Analysis of non-isothermal mold filling process in resin transfer molding (RTM) and structural reaction injection molding (SRIM)

In this paper, we present a modeling and numerical simulation of a mold filling process in resin transfer molding/structural reaction injection molding utilizing the homogenization method. Conventionally, most of the mold filling analyses have been based on a macroscopic flow model utilizing Darcy's law. While Darcy's law is successful in describing the averaged flow field within the mold cavity packed with a porous fiber preform, it requires experiments to obtain the permeability tensor and is limited to the case of porous fiber preform-it can not be used to model the resin flow through a double porous fiber preform. In the current approach, the actual flow field is considered, to which the homogenization method is applied to obtain the averaged flow model. The advantages of the current approach are: parameters such as the permeability and effective heat conductivity of the impregnanted fiber preform can be calculated; the actual flow field as well as averaged flow field can be obtained; and the resin flow through a double porous fiber preform can be modelled. In the presentation, we first derive the averaged flow model for the resin flow through a porous fiber preform and compare it with that of other methods. Next, we extend the result to the case of double porous fiber preform. An averaged flow model for the resin flow through a double porous fiber preform is derived, and a simulation program is developed which is capable of predicting the flow pattern and temperature distribution in the mold filling process. Finally, an example of a three dimensional part is provided.     

Application of calcium carbonate in resin transfer molding process: An experimental investigation

The resin transfer molding (RTM) process is used to manufacture advanced composite materials made of continuous glass or carbon fibers embedded in a thermoset polymer matrix. In this process, a fabric preform is prepared, and is then placed into a mold cavity. After the preform is compacted between the mold parts, thermoset polymer is transferred from an injection machine to the mold cavity through injection gate(s). Resin flows through the porous fabric, and eventually flows out through the ventilation port(s). After the resin cure process (cross‐linking of the polymer), the mold is opened and the part is removed. The objective of this study is to verify the application of calcium carbonate mixed in resin in the RTM process. Several rectilinear infiltration experiments were conducted using glass fiber mat molded in a RTM system with cavity dimensions of 320 × 150 × 3.6 mm, room temperature, maximum injection pressure 0.202 bar and different content of CaCO₃ (10 and 40%) and particle size (mesh opening 38 and 75 µm). The results show that the use of filled resin with CaCO₃ influences the preform impregnation during the RTM molding, changing the filling time and flow front position, however it is possible to make composite with a good quality and low cost.     

Mechanical properties of composite structures fabricated with the vacuum induced preform relaxation process

A new technique called vacuum induced preform relaxation (VIPR) can be used to improve the processing of composite parts manufactured using vacuum resin infusion methods. The VIPR process is a method for manipulating and guiding the resin filling pattern during a vacuum assisted resin transfer molding (VARTM) manufacturing process with a relatively small external vacuum chamber. This VIPR chamber can be sealed against the flexible molding surface of a VARTM mold and used to create vacuum above the preform. This causes the compressive forces compacting the fabric to decrease allowing the resin to flow faster in the effected region under the chamber. Thus the chamber can influence the resin flow pattern as well as avoid the formation of voids due to merging flow fronts. When the regulated vacuum in the chamber is applied it temporarily decreases the fiber volume fraction of the preform. It is important to investigate if this relaxation has a permanent adverse effect on the mechanical properties of the composite. The results of these tests strongly suggest that the use of the VIPR process does not compromise the mechanical properties of composite structures.     

Flow sensing and control strategies to address race-tracking disturbances in resin transfer molding. Part I: design and algorithm development

In the resin transfer molding process for advanced polymer composites manufacturing, the fiber preform is placed in the mold cavity and a thermoset resin is injected into the mold to impregnate the stationary preform. The resin displaces the air in the mold through openings called vents. Once the resin emerges out of the vents, the injection is discontinued. The near net-shaped composite part can be demolded after the resin cures. Ideally, the vents should be placed at the locations where the resin arrives last to ensure the complete saturation of the preform. However, the racetracking phenomenon, in which the resin flows faster along the minuscule channels induced by imperfect fits between the preform edges and the mold walls, can dramatically change the resin infiltration process. The ramifications of racetracking are that the resin may arrive at the vents before completely impregnating the preform and create undesired dry spots, which are fiber regions devoid of resin. The racetracking strength is not repeatable and may vary from one injection trial to next. Hence, the online strategic flow control can be useful in improving the processing reliability and the parts quality by re-directing the flow to arrive last at the vents. In this article, an online strategic flow control system consisting of a flow sensing network and a flow actuation network is proposed. A flow pattern recognition technique, which is based on the dimensionless time vector collected by the flow-sensing network, is developed in order to perform the online flow characterization effectively. Flow simulations are utilized to off-line design the flow control system. An evaluation function is formulated to optimize the flow sensing network design. A multi-tier genetic algorithm is implemented to optimize the locations of vents and gates along with the necessary control actions. A numerical case study for testing the computer-generated flow control solutions is presented. It was found that there was significant improvement in the success rate (fewer dry spot regions) due to the use of the strategic flow control and the automated design approach.     

