Analysis of resin injection molding in molds with preplaced fiber mats. II: Numerical simulation and experiments of mold filling

This work presents the results of numerical simulation and experimental visualization of the mold filling process in resin injection molding with preplaced fiber mats. The mold filling experiments were conducted with various mat stacks consisting of continuous random glass fiber mats and bidirectional stitched glass fiber mats. The use of two different mat types in the mat stack created porosity and permeability variations. The effect of these permeability variations was studied by taking flow pressure measurements and observing the progress of the flow front of a non-reactive fluid filling a clear acrylic mold that contained the reinforcement mat stack. Numerical simulation corresponding to each experiment was also carried out. The numerical results were compared to the experimental measurements.     

Measurement of effective permeability of reinforcement mats using sensitivity analysis

A methodology using sensitivity analysis is proposed to measure the effective permeability which includes the interaction of the resin and the reinforcement. Initially, mold‐filling experiments were performed at isothermal conditions on the test specimen and the positions of the flow front were tracked with time using a flow visualization method. Following this, mold‐filling experiments were simulated using a commercial software to obtain the positions of the flow front with time at the process conditions used for experiments. Several iterations were performed using different trial values of the permeability until the experimentally tracked and simulated positions of the flow front with time were matched. Finally, the value of the permeability thus obtained was validated by comparing the positions obtained by performing the experiments at different process conditions with the positions obtained by simulating the experiments. In this study, woven roving and chopped strand mats of E‐class glass fiber and unsaturated polyester resin were used for the experiments. From the results, it was found that the measured permeabilities were consistent with varying process conditions. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Reactive mold filling in resin transfer molding processes with edge effects

Reactive mold filling is one of the important stages in resin transfer molding processes, in which resin curing and edge effects are important characteristics. On the basis of previous work, volume‐averaging momentum equations involving viscous and inertia terms were adopted to describe the resin flow in fiber preform, and modified governing equations derived from the Navier–Stokes equations are introduced to describe the resin flow in the edge channel. A dual‐Arrhenius viscosity model is newly introduced to describe the chemorheological behavior of a modified bismaleimide resin. The influence of the curing reaction and processing parameters on the resin flow patterns was investigated. The results indicate that, under constant‐flow velocity conditions, the curing reaction caused an obvious increase in the injection pressure and its influencing degree was greater with increasing resin temperature or preform permeability. Both a small change in the resin viscosity and the alteration of the injection flow velocity hardly affected the resin flow front. However, the variation of the preform permeability caused an obvious shape change in the resin flow front. The simulated results were in agreement with the experimental results. This study was helpful for optimizing the reactive mold‐filling conditions. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Numerical simulation of the resin transfer mold filling process using the boundary element method

A numerical simulation of the mold filling process during resin transfer molding with a heated die was performed using the boundary element method. The governing differential equation with a variable coefficient was rearranged into a system of Poisson equations using the perturbation technique. The boundary element method was employed to solve the resulting equations. The resin viscosity was calculated by introducing markers at the resin inlet and tracing them. As the calculation domain changes because of the proceeding resin front, numerical calculation nodes on the boundary were rearranged for each time step and integration was performed only for the meshes in the calculation domain among the fixed meshes over the mold. Sample calculations were performed for two molds with different shapes. To check the validity of the numerical scheme, the calculated mass flow rate at the resin front was compared with the mass flux at the inlet. Close agreement was observed.     

Analysis of resin injection molding in molds with preplaced fiber mats. I: Permeability and compressibility measurements

This work presents the characterization of fibrous reinforcement mats in resin injection molding. The fiber mat characterization involved determining the mat permeability and compressibility. Mold filling experiments were conducted using two or more different fiber types in the mat stack, which created transverse porosity, permeability, and compressibility variations. The effect of these variations was studied by taking flow pressure measurements and observing the progress of the flow front of a non-reactive fluid filling a clear acrylic mold that contained the reinforcement mat stack.     

