注塑模充填过程动态模拟

本文基于注塑模型腔内充填机理和流体力学基本方程，视聚合物熔体在型腔中的流动为平行板中的流动，在浇注系统中的流动为等效圆柱管内的流动，在此假定的基础上进行合理简化，建立了三维薄壁型腔的基于非弹性、非牛顿流体在非等温下的Ｈｅｌｅ－Ｓｈａｗ流动的数学模型及浇注系统的数学模型，并采用混合有限元／有限差分数值方法耦合求解压力方程和能量方程，采用控制体积法自动跟踪熔体前峰面，从而实现充填过程的动态模拟。模拟结     

注塑成型充填过程的可压缩流动分析

注塑成型过程中,熔体在型腔中的流动和传热对制品质量性能有重要的影响.为了预测注塑制品的收缩、翘曲和力学性能,精确预测充填过程的流动及传热历史是十分必要的.本文考虑熔体的可压缩性及相变的影响,将充填过程中熔体的流动视为非牛顿可压流体在非等温状态下的广义Hele-Shaw流动.采用有限元/有限差分混合方法求解压力场和温度场,采用控制体积法跟踪熔体流动前沿,并应用Visual C++实现了注塑充填过程的可压缩流动分析.为了保证能量方程各项在单元内边界的连续性,结点能量方程各项由单元形心处的离散值加权平均获得,因而,能量方程在计算区域内整体求解.对两个算例进行了分析,模拟结果与实验结果的对比,验证了本文数值算法及程序.     

气体辅助注射成型过程的数值模拟技术

本文描述了气体辅助注射成型过程中熔体充填及气体穿入的数学模型，采用有限元／控制体积法计算充填阶段的压力场，确定两类移动边界，熔体前沿和熔体－气体边界。并对典型制件进行模拟验证了模型的可行性。对不同成型参数如熔体充填百分比及气道直径的影响进行了研究，结果表明熔体充填百分比不够，使气体吹入薄壁，较高的充填比又会阻止气体进入气道；较大直径的气道较小直径的气体不易进入薄壁处。     

气体辅助注射成型充填过程的数值模拟

描述了气体辅助注射成型的工艺过程及熔体充填和气体穿入的数学模型,采用有限元/有限差分/控制体积法计算充填阶段的压力场和温度场,确定熔体前沿和熔体/气体界面两类移动边界,并对典型制件充模过程进行了模拟.     

高速微注射成型中熔体充填模式及裹气机理研究

塑料为了准确模拟聚合物熔体在型腔中的流动及前沿位置和形态,建立了熔体、气体两相流流动模型,构造了熔体流动的黏弹性本构关系,用无量纲方法建立了熔体流动前沿的气体、熔体流动的统一控制方程和本构方程,并采用水平集方法预测和跟踪熔体流动前沿,模拟了熔体在低速、中速、高速条件下的流动状态和充填模式,分析了高速微注射成型中气孔产生的原因和可能出现的位置,开展了实际产品的高速微注射成型实验,比较了模拟结果和实验结果。研究表明,熔体充填模式与注射速度、材料特性、型腔尺寸密切相关,在喷射充填模式下可能产生裹气。     

气辅共注成型工艺中气道截面影响的CAE研究

基于气辅共注成型充填过程控制方程和7参数Cross—WLF黏度模型，采用数值模拟的方法研究了气辅共注成型工艺中气道截面的大小对熔体流动、气体穿透与压力分布的影响。采用改进的控制体积／有限元／有限差分法实现对充填过程中多重运动界面的追踪以及压力、温度等场量分布的预测，编写了相应的模拟程序。对气道等效直径分别为5mm、8mm和12mm的矩形板的气辅共注成型充填过程进行了数值模拟。通过对模拟结果的比较发现：随着气道等效直径的增大，气道中的熔体与薄壁区的熔体流速差越来越大，熔体流动的“跑道”效应越来越突出；“薄壁穿透”缺陷由明显到缓解直至基本消除；压力损失越小，压力分布也变得更为均匀。因而在制件设计时，气道截面尺寸宜稍大而不宜过小。     

