Modelling the flow of particle-filled resin through a fibrous preform in liquid composite molding technologies

During the manufacturing of particle-filled resin composite parts with a liquid composite molding (LCM) process, undesirable issues arise like resin viscosity increase or particles filtration. As the filled resin flow is taking place, the fibrous preform may act as a filter and hinder the even repartition of the fillers throughout the part or even stop the mold filling. The present paper proposes an experimental investigation of the particle filtration during the injection of a composite part. The model proposed by Erdal et al. is analysed and improved in order to take liquid retention phenomenon into account. Finally, simulated and experimental data are compared.     

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.     

Numerical prediction and experimental characterisation of meso-scale-voids in liquid composite moulding

Pressure gradients that drive the resin flow during liquid composite moulding (LCM) processes can be very low while manufacturing large composite parts. Capillary pressure becomes the predominant force for tow impregnation and thus meso-scale-voids can be generated, reducing the part quality. In contrast, micro-voids are created at high resin pressure gradients. In this work, a numerical method is presented to predict the creation of meso-scale-voids and their evolution. Experimental validation is conducted by measuring void content of produced composite parts with micro-computed tomography (μ-CT). Additionally, the void content as a function of the modified capillary number Ca^* is determined and the influence of the fibre volume content in the bundles on the meso-scale- and micro-void content is studied.     

An integrated optimisation for the weight,the structural performance and the cost of composite structures

An integrated optimisation methodology is proposed to optimise the manufacturing cost as well as the structural performance and the weight of composite laminated plates manufactured by the resin transfer moulding (RTM) process. In the present approach, the fibre type, the number of fabrics, the layer stacking sequence and the fibre volume fraction are optimised to minimise the structural weight and the material cost of composite structure under the stiffness constraint and the mould filling time constraint which is a part of process cycle time. With the results obtained, it is investigated how the weight and the material cost are traded-off. The optimisation methodology suggests a guide to cost-effective material selection in the preliminary conceptual design stage.     

Compaction and in plane permeability characteristics of quasi unidirectional and continuous random reinforcements

Compaction and permeability behaviour are important influences on the processing of composites for a range of manufacturing techniques including liquid composite moulding (LCM) and compression moulding. This paper describes an experimental study of the factors influencing these characteristics for two glass fibre reinforcement media used in LCM: continuous strand mat and an aligned fabric produced using warp knitting technology. The compaction relationships for the materials are presented as functions of process variables such as pressure, temperature, and forming rate. In plane permeability relationships measured using rectilinear and radial flow tests are presented for different fibre orientations and packing fractions. The results are discussed with reference to processing by resin transfer and structural reaction injection moulding.

MST/3248     

Three-dimensional reconstruction of resin flow using capacitance sensor data assimilation during a liquid composite molding process: A numerical study

Liquid composite molding (LCM) is a method to manufacture fiber-reinforced composites, where dry fabric reinforcement is impregnated with a resin in a molding apparatus. However, the inherent process variability changes resin flow patterns during mold filling, which in turn may cause void formation. We propose a method to reconstruct three-dimensional resin flow in LCM, without embedding sensors into the composite structure. Capacitance measured from pairs of electrodes on molding tools and the stochastic simulation of resin flow during an LCM process are integrated by a sequential data assimilation method based on the ensemble Kalman filter; then, three-dimensional resin flow and permeability distribution are estimated simultaneously. The applicability of this method is investigated by numerical experiments, characterized by different spatial distributions of permeability. We confirmed that changes in resin flow caused by spatial permeability variations could be captured and the spatial distribution of permeability could be estimated by the proposed method.     

