快速成形工艺软件的若干关键技术研究
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摘要
本文对快速成形软件的体系结构、核心算法和工艺算法进行了研究，内容包括离线式快速成形软件体系结构、STL(Stereo Lithography)模型布尔运算算法、快速成形工艺算法以及提高选择性激光烧结制件精度的方法四个方面。其理论和成果，不仅为华中科技大学快速成形软件和设备水平的提高作出了贡献，而且为快速成形技术在我国的发展提供了有力的支持。
为了适应快速成形技术的进步和加工方法的革新，本文按照领域工程、面向构件的思想，研究并开发了快速成形软件的统一体系结构，它基于分层架构+支撑平台和领域细化逐步求精的思想，包括工艺规划软件和系统控制软件。采用该软件系统可以缩短因快速成形加工方法和设备型号改变而引起的软件开发的周期，同时可以提高软件的开发质量。目前该软件系统已经成功地应用在华中科技大学研发的多种快速成形设备上，取得了良好的效果。
快速成形工艺规划软件的许多高级和核心功能都必须借助于对STL模型的布尔运算来实现，然而国内的学者对于这一问题的研究还处在初级阶段，算法的效率和稳定性都比较差。文通过精确的空间离散化的方法快速地找到可能参与相交运算的三角面片的最小集合，大大降低了三角面片之间的求交次数，显著地提高了算法的效率。采用七元组的数据结构记录交线段。依据STL文件的拓扑关系快速、准确地提取交线链和交线环，避免了采用容差法进行提取。通过判断射线到三角面片的有向距离来确定三角面片和每个区域的位置关系。采用规避歧义位置的方法，解决了判断位置关系时出现的歧义情况。最后，重新三角化相应的区域实现布尔运算。该算法的效率和稳定性均已达到RP业内最优秀的软件Magics RP的水平。
根据快速成形工艺发展的需求，实现了变扫描矢工艺路径算法，该扫描方式使得选择性激光熔化成形过程中制件充分熔化，提高了制件的成形性能和质量。基于路径追踪算法的分形扫描工艺实现了二维任意轮廓的分形曲线填充，该扫描方式使得选择性激光烧结零件的致密度得到较大提高。实现了基于PowerRP软件系统的掩膜法光固化工艺规划软件并解决了相应的工艺问题。
制件的翘曲是高分子聚合物选择性激光烧结过程中普遍存在的破坏性现象，对这种现象的合理抑制是提高SLS成形精度的关键。本文分析和总结了翘曲产生的原因以及翘曲现象的发展规律，提出了采用热源平衡支撑的方法来解决加工过程中由于温度场的不均匀进而导致应力场不均最终引起制件翘曲变形的问题。热源平衡支撑在样式和参数的选择上需要考虑材料的物理性能，化学性能，材料与激光相互作用后产生的收缩和热传导等因素，更重要的是热源平衡支撑要尽可能保持温度场的均匀性而且较容易地被清除掉。该方法已经成功地应用在聚苯乙烯和尼龙两种材料的成形中，取得了非常好的效果。
The architecture, the core algorithms and the technical algorithms of rapid prototyping (RP) software have been studied in this dissertation, which include off-line RP software architecture, algorithms of Boolean operations on STL (Stereo Lithography) models, RP technology algorithms and method of improving parts accuracy in selective laser sintering (SLS). The theories and achievements contribute to the development of the RP software and equipment of Huazhong University of Science and Technology, and provide a versatile software tool for the domestic promotion of Rapid Prototyping.
In order to meet the development of RP technology and the innovation of RP processing method, this dissertation, in accordance with domain engineering and thought of component-oriented programming, researches and designs a unified architecture of RP software, which is based on hierarchical architecture + support platform and the idea of the domain stepwise refinement, including technical processing software and system control software. The use of the software system can shorten the software development cycle, which is caused by changes in processing methods and equipment types, and at the same time it can improve the quality of software development. At present, the software system has been successfully applied to a variety of RP equipments which are researched and designed by Huazhong University of Science and Technology, and achieved good results.
