Quantifying residual orientation and thermal stress contributions to birefringence in the material extrusion of polylactide

Abstract

Material extrusion is a common additive manufacturing process that subjects polymers to non-steady deformation and thermal processing to build a customized part. The mechanical properties of these parts are often worse than those of injection-molded specimens due to failures at or near the weld zone between extrudate layers. Chain orientation is often cited as a contribution to mechanical weakness at the weld, and it is therefore of critical importance to develop strategies to quantify the magnitude and location of residual chain orientation as a function of printing conditions. Here we use birefringence imaging to characterize the spatial variation in residual stress and residual chain orientation in a glassy polylactide. A combination of retardance measurements and sample thickness measurements provide a measure of birefringence as a function of position. As-printed samples show a nearly uniform birefringence background of approximately $7\times 10^{-5}$ and higher birefringence near the weld region at lower nozzle temperatures and faster printing speeds. We propose two origins to the birefringence: one due to residual chain orientation, and the other due to residual stresses that occur when the sample cools non-uniformly on the build plate. Annealing the sample at 65 °C (slightly above the glass transition temperature) allows us to relax the residual stress without removing the orientation-based birefringence or crystallizing the sample. The residual orientation shows a strong power-law dependence on the Weissenberg number based on the characteristic timescales for flow in the nozzle and polymer chain reptation.