Probing Material Transformation Volumetrically in Computed Axial Lithography

Volumetric additive manufacturing processes such as computed axial lithography (CAL) are promising for the rapid production of 3D geometries in a wide range of photopolymer and nanocomposite materials. In CAL, light dose is reconstructed tomographically by projecting a video image into a rotating volume of precursor material. Practically, this reconstruction approach exposes the bulk of the precursor (and not just the target geometry) to background illumination. The reconstructed dose distribution is also not perfectly sharp at the boundary of the component. Consequently, there is a range of degrees of conversion throughout the volume at any given moment, and it is very important to terminate the printing process accurately to be able to recover the desired geometry. In-process volumetric monitoring of refractive index can be valuable for this purpose, because most precursor materials exhibit refractive index increases during conversion to the solid state. We will review a set of Schlieren-based techniques we previously introduced for real-time material conversion observations. We will also discuss recent work developing a volumetric temperature monitoring technique based on fluorescence imaging. The evolution of temperature within a CAL volume can serve as an indicator of material conversion dynamics and can influence the quality of printed components.