Simulation of reacting flow during filling in reaction injection molding (RIM)

This paper deals with computer simulation of the filling stage of the Reaction Injection Molding (RIM) process for cavities of rectangular, cylindrical, and disc shapes. The computer model is in two parts: the main flow and the flow by the moving front. In the main flow part, the transient equations of axial momentum, energy and species conservation and also the continuity equation are solved numerically by finite-difference methods using a moving, changing mesh. In the flow front part, which is quite novel, the transient (parabolic) vorticity, energy and species conservation equations and the elliptic streamfunction equation are again solved by finite-difference methods. An important feature of both parts is that convection along and across the flow is included in all the transient equations. Results are presented for all three cavity shapes and those for rectangular cavities are compared with the experimental results of previous investigators.     

Numerical simulation of mold filling in reaction injection molding

A two dimensional finite element model for the simulation of the advancing front in reaction injection molding (RIM) is presented. The model is based on the solution of the full Navier-Stokes equation for the computation of the velocity and pressure. The arbitrary Lagrange-Euler method is used for the moving front. The method of characteristics is used for the solution of the mass-and energy equations. An automatic remeshing algorithm is used to prevent element distortion and to optimize element size and number. Numerical results are presented for flow into a complex domain in order to illustrate the versatility of the method.     

Simulation of foaming in reaction injection molding

Blowing agents are often used in the reaction injection molding process to compensate for the shrinkage that occurs upon polymerization, thereby creating a structural foam part. Controlling the thickness of the solid skin and foamed core is essential to achieve the intended mechanical properties of the molded part. A numerical model was developed to predict the foaming behavior in reaction injection molding. This algorithm employs a novel primitive cell construction to enable it to analyze complex rectangular geometries, including inserts, with a two-dimensional, finite difference solution method. The analysis was applied to foaming of polyurethane in a rectangular cavity. The predicted skin thickness was found to be in good agreement with actual structural foam parts. Foaming as a function of cavity thickness was also treated. The algorithm Is useful for understanding and interpreting nonlinear phenomena of rapid, exothermic polymerizations such as foam formation adjacent to the mold wall or around a metal insert. The results can be used to formulate design guidelines for achieving desired skin/core thicknesses as a function of design, material, and process parameters.     

The process of cavity filling including the fountain flow in injection molding

This work deals with the simulation of the filling of a cavity utilizing the Marker-and-Cell numerical technique in solving the transient problem involved. The cavity is confined by two parallel plates, and is “end fed.” The flow was assumed isothermal and the fluid incompressible, obeying the power law model. Special attention was given to the flow region near the advancing melt front, in order to obtain a better insight of the “fountain effect,” during which the fluid flows from the center to the walls of the cavity. The results of the simulation of the front flow region are supported by and in qualitative agreement with experimental results involving “tracer resins” during cavity filling. Although the flows considered were slow and isothermal, this study has significant practical ramifications on industrial mold filling during injection molding.     

Modeling fountain flow and filling front shape in reaction injection molding

A numerical model of the reaction injection molding process was developed to test front shape and flow approximations employed in previous models. The model was two-dimensional and simulated the flow, reaction, and heat transfer in the typically long axial dimension and the typically small thickness dimension of a mold. The filling front shape and the velocity profiles in the filling fluid were determined by numerical solution of the momentum equation with the appropriate stress boundary conditions using the method of Patankar (1980). The predicted temperature and conversion results agreed with calculations assuming that the front was flat perpendicular to the flow and that a parabolic velocity profile existed behind the fountain flow region at the front. Thus, simple assumptions about front shape and velocity in the thin dimension of a reaction injection mold can be employed without significant loss of accuracy in modeling reaction injection molding.     

A numerical simulation of the cavity filling process with PVC in injection molding