Post-filling flow in vacuum assisted resin transfer molding processes: Theoretical analysis

For rigid mold filling processes such as resin transfer molding, the resin flow stops when the preform is fully saturated with the resin. However, in vacuum assisted resin transfer molding process (VARTM), due to preform deformation the resin flow continues after the filling stage is complete as it does take a finite time for the pressure field to become uniform during this post-filling period. In this paper, the post-filling flow in the VARTM process with and without the membrane is examined. The governing equations for post-filling flow, in which the preform is allowed to deform, are developed with simplifying assumptions. A one-dimensional flow and deformation coupled process model is developed to simulate the time dependent pressure distribution during the post-filling stage. The model is implemented using finite differences, both in time and space, and utilizes the explicit time integration which is found to be conditionally stable. The change in pressure inside the mold during the post-filling stage is predicted for three different injection scenarios. The influence of the pressure distribution at the end of filling on the dwell time for the pressure to equilibrate and on the final thickness of the part is discussed. The effects of change in preform permeability and compliance on the dwell time and thickness are demonstrated and the extension of the model to more complex geometries and systems is outlined.     

In situ measurement and monitoring of whole-field permeability profile of fiber preform for liquid composite molding processes

For liquid composite molding (LCM) processes, such as resin transfer molding (RTM), the quality of final parts is heavily dependent on the uniformity of the fiber preform. However, the conventional permeability measurement method, which uses liquid (oil or resin) as its working fluid, only measures the average preform permeability in an off-line mode. This method cannot be used to create an in situ permeability profile because of fiber pollution. Further, the conventional method cannot be used to reveal preform's local permeability variations. This paper introduces a new permeability characterization method that uses gas flow to detect and measure preform permeability variations in a closed mold assembly before resin injection. This method is based upon two research findings: (1) resin permeability is highly correlated with air permeability for the same fiber preform with well-controlled gas flow, and (2) the whole-field air permeability profile of a preform can be obtained through measuring the pressure field of gas flow.In this study, first the validity of the gas-assisted, in situ permeability measurement technique was established. Then the technique was demonstrated as effective by qualitatively detecting non-uniformities and permeability variations in fiber performs. Finally, a two-dimensional flow model, based on the finite difference scheme, was developed to quantitatively estimate the whole-field preform permeability profile using predetermined pressure distribution. The efficacy of the new method was illustrated through experimental results.     

Analysis of vacuum bag resin transfer molding process

An analytical model is developed to analyze the resin flow through a deformable fiber preform during vacuum bag resin transfer molding (VBRTM) process. The force balance between the resin and the fiber preform is used to account for the swelling of fiber preform inside a flexible vacuum bag. Mold filling through multiple resin inlets is analyzed under different vacuum conditions. The formation of dry spots is demonstrated in the presence of residual air. Molding of a three-dimensional ship hull with lateral and longitudinal stiffeners is simulated to demonstrate the applicability of the model.     

An approach to couple mold design and on-line control to manufacture complex composite parts by resin transfer molding

During impregnation of the resin into a closed mold containing the preform in the resin transfer molding (RTM) process, increased yield of successful parts can be achieved if one could account for the inherent disturbances such as race-tracking and preform variability. One way to address this is to use sensors and actuators to control the resin flow dynamics during the impregnation process to counteract the disturbances. In this paper, we use a mold filling simulation tool to develop a design and control methodology that, with the help of sensors and actuators, identifies the flow disturbance and redirects the resin flow to successfully complete the mold filling process without any voids. The methodology is implemented and experimentally validated for a mold geometry that contains complex features such as tapered regions, rib structures, and thick regions. The flow modeling for features such as ribs and tapered sections are validated independently before integrating them into the mold geometry. The approach encompasses creation of software tools that find the position of the sensors in the mold to identify anticipated disturbances and suggest flow control actions for additional actuators at auxiliary locations to redirect the flow. Laboratory hardware is selected and integrated to automate the filling process. The effectiveness of the methodology is demonstrated by conducting experiments that, with feedback from the sensors, can automate and actively control the flow of the resin to consistently impregnate all the fibers completely despite disturbances in the process.     

