A rheological approach for measuring cure depth of filled and unfilled photopolymers at additive manufacturing relevant length scales

Abstract

We introduce a novel experimental method that uses an ultraviolet (UV) photorheometer to enable accurate and repeatable measurements of cure depth for a wide range of photopolymers. When exposed to UV irradiation within the rheometer, the cured thickness of the photoresin is measured by lowering the upper plate while monitoring the commensurate axial force. The encoder-measured distance is correlated with axial force to measure cure depth under different UV curing conditions. This technique enables precise measurements of photocured films thinner than 50 µm, which can then be used to establish working curve equations for photopolymerization-based additive manufacturing (AM) processes. This process is validated through the evaluation of three unique photoresins, including a commercially available VP stiff photoresin, a highly-viscous photoresin that cures into a “soft” gel, and a highly solids-loaded photoresin with particles that greatly limit maximum cure depth. In addition, the technique is employed to explore the impact of UV irradiance on curing behavior of photoresins. This technique enables measurement of AM process-relevant cure depths with process-relevant irradiance and wavelengths and has applications in advanced material development for a range of AM and traditional UV processing technologies. This technique meets identified gaps in current cure depth characterization techniques, including the need for automated instrumental approaches to reduce human error in measurements, the need to directly measure cure depths less than 150 µm, and the need to measure curing behavior at different UV irradiances/wavelengths. Due to the non-linear cure depth versus exposure behavior of some photoresins, it is important to directly measure cure depths at AM process-relevant sizes ( µm), as a working curve made from measurements of thicker films leads to more inaccurate predictions than those made from measuring thinner films.