Constitutive modeling of UV-curing printed polymer composites

3D打印是一种用于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，加工材料可以是金属、陶瓷或聚合物。在逐点或逐层打印期间，材料经历从液体到固体的转变以及伴随着热机械和热材料行为的变化的温度变化。该过程导致材料性质的梯度和残余应力，其以期望或不期望的方式影响结构的机械行为和形状。由于加工材料是非弹性的，这些效果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，缺少本构模型和模拟工具，这类问题通常是昂贵的试错方法所面临的。为了保持成本，该项目专注于3D打印在紫外线辐射下固化的填料改性聚合物。我们的主要目标是了解，建模，模拟和优化填料改性聚合物结构的打印和打印后工艺。因此，实验开始调查的紫外线诱导固化的填料改性和未填充的聚合物：量热，流变和体积实验下的紫外线和温度控制的计划。印刷和后处理的复合材料拉伸棒进行了分析，依赖于工艺参数，如温度，紫外线强度，层厚度或时间尺度。使用未填充聚合物的数据，将开发的固化依赖的热粘弹性模型的程度。它描述了固化引起的材料性能的变化，是适合的实验数据，并实施到一个有限元代码。建立了一个参数与紫外光强度相关的微分方程来描述固化度的变化。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。通过电子显微镜研究了印刷样品中填料颗粒的分布和几何形状，并通过力学测试研究了填料对复合材料材料性能的影响。结合这些信息，创建一个参考体积单元，其均匀化的行为进行计算，在进一步的假设下与有限元实现的热粘弹性模型的矩阵，并与experiments.Finally，模拟链应用于模拟打印过程和后处理的晶格结构。在后处理期间的不同时间，比较测量和模拟的几何形状和残余应力。如果验证成功，模拟链将提供最佳工艺参数，最大限度地减少残余应力，并保持打印形状稳定并在容许公差范围内。

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.