number of layers above the glass transition temperature, (ii) residual stress development, and (iii) positional offset between the nozzle and the structure. The proposed approach provides an efficient tool to optimize process parameters and nozzle trajectories, thereby enhancing the quality and manufacturability of LFAM-produced parts.