Digital Light Projection (DLP) 3D printing of highly filled photopolymer systems is a rapidly expanding area due to its ability to achieve extreme mechanical or other functional properties. Recently, DLP printing syntactic foams with a high-volume fraction of low-density, hollow microspheres has achieved lightweight, high-modulus materials for applications requiring low mass without compromising mechanical properties. However, the resolution of printing is degraded by the significant scattering during light exposure. Moreover, the scatter for a three-phase hollow microsphere system in photopolymer resin may not conform to the scatter predicted in two-phase systems. Light scattering in systems with solid filler particles is well established using various empirical models, in which features such as cured depth (CD), and cured width (CW) can be correlated with the filler properties, such as its particle size, refractive index, and volume fraction. The addition of hollow microspheres, however, introduces a third optically active phase in form of air within the ceramic shell, which is surrounded by the photopolymer medium. This third phase is not accounted for in existing theories for light scattering in filled systems. This work aims to investigate, for the first time, the UV-light scattering in a three-phase photopolymer system using a first principles approach and developing a generalized mathematical framework. Mie scattering theory is utilized to understand the scattering behavior of a hollow microsphere particle at a specific wavelength. The photon transport within the resin system is then scaled using a probabilistic Monte Carlo method. The scattering of large photon populations (~108) within a filled photopolymer system is simulated for a 300 × 300 µm target geometry. An inversion parameter is proposed to predict the film dimensions. Photopolymer CD and CW is then predicted at 10, 30, and 50 vol% of hollow glass microsphere filler concentration by calculating volumetric photon density in 3D space, and experimentally verified with printed comparison geometries. A counter-intuitive behavior of higher CD for filled systems compared to neat resin is observed in experiments and is predicted by the theoretical framework.

Predicting UV-light scattering in a three-phase system using the Monte Carlo method

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