Non-destructive evaluation of additively manufactured polymer objects using X-ray interferometry

X-ray interferometry provides a dark-field image, essentially a small-angle X-ray scattering image, of the voids and print defects in an additively manufactured polymer object. The interferometers used were tuned to scattering length 2–5 μm and configured to measure scattering along both vertical and horizontal directions. The samples studied included Stanford Bunnies, fabricated from acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA), and a quadratic test object fabricated from PLA. The dark-field projection images show orientation-dependent X-ray scattering which is due to anisotropic voids and gaps at the filament-to-filament interface in these fused deposition modeling additive manufacturing objects. SEM corroborates the existence of gaps between filaments. The absorption and dark-field volumes are used to correlate printhead trajectory with print defect density. The absorption volume is used to generate perimeter points slice-by-slice, and from these points, the 2D curvature is calculated. There is a slight increase in X-ray scattering, hence print defect density, at regions with high curvature. Two X-ray interferometry techniques were used: stepped-grating and single-shot. As currently developed, stepped-grating has the larger field-of-view—examination of an entire test object—whilst single-shot has the potential for real-time, in situ measurement of the printing process within 1 mm of the printhead.