Infiltration Condition and Mouldability Diagram in Resin Injection Moulding

This paper deals with the modelling of injection moulding processes taking into account the deformability of the preform and the polymerisation of the resin. The coupled flow-deformation problem in the infiltrated and dry region is formulated with the corresponding boundary conditions and with the proper evolution equations determining the motion of the boundaries. An approximated analytical discussion is performed to obtain some estimates on the infiltration velocity, helping in identifying a window of applicability in the parameters space (i.e., the mouldability diagram), which fits well with the numerical simulations

Modeling and Experimental Validation of the VARTM Process for Thin-Walled Preforms

In this paper, recent shell model is advanced towards the calibration and validation of the Vacuum-assisted Resin Transfer Molding (VARTM) process in a novel way. The model solves the nonlinear and strongly coupled resin flow and preform deformation when the 3-D flow and stress problem is simplified to a corresponding 2-D problem. In this way, the computational efficiency is enhanced dramatically, which allows for simulations of the VARTM process of large scale thin-walled structures. The main novelty is that the assumptions of the neglected through-thickness flow and the restricted preform deformation along the normal of preform surface suffice well for the thin-walled VARTM process. The model shows excellent agreement with the VARTM process experiment. With good accuracy and high computational efficiency, the shell model provides an insight into the simulation-based optimization of the VARTM process. It can be applied to either determine locations of the gate and vents or optimize process parameters to reduce the deformation

Process modeling of liquid composite molding processes

The polymer matrix composites (PMCs) are carving out a niche amid the keen market competition to replace the other material counterparts, e.g., metals. Due to the low weight and the corrosion resistance, the PMCs are wildly utilized from aerospace to automobile industries, both in the sectors of civilian and defense. To obtain high-quality products at low cost, the composites industry continues seeking for numerical simulation tools to predict the manufacturing processes instead of prototype testing and trials. Regarding the attractive liquid composite molding (LCM) process, it provides the possibility to produce net shape parts from composites. The challenges are how to identify the primary physics of LCM processes and develop mathematical models to represent them. Models need to be both accurate and efficient, which is not easy to achieve.To model LCM processes, we have one option that describes all physics at the macroscopic scale. The fundamental continuum mechanics principles, e.g., mass balance, momentum balance, energy balance, and entropy inequality, help us developing models. In this regard, the theory of porous media (TPM), which relies on the concept of volume fractions, can explain the problems of the saturated/unsaturated multi-phase materials. Darcy\u27s law describes the relation between the flow velocity and the pressure gradient, without accounting for the dual-scale flow. The air and resin compose the homogenized flow at the infusion stage. The existence of the capillary pressure influences the flow front, which has been revealed in this thesis. The finite element method is employed to solve for the homogenized flow pressure, and the degree of saturation with the staggered approach, especially the Streamline-Upwind/Petrov-Galerkin (SUPG) method is implemented to eradicate the stability problem.As to the fiber preform response, an assumption of shell kinematics is made to reduce the model from a full 3-D problem to a shell-like problem. Given this, an explicit formulation is obtained to express the normal directional stretch as a function of homogenized flow pressure. This model has been verified and validated by a resin infusion experiment. The model mimics the preform relaxation and lubrication mechanisms successfully and efficiently.So far, the works mentioned above aimed at the isothermal infusion stage. However, resin flow development, heat transfer, and resin curing are strongly interrelated during the whole LCM process. The holistic simulation of both the infusion stage and the curing stage is carried out in this thesis. Finally, we propose a system of coupled models to help process engineers to design and control process parameters by using virtual numerical experiments instead of the traditional trial-and-error approach

Numerical and experimental analyses of resin infusion manufacturing processes of composite materials

Liquid resin infusion (LRI) processes are promising manufacturing routes to produce large, thick, or complex structural parts. They are based on the resin flow induced, across its thickness, by a pressure applied onto a preform/resin stacking. However, both thickness and fiber volume fraction of the final piece are not well controlled since they result from complex mechanisms which drive the transient mechanical equilibrium leading to the final geometrical configuration. In order to optimize both design and manufacturing parameters, but also to monitor the LRI process, an isothermal numerical model has been developed which describes the mechanical interaction between the deformations of the porous medium and the resin flow during infusion.1, 2 With this numerical model, it is possible to investigate the LRI process of classical industrial part shapes. To validate the numerical model, first in 2D, and to improve the knowledge of the LRI process, this study details a comparison between numerical simulations and an experimental study of a plate infusion test carried out by LRI process under industrial conditions. From the numerical prediction, the filling time, the resin mass and the thickness of the preform can be determined. On another hand, the resin flow and the preform response can be monitored by experimental methods during the filling stage. One key issue of this research study is to highlight the changes in major process parameters during the resin infusion stage, such as the temperature of the preform and resin, and the variations of both thickness and fiber volume fraction of the preform. Moreover, this numerical/experimental approach is the best way to improve our knowledge on the resin infusion processes, and finally, to develop simulation tools for the design of advanced composite parts