板类制件水辅助注射成型流动过程的数值模拟研究

基于Hde-Shaw流动模型,采用广义牛顿流体本构方程,建立了三维薄壁制件的水辅助注射成型充填阶段的数学模型,采用黏性、可压缩非牛顿流体基本方程,建立了水辅助注射成型保压补缩阶段的数学模型。采用有限元／有限差分／控制体积法数值求解。最后对一具有梯形截面水道的薄壁平板零件的水辅助注射成型流动过程进行了模拟,模拟所得到的熔体前沿位置与前人实验结果吻合较好,浇口处压力变化、水的厚度分布及平均温度分布合理。     

三维熔体前沿界面的Level Set追踪

给出三维Level Set方程,采用五阶加权本质无振荡格式进行空间离散,通过算例验证了该算法的正确性及追踪三维运动界面的准确性。进而将Level Set算法和同位网格有限体积法进行耦合,模拟了注塑成型充填阶段的三维流动过程,准确追踪到了不同时刻熔体前沿界面,预测并分析了流动过程中不同时刻的压力、速度等重要流动特征。数值结果表明,该方法可追踪三维熔体前沿界面,预测充填过程中的重要流动特征。     

气辅注射成型矩形腔中熔体流动的数值模拟

应用Hele-Shaw物理模型和改进的Ｃross流变模型对辅助射成型过程中充填区域内熔体的流动进行数值模拟,采用控制体积法对充模过程中的熔体前沿、熔体－气体边界进行跟踪,运用有限元／有限差分混合数值方法求解气体注射阶段的速度场、压力场、温度场,以图表的形式列举了不同时刻压力场的分布和充模过程中的流线图。在计算过程中,采用压力场和温度场耦合的方法。     

基于数值模拟的气体辅助注射成型工艺控制研究

基于广叉Hele-Shaw流动模型描述了气体辅助注射成型时气体的穿透过程。采用有限元／有限差分／控制体积法计算充填阶段的压力场、温度场,确定熔体前沿和气熔界面,并以实际产品为例分析工艺参数对气体辅助注射成型产品质量的影响。     

On a pure finite-element-based methodology for resin transfer molds filling simulations

A physically accurate and computationally effective pure finite-element-based methodology for Resin Transfer Molding (RTM) process simulations is presented. The formulations are developed starting with the time-dependent mass conservation equation for the resin flow. Darcy's flow approximations are invoked for the velocity field, thereby forming a transient governing equation involving the pressure field and the resin saturation fill factor which tracks the location of the resin front surface. Finite element approximations are then introduced for both the fill factor and the pressure field, and the resulting transient discrete equations are solved in an iterative manner for both the pressures and the fill factors for tracking the progression of the resin front in an Eulerian mold cavity. The formulation involves only a pure finite-element Eulerian mesh discretization of the mold cavity and does not require specification of control volume regions and has no time increment restrictions that exist as in the traditional explicit finite-element-control volume based formulations. The present formulations accurately account for and capture the physical transient nature of the mold-filling process while maintaining improved numerical and computational attributes. Mold-filling simulations involving various geometrically complex mold configurations are presented, demonstrating the applicability of the developments for practical manufacturing process simulations.     

Nodal positioning analysis (NPA)—A numerical method for determining flow behavior of thermoplastic materials

A finite difference method is presented for the solution of two dimensional flow problems in polymer processing. The method is applicable to narrow gaps of any shape and variable thickness. NPA was developed for analyzing the filling stage of the injection molding cycle, but it could be used in extrusion, blown film, and other polymer processing operations. In NPA the position of the flow front is calculated at the end of each time increment, and an axial node is placed at the newest location of the flow front. Each axial node is then divided into a determined number of radial nodes. The velocity and temperature profiles are obtained from the simultaneous solution of the momentum and energy equations. The use of finite differences transforms the continuity, momentum, and energy equations into a system of linear equations which can be solved by any direct or iterative technique. The procedure is repeated until axial nodes have been placed throughout the whole flow channel or until the flow front stops due to polymer solidification. The main advantage of this technique, when compared to the use of a fixed finite difference grid, is that computation time is saved by considering only nodes filled with the fluid. Empty nodes are not considered and corrections for partially filled nodes are not needed. No complications due to the parabolic-shape of the flow front profile are introduced because the axial nodes are placed at average front locations determined by the average velocity at the particular time interval under consideration.     