In‐plane anisotropic permeability characterization of deformed woven fabrics by unidirectional injection. Part I: Experimental results

Because of the increasing use of polymer composites in a wide variety of industrial applications, the manufacturing of complex composite parts has become an important research topic. When a part is manufactured by liquid composite molding (LCM), the reinforcement undergoes a certain amount of deformation after closure and sealing of the mold. In the case of bidirectional woven fabrics, this deformation may significantly affect the resin flow and mold filling because of changes in the values of permeability. Among other considerations that govern the accuracy of numerical simulations of mold filling, it is important to predict the changes of permeability as a function of the local shearing angle of the preform. The resin flow through a fibrous reinforcement is governed by Darcy's law, which states that the fluid flow rate is proportional to the pressure gradient. The shape of the flow front in a point‐wise injection through an anisotropic preform is an ellipse. Part I of this article describes a new methodology based on the ellipse equation to derive the in‐plane permeability tensor from unidirectional injection experiments in deformed woven fabrics. Part II presents a mathematical model that predicts the principal permeabilities and their orientation for sheared fabrics from the permeability characterization of unsheared fabrics. Unidirectional flow experiments were conducted for a nonstitched, balanced, woven fabric for different shearing angles and fiber volume fractions. This article presents experimental results for deformed and undeformed fabrics obtained by unidirectional flow measurements. A comparison of the proposed characterization methodology with radial flow experiments is also included. POLYM. COMPOS. 28:797–811, 2007. © 2007 Society of Plastics Engineers.     

Experimental analysis and numerical modeling of flow channel effects in resin transfer molding

Composite manufacturing by Liquid Composite Molding (LCM) processes such as Resin Transfer Molding involve the impregnation of a net‐shape fiber reinforcing perform a mold cavity by a polymeric resin. The success of the process and part manufacture depends on the complete impregnation of the dry fiber preform. Race tracking refers to the common phenomenon occurring near corners, bends, airgaps and other geometrical complexities involving sharp curvatures within a mold cavity creating fiber free and highly porous regions. These regions provide paths of low flow resistance to the resin filling the mold, and may drastically affect flow front advancement, injection and mold pressures. While racetracking has traditionally been viewed as an unwanted effect, pre‐determined racetracking due to flow channels can be used to enhance the mold filling process. Advantages obtained through controlled use of racetracking include, reduction of injection and mold pressures required to fill a mold, for constant flow rate injection, or shorter mold filling times for constant pressure injection. Flow channels may also allow for the resin to be channeled to areas of the mold that need to be filled early in the process. Modeling and integration of the flow channel effects in the available LCM flow simulations based on Darcian flow equations require the determination of equivalent permeabilities to define the resistance to flow through well‐defined flow channels. These permeabilities can then be applied directly within existing LCM flow simulations. The present work experimentally investigates mold filling during resin transfer molding in the presence of flow channels within a simple mold configuration. Experimental flow frot and pressure data measurements are employed to experimentally validate and demonstrate the positive effect of flow channels. Transient flow progression and pressure data obtained during the experiments are employed to investigate and validate the analytical predictions of equivalent permeability for a rectangular flow channel. Both experimental data and numerical simulations are presented to validate and characterize the equivalent permeability model and approach, while demonstrating the role of flow channels in reducing the injection and mold pressures and redistributing the flow.     

Fiber mat deformation in liquid composite molding. I: Experimental analysis

When a resin in injected into the mold in liquid composite molding, the preplaced fiber mat may deform near the inlet gate because of the high momentum carried by the injected fluid. A fiber free region near the gate followed by the fiber mat deformation may emerge. This phenomenon is most likely to occur when the stacked fiber mats have low permeability and the resin has high viscosity. A set of mold filling experiments were carried out using an instrumented metal mold and a small transparent mold to investigate the fiber mat deformation during mold filling. Experimental results showed that the fiber mat deformation was limited to a small region near the gate and that deformation greatly reduced the molding pressure. A forced fiber mat deformation employing a modified gate design was proposed to facilitate mold filling in liquid composite molding.     

