Residual orientation mapping in material extrusion processes

Material extrusion is a common additive manufacturing process that subjects molten polymers to non-steady deformation and thermal processing to build customizable parts. Often, the complex processing conditions result in localized residual orientation, which is associated with variations in semicrystalline microstructure<span style="font-size:10.8333px"> and decreased weld strength between layers<span style="font-size:10.8333px"> in the printed part. There is a critical need to identify and characterize the spatial variation in polymer chain orientation due to these melt flow processes. Here, we use polarization imaging to characterize the spatial variation in residual stress and residual chain orientation in a glassy polylactide. By imaging the sample in a refractive index-matched fluid using a four-quadrant polarization camera we can measure the spatial variation in retardance in printed parts as a function of the Weissenberg number (Wi) in the printer nozzle. When combined with micro-computed tomography, we identify birefringence variations across the printed part that can be decomposed into residual orientation effects from the extrusion-deposition process and residual stress from spatial variations in temperature during cooling. The resulting analysis shows that residual orientation exhibits an approximate power-law dependence on Wi, which can be related back to the melt flow and relaxation that occurs in the extrusion-deposition process. We show that the residual orientation can be scaled as a function of processing and material parameters to generate a master plot of residual orientation. Our results show that a controlled amount of residual orientation can be templated into materials by controlling nozzle temperature and print speed.