Numerical simulations for class A surface finish in resin transfer moulding process

Simulation tools for Liquid Composite Moulding (LCM) processes are a key to predict and solve manufacturing issues. Despite the fact that numerical process analyses are commonly used to predict mould filling, resin cure and exothermic temperatures, more comprehensive computational tools are still required. Resin additives such as low profile additives (LPA) show a significant impact on process performance and part quality. In this work, mould pre-heating experiments were compared to numerical predictions using commercial simulation software. Non-isothermal simulations were then carried out and the predicted flow and degree-of-cure evolution were compared to experiments. Finally, a volume change model, previously developed, was implemented in this work to calculate mould pressure increases in RTM of resins with four different LPA contents (0%, 5%, 10% and 40%). The predictions were compared to the results from the mould pressure transducers in the mould cavity. Simulation results matched closely with the experimental results. Pressure evolution of low profile resins was found to be very sensitive to the model parameters.     

Simulating the effect of temperature elevation on clamping force requirements during rigid-tool Liquid Composite Moulding processes

Moulds used for rigid-tool Liquid Composite Moulding (LCM) processes, namely Resin Transfer Moulding (RTM) and Compression RTM, are often subjected to large internal forces which originate due to resin injection and from the compaction of fibre reinforcements. Appropriate clamping equipment (e.g. press or perimeter clamps) is necessary to equilibrate these forces. An optimal selection (or design) of such clamping equipment calls for an accurate prediction of the tooling forces generated. This work aims to introduce a comprehensive numerical scheme which addresses this issue, including the case of non-isothermal mould filling. A hybrid Finite Element/Finite Difference (FE/FD) methodology is utilised for solving the coupled flow/energy/species equations. A new fibre compaction model, developed in order to reduce computational complexity while maintaining solution accuracy, is implemented into the simulation algorithm. The force predictions obtained for a planar axisymmetric part reveal that the chosen combination of mould and resin temperatures, together with other process variables, plays a crucial role in allowing fast fill times while keeping setup costs low.     

Measurements of normal stress distributions experienced by rigid liquid composite moulding tools

Mould tools used for LCM processes such as Resin Transfer Moulding (RTM) and Injection/Compression Moulding (I/CM) must withstand local forces due to compaction of the fibre reinforcement, and due to resin pressure generated within the laminate. A series of RTM and I/CM experiments have been carried out, with the focus placed on measurement of normal stress distributions exerted on the mould surface. In addition, total mould clamping force and injection gate pressure histories have been recorded. I/CM experiments using force-controlled secondary compaction were also undertaken, and compared to the velocity-controlled cases. Observed fluid pressure fields showed good agreement with theory, namely a logarithmic distribution during fluid injection and a quadratic distribution during the compression driven filling phase of I/CM. Significant spatial variation in normal stress due to reinforcement compaction was observed. The influence of the fluid pressure on the total stress experienced by the mould was observed to be a function of both the fibre volume fraction of the part and the applied injection pressure, the latter being more pronounced at lower part volume fractions.     

Optimization of Resin Infusion Processing for Composite Pipe Key-Part and K/T Type Joints Using Vacuum-Assisted Resin Transfer Molding

In present study, the optimization injection processes for manufacturing the composite pipe key-part and K/T type joints in vacuum-assisted resin transfer molding (VARTM) were determined by estimating the filling time and flow front shape of four kinds of injection methods. Validity of the determined process was proved with the results of a scaling-down composite pipe key-part containing of the carbon fiber four axial fabrics and a steel core with a complex surface. In addition, an expanded-size composite pipe part was also produced to further estimate the effective of the determined injection process. Moreover, the resin injection method for producing the K/T type joints via VARTM was also optimized with the simulation method, and then manufactured on a special integrated mould by the determined injection process. The flow front pattern and filling time of the experiments show good agreement with that from simulation. Cross-section images of the cured composite pipe and K/T type joints parts prove the validity of the optimized injection process, which verify the efficiency of simulation method in obtaining a suitable injection process of VARTM.     

Permeability measurement of textile reinforcements with several test fluids

In liquid composite moulding (LCM) techniques, the liquid resin has to flow a long distance to impregnate the dry fibres. The measure for the resistance of the fibre preform to the resin flow is the permeability of the fibre preform. Because of the dual-scale porous structure of the textile preforms, test fluid can influence the unsaturated permeability values through the interaction of the fluid and fibres. In this study, the influence of test fluid on the permeability measurement of several types of textile reinforcements is investigated. First the contact angle of various fluids and fibres was measured. Then the permeability measurement of the textile reinforcements was carried out. The results showed that the influence of test fluid is small under the test conditions.     

