Constitutive modeling of UV-curing printed polymer composites

3d打印是一种制造具有复杂形状的三维物体的创新技术。根据打印机的工作原理，加工的材料是金属、陶瓷或聚合物。在点或层印刷过程中，材料经历了从液体到固体的转变，温度变化伴随着热机械和热材料行为的变化。该过程导致材料性能的梯度和残余应力，这些应力以期望或不希望的方式影响机械行为和结构形状。由于加工的材料是无弹性的，这些影响取决于时间和温度，以及印刷和后处理过程的参数。由于缺乏理解，缺少本构模型和仿真工具，这些问题通常是通过昂贵的试错方法来解决的。考虑到成本，该项目专注于在紫外线辐射下放热固化的填料改性聚合物的3d打印。我们的主要目标是理解、建模、模拟和优化填料改性聚合物结构的打印和打印后过程。因此，实验从研究填料改性和未填充聚合物的紫外光诱导固化开始：计划在紫外光和温度控制下进行量热、流变和体积实验。打印和后处理的复合材料拉伸棒的分析依赖于工艺参数，如温度，紫外线强度，层厚度或时间尺度。利用未填充聚合物的数据，建立了与固化程度相关的热粘弹性模型。它描述了固化引起的材料性能变化，与实验数据相吻合，并实现为有限元代码。建立了一个与UV强度相关的参数的微分方程来描述固化程度的演变。如果完全固化聚合物的玻璃化转变温度高于固化温度，则需要考虑扩散控制。利用电子显微镜研究了填料颗粒在印刷样品中的分布和几何形状，并通过力学测试研究了填料对复合材料性能的影响。合并这些信息，创建一个参考体积单元，在进一步的假设下计算其均匀行为，并与矩阵的热粘弹性模型的有限元实现进行比较。最后，应用仿真链对点阵结构的打印过程和后处理进行了仿真。在后处理的不同时间，对实测和模拟的几何形状和残余应力进行了比较。如果验证成功，仿真链提供最佳工艺参数，最大限度地减少残余应力，并保持打印形状稳定并在可接受的公差范围内。

3D-printing is an innovative technique to manufacture three-dimensional objects with complex shape. Processed materials are metals, ceramics or polymers depending on the working principle of the printer. During the point- or layer-wise printing the material experiences a transition from a liquid to a solid and a temperature change accompanied by changes in the thermomechanical and caloric material behavior. The process leads to gradients in the material properties and to residual stresses which influence the mechanical behavior and the shape of the structure in desired or undesired manners. Since the processed materials are inelastic these effects depend on time and temperature and on the parameters of the printing and post-treatment process. Due to the lack of understanding, missing constitutive models and simulation tools, such problems are usually faced by costly trial-and-error methods. To keep the costs in view, this project is focused to 3D-printing of filler-modified polymers which are exothermally curing under UV radiation. Our main objectives are to understand, to model, to simulate and to optimize the printing and post-printer processes of filler-modified polymer structures. Therefore, the experiments start with investigations of the UV-induced curing of filler-modified and unfilled polymers: calorimetric, rheometric and volumetric experiments under UV- and temperature-control are planned. Printed and post-treated tensile bars of composites are analysed in dependence on the process-parameters like temperature, UV-intensity, layer thickness or time-scales. Using the data of the unfilled polymer, a degree of cure-dependent model of thermoviscoelasticity will be developed. It describes curing-induced changes in the material properties, is fitted to the experimental data and implemented into a finite element code. A differential equation with UV intensity-dependent parameters is developed to describe the evolution of the degree of cure. If the glass transition temperature of the fully cured polymer is above the curing temperature, diffusion control is taken into account. The distribution and the geometry of the filler particles in printed samples are studied by electron microscopy and the influence of the filler to the material behavior of the composites by mechanical testing. Merging this information, a reference volume element is created whose homogenized behavior is computed under further assumptions with the finite element implementation of the thermoviscoelastic model for the matrix and compared with experiments.Lastly, the simulation chain is applied to simulate the printing process and the post treatment of lattice structures. At different times during the post treatment, the measured and simulated geometries and residual stresses are compared. If the validation is successful, the simulation chain provides optimal process parameters minimizing residual stresses and keeping the printed shape stable and within admissible tolerances.