Filling cold mold cavities with hot polymer melts at high pressures is of great practical interest. The transport approach to this process of solving the general equations of change with suitable equations of state to describe the flowing material has been largely ignored. No analytic solution is possible, and the non-steady state flow adds a dimension which makes digital computation discouraging because of the core storage and execution time requirements. The mold filled in this simulation is a disk which hot polymer melt enters through a tubular entrance located at the center of the top plate. The tube is 2.54 cm. long and has a radius of 0.24 cm. The plate separation and outer radius of the disk cavity may be varied. A constant pressure applied at the entrance of the tube causes the flow. The cavity walls are kept at various low temperatures. The reported results are for rigid polyvinyl chloride (PVC). The general transport equations, i.e. continuity, momentum, and energy, for a constant density power law fluid are used to solve the flow problem. Convergence to the differential solutions is guaranteed but since a lower limit was imposed on the time increment by the core storage limit of the computer facilities (27K) and long execution times, all results are semiquantitative for the problem as stated. Using the results obtained it is possible to predict “fill times”. The formation of a frozen polymer skin as the cavity fills may be followed via the velocity profiles. The temperature profiles which reflect cooling and the amount of viscous heat generated provide the basis for studying resin thermal degradation effects. Finally, because so much of the total pressure drop is disispated in the entrance tube, and so much viscous heat is generated there, this study indicates that the design of the gate and runner system is perhaps the most important facet of success in mold filling.     

Mold filling and curing analysis in liquid composite molding

Non-isothermal mold filing and curing experiments of liquid composite molding were carried out in this work. To compare the experimental results with a previously developed numerical simulation model, measurements of volumetric heat transfer coefficient between the resin and the fiber, and characterization of resin kinetics and rheological changes were also conducted. Combined with the previously measured fiber perform permeability, the numerical model provided a good prediction of temperature profiles during molding for a polyurethane/glass fiber composite.     

注塑充模过程中温度场的全三维数值模拟

注塑充填过程中,聚合物温度的变化直接影响到成型过程的各个方面,因此,对注塑流动过程中温度场的准确预测具有重要意义.采用有限元方法对注塑充填过程中温度场的三维数值分析进行了研究.没有采用传统的Hele-Shaw薄壁假设,而是全面考虑了对流项在各个方向的影响,提出了一种全三维的温度场计算的数学模型和数值实现方法.采用Galerkin法对能量方程进行了三维有限元离散,同时,为保证数值计算过程的稳定性,采用上风法来处理对流项和黏性热项.模型可预测非牛顿非等温流体在任意厚度型腔内流动时的温度场.和二维模型相比,它具有更加广泛的适用范围,计算结果也更准确.数值算例证明了给出的模型和算法的可靠性.     

Impingement mixing in reaction injection molding

Impingement mixing is the unique feature of the reaction injection molding (RIM) process but mixheads are largely designed by trial and error. To visualize the impingement process we have taken high speed photographs. To characterize mixing quality we have followed adiabatic temperature rise of mixtures, varying Reynolds number, mixhead geometry and the reaction rate. These results are examined in terms of a simplified model which includes both fluid mechanical and polymerization aspects of the problem.     

A proposed technique to improve the filling of the mold cavity by polymer during injection molding

In this study, a technique is proposed to improve the filling process of the injection molding and minimize the solidification during the filling to achieve a complete filling of the mold cavity. Two methods are proposed: stopping the flow rate of the cooling fluid or passing cooling fluid inside the cooling channels at higher temperature during a period of the injection molding cycle. The configuration studied consists of the mold with cuboids‐shape cavity having two different thicknesses. A validation of the numerical model used by an experimental work is presented. The results show that, stopping of cooling fluid on a period of injection cycle has not great effect on the improvement of injection cycle. The results indicate that passing coolant fluid at higher temperature during the ejection stage decreases the solidification of the polymer during the filling stage by about 40%. © POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers.     

压延、挤出、硫化、注压

介绍了压延、挤出、硫化和注压4种橡胶加工工艺的基本设备组成、重点结构部件、工艺原理及过程、加工中的注意事项、技术参数、各种常见故障及消除措施等。另外对这4种加工工艺的发展趋势和优缺点作了介绍和评价。     

Simulation of reactive injection molding

This paper deals with the computer simulation of those aspects of Reactive Injection Molding (RIM) dealing with the non-isothermal and transient flow of a chemically reacting mixture into a mold cavity, and the in situ polymerization (“curing”) which follows mold filling. Linear polyurethane systems were considered. The purpose of this simulation work is to investigate the effects of the operating, chemical, and rheological variables on the length and the stability of the RIM process, as well as the quality of the resulting product. Since the flowing fluid mixture is reactive, there is a need to know the thermal history of each of the flowing fluid particles. For this reason the “fountain” flow at the fluid-air interface is considered in a heuristic fashion.     