Robust design of composites manufacturing processes with process simulation and optimisation methods

Due to the increasing variations in raw materials and manufacturing processes, composite manufacturing processes have more part-to-part variations compared with the metal manufacturing processes. To improve part quality consistency, tooling design optimisation is an imperative step for addressing the stochastic behaviour of composite manufacturing processes. This paper presents an optimisation approach for the typical composite manufacturing technique of resin transfer moulding (RTM), which minimises the sensitivity of the mould design to uncertain material properties by choosing appropriate locations of injection gates and vents. This paper proposes a stochastic simulation based approach for the RTM processes. Normal distribution and Weibull distribution were utilised as the statistical models for representing the permeability values for the main region and race-tracking, respectively. Based on the statistical properties of the permeability, a graph-based two-phase heuristic (GTPH) was adopted to minimise the flow dispersion value (a quantitative measure for part quality consistency) such that the process design is not sensitive to the material and process parameter variations.     

Characterising wood fibre mats as reinforcements for liquid composite moulding processes

Liquid composite moulding (LCM) processes are commonly used techniques for the manufacture of advanced composite structures. This study explores the potential of wood fibres as reinforcement for LCM preforms, considering discontinuous fibre mats produced using four different methods. Modified paper manufacturing techniques were employed to produce two types of wet formed mats, the other two being manufactured using dry methods. The dry compaction response of these mats has been investigated, required compression loads being measured up to a fibre volume fraction of 0.4. A complex non-elastic compression response was observed which has significant influence on forces generated within moulds. Saturated compaction tests were also carried out, the samples infiltrated with two different test fluids. A significant reduction in compaction load was observed due to wood softening when using a water based fluid. On the other hand, a non-water-based solution had little less influence on the compaction of the wood fibre mats. In addition, permeability of all four types of mats was measured as a function of fibre volume fraction. Reinforcement permeability and compaction response data are required to simulate LCM processes.     

Non‐isothermal ‘2‐D flow/3‐D thermal’ developments encompassing process modelling of composites: flow/thermal/cure formulations and validations

In the manufacturing process of large geometrically complex components comprising of fibre‐reinforced composite materials by resin transfer molding (RTM), the process involves injection of resin into a mold cavity filled with porous fibre preforms. The overall success of the RTM manufacturing process depends on the complete impregnation of the fibre mat by the polymer resin, prevention of polymer gelation during filling, and subsequent avoidance of dry spots. Since a cold resin is injected into a hot mold, the associated physics encompasses a moving boundary value problem in conjunction with the multi‐disciplinary study of flow/thermal and cure kinetics inside the mold cavity. Although experimental validations are indispensable, routine manufacture of large complex structural geometries can only be enhanced via computational simulations, thus eliminating costly trial runs and helping the designer in the set‐up of the manufacturing process. This study describes the computational developments towards formulating an effective simulation‐based design methodology using the finite element method. The specific application is for thin shell‐like geometries with the thickness being much smaller than the other dimensions of the part. Due to the highly advective nature of the non‐isothermal conditions involving thermal and polymerization reactions, special computational considerations and stabilization techniques are also proposed. Validations and comparisons with experimental results are presented whenever available. Copyright © 2001 John Wiley & Sons, Ltd.     

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.     

