塑料微注射成型流动裹气机理研究

The air entrapment formed by the unsteady melt front during micro-injection molding is hard detected by usual instruments. The existing theory and method for micro-injection molding are unable to predict this phenomenon either. In this project, we will establish the multi-mode nonlinear viscoelastic constitutive model and governing equations for melt flow by rheological experiments and dimensionless method, and the coupled viscous flow equations and interface conditions for air according to two-phase flow principle, construct numerical method to solve the flow problem with combining the Streamline Up-Wind Scheme, Elastic Viscous Stress Split and Successive Over-Relaxation methods, and develop the simulating software to predict the flow field and the air entrapment. Combining the short shot experiments for typical products and simulated results to master the triggering mechanism for unsteady melt front and jetting swerve, explore the influence of boundary condition and surface tension on the evolution of air-melt interface, disclose the relationship between the air entrapment and filling velocity, part shape, material properties and melt entrance direction to cavity. In addition, we will establish the judge rule in terms of Re, Wi for the unsteady melt front and jetting swerve, and propose the mold design and processing conditions to prevent the air entrapments during melt flowing. Threfore this project will provide the theoretical support for studying the complex physical problems in micro-injection molding.

微注射成型中流动前沿失稳形成的裹气是用普通仪器难以检测的质量缺陷，已有的微注射成型理论和方法也不能预测这种现象。本项目根据流变实验和无量纲方法建立熔体流动的多模态非线性黏弹性本构方程和控制方程，基于两项流原理建立与之耦合的黏性气体流动的控制方程和界面边界条件，结合流线上风法、黏弹性分裂算法、松弛迭代法构造数值求解方法，开发模拟程序预测流场变化、裹气位置及尺寸。开展典型微制品缺料注射试验，结合模拟结果掌握流动前沿失稳及射流偏转的触发机理，探索边界条件及表面张力对气- - 熔界面演化及熔体裹气的影响，揭示裹气与充填速度、制品形状、材料特性及熔体流入型腔方向等因素的关系，建立前沿失稳及射流偏转的Re数、Wi数判别准则，提出避免熔体流动裹气的模具设计、工艺设计方案，为研究微注射成型中的复杂物理问题提供理论支撑。

根据两相流原理建立了微注射成型熔体、气体耦合流动模型，构造了熔体流动前沿演化的输运方程，提出了压强和速度场及速度和温度的解耦合方案，阐明了惯性力、黏性力竞争优势是诱发喷射的关键，发现了射流长度与注射速度、熔体温度的依赖关系，预测了射流转入蛇形流后裹入气体的位置及气孔大小；开展了黏弹性流动仿真分析，分析了Weissenberg数对射流形态演化的影响，明确了流动应力与惯性力的竞争关系对喷射产生的影响；基于黏弹性及ALE原理建立了熔体三维压缩流动理论模型，构造了有限元求解的变分方程，提出了双重迭代解耦合方法分别求解连续方程、动量方程、本构方程和能量方程，模拟了物理场尤其是压力场的演化规律，分析了注压成型出现的压力平台现象的原因，结合实验与数值模拟发现注压成型流动诱导残余应力比传统注射成型小一个数量级，并且显著减小了制品的双折射；系统评价了聚合物熔体非线性流变本构模型，发现XPP模型能较好表征压缩流变特征；针对多个金属嵌件放置、固定困难，发明了带多个金属嵌件定位切断机构，实现了多个金属嵌件的自动切断，获得了发明专利；针对带有导电片嵌件在模具上无法实现定位压紧、熔体冲击而发生位移及配合困难，提出了导电片嵌件的二次成型方法及注射模二次顶出方法，解决了直接放入金属嵌件易发生侧向偏移的技术难题。项目共发表标注资助学术论文17篇，其中SCI论文4篇，获得1项发明专利和2项软件著作权，毕业硕士研究生3名，1名博士生、4名硕士生在读。