Recent patents on in-plane permeability measurement of LCM composite reinforcements

International audienceOptimization of Liquid Composite Molding (LCM) processes using Darcy flow numerical simulation requires inputting accurate reinforcement permeability data. Historically introduced by its author to describe infiltration phenomenon, permeability coming from Darcy's law is usually used in LCM processes as a rheological parameter in order to predict the macroscopic resin motion during the filling stage. Resulting from the flow through a complex fibrous architecture, its measurement is very sensitive to the test conditions due to the high filaments flexibility and meso-structure heterogeneity. Reinforcements are currently anisotropic fibrous media and their in-plane permeability measurement requires specific facilities. Measurements can be performed in transient or steady state conditions, and in one-, two-or three-dimensional configurations. This paper describes the different existing experimental configurations, identification procedures and instrumentation techniques. Advantages and drawbacks of each method are discussed, in the particular case of 2D transient measurements, which are the most representative of Resin Transfer Molding process. Two recent patents using different instrumentation techniques are detailed. They are focused on the simultaneous identification of the in-plane principal permeability values in an anisotropic fibrous reinforcement. Some perspectives are suggested to improve the repeatability of such measurement results

Multi-scale modeling of complex fluids and deformable fibrous media for liquid composite molding

In the last few years, the interest of the aerial and terrestrial transport industry in the fabrication of textile-reinforced composite materials has sensibly grown. This is basically due to the remarkable properties of these materials, which combine high mechanical strength with reduced weight. The manufacturing techniques that provide better control on the final quality of the components rely on autoclave curing: heat and pressure are applied on vacuum bags to achieve high volume fractions of the reinforcement and low number of defects due to the presence of voids. Nevertheless, autoclave curing implies high costs for the acquisition of the vessel and the process is energy and time consuming. To reduce the production costs, the industry has increased its interest in out-of-autoclave processing technologies, that is, liquid composite molding (LCM) techniques. In its most basic version, the technique consists in the injection of a catalyzed resin into a closed cavity, where a pre-placed fiber stack lies. When the resin has completely permeated the preform, the mold is subject to high temperatures to induce the curing of the resin to obtain the composite. The current challenge for this technology is to achieve the same quality standards for the final component as those achievable with in-autoclave processing. In LCM processes, the final quality of the component depends on several factors, such as: the structure of the textile, the arrangement of the layers, the adaption to the mold, the compaction process, the operating conditions, the geometry of the component, the configuration of the injection points for the resin, the physical and chemical interactions between the resin and the textile. All these factors affect the correct saturation of the reinforcement, and therefore process parameters must be adequately controlled in order to guarantee the required quality standards for the composite. In this sense, mold filling simulation software is a valuable tool for the process optimization; however the permeability of the reinforcement is required as an input parameter. An accurate evaluation of the permeability of the reinforcement however, represents a challenging task. Fibrous preforms for LCM generally present a hierarchical structure: the fibers are bunched in yarns, which in turn are bundled in a fabric. This structure, undergoes complex deformations during the production process: 1) during the compaction in the mold and 2) during the injection of the resin. This issue remarkably complicates an accurate evaluation of the permeability of the reinforcement and may be at the origin of the scatter observed in the experimental measurements. From a modeling point of view, the different length scales to be taken into account (typically ranging between one and three orders of magnitude) hinders a proper simulation of the deformation of the textile. The typical diameter of the fibers ranges indeed in few micrometers, while the characteristic dimension of the yarns is in the order of the millimeter. This issue represents a constraint for standard numerical approaches due to computational limits. In order to account for the effect on the permeability of the deformation of the hierarchical structure of the preform, multi-scale modeling techniques must be adopted. The objective of the thesis is the development of novel theoretical and numerical frameworks to account for the effect on the permeability of the multi-scale deformations that the textile undergoes during the two aforementioned stages of the process. The development focuses on the fiber-yarn level in 2D, where the yarn is always modeled as suspension of fibers by analogy with a complex fluid. The numerical implementations use computational fluid dynamic (CFD) tools. In order to address the problem, the permeability of a textile preform for LCM is first analyzed by experimental means. A standard CFD approach is then adopted for the simulation of a representative elementary volume of the textile; it is shown that, by means of this approach, the experimental permeability cannot be recovered over the full range of porosities. An X-ray computed microtomography of the textile is then performed. The obtained data are used for the virtual reconstruction of the exact geometry of the textile after its use for LCM. The simulations with this latter geometry provide better results; however the uncertainties on permeability still hold, and the permeability is always overestimated. These uncertainties are discussed in detail and motivate the work described hereafter. The first modeling block of the thesis concerns the analysis of the deformation that the textiles undergo during the compaction in the mold. A continuum model is first developed and validated for the squeeze flow of epoxy-based materials, the rheology of which is given by a viscoplastic constitutive law. The model is then applied to the compaction of yarns, where a viscoplastic behavior for the fiber bundle is assumed in the quasi-static regime of compression and by an analogy with flowing granular media. The rheological parameters are obtained from experimental data by a simplified analytical model for the deformation of the yarns under compaction. The commercial CFD code ANSYS Fluent is adopted for the numerical solution. The model yields information about the evolution of the fiber volume fraction during the compaction and is found to correctly recover the experimental force for high compression ratios. The second modeling block of the thesis concerns the analysis of the deformation that the textiles undergo during the injection of the resin. A numerical framework is first developed and validated for the direct numerical simulation of dilute colloidal suspensions of polymeric molecules. The numerical method consists in a coupled finite-volume/lattice-Boltzmann solution: finite volume method for hydrodynamics and lattice Boltzmann method for the sub-grid-scale physics. For computational efficiency, the lattice Boltzmann solution is accelerated on a graphic processing unit (GPGPU) with a tailored implementation and efficiently coupled with the macroscopic solver (ANSYS Fluent). The numerical method is then exploited for the solution of a mesoscopic model for the flow-induced fiber dynamics during the injection. A statistical model for the fiber dynamics is derived, based on analogy of the yarn with a non-Brownian suspension of particles with confining potentials. The fiber topology during the injection is recovered by a topological invariant and yields information about the change in permeability due to the clustering of fibers in steady-state, fully-saturated conditions. The results are presented in the form of phase diagrams, which show that in the deformable case the permeability can be up to one order of magnitude lower than in the rigid case. On the basis of the results obtained, the following main conclusions can be drawn: 1. The model developed for the compaction in the mold showed to be appropriate for a phenomenological analysis of the deformation of the yarns under compression. The model allows to analyze quantitatively the evolution of the fiber volume fraction, which yields useful information for a better understanding of the distribution of the fibers before the injection. 2. The model developed for the fiber dynamics during the injection, allows to analyze their topology induced by the fluid flow. The clustering of fibers significantly reduces the permeability at the fiber level, which could explain the overestimation obtained with simplified numerical approaches. The phase diagrams obtained for the permeability, both at the yarn and fiber level, allow to identify the best operating conditions for the infiltration of the resin. The proposed models have been developed using fluid dynamic techniques, which opens the possibility for a unified framework for the analysis, and ultimately, for a more precise estimation of the permeability. This work aims to represent a first tentative in this direction

