Constitutive modeling of UV-curing printed polymer composites

3D打印是一种制造形状复杂的三维物体的创新技术。根据打印机的工作原理，加工材料可以是金属、陶瓷或聚合物。在点或层打印期间，材料经历了从液体到固体的转变和温度变化，并伴随着热机械和热量材料行为的变化。这一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期望的方式影响结构的机械行为和形状。由于加工的材料是非弹性的，这些影响取决于时间和温度以及印刷和后处理工艺的参数。由于缺乏了解，缺乏本构模型和模拟工具，这类问题通常面临昂贵的试错法。为了控制成本，这个项目专注于3D打印填料改性聚合物，这些聚合物在紫外线辐射下放热固化。我们的主要目标是了解、建模、模拟和优化填充改性聚合物结构的印刷和印刷后工艺。因此，实验从研究填料改性和非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、流变学和体积实验。根据工艺参数，如温度、紫外线强度、层厚度或时间刻度，对复合材料的印刷和后处理拉伸棒进行分析。利用未填充聚合物的数据，建立了热粘弹性与固化程度相关的模型。它描述了固化引起的材料性能的变化，与实验数据相拟合，并被实施到有限元程序中。建立了具有紫外光强度相关参数的微分方程式来描述固化度的演化。如果完全固化的聚合物的玻璃化转变温度高于固化温度，则考虑扩散控制。用电子显微镜研究了印花样品中填料颗粒的分布和几何形状，并通过力学性能测试研究了填料对复合材料材料性能的影响。结合这些信息，建立了一个参考体积单元，在进一步假设下计算了该参考体积单元的均匀化行为，并与实验结果进行了比较。最后，将模拟链应用于模拟印刷过程和点阵结构的后处理。在后处理过程中的不同时刻，对测量和模拟的几何形状和残余应力进行了比较。如果验证成功，模拟链将提供最优的工艺参数，最大限度地减少残余应力，并保持印刷形状稳定并在允许的公差范围内。

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.