A novel approach for understanding laser sintering of polymers
A novel approach for understanding laser sintering of polymers
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DOI：
10.1016/j.addma.2019.03.012
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发表时间：
2019-05-01
影响因子：
11
通讯作者：
Wudy, K.
中科院分区：
文献类型：
--
作者：
Drummer, D.;Greiner, S.;Wudy, K.
与其他增材制造方法相比，热塑性粉末的选择性激光烧结 (LS) 可以构建具有更高机械性能和耐用性的复杂零件。根据当前的等温激光烧结模型，半结晶热塑性塑料需要在一定的温度范围内进行加工，导致材料同时以熔融状态和固态存在，这在零件制造过程中是存在的。基于该工艺模型，从几小时到几天的高循环时间被认为是避免翘曲的必要条件。在本文中，各种实验表明等温激光烧结模型的有效性有限，因为可以在粉末床表面下方的几层处检测到正在进行的凝固。结果表明，结晶和材料凝固在高温下开始，并在 z 方向的整个零件成型过程中进一步进行。因此，进行了适应工艺的材料表征，以确定加工温度下的等温结晶动力学，并跟踪材料状态随时间的变化。采用红外热成像测量表面温度和 z 方向附加热电偶测量的双重方法，以确定对材料凝固的进一步影响。模型实验表明，在地表以下几毫米处，LS 生产的部件已经凝固。基于这些结果，作者提出了等温激光烧结的增强工艺模型，该模型考虑了零件构建过程中 z 方向的材料凝固。此外，还导出了一种新的处理策略，可显着提高LS处理的效率。
Selective laser sintering (LS) of thermoplastic powders allows for the construction of complex parts with higher mechanical properties and durability compared to other additive manufacturing methods. According to the current model of isothermal laser sintering, semi-crystalline thermoplastics need to be processed within a certain temperature range, resulting in the simultaneous presence of the material both in a molten and solid state, which is present during part building. Based on this process model, high cycle times ranging from hours to days are a thought to be a necessity to avoid warpage.In this paper, the limited validity of the model of isothermal laser sintering is shown by various experiments, as ongoing solidification could be detected a few layers below the powder bed surface. The results indicate that crystallization and material solidification is initiated at high temperatures and further progresses throughout part build-up in z-direction. Therefore, a process-adapted material characterization was performed to identify the isothermal crystallization kinetics at processing temperature and to track changes of the material state over time. A dual approach on measuring surface temperatures by infrared thermography and additional thermocouple measurements in z-direction was performed to identify further influences on the material solidification. A model experiment revealed that a few millimeters below the surface, components produced by LS are already solidified. Based on these results, the authors present an enhanced process model of isothermal laser sintering, which considers material solidification in z-direction during part build-up. In addition, a new processing strategy is derived to increase the efficiency of LS processes significantly.