The dynamics of peak cavity pressure in injection molding

The modification of the injection-molding machine, incorporating additional filters, an accumulator, and a servovalve, was shown to reduce the peak cavity pressure variations, in as much as the variations using the microprocessor system were random in comparison to the variations using the normal mode of operation, which exhibited nonstationarity and autoregressive behavior. If the peak hydraulic pressure could be randomized, then the peak cavity pressure variations also would be random. No closed-loop control would be needed to control the peak cavity pressure.     

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.     

Laminar shear mixing in reaction injection molding

Turbulent flow in the impingement mixing head of a reactions injection molding machine produces a laminated mixture, whose striations are mostly so thick that any subsequent copolymerization would be hindered by diffusion. However, further reduction in striation thickness occurs during laminar flow through the runner. This is calculated here and is a function of the radial position in the runner and of its length/diameter ratio. So, for example, when L/D = 10, it is predicted that at least 79 percent of, the reagent striations would be thin enough to permit reaction under kinetically controlled conditions.     

纤维增强塑料注射充模过程中纤维对模具的冲蚀磨损模拟

针对塑料模具在玻璃纤维冲蚀下造成的表面磨损问题,运用有限元软件ANSYS/LS-DYNA建立了模具材料多颗粒冲蚀有限元模型,进行三维显式冲击动力学计算,研究纤维在不同冲蚀速度和冲蚀角度下对模具的冲蚀规律,分析冲蚀机理。结果表明,玻璃纤维对模具的冲蚀过程以微切削为主;玻璃纤维对模具的冲蚀磨损率随着冲蚀速度的增加而增加,随着冲蚀角度的增加而呈现先增加后减小的趋势。     

Modeling of mold filling process for powder injection molding

The mold filling process has been modeled for the injection molding of different polymer-based binders and powder-polymer mixtures. It is essentially a two dimensional non-Newtonian fluid flow analysis in a non-isothermal environment. A complete analysis is accomplished by combining a finite element method and control volume technique to describe an increment of flow front movement, whereas a finite difference method is used to solve the energy equation to characterize the temperature distribution. Numerical results are compared to exact solutions for a circular ring cavity using a power law fluid model under an isothermal condition. Comparison of computed results against published data for a simple circular disk shows good agreement between the two analysis methods. After making selected comparison studies, it is demonstrated that the filling process in Powder Injection Modeling with different combination of powder-polymer mixtures is markedly dependent on specific combinations of powder; and polymer based binders. Computed flow front results for a rectangular cavity also compared favorably against the data for a power law fluid model under non-isothermal conditions.     

基于模腔压力的注塑过程注射速度设定曲线优化

注塑成型中,把熔前速度恒定(熔体均匀填充模具)作为设定注射速度参数的重要依据。针对熔前速度无法测量问题,提出对于一类注塑制品,用模腔压力随时间线性增加来近似熔前速度恒定。其次,利用实验数据,设计并建立了模腔压力递归神经网络(RNN)模型。在此基础上,提出一种注射速度设定曲线的分级优化策略,其中曲线分级是利用Douglas-Peucker方法自动确定速度分级数目和位置,优化过程则采取基于滑动窗口的迭代优化方法来实现。实验结果表明,该优化方法可以根据模腔复杂程度自动选择合适的注射曲线分级数目,并且其优化精度较常规等间隔分级优化方法更高。     

Automatic velocity profile determination for uniform filling in injection molding

The filling phase plays an important role in determining the quality of injection molded parts. Uniform melt‐front velocity has been recommended for minimizing the part non‐uniformity. On the basis of a capacitive transducer that has been developed for online measurement of melt front velocity, we propose an intelligent strategy for profiling injection velocity. The nonlinear response of melt development in a mold cavity is first acquired with a constant screw injection velocity. On the basis of this response, a stage division method is proposed to determine an appropriate stage number of a piecewise linear profile and the locations of the switch points. An optimization based on sequential simplex search is then conducted to search for the optimal profile of injection velocity. Online experimental tests on molds with different geometries show that the proposed method is effective in achieving a uniform mold filling. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

反应注射成型尼龙6的研究进展

介绍了反应注射成型尼龙6（nylon 6-RIM）、增强反应注射成型尼龙6（nylon 6-RRIM）、毡片模塑反应注射成型尼龙6（nylon 6-MMRIM）以及反应注射成型尼龙6嵌段共聚物（NBC-RIM）材料的性能特点以及研究进展。并讨论了nylon 6-RIM材料在各个领域的应用优势和前景。