Transient gas flow technique for inspection of fiber preforms in resin transfer molding

A transient gas flow method was developed to determine the quality of fibrous preforms in resin transfer molding (RTM) prior to resin injection. The method aims at detecting defects resulting from preform misplacement in the mold, accidental inclusions, preform density variations, race tracking, shearing, etc. Unlike the previously developed method based on steady-state gas flow, the new method allows for the acquisition of continuous time-varying pressure data from multiple ports during a single test. The validity of the method was confirmed by one-dimensional flow experiments.     

Resin flow analysis with fiber preform deformation in through thickness direction during Compression Resin Transfer Molding

Resin flow during Compression Resin Transfer Molding (CRTM) can be best described and analyzed in three phases. In the first phase, a gap is created by holding the upper mold platen parallel to the preform surface at a fixed distance from it. The desired amount of resin injected into the gap quickly flows primarily over the preform. The second phase initiates when the injection is discontinued and the upper mold platen moves down squeezing the resin into the deforming preform until the mold surface comes in contact with the preform. Further mold closure during the final phase will compact the preform to the desired thickness and redistribute the resin to fill all empty spaces. This paper describes the second phase of the infusion. We assume that at the end of phase one; there is a uniform resin layer that covers the entire preform surface. This constrains the resin to flow in through the thickness direction during the second phase. We model this through the thickness flow as the load on the upper mold forces the resin into the preform, simultaneously compacting the preform. The constitutive equations describing the compaction of the fabric as well as its permeability are included in the analysis. A numerical solution predicting the flow front progression and the deformation is developed and experimentally verified. Non-dimensional analysis is carried out and the role of important non-dimensional parameters is investigated to identify their correlations for process optimization.     

Numerical simulation of injection/compression liquid composite molding. Part 1. Mesh generation

This paper presents a numerical simulation of injection/compression liquid composite molding, where the fiber preform is compressed to a desired degree after an initial charge of resin has been injected into the mold. Due to the possibility of an initial gap at the top of the preform and out-of-plane heterogeneity in the multi-layered fiber preform, a full three-dimensional (3D) flow simulation is essential. We propose an algorithm to generate a suitable 3D finite element mesh, starting from a two-dimensional shell mesh representing the geometry of the mold cavity. Since different layers of the preform have different compressibilities, and since properties such as permeability are a strong function of the degree of compression, a simultaneous prediction of preform compression along with the resin flow is necessary for accurate mold-filling simulation. The algorithm creates a coarser mechanical mesh to simulate compression of the preform, and a finer flow mesh to simulate the motion of the resin in the preform and gap. Lines connected to the top and bottom plates of the mold, called spines, are used as conduits for the nodes. A method to generate a surface parallel to a given surface, thereby maintaining the thickness of the intermediate space, is used to construct the layers of the preform in the mechanical mesh. The mechanical mesh is further subdivided along the spines to create the flow mesh. Examples of the three-dimensional meshes generated by the algorithm are presented.     

Analysis and minimization of void formation during resin transfer molding process

In the resin transfer molding process, residual air in the pores of fiber preform results in dry spots and microvoids in the finished product. The dry spots are usually formed due to irregular permeability of fiber mat and improper injection locations. The microvoids result from non-uniform microarchitecture of the fiber preform, and they are transported through the gap between fiber tows during infiltration of the resin. In this study, a real-time simulation/control method was proposed to actively control the formation and the transport of air voids during the mold filling. The flow equations were solved in real time to predict the change of the flow front shape. The flow front was detected by optical sensors and the control actions were taken based on the sensor signals. Through this automated simulation/control scheme, a real-time control of resin flow could effectively avoid the dry spots and minimize the formation of microvoids by modulating the injection pressure.     