Optimization of filling process in RTM using a genetic algorithm and experimental design method

In the resin transfer molding (RTM) process, preplaced fiber mat is set up in a mold and thermoset resin is injected into the mold. An important issue in RTM processing is minimizing the cycle time without sacrificing part quality or increasing the cost. In this study, a numerical simulation and optimization process for the filling stage was conducted in order to determine the optimum gate locations. The control volume finite element method (CVFEM), modeled as a 2‐dimensional flow, was used in this numerical analysis along with the coordinate transformation method to analyze a complex 3‐dimensional structure. Experiments were performed to monitor the flow front to validate the simulation results. The results of the numerical simulation corresponded with that of the experimental quite well for every single, simultaneous, and sequential injection procedure. The optimization analysis of the sequential injection procedure was performed to minimize fill time. The complex geometry of an automobile bumper core was chosen. A genetic algorithm was used to determine the optimum gate locations in the 3‐step sequential injection case. Taguchi's experimental design method was also used for determining the pressure contribution of each gate. These results could provide the information on the optimum gate locations and injection pressure in each injection step and predict the filling time and flow front.     

An interface‐update‐based implicit algorithm for mold filling simulation of liquid composite molding

In this article, a new implicit numerical algorithm has been presented for mold filling simulation in liquid composite molding process. The new implicit numerical algorithm is based on the update of flow interface to track flow front for each time step. Nodes of mesh are divided into three groups, i.e. filled nodes, interface nodes (or partially filled nodes), and empty nodes. Governing equations of filled nodes are solved to obtain pressure distribution; filling fractions of interface nodes are checked to determine if any node is needed to be added to filled nodes or be removed from filled nodes. The local LU factorization solver and preconditioned conjugated gradient iterative solver were employed to investigate the new implicit algorithm. Case studies demonstrated the performance of the proposed implicit algorithm. The new implicit algorithm reduced the computational complexity to below 2.0 power order of problem sizes. POLYM. COMPOS., 27:271–281, 2006. © 2006 Society of Plastics Engineers.     

水辅助熔体充模流动的熔体流痕

构建了用于研究水辅助熔体充模的仿真模具,采用红色和绿色着色剂作为示踪剂,通过水辅助着色的聚丙烯(PP)熔体在柱状模腔里充模,获得了能够反映熔体流动痕迹的样品。通过观察流痕,对水作用下熔体的充模流动进行了研究。实验结果表明:在一次穿透中,与注水喷嘴接触的高黏度熔体造成水从喷嘴射入熔体的不稳定,水的穿透导致模壁附近熔体可能产生回流现象,回流沿水的穿透方向呈减弱趋势。在二次穿透中,水前缘熔体黏度和黏度分布对水的穿透影响较大,熔体体积的收缩是近似线性的减小过程,熔体的剪切流动弱于一次穿透。实验中还发现,水前缘的熔体也会产生"喷泉流"。     

Online simulation‐based process control for injection molding

An online numerical simulation is presented that is capable of predicting state variables such as flow rate, melt temperature, shear rate, and melt viscosity by using real time data from a nozzle pressure sensor. The simulation solves the non‐Newtonian nonisothermal polymer flow into multicavity tools while executing rapidly enough for real time process control. Numerical accuracy and stability were first validated offline by comparing the online simulation to results obtained from a commercial mold filling simulation. Simulation‐based process control was then demonstrated by transferring a molding machine from fill to pack‐based on the predicted flow front position. The simulation‐based controller dynamically determined the appropriate transfer position for each part and transferred the machine at the correct time, thereby eliminating flash. The simulation, however, did increase process variability slightly due to delay times associated with the controller‐machine interface. A full factorial design of experiments (DOE) was performed varying injection velocity, mold temperature, and melt temperature. Results show that while the simulation dynamically adjusted the process on a part‐by‐part basis, it did not fully account for the process changes. Accuracy could potentially be improved by incorporating data from additional process sensors, by developing adaptive viscosity models, and by accounting for the melt compressibility. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Automated Design for the Runner System of Injection Molds Based On Packing Simulation

