The role of optical tomography in in-situ monitoring of the 3D photopolymerization
Abstract
Tomographic additive manufacturing physically reverses computed tomography for 3D printing.
The reactive transport of oxygen during photopolymerization controls the quality of the printout and in most cases, there exists a short time window for having the optimum light exposure.
Optical tomography as a non-invasive 3D imaging technique can be used for in-situ monitoring of the refractive index variations to find the window for termination of the light exposure to achieve the optimum print quality.
We have designed a visible light optical tomography system equipped with an in-house image reconstruction toolbox to monitor the density variations during tomographic volumetric printing (TVP).
In our experiments, a cylindrical test tube containing the reactive resin is mounted to a motorized rotation stage and the tube is then immersed into a matching index fluid.
2D projections are recorded by a CCD camera at various angles which are later used in dynamic volumetric reconstruction of the printed object.
Developed reconstruction software follows an automated two-step workflow including image processing and dynamic 3D reconstruction using filtered backprojection.
The image processing step involves registration of 2D projections and center of rotation optimization to correct for the off-axis tube rotation, and intensity inhomogeneity correction for ring artifact removal.
A moving window approach is also employed for dynamic reconstruction combined with an offset correction approach to align the reconstructed geometries.
The software partially reconstructs the geometrical evolution of the printed materials by considering the light absorption as the dominant mechanism in image formation.
However, to obtain a more accurate reconstruction, understanding the effect of refraction and scattering of the light rays propagating through the material is also important.
A high-fidelity reconstruction is desirable to evaluate the print quality and to understand the dynamics of photochemistry in TVP.