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.     

Flow front advancement during composite processing: predictions from numerical filling simulation tools in comparison with real‐world experiments

Liquid composite molding (LCM) techniques are innovative manufacturing processes for processing fiber reinforced polymer parts used e.g. for aerospace structures. Thereby the reinforcing material is placed in a mold and infiltrated with a low viscosity polymer matrix. Increasing production rates as well as part complexity lead to high production risks such as air inclusions or incomplete mold filling. Numerical mold filling simulations are promising tools enabling the composite manufacturing engineer to detect dry spots in the mold and find the optimal positions of the resin entry and ventilation system at an early process development stage. Today, different numerical models and software packages are available for modeling the flow through the reinforcing structure for visualization of the flow behavior. The goal of this study is the systematic comparison of two different software packages, namely PAM‐RTM^® and OpenFOAM. Both software tools are operated as they are commonly foreseen. Real world experiments under real process conditions are the basis for the assessment of the numerical predictions. The resin transfer molding (RTM) experiments are executed in a tool with a transparent upper mold half in order to see the flow front advancement. POLYM. COMPOS., 37:2782–2793, 2016. © 2015 Society of Plastics Engineers     

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.     

Analysis of resin transfer/compression molding process

Resin transfer/compression molding (RT/CM) is a two-step process in which resin injection is followed by mold closing. This process can enhance the resin flow speed and the fiber volume fraction, as well as reducing the mold filling time. In this study, a simulation program for the mold filling process during RT/CM was developed using the modified control volume finite element method (CVFEM) along with the fixed grid method. The developed numerical code can predict the resin flow, temperature, pressure, and degree of cure distribution during RT/CM. The compression force required for squeezing the impregnated preform can also be calculated. Experiments were performed for a complicated three-dimensional shell to verify the feasibility of the RT/CM process and the numerical scheme. The compression force and the compression speed were measured. A close agreement was found between the experimental data and the numerical results. The resin front location obtained from a short shot experiment was compared with the numerical prediction. Again, a close agreement was observed. In order to demonstrate the effectiveness of the numerical code, simulations were performed for more complicated process conditions with anisotropic permeability of the preform at higher fiber volume fractions.     

Injection molding of unsaturated polyester compounds

A bulk-molding compound made of unsaturated polyester resin, glass fiber, calcium carbonate fillers, and low profile additives is studied. The viscosity of the compound in the absence of cure reaction is measured by capillary rheometry. The compound exhibits a shear-thinning behavior. Injection molding in a rectangular plaque equipped with pressure transducers shows that the crosslinking reaction can begin during mold filling for low flow rate or high mold temperature. Fiber orientation in the plaque is complex as the reinforcement appears under two aspects, bundles or filaments. Their lengths and orientations are different. A layered structure throughout the thickness is observed at the mold entrance, whereas the orientation becomes progressively unidirectional in the plaque. Two fiber-free layers near the the mold walls are observed. A numerical simulation of mold filling assuming inelastic non-Newtonian kinetic dependent behavior is presented. The results agree well with pressure measurements. A simplified decoupled fiber motion calculation is finally proposed. A qualitative explanation of the basic phenomena which induce fiber orientation is presented.     

Edge effect in RTM processes under constant pressure injection conditions

In resin transfer molding processes, the edge effect caused by the nonuniformity of permeability between fiber preform and edge channel may disrupt resin flow patterns and often results in the incomplete wetting of fiber preform, the formation of dry spots, and other defects in final composite materials. So a numerical simulation algorithm is developed to analyze the complex mold‐filling process with edge effect. The newly modified governing equations involving the effect of mold cavity thickness on flow patterns and the volume‐averaging momentum equations containing viscous and inertia terms are adopted to describe the fluid flow in the edge area and in the fiber preform, respectively. The volume of fluid (VOF) method is applied to tracking the free interface between the two types of fluids, namely the resin and the air. Under constant pressure injection conditions, the effects of transverse permeability, edge channel width, and mold cavity thickness on flow patterns are analyzed. The results demonstrate that the transverse flow is not only affected by the transverse permeability and the edge channel width but also by the mold cavity thickness. The simulated results are in agreement with the experimental results. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

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.     

