Three-dimensional nonisothermal mold filling simulations in resin transfer molding

The manufacture of polymer composites through the process of resin transfer molding (RTM) involves the impregnation of the reactive polymer resing into a mold with preplaced fibrous reinforcements. Determination of RTM processing conditions requires the understanding of various parameters, such as material properties, mold geometry, and mold filling conditions. Modeling of the entire RTM process provides a tool for analyzing the relationship of the important parameters. This study developed a nonisothermal 3-D computer simulation model for the mold filling process of RTM based on the control volume finite element method. The model will be able to simulate mold filing in molds with complicated 3-D geometry. Results of some numerical studies in RTM show the applications of the proposed model.     

Modeling of the impregnation process during resin transfer molding

A model has developed for simulating isothermal mold filling during resin transfer molding (RTM) of polymeric composites. The model takes into account the anisotropic nature of the fibrous reinforcement and change in viscosity of the polymer resin as a result of chemical reaction. The flow of impregnating resin through the fibrous network is described in terms of Darcy's law. The differential equations in the model are solved numerically using the finite element technique. The Galerkin finite element method is used for obtaining the pressure distribution. A characteristics based method is used to solve the non-linear hyperbolic mass balance equation. The finite element formulation facilitates computations involving the motion of the polymer resin front characterized by a free surface flow phenomenon.     

Flow simulation in molds with preplaced fiber mats

The primary goal of this study was to develop 2-D and 3-D computer simulation schemes for the mold filling processes of structural reaction injection molding (SRIM) and resin transfer molding (RTM) under isothermal conditions. The developed computer code was able to simulate the mold filling in molds with complicated geometry, Experiments were also carried out based on flow vitalizations. Experimental results were compared with the numerical simulations.     

Flow and cure phenomena in liquid composite molding

Liquid molding processes including resin transfer molding (RTM) and structural reaction injection molding (SRIM) continue to attract attention due to their potential for high volume manufacture. This paper examines and compares the pressuare and temperature histories observed in mold cavities during impregnation, heating, and polymerization for both RTM and SRIM using polyester, vinyl ester, and polyurethane resins in combination with continuous strand mats. Experimental results are related to thermal, chemical and rheological effects. Factors which influence materials behavior and process control and the implications for mold design are discussed.     

A finite element/control volume approach to mold filling in anisotropic porous media

Mold filling in anisotropic porous media is the governing phenomena in a number of composite manufacturing processes, such as resin transfer molding (RTM) and structural reaction injection molding (SRIM). In this paper we present a numerical simulation to predict the flow of a viscous fluid through a fiber network. The simulation is based on the finite element/control volume method. It can predict the movement of a free surface flow front in a thin shell mold geometry of arbitrary shape and with varying thickness. The flow through the fiber network is modeled using Darcy's law. Different permeabilities may be specified in the principal directions of the preform. The simulation permits the permeabilities to vary in magnitude and direction throughout the medium. Experiments were carried out to measure the characteristic permeabilities of fiber preforms. The results of the simulation are compared with experiments performed in a flat rectangular mold using a Newtonian fluid. A variety of preforms and processing conditions were used to verify the numerical model.     

Optimization method to select temperature based on chemorheological and exothermal reaction of RTM

Heating mold and resin have been widely used in resin transfer molding (RTM) to improve injection and manufacturing efficiency. The unreasonable mold/resin temperatures sometimes lead to excessive viscosity of resin and premature curing, which will result in failure of the filling process. Selection of optimal mold and resin temperature has become a source of concern in the polymer industry. This article presents an optimization method to select mold and injection resin temperatures by using numerical simulation based on chemorheological and exothermal reaction of the RTM process. The results show that the optimization method has high computational efficiency for three-dimensional parts with different shapes. The selected mold/resin temperature ensures the smooth filling process, which provides a powerful tool for parameter design in polymer industry. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48245.     

Modeling and simulation approaches in the resin transfer molding process: A review

A review of current approaches in modeling and simulation of the resin transfer molding (RTM) process is presented. The processing technology of RTM is discussed and some available experimental techniques to monitor the process cycle are presented. A master model is proposed for the entire process cycle consisting of mold filling and curing stages. This master model contains the fundamental and constitutive sub‐models for both stages. The key elements of the master model discussed in this study are: flow, heat and mass balance equations for fundamental sub‐models, permeability, cure kinetics, resin viscosity and void formation for constitutive sub‐models. At the end, numerical methods widely used to simulate the filling process are presented and published simulation results of mold filling and process cycle are reviewed.     