Many advanced and core functions of rapid prototyping softwares for technology processing must be implemented with the aid of algorithms of Boolean operations on STL models. However, the research on this issue by scholars at home stays at a primary stage and the algorithm is of relatively low inefficiency and poor stability. A method of precise space decomposition is employed to limit the scope of searching for candidate triangles that may intersect, which can greatly reduce the times of calculation intersecting triangles and significantly improve the efficiency of the algorithm. A kind of data structure of seven-tuple is choosen to record the intersecting straight-line segment. Based on the topological structure of STL models, the intersection chains/loops can be quickly and accurately determined, and meanwhile the method of tolerance extraction can be avoided. The directional distance between a ray and a triangular facet can be used to determine the locations of different triangular facets and regions. In order to avoid the case of ambiguity when judge the locations, the way of avoiding ambiguous positions has been choosen. Finally, re-triangulate the corresponding regions in order that the Boolean operations can be realized. The efficiency and stability of the algorithm have achieved the level of MagicsRP.
According to the needs of development of RP technology, consecutive subsector scan mode with adjustable scan length and direction has been achieved, which can make the parts full melt and improve the forming capability and quality of the parts in selective laser melting (SLM). Based on the algorithm of path tracking, the fractal scanning mode can fill arbitrary two-dimensional contours with fractal curves, which greatly enhance the density of the parts in SLS. Based on the PowerRP software system, the technice processing software for mask Stereo Lithography (SL) has been carried out and the corresponding technical problems have been solved.
It is a crucial task to inhibit the curling of the parts, which is a ubiquitous destructive phenomenon in the process of selective laser sintering the polymer materials. Concerning the disadvantages of controlling the preheating temperature by key-layers, the causes of warpage and the development of warpage phenomenon have been analyzed and summarized. In order to prevent parts from warpage, which is caused by the non uniform temperature field and stress field, a method of adding Heat Balance Support (HBS) has been proposed. The selection of styles and parameters of HBS should consider the physical and chemical properties of materials as well as the deformation and heat conduction after the laser sintering the materials; moreover, the HBS should be easily removed from the parts. The method has been successfully used in two kinds of materials which are PS and nylon, and achieved very good results.
为了适应快速成形技术的进步和加工方法的革新，本文按照领域工程、面向构件的思想，研究并开发了快速成形软件的统一体系结构，它基于分层架构+支撑平台和领域细化逐步求精的思想，包括工艺规划软件和系统控制软件。采用该软件系统可以缩短因快速成形加工方法和设备型号改变而引起的软件开发的周期，同时可以提高软件的开发质量。目前该软件系统已经成功地应用在华中科技大学研发的多种快速成形设备上，取得了良好的效果。
快速成形工艺规划软件的许多高级和核心功能都必须借助于对STL模型的布尔运算来实现，然而国内的学者对于这一问题的研究还处在初级阶段，算法的效率和稳定性都比较差。文通过精确的空间离散化的方法快速地找到可能参与相交运算的三角面片的最小集合，大大降低了三角面片之间的求交次数，显著地提高了算法的效率。采用七元组的数据结构记录交线段。依据STL文件的拓扑关系快速、准确地提取交线链和交线环，避免了采用容差法进行提取。通过判断射线到三角面片的有向距离来确定三角面片和每个区域的位置关系。采用规避歧义位置的方法，解决了判断位置关系时出现的歧义情况。最后，重新三角化相应的区域实现布尔运算。该算法的效率和稳定性均已达到RP业内最优秀的软件Magics RP的水平。
根据快速成形工艺发展的需求，实现了变扫描矢工艺路径算法，该扫描方式使得选择性激光熔化成形过程中制件充分熔化，提高了制件的成形性能和质量。基于路径追踪算法的分形扫描工艺实现了二维任意轮廓的分形曲线填充，该扫描方式使得选择性激光烧结零件的致密度得到较大提高。实现了基于PowerRP软件系统的掩膜法光固化工艺规划软件并解决了相应的工艺问题。
制件的翘曲是高分子聚合物选择性激光烧结过程中普遍存在的破坏性现象，对这种现象的合理抑制是提高SLS成形精度的关键。本文分析和总结了翘曲产生的原因以及翘曲现象的发展规律，提出了采用热源平衡支撑的方法来解决加工过程中由于温度场的不均匀进而导致应力场不均最终引起制件翘曲变形的问题。热源平衡支撑在样式和参数的选择上需要考虑材料的物理性能，化学性能，材料与激光相互作用后产生的收缩和热传导等因素，更重要的是热源平衡支撑要尽可能保持温度场的均匀性而且较容易地被清除掉。该方法已经成功地应用在聚苯乙烯和尼龙两种材料的成形中，取得了非常好的效果。
