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

Published

January 17, 2025

Author(s)

Anthony Kotula, Benjamin Dolata, Yoontae Kim, Sara Orski, Jonathan Seppala

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 7×10−5 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.

Citation

Additive Manufacturing

Pub Type

Journals

Download Paper

https://doi.org/10.1016/j.addma.2025.104652

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Keywords

material extrusion, orientation, birefringence, processing

Citation

Kotula, A. , Dolata, B. , Kim, Y. , Orski, S. and Seppala, J. (2025), Quantifying residual orientation and thermal stress contributions to birefringence in the material extrusion of polylactide, Additive Manufacturing, [online], https://doi.org/10.1016/j.addma.2025.104652, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=958260 (Accessed April 4, 2025)