Optimization of cure kinetics parameter estimation for structural reaction injection molding/resin transfer molding

A numerical method is proposed for polymer kinetic parameter estimation of either Structural Reaction Injection Molding (SRIM) or Resin Transfer Molding (RTM). The method simulates either radial flow or axial flow of reactive resins through a fiber preform inside a mold cavity. This method considers a non‐isothermal environment with different inlet boundary conditions. Based on the molding conditions, this method can find the best values of chemical kinetic parameters by comparing the simulated temperature history and the experimental temperature history. Since the kinetic parameters are estimated with the real molding conditions, the simulations using these parameter values can have better agreement with molding data than those parameters which are obtained from idealized conditions such as Differential Scanning Calorimeter (DSC). The optimization approach was verified by estimating kinetics parameters for RTM data available in the literature. Temperatures predicted by the optimized kinetics parameters are compared with experimental data for two different molding conditions: injection of a thermally activated resin into a radial mold under constant pressure flow, and injection of a mix activated resin into a radial mold under constant volume. In both cases, the optimized kinetics parameters fit the temperature data well.     

Key issues in numerical simulation for liquid composite molding processes

Polymer-based composites have a great potential for the manufacture of energy-efficient vehicles. Because of this growing usage and also because mold cost increases with part complexity, numerical simulation of Liquid Composite Molding Processes such as Resin Transfer Molding (RTM) and Structural Reaction Injection Molding (SRIM) are becoming more important. To succeed in that venture, reliable input data as well as a numerical model able to simulate specific molding difficulties and complex shapes must be used. In this paper, several issues are discussed and a computer software is presented. Among them, permeability measurement is discussed. Concerning specific molding difficulties, simulation results compared with experimental data are presented for edge effects, flow in multilayers and flow in ribbed structures. Finally, nonisothermal filling is discussed. Experimental data showing how the temperature boundary layer is developed during the filling of a heated mold are presented.     

树脂传递模塑充填过程的数值模拟

树脂传递模塑成型（RTM）工艺是在一定温度及压力下把低黏度的树脂注入预先置有增强纤维的模具中,然后固化成型的一种复合材料液体成型方法。本文建立了RTM工艺充模过程的数学模型,并采用有限元/控制体积法实现了对复杂薄壁构件的充填模式、压力场和速度场的模拟。     

RTM工艺充填过程的动态仿真

建立了树脂传递模塑成型(RTM)工艺充模过程的数学模型,并采用有限元/控制体积法实现了对复杂薄壁构件的充填模式、压力场和速度场的动态仿真.算例表明,该法可以快速有效地描述RTM工艺的充填过程.     

Sensor system for monitoring impregnation and cure during resin transfer molding

In the resin transfer molding (RTM) fabrication of composites, knowledge of the position of a moving resin front during impregnation is important for process optimization. We describe here a simple, inexpensive, multi-point sensor system based on DC conductometry for determination of resin position in an RTM mold. This Resin Position Sensor (RPS) system consists of a matrix of small sensors embedded in the RTM tool, whose combined output can be used to produce a resin flow pattern at any given time after the start of impregnation. As it cures, the resin resistance increases and the sensor can then function as a cure monitor. A large, 24-sensor RTM tool was fabricated for demonstration of the RPS. Flow contour maps generated from sensor data during impregnation of both E-glass and carbon fiber preforms are shown.     

In situ sensor monitoring and intelligent control of the resin transfer molding process

An intelligent closed-loop expert control system has been developed for automated control of the resin transfer molding process of a graphite fiber preform using an epoxy resin, E905L. The sensor model system has been developed to make intelligent decisions based on the achievement of landmarks in the cure process, such as full preform impregnation, the viscosity, and the degree of cure of the resin rather than time or temperature. In-situ frequency dependent electromagnetic sensor (FDEMS) and the Loos resin transfer model are used to monitor and control the processing properties of the epoxy resin during RTM impregnation and cure of an advanced fiber architecture stitched preform. Once correlated with viscosity (η) and degree of cure (α), the FDEMS sensor monitors and the RTM processing model predicts the reaction advancement of the resin, viscosity and the impregnation of the fabric. This provides a direct means for monitoring, evaluating, and controlling intelligently the progress of the RTM process in situ in the mold throughout the fabrication process and for verification of the quality of the composites.     

Liquid flow in molds with prelocated fiber mats

The mechanism associated with mold filling in the manufacture of structural RIM (SRIM) and resin transfer molding (RTM) composites is studied by means of flow visualization and pressure drop measurements. To facilitate this study, an acrylic mold with a variable cavity was constructed and the flow patterns of nonreactive fluid flowing through various layers, types, and combinations of preplaced glass fiber reinforcement mats were photographed for both evacuated and nonevacuated molds. The pressure drops in the flow through a single type of reinforcement (e.g., a continuous strand random fiber mat) and also a combination of reinforcement types (e.g., a stitched bidirectional mat in combination with a random fiber mat) were recorded at various flow rates to simulate high-speed feeding processes (e.g., SRIM) and low-speed feeding processes (e.g., RTM). By changing the amount of reinforcement placed into the mold, the permeabilities of the different types and combinations of glass fiber mats were obtained as a function of porosity. It is shown that partially evacuating the mold cavity decreases the size of bubbles or voids in the liquid, but ultimately increases the maximum pressure during filling. The results also show that glass fiber mats exhibit anisotropic permeabilities with the thickness permeability, K_z, being extremely important and often the determining factor in the pressure generated in the mold during filling.     