Multiple injection cavities are automatically balanced by adjusting runner and gate sizes based on an iterative redesign methodology integrated with computer-aided engineering (CAE) packing simulation. For runner balancing, each cavity must be filled simultaneously at uniform pressure. In addition, the time-pressure history of the polymer melt over the entire molding cycle should be considered. Based on the proposed methodology, a multicavity mold with identical cavities is balanced to minimize entrance pressure differences among various cavities at discrete time steps of the molding cycle. The results have shown more than a 95% reduction of the entrance pressure differences over other related studies, and also have demonstrated increased searching performance over other optimization techniques. A family mold with different cavity volumes and geometries is also balanced to minimize pressure differences at the end of the melt flow path in each cavity on a basis of discrete time steps of the molding cycle. The methodology has shown uniform pressure distributions among the cavities during the entire molding cycle.     

Numerical simulation of thickness variation effect on resin transfer molding process

In this work, a computer model has been developed to investigate the effect of reinforcement thickness variation and edge effect on infiltration and mold filling in resin transfer molding (RTM) process. The developed code is able to predict the flow front location of the resin, the pressure, and the temperature distribution at each time step in a mold with complex geometries. It can also optimize the positioning of injection ports and vents. The filling stage is simulated in a full two‐dimensional space by using control volume/finite element method CV/FEM and based upon an appropriate filling algorithm. Results show that the injection time as well as flow front progression depends on the edge effect, the variation of reinforcement thickness, and the position of injection ports; this highlights that the inclusion of these effects in RTM simulation is of definite need for the better prediction and optimization of the process parameters. The validity of our developed model is evaluated in comparison with analytical solutions for simple geometries, and excellent agreements are observed. POLYM. COMPOS., 2012. © 2011 Society of Plastics Engineers     

Enhancement of resin transfer molding using articulated tooling

Mold articulation is introduced in this concept for resin transfer molding (RTM) to increase mold fill times and potentially allow for the use of high viscosity, hot melt resin systems, or thermoplastics. Following a brief review of conventional RTM and a discussion of the limitations on the factors that control fluid flow through porous media, the articulated concept is described. This is followed by an explanation of the sequence of motion of an articulated segmented mold necessary for consolidation, void removal and accelerated fluid flow through a fibrous preform. An analysis of the process using a fiber preform with orthotropic permeability is outlined from which mold fill time is obtained. This is compared with conventional RTM mold fill times using typical resin properties and fiber volume fractions. For the conservative assumptions used, an improvement by a factor of ten in mold fill time is achieved using the articulated process relative to conventional RTM.     

Flow front measurements and model validation in the vacuum assisted resin transfer molding process

Through‐thickness measurements were recorded to experimentally investigate the through thickness flow and to validate a closed form solution of the resin flow during the vacuum assisted resin transfer molding process (VARFM). During the VART'M process, a highly permeable distribution medium is incorporated into the preform as a surface layer and resin is inftised Into the mold, under vacuum. During Infusion, the resin flaws preferentially across the surface and simultaneously through the thickness of the preform, giving rise to a three dimensional‐flow front. The time to fill the mold and the shape of the flow front, which plays a key role in dry spot formation, are critical for the optimal manufacture of large composite parts. An analytical model predicts the flow times and flow front shapes as a function of the properties of the preform, distribution media and resin. It was found that the flow front profile reaches a parabolic steady state shape and the length of the region saturated by resin is proportional to the square root of the time elapsed. Experimental measurements of the flow front in the process were carried out using embedded sensors to detect the flow of resin through the thickness of the preform layer and the progression of flow along the length of the part. The time to fill the part, the length of flow front and its shapes show good agreement between experiments and the analytical model. The experimental study demonstrates the need for control and optimization of resin injection during the manufacture of large parts by VARTM.