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BIT’s progress in the field of UV-curing 3D printing of energetic materials

News Source: School of Materials

Editor: News Agency of BIT

Translator: Yuan Xiangqing, News Agency of BIT

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Recently, the research group led by Professor Xia Min and Professor Luo Yunjun in School of Materials, BIT, has made important progress in the field of UV-curing 3D printing process control of energetic materials. The research results have been published in Additive manufacturing, top international journal in the field of additive manufacturing, entitled Model of UV-curable 3D printing for new thiol-ene resin Additive manufacturing of energetic materials(IF="""11.0)(DOI:10.1016/j.addma.2022.102716). The first author is Guo Tao who is a graduate student of school of Materials, BIT, and the corresponding authors are Professor Xia Min and Professor Luo Yunjun.

It is an effective technical approach to realize the additive manufacturing of energetic materials by combining the photocuring resin with energetic solid components and rapid photocuring of resin. Digital light processing (DLP) is an additive manufacturing method with high precision and high efficiency which is formed by layer-by-layer exposure curing of photocurable resin. After finishing one layer of printing, the forming platform rises H height, and ultraviolet light from the digital light source for a sustained period of time irradiates on the resin located between the bottom of the resin box and the sample. It begins to cure from the bottom of the box, and the thickness gradually increases until reaching H. The key to the success of printing is the matching degree between the height H and the exposure time. Only in the set exposure time which makes the curing layer thickness zp is greater than or equal to H, the printing process can be completed, otherwise delamination will occur. During the exposure process, the curing thickness of the resin increases nonlinearly with the exposure time, which is closely related to the exposure intensity and the properties of the resin itself. Finding a method that can describe this process and predict curing thickness is of great significance for the fine control of DLP process..

Based on this requirement, the research group designed an analysis model for curing thickness. And they prepared photocuring binder by end-base modification from traditional energetic material binder and the relationship between the curing thickness and exposure time of the binder was analyzed.It finally corroborated the derived formula and the parameters were calculated to obtain a mathematical model that predicts the curing thickness of the photocuring resin laying the foundation for process regulation of 3D printing of energetic materials.

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Fig.1 Resin curing principle and thickness test method(a);Analysis of relationship between curing thickness and exposure time(b);Model conclusion(c)

Results show that the uv-curable resin in the process of 3 d print, the thickness of solidified time variation depends on two key parameters: one is the characteristics of penetration depth ha, the second is characteristic cure time Tc, In the early stage of radical polymerization, the concentration of initiator and monomer could be deemed as constants, Tc and 1/I00.5 and 1/[I]0.5. The experiment result is shown in Fig.2 a、b、c, this is consistent with what the model predicts.

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Fig. 2 Characteristic penetration depth ha as a function of initiator concentration [I] (a). Critical time as a function of 1/[I]0.5 (b) and 1/I0 0.5(c). The thickness-time relationship obtained by the model and experiments at two light strengths (d).

Theoretical models and experimental results show that The relationship between curing thickness and exposure time is mainly affected by initiator concentration and light intensity. The characteristic penetration depth is inversely proportional to the initiator concentration, while the critical time is positively proportional to the reciprocal of the initiator concentration and the square root of light intensity product. High concentration of initiator means high light coefficient ,which will significantly increase the curing time when print at high layer. However, the low concentration of initiator will make the light intensity distribution in the resin too uniform, and it is difficult to control curing thickness by controlling the exposure time. For thiol-ene UV curable resin system, 2 wt% to 3 wt% of the TPO concentration is feasible.

Paper link: https://doi.org/10.1016/j.addma.2022.102716