Three-dimensional modeling of reaction injection molding. I

In this research a model to simulate both the filling the curing stages of a reaction injection molding (RIM) process in complex three-dimensional molds is developed. This model can be used to predict not only the temperature and conversion changes with time but also the front position during filling. Using given physical and chemical properties of the RIM system, moldability can be determined in advance. The numerical techniques used in this research include adaptation of the SIMPLE algorithm developed by Patankar for a moving-front, two-phase system with non-negligible inertial effects, and exothermic chemical reaction. The model predictions of temperature and conversion compare favorably with available data on simple two-dimensional molds. The ability of the model to predict the dynamics of filling in more complicated molds was verified by comparison to mold filling experiments with water and a polyurethane foam.     

Numerical simulation of unsaturated flow in woven fiber preforms during the resin transfer molding process

In this paper, the unsaturated flow encountered in the woven or stitched fiber mats used in RTM is simulated using an adaptation of the Finite Element Method/Control Volume (FEM/CV) technique. The movement of resin through such fiber mats is modeled as flow through dual scale porous media and the mass balance in such media creates a sink term in the equation of continuity of the macroscopic flows. Combining this equation with Darcy's law leads to a non-homogeneous non-linear elliptic partial differential equation for pressure that is solved iteratively. First the simulation is used to study simple flows encountered during the characterization of preforms, such as the constant injection pressure 1-D flow and the constant flow rate radial injection flow. Previously observed experimental results of relatively flatter pressure histories for the latter type of flows in wove fiber mats are replicated, both numerically and analytically, by the pressure equation with the sink term. A quantity called pore volume ratio is shown to play an important role in such flows. Finally, the unsaturated flow in a typical RTM mold, packed with woven fiber mats, is simulated numerically, and inlet pressures, fill times, and mat saturation are studied.     

The effects of preforming induced variable permeabilities on the rtm molding flow

Forming bidirectional woven fabric into complex shapes consisting of double curvature surfaces leads to rearrangement of the network structure of fiber mats. In this study, the geometric configuration of the fabrics after deformation is modeled using a modified pin-jointed net model. Deformation of the fabric can result in the local change of the permeability in the reinforcements. In the flow simulation, the prediction of the filling flow highly depends on the correct values of permeabilities. The primary goal of this study is to investigate the effects on filling flow phenomena caused by the deformation of the fiber mats. A general integrated procedure of computer simulation with variable permeabilities for the filling process of arbitrary mold shape is also presented.     

Conformal mapping analysis of infection mold filling

A conformal mapping analysis of the mold filling behavior in rectangular cavities is developed. The polymer melt is assumed to behave as a purely viscous Generalized Newtonian Fluid. The shape of the advancing flow front, the pressure distribution in the mold cavity, streamlines, constant temperature lines, and the required filling time may be readily determined using the techniques described. The theoretical results are in good agreement with experimental data previously reported in the literature.     

Mold filling analysis in vacuum-assisted resin transfer molding. Part II: SCRIMP based on grooves

Compared with SCRIMP based on the high-permeable medium, SCRIMP based on grooves has the advantage of a much higher mold filling rate. This paper analyzes the influences of various molding conditions on mold filling and presents models that can be used to predict the filling time and flow pattern in SCRIMP based on grooves. Mold filling experiments were carried out to investigate the effect of various factors such as the size of the groove, groove spacing, number of fiber layers and resin viscosity on mold filling. A leakage flow model was developed to simplify the simulation of the mold filling process in SCRIMP based on grooves. An “equivalent permeability” was introduced to represent the average flow capacity in the grooves. Compared with the Control Volume/Finite Element Method (CV/FEM) model, the leakage flow model greatly reduced computation time and yet provided simulation results that were in good agreement with experimental observations.