Process modeling of laser scanning vat-photopolymerization operating under intermittent exposure conditions

Abstract

Process modeling is an important tool in additive manufacturing (AM) because it elucidates the relationship between processing parameters and material properties. Ultraviolet laser scanning vat-photopolymerization (VPP-UVL) is a common additive manufacturing technique, with a strong physical modeling based on the Jacobs work curve framework. The work curve model was developed for continuous UV exposure (CE) systems, however most VPP-UVL systems to date operate under intermittent exposure (IE) conditions due to the wide adoption of solid-state pulsed-laser sources. Despite their fundamental differences, the Jacobs work curve model is still successfully employed in IE systems under high overlap conditions between pulses, however there are few investigations regarding the limiting conditions for the applicability of the Jacobs work curve model to describe the total exposure profile in intermittent exposure (IE) setups. To address this issue, a model describing a VPP-UVL-IE process was developed. The VPP-UVL-IE model presented here has the same logarithmic dependence as the Jacobs work curve model, but with the addition of a multiplying factor carrying information on the spatial distribution of energy, that is, the overlap of individual pulses. The model is adaptable for non-gaussian beam profiles and can be solved for bidimensional analysis representing layer fabrication. To test the model, a custom-built VPP-UVL-IE additive manufacturing system was used, and structures were printed and characterized using SEM and stylus profilometry. The model`s reliability was investigated under constant overlap and exposure conditions. The model is very robust under constant overlap conditions, with good agreement between experimental data and simulation. For constant exposure, there is good qualitative agreement, but discrepancies occur due to insufficient modeling of the spatial distribution of energy. The 3D printing of complex structures is demonstrated, and the model indicates that VPP-UVL-IE can offer, theoretically, faster printing compared to VPP-UVL-CE for large beam radius (>275 µm), under the assumption of Gaussian beam profile, constant irradiance during the pulses, and dwell time longer than beam travel time. Conversely, same levels of exposure can be achieved at higher scan speed using CW laser sources instead of pulsed sources for small beam radius (<275 µm).