3D混凝土打印成形质量分析与路径优化研究

徐捷

摘要

3D混凝土（水泥砂浆）打印技术因其无模板化施工、高效自动化、可实现复杂几何形状等特点已经成为建造行业研究应用的前沿与热点。然而，目前的3D混凝土打印技术始终存在一定程度的打印构件质量问题，包括成形质量问题、力学（强度及耐久性能等）质量问题等。其中，成形质量问题主要表现在两个方面：一是打印构件的形状、尺寸等方面的几何误差，即几何质量问题；二是打印构件表面的粗糙度欠佳，即表面质量问题。这种成形质量问题一定程度上制约了3D混凝土打印技术的实际应用与发展。为此，本文针对在应用实践中对3D混凝土打印构件优良成形质量的需求开展研究，主要研究成果如下：

（1）初步建立了3D混凝土打印构件成形质量的影响因素作用机制及评价方法3D混凝土（水泥砂浆）打印技术因其无模板化施工、高效自动化、可实现复杂几何形状等特点已经成为建造行业研究应用的前沿与热点。然而，目前的3D混凝土打印技术始终存在一定程度的打印构件质量问题，包括成形质量问题、力学（强度及耐久性能等）质量问题等。其中，成形质量问题主要表现在两个方面：一是打印构件的形状、尺寸等方面的几何误差，即几何质量问题；二是打印构件表面的粗糙度欠佳，即表面质量问题。这种成形质量问题一定程度上制约了3D混凝土打印技术的实际应用与发展。为此，本文针对在应用实践中对3D混凝土打印构件优良成形质量的需求开展研究，主要研究成果如下：

定性分析了材料性能、工艺参数、规划路径等三类决定性因素和硬件性能、构件设计参数等两类约束性因素对打印构件成形质量的耦合作用，初步建立成形质量影响因素作用机制；基于GD&T体系设计影响因素试验对平面和角的成形质量影响因素作用机制进行定量研究，揭示成形质量影响因素作用机制的四个特性；最后，基于构件关键几何元素构建了打印过程成形能力对成形质量进行预测，并建立了打印构件成形质量评价模型对实际成形质量进行定量评价。

（2）提出了体积成形3D混凝土打印理论及路径优化算法

针对规划路径对复杂几何构件成形质量的影响，提出了新型体积成形3D混凝土打印理论，阐明以可变截面体素的有序组合来代替等截面线条累加的原理，设计了针对单道次挤出体积成形的条纹状图案构件和多道次挤出体积成形的实体构件的路径优化算法，包含基于体素定位线的可变层厚模型曲面切片优化算法，和基于Voronoi图生成的可变宽度路径规划优化算法，完成了方形（矩形）截面体素的体积成形构建。

（3）研发了体积成形3D混凝土打印多参数协同控制优化工艺

研发出一套具有自主知识产权的可实现方形截面体素体积成形的3D混凝土打印原型软硬件系统，该系统可多维联动控制运动模块的XYZ三轴和挤出模块的物料挤出轴、喷嘴转向轴、喷嘴变径轴等六轴；在此基础上，基于满足打印混凝土工作性能要求的材料配合比完成喷嘴尺寸、喷嘴移动速度、物料挤出速率和路径曲率半径等关键工艺参数匹配关系的试验拟合，构建了多参数协同控制机制，同时对设备运动卡顿、单道次路径关键点、多道次多层层面光滑度等关键工艺细节进行了优化处理。

本文围绕提升3D混凝土打印成形质量开展了系统性研究，研究成果将为推动3D混凝土打印技术的推广应用、促进建筑业高质量发展提供有效参考。

关键词：3D混凝土打印；成形质量；影响因素；体积成形；路径优化；多参数协同

Abstract

3D Concrete (cement mortar) Printing (3DCP) has become the research frontier and hot spot of the construction industry due to its characteristics as non-template construction, efficient automation and more freedom of achievable complex geometries. However, the current 3DCP technology has always been recognized for the quality problems of printed components, including forming quality, mechanical (strength, durability, etc.) quality, etc. The mentioned forming quality problem generally refers to two aspects - a) The geometric error in the shape and size of printed components, that is, the geometric quality issue; and b) The imperfection in the surface roughness of printed components, that is, the surface quality issue. This forming quality problem has restricted the practical application and development of the 3DCP technology to some extent. This dissertation performs research for the practical demand of good 3DCP forming quality with the main research outputs described as follows:

(1) Preliminary establishment of the factor influence mechanism and assessment method of the 3DCP forming quality

The coupling influences of factors on the forming quality of 3DCP components were qualitatively analysed. There were three decisive factors - material property, process parameter and planned toolpath, and two restrictive factors - hardware performance and component design parameter. The factor influence mechanism of the forming quality was preliminarily established. Based on the Geometric Dimensioning and Tolerancing (GD&T) system, a control-variable experiment was designed to quantitively investigate the factor influences on the forming quality of planes and corners. Four characteristics of the factor influence mechanism were observed. Consequently, the printing process capability was established according to key features of a component to predict the forming quality, and the forming quality assessment model of physically printed components was also established for quantitive assessment.

(2) Development of the theory with toolpath optimization algorithms of volumetric 3DCP

Considering the influence of toolpath on the forming quality of complex geometries, an innovative volumetric 3DCP theory was proposed. This theory replaced the accumulation of lines of constant cross-sections with an ordered combination of voxels with variable cross-sections. Volumetric toolpath optimization algorithms were then designed for both the single-path volumetric pattern parts and the multi-path volumetric solid parts. These included a voxel positioning line - based variable layer-thickness slicing optimization algorithm, and a Voronoi graph - based variable width toolpath planning optimization algorithm. The mathematical solution was accomplished for the voxels of square (rectangle) cross-sections.

(3) Development of an optimised process under multi-parameter collaborative control mechanism for volumetric 3DCP

A volumetric 3DCP software and hardware prototype system with independent intellectual property rights was developed for the voxels of square cross-section. The system could achieve a multi-dimensional linkage control of the XYZ motions of the motion module with the material extrusion, the nozzle steering, and the nozzle size varying of the extrusion module. To put to use, matching relationships of selected key process parameters were experimentally fitted to develop a multi-parameter collaborative control mechanism. These key process parameters referred to nozzle size, nozzle travel speed, material extrusion rate and toolpath curvature radius. During the process experiments, a material mix was applied in consistence with the common workability requirements of printed concrete. Key process details as gantry motion jitter, single-path key points, multi-path surface smoothness were also optimised.

This dissertation has finished a systematic research on the improvement of the 3DCP forming quality. The obtained research outputs will provide effective references for promoting practical applications of the 3DCP technology and the high-quality development of the construction industry.

Keywords:3D Concrete Printing; forming quality; influence factor; volumetric forming; toolpath optimization; multi-parameter collaboration