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Constitutive modeling of UV-curing printed polymer composites

Subject Area Mechanics
Term from 2018 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 406819523
 
Final Report Year 2023

Final Report Abstract

Within this project, a material model was developed for the description of the crosslinking process of photopolymers used in additive manufacturing processes. Employing the concept of continuum mechanics, the total deformation is separated into mechanical, thermal, and chemical parts. The developed model describes both the temperature- and degree of cure-dependent viscoelastic and the viscoplastic material behavior of the photopolymer at hand. For this purpose, time-temperature and time-cure superposition principles are applied. Furthermore, the material model was discretized and implemented as a material subroutine into the commercial finite element program LS-DYNA® in order to conduct threedimensional simulations. However, the conducted simulations are only restricted to the purely mechanical case and constant conversion states of the photopolymer. Overall, good accordance is achieved between the experiments and simulations by means of the developed model equations and the identified values for the parameters. Photo-DSC measurements were conducted for the experimental characterization of the crosslinking progress. As a result, the temperature of the photopolymer has a more substantial influence on the crosslinking speed than the irradiance. Therefore, considerable efficiency gains regarding the print job duration can be obtained by increasing the temperature of the photopolymer. Additionally, this phenomenon was confirmed by an innovative experimental method for the determination of temperature-dependent working curves. A conventional rheometer was modified in order to record the viscoelastic properties depending on the crosslinking progress. Unfortunately, the quality of the measurements contains room for improvement. However, the measurements show the expected qualitative development of the viscoelastic properties upon irradiation. Last but not least, the tensile tests show that the temperature and the degree of cure have a strong influence on the material properties of printed parts. Considering the tensile tests subjected to cyclic loading and unloading, viscoelastic and viscoplastic deformations overlap each other. Naturally, the conducted investigations do not cover all of the arisen questions. Primarily, no simulations considering the shrinkage of the photopolymer layer are conducted in this thesis. Therefore, simulations should be conducted taking into account the coupling between the mechanical, thermal, and chemical parts of the material model in order to predict the chemical shrinkage and potential residual stresses. Additionally, the heat generation caused by the exothermic crosslinking reaction could be investigated. Furthermore, the thermomechanical consistency of the material model should be proven in order to avoid unphysical behavior of the model. Although three different conversion states of the photopolymer are investigated in the tensile tests, the underlying database can be easily extended by testing more specimens with lower degrees of cure. For this purpose, one can also change the chosen layer thickness, exposure time, and irradiance during the printing process. This would also give insight into the dependencies of the tensile behavior on the mentioned printing parameters. Following this, the introduced equations describing the temperature- and degree of cure-dependent parameters can be extended to a broader range of possible conversion states and temperatures.

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