The architecture, the core algorithms and the technical algorithms of rapid prototyping (RP) software have been studied in this dissertation, which include off-line RP software architecture, algorithms of Boolean operations on STL (Stereo Lithography) models, RP technology algorithms and method of improving parts accuracy in selective laser sintering (SLS). The theories and achievements contribute to the development of the RP software and equipment of Huazhong University of Science and Technology, and provide a versatile software tool for the domestic promotion of Rapid Prototyping.
In order to meet the development of RP technology and the innovation of RP processing method, this dissertation, in accordance with domain engineering and thought of component-oriented programming, researches and designs a unified architecture of RP software, which is based on hierarchical architecture + support platform and the idea of the domain stepwise refinement, including technical processing software and system control software. The use of the software system can shorten the software development cycle, which is caused by changes in processing methods and equipment types, and at the same time it can improve the quality of software development. At present, the software system has been successfully applied to a variety of RP equipments which are researched and designed by Huazhong University of Science and Technology, and achieved good results.
Many advanced and core functions of rapid prototyping softwares for technology processing must be implemented with the aid of algorithms of Boolean operations on STL models. However, the research on this issue by scholars at home stays at a primary stage and the algorithm is of relatively low inefficiency and poor stability. A method of precise space decomposition is employed to limit the scope of searching for candidate triangles that may intersect, which can greatly reduce the times of calculation intersecting triangles and significantly improve the efficiency of the algorithm. A kind of data structure of seven-tuple is choosen to record the intersecting straight-line segment. Based on the topological structure of STL models, the intersection chains/loops can be quickly and accurately determined, and meanwhile the method of tolerance extraction can be avoided. The directional distance between a ray and a triangular facet can be used to determine the locations of different triangular facets and regions. In order to avoid the case of ambiguity when judge the locations, the way of avoiding ambiguous positions has been choosen. Finally, re-triangulate the corresponding regions in order that the Boolean operations can be realized. The efficiency and stability of the algorithm have achieved the level of MagicsRP.
According to the needs of development of RP technology, consecutive subsector scan mode with adjustable scan length and direction has been achieved, which can make the parts full melt and improve the forming capability and quality of the parts in selective laser melting (SLM). Based on the algorithm of path tracking, the fractal scanning mode can fill arbitrary two-dimensional contours with fractal curves, which greatly enhance the density of the parts in SLS. Based on the PowerRP software system, the technice processing software for mask Stereo Lithography (SL) has been carried out and the corresponding technical problems have been solved.
It is a crucial task to inhibit the curling of the parts, which is a ubiquitous destructive phenomenon in the process of selective laser sintering the polymer materials. Concerning the disadvantages of controlling the preheating temperature by key-layers, the causes of warpage and the development of warpage phenomenon have been analyzed and summarized. In order to prevent parts from warpage, which is caused by the non uniform temperature field and stress field, a method of adding Heat Balance Support (HBS) has been proposed. The selection of styles and parameters of HBS should consider the physical and chemical properties of materials as well as the deformation and heat conduction after the laser sintering the materials; moreover, the HBS should be easily removed from the parts. The method has been successfully used in two kinds of materials which are PS and nylon, and achieved very good results.
引文
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