An efficient solver of the saturation equation in liquid composite molding processes

A major issue in Liquid Composite Molding Process (LCM) concerns the reduction of voids formed during the resin filling process. Reducing the void content increases the quality of the composite and improves its mechanical properties. Most of modeling efforts on process simulation of mold filling has been focused on the single phase Darcy’s law, with resin as the only phase, ignoring the formation and transport of voids. The resin flow in a partially saturated region can be characterized as two phase flow through a porous medium. The mathematical formulation of saturation in LCM takes into account the interaction between resin and air as it occurs in a two phase flow. This model leads to the introduction of relative permeabilities as a function of saturation. The modified saturation equation is obtained as a result, which is a non-linear advection-diffusion equation with viscous and capillary phenomena. In this work, a flux limiter technique has been used to solve a modified saturation equation for the LCM process. The implemented algorithm allows a numerical optimization of the injected flow rate which minimizes the micro/macroscopic void formation during mold filling. Some preliminary numerical results are presented here in order to validate the proposed mathematical model and the numerical scheme. This formulation opens up new opportunities to improve LCM flow simulations and optimize injection molds.     

A methodology for flow-front estimation in LCM processes based on pressure sensors

Liquid Composite Molding (LCM) processes offer nowadays considerable advantages for composite manufacturers; nevertheless, their robustness is still an open issue, since part quality is sensitive to slight material and process variations, mainly arising during preform impregnation. An accurate monitoring system is thus required for early identification of filling troubles and potentially adoption of control actions. This work proposes an approach for resin flow monitoring, based on a combination of a sensing system and numerical modeling, which can be easily implemented into a generic LCM process. Using pressure data provided by few sensors placed in strategic positions inside the mold cavity, the developed methodology enables reconstruction of flow-front patterns at any impregnation time, without simulation of cavity filling. The effectiveness of the methodology is demonstrated by two test cases, which validate the approach through comparison between real and estimated flow-front profiles. Potential and limitations of the method in presence of flow disturbances have been studied through several virtual experiments, defining the range of applicability and key issues for future development.     

Investigation of the dimensional stability of carbon epoxy cylinders manufactured by resin transfer moulding

The prediction of process-induced dimensional variability and residual stresses occurring during the manufacturing of composite structures is critical to produce parts where tight tolerances are required. Therefore, the development of material constitutive models and processing properties, and the validation of these models, are two essential steps in order to accurately simulate the behaviour of the materials involved. In this paper, the material constitutive models of a one-part epoxy resin were implemented in a three-dimensional finite element software based on the ABAQUS/COMPRO platform to investigate the dimensional stability of a composite structure manufactured by resin transfer moulding (RTM). A simplified geometry was used as a representative structural component with different layup configurations. Both heat transfer analysis and stress analysis were conducted. Contact interactions were implemented in the stress analysis to simulate the tool–part interaction. The presented analysis predicted the angle variation and the composite debonding caused by the coefficient of thermal expansion mismatch between the mould and the composite part and the resin volumetric chemical shrinkage.     

Experimental investigation and flow visualisation of the resin transfer mould filling process for non-woven hemp reinforced phenolic composites

Resin transfer moulding (RTM) of glass fibre reinforced polymeric composites offers the advantages of automation, low cost and versatile design of fibre reinforcement. A replacement of glass fibres with natural plant fibres as reinforcement in polymeric composites provides additional technological, economical, ecological and environmental benefits. The resin transfer mould filling process has significant effects on different aspects, such as fibre wetting out and impregnation, injection gate design, “dry patch” and void formation. Flow visualisation experiments were carried out using a transparent RTM mould to develop a better understanding of the mould filling process for hemp mat reinforced phenolic composites. The mould filling of unreinforced phenolics was characterised by a “quasi-one-dimensional steady state” flow. In the case of hemp non-woven reinforced system, the mould filling process can be considered as the flow of fluids through porous media. “Fibre washing” was a typical problem encountered during the injection process, leading to poor property uniformity. In addition, a preferential flow path was usually created near the edges and corners of the mould. The path exhibited low flow resistance and caused the resin flow front to advance much faster in these regions. The edge flow disturbed the steady flow, leading to difficulties in venting arrangement and “dry patch” formation. The edge flow and fibre washing were alleviated by reinforcement manipulation so steady state flow could be achieved. The relationships between the filling time and injection pressure and between filling time and different fibre weight fractions have been established for certain specific injection strategies.