Numerical simulation of injection/compression liquid composite molding. Part 2: preform compression

In the injection/compression liquid composite molding process (I/C-LCM), a liquid polymer resin is injected into a partially open mold, which contains a preform of reinforcing fibers. After some or all of the resin has been injected, the mold is closed, compressing the preform and causing additional resin flow. This paper addresses compression of the preform, with particular emphasis on modeling three-dimensional mold geometries and multi-layer preforms in which the layers have different mechanical responses. First, a new constitutive relation is developed to model the mechanical response of fiber mats during compression. We introduce a new form of nonlinear elasticity for transversely isotropic materials. A special case of this form is chosen that includes the compressive stress generated by changes in mat thickness, but suppresses all other responses. This avoids the need to model slip of the preform along the mold surface. Second, a finite element method, based on the principle of virtual displacement, is developed to solve for the deformation of the preform at any stage of mold closing. The formulation includes both geometric and material nonlinearities, and uses a full Newton–Raphson iteration in the solution. An open gap above the preform can be incorporated by treating the gap as a distinct material layer with a very small stiffness. Examples show that this approach successfully predicts compression in dry preforms for three-dimensional I/C-LCM molds.     

Investigation on the spring-in phenomenon of carbon nanofiber-glass fiber/polyester composites manufactured with vacuum assisted resin transfer molding

Process-induced residual stress arises in polymer composites as a result of mismatched resin contraction and fiber contraction during the cure stage. When a curved shell-like composite part is de-molded, the residual stress causes the spring-in phenomenon, in which the enclosed angle of the part becomes smaller than the angle of its mold. In this paper, a new approach is presented to control and reduce the spring-in angle by infusing a small amount of carbon nanofibers (CNFs) together with liquid resin into the glass fiber preform using vacuum assisted resin transfer molding (VARTM) process. The experimental results showed that the spring-in angles of the L-shaped composite specimens were effectively restrained by the CNFs. An analytical model and a 3-D FEA model were developed to predict the spring-in phenomenon and to understand the role of CNFs in reducing the spring-in angle. The models agreed with the experimental results reasonably well. Furthermore, the analytical model explains how the CNF-enhanced dimensional tolerance control is accomplished through the reductions in the matrix’s equivalent coefficient of thermal expansion and linear crosslinking shrinkage.     

Analysis of the Edge Effect on Flow Patterns in Liquid Composites Molding

In liquid composites molding, the cutting of the fiber preform is often not sufficiently precise leaving a small clearance between the reinforcement and the mold edges. This clearance creates a preferential flow path for the resin which may disrupt the filling of the mold cavity. Experience has shown that even a small clearance of 1 or 2 mm could have a significant effect, especially if the preform has a high fiber content. A model is thus needed to predict this channeling effect in order to take it into account in computer simulations of the mold filling process.This paper presents a model to describe this phenomenon. The idea is to characterize simultaneously the flow in the channel and through the reinforcement. The model is derived using Navier–Stokes equations in the open channel and Darcy's law in the porous preform. From this model, an equivalent porous medium was defined for which an equivalent permeability tensor can be computed as a function of the channel geometry. Numerical simulations performed with the computer software RTMFLOT developed in our laboratory have shown that in some limit cases, i.e., a large channel (5–6 mm) or for very low porosity reinforcements, the transverse flow from the channel to the preform can be neglected in the model while still obtaining quite a good prediction of the flow edge effect. In other cases, however, namely for a clearance of intermediate size (2–3 mm) which is the most common case in RTM, or for a higher porosity of the reinforcement, the transverse flow from the channel to the rest of the preform must be taken into account. Experimental data to validate the proposed model are also presented.     

A numerical study of filling process through multilayer preforms in resin injection/compression molding

In resin injection/compression molding (RI/CM), a preform often comprises layers of different fiber reinforcements. Each fiber reinforcement has unique through thickness and in-plane permeabilities as well as compressibility, creating a heterogeneous porous medium in the mold cavity. In the present article, numerical simulation is utilized to investigate the filling process of RI/CM in such a heterogeneous porous medium. The filling stage is simulated in a full three-dimensional space by using control volume/finite element method and based upon an appropriate filling algorithm. The flow in the open gap which may be present in the mold cavity is modeled by Darcy’s law using an equivalent permeability. Numerical simulations of filling process for preforms containing two and three layers of different reinforcements in various stacking sequences are conducted with the aid of computer code developed in this study. Results show that the injection time as well as flow front progression depends on fiber types in the whole preform, fiber stacking sequence and open gap provided in the mold cavity. Simulated results also suggest that the presence of open gap at top of reinforcement can lead to both low injection time and uniform flow pattern.     