Thermal dispersion in resin transfer molding

This investigation focuses on the effects of thermal dispersion in resin transfer molding (RTM). A set of volume-average balance equations suitable for modeling mold filling in RTM is described and implemented in a numerical mold filling simulation. The energy equation is based on the assumption of local thermal equilibrium and includes a dispersion term. Thermal dispersion is an enhanced transport of heat due to local fluctuations in the fluid velocity and temperature away from their average values. Nonisothermal mold filling experiments are performed on a center-gated disk mold to investigate and quantify dispersion effects. Good agreement is found between the experimentally measured and numerically predicted temperatures, and a function for the transverse dispersion coefficient in a random glass fiber mat is determined. The results indicate that thermal dispersion is important in RTM processes and must be included in simulations to obtain accurate predictions.     

Tow impregnation during resin transfer molding of bi-directional nonwoven fabrics

A model has been developed for analyzing resin impregnation of fiber tows during resin transfer molding of bi-directional nonwoven fiber performs. The model is based on the existence of two main regions of resin flow: the macropore space formed among fiber tows and the micropore space formed among individual fiber filaments within a tow. The large difference in permeability between these two regions of flow leads to the potential for void formation during resin transfer molding. The model was formulated for both constant flow rate and constant pressure mold filling. For ambient pressure mold filling, the model predicts a difference in the size of the voids and distribution between axial tows (oriented along the flow direction) and transverse tows (oriented in the transverse direction). When vacuum is imposed on the mold, the model predicts the same resin impregnation behavior for both axial and transverse tows. Furthermore, given sufficient time, voids generated under vacuum mold filling will eventually collapse because of the absence of an opposing internal void pressure. In addition to insights on void formation, the model also provides a basis for the study of the relationship between resin transfer molding parameters and the resin impregnation process.     

Physical and numerical modeling of mold filling in resin transfer molding

Finite element modeling and experimental investigation of mold filling in resin transfer molding (RTM) have been performed. Flow experiments in the molds were performed to investigate resin flow behavior into molds of rectangular and irregular shapes. Silicone fluids with viscosity of 50 and 100 centistoke as well as EPON 826 epoxy resin were used in the mold filling experiments. The reinforcements consisted of several layers of woven fiberglass and carbon fiber mats. The effects of injection pressure, fluid viscosity, type of reinforcement, and mold geometry on mold filling times were investigated. Fiber mat permeability was determined experimentally for the five-harness and eight-harness woven mats. Resin flow through fiber mats was modeled as flow through porous media. Pressure distributions inside both types of molds were also determined numerically. In the case of resin flow into rectangular molds, numerical results agreed well with experimental measurements. Comparison between the experimental and numerical results of the resin front position indicated the importance of edge effects in resin flow behavior in small cavities with larger boundary areas. Reducing the resistance to resin flow at the edge region in the numerical model allowed for good agreement between the numerical simulation and the physical observations of the resin front position and mold filling time.     

Micro scale flow behavior and void formation mechanism during impregnation through a unidirectional stitched fiberglass mat

Liquid composite molding (LCM) processes such as resin transfer molding (RTM) and structural reaction injection molding (SRIM) have been perceived as high potential processes for the near-net-shape manufacturing of composite parts. This paper addresses two major issues in LCM technology: fiber wetting and void formation during mold filling. Flow visualization experiments were carried out to develop a better understanding of the flow induced voids. The formation and elimination of voids were studied using several liquids and a unidirectional stitched fiberglass mat. Void formation was correlated to capillary number and liquid-fiber-air contact angle.     

Numerical analysis of promoted polyester and vinylester reinforced composites in RTM molds

This article presents a one-dimensional simulation of the post-filling period in the resin transfer molding (RTM) process. In this work, transient heat conduction and kinetic of cure equations were solved simultaneously using the Galerkin finite element method. The kinetic models employed in this study were developed with regard to the effects of additives frequently used in the formulation of composite parts (e.g., promoting agents and reinforcements). These specialized kinetic models enabled us to predict the influence of incorporated reinforcement on the process variables, e.g., gel time and demold time, and on the distributions of temperature and extent of cure in through-the-thickness direction of the polyester-based and vinylester-based composites. It was shown that in the case of vinylester, the incorporation of glass fibers gave rise to a severe inhibition of the cure reactions at high conversions. The inhibiting effect of glass fibers in polyester-based formulation was found less significant, and the delay in the exotherm temperature peak was mainly attributed to the thermal effects induced in the presence of glass fibers. The numerical predictions were compared with the experimental data obtained in a heated mold for a reinforced polyester part.     

Manufacturing of composite links by structural reaction injection molding

An innovative approach has been developed to fabricate composite parts with complicated geometry. The part is first divided into several simple substructures and each substructure is produced by an optimal process. These preformed substructures are then assembled on a specially designed tool and transferred into a mold cavity for resin impregnation by structural reaction injection molding (SRIM). A composite link is used as an example in this study. The mold filling pattern is simulated using a 3-D computer model. The simulated results compared well with the experimental results when race tracking and fiber impingement effects were considered in the model.