In-plane permeability characterization of the vacuum infusion processes with fiber relaxation

In vacuum infusion processes fiber preforms are placed onto the single molding surface and enveloped with a non-rigid polymer bag which is sealed to the molding surface. The flexible bagging film does deform during the resin infusion process thus changing the compaction of the fabric. However, one can also relax the preform by drawing a partial vacuum in a rigid chamber placed on top of the flexible bag which will increase the permeability of the fabric under the chamber. A numerical model is presented to characterize the change in permeability and describe the mold filling for such processes in which the fabrics undergo controlled relaxation by external stimuli. The predictions from the simplified model agreed reasonably well with the experiments. This characterization and resin flow front prediction with time method should prove useful in processes such as Vacuum Induced Preform Relaxation (VIPR) process which can be used to actively manipulate flow in a vacuum infusion process.     

A Two-layer Model for the Simulation of the VARTM Process with Resin Distribution Layer

Vacuum assisted resin transfer molding (VARTM) is one of the important processes to fabricate high performance composites. In this process, resin is drawn into the mold to impregnate the fiber reinforcement to a form composite. A resin distribution layer with high permeability was often introduced on top of the fiber reinforcement to accelerate the filling speed. Due to the difference of the flow resistance in the resin distribution layer and the reinforcement as well as the resulting through thickness transverse flow, the filling flow field is intrinsically three-dimensional. This study developed a two-layer model with two-dimensional formulation to simulate the filling flow of the VARTM process with a resin distribution layer. Two-dimensional flow was considered in each layer and a transverse flow in the thickness direction was estimated between the two layers. Thermal analysis including the transverse convection was also performed to better simulate the temperature distribution.     

The effect of fabric and fiber tow shear on dual scale flow and fiber bundle saturation during liquid molding of textile composites

In Liquid Composite Molding (LCM) processes, a fibrous reinforcement preform is placed or draped over a mold surface, the mold is closed and a resin is either injected under pressure or infused under vacuum to cover all the spaces in between the fibers of the preform to create a composite part. LCM is used in a variety of manufacturing applications, from the aerospace to the medical industries. In this manufacturing process, the properties of the fibrous reinforcement inside the closed mold is of great concern. Preform structure, volume fraction, and permeability all influence the processing characteristics and final part integrity. When preform fabrics are draped over a mold surface, the geometry and characteristics of both the bulk fabric and fiber tow bundles change as the fabric shears to conform to the mold curvature. Numerical simulations can predict resin flow in dual scale fabrics in which one can separately track the filling of the fiber tows in addition to flow of resin within the bulk fabric. The effect of the deformation of the bulk fabric due to draping over the tool surface has been previously addressed by accounting for the change in fiber volume fraction and permeability during the filling of a mold. In this work, we investigate the effect of shearing of the fiber tows in addition to bulk deformation during the dual scale filling. We model the influence of change in fiber tow characteristics due to draping and deformation on mold filling and compare it with the results when the fiber tow deformation effect is ignored. Model experiments are designed and conducted with a dual scale fabric to characterize the change in permeability of fiber tow with deformation angle. Simulations which account for dual scale shear demonstrate that the tow saturation rate is affected, requiring longer fill times, or higher pressures to completely saturate fiber tows in areas of a mold with high local shear. This should prove useful in design of components for applications in which it is imperative to ensure that there are no unfilled fiber tows in the final fabricated component.     

Resin impregnation during composites manufacturing: Theory and experimentation

An experimental investigation into the area of resin impregnation during the manufacturing of composite materials is undertaken. The study is specifically directed at furthering the degree of understanding of the resin transfer molding and resin film stacking manufacturing processes. An additional goal of the present work is the testing of a previously developed numerical model for simulating such processes. The experimentation performed consists of the fully monitored impregnation of resin into a thin mold containing a three-dimensional reinforcement fiber composite preform material. Experimental results for two cases involving highly anisotopic resin impregnation are compared to corresponding numerical results. Reasonable agreement between the two sets of results is found to exist and suggestions are made as to future work which could further increase the technical understanding of such processes.