Effect of Processing Parameters and Matrix Shrinkage on Porosity Formation During Synthesis of Metal Matrix Composites with Dual-scale Fiber Reinforcements Using Pressure Infiltration Process

This is first such study on porosity formation phenomena observed in dual-scale fiber preforms during the synthesis of metal matrix composites (MMCs) using the gas-based pressure infiltration process (gas PIP). In this thesis, different mechanisms of porosity formation during pressure infiltration of Al-Si alloys into Nextel\u27s 3D woven ceramic-fabric reinforcements (a dual-porosity or dual-scale porous medium) are studied. The effect of processing conditions in terms of the infiltration temperature and pressure on porosity content of the ceramic fabric infiltrated by the alloys through the gas PIP is investigated. Relative density (RD), defined as the ratio of the actual MMC density and the density obtained at ideal 100% saturation of the preform, was used to quantify overall porosity. Increasing the infiltration temperature led to an increase in RD (and reduction in porosity) due to reduced viscosity and enhanced wettability leading to improved feedability of the liquid metal. Similarly, increasing the infiltration pressure led to enhanced penetration of fiber tows and led to higher RD and reduced porosity. For the first time, the modified Capillary number (Ca*), which is found to predict formation of porosity in polymer matrix composites quite well, is employed to study porosity in MMCs made using PIP. It is observed that in the high Ca* regime used in the present study (and common in PIP), the overall porosity shows a strong downward trend with increasing Ca* due to a decrease in the size of trapped air pockets inside fiber tows due to increased infiltration pressures. This contradicts the well-known result of increasing porosity with Ca* observed by Patel et al. in [1]. In addition, the effect of matrix shrinkage on porosity content of the samples is studied through using a zero-shrinkage Al-Si alloy as the matrix: usage of this alloy as the matrix led to a reduction in porosity content

Analytical modeling and sensor monitoring for optimal processing of advanced textile structural composites by resin transfer molding

A two-dimensional model of the resin transfer molding (RTM) process was developed which can be used to simulate the infiltration of resin into an anisotropic fibrous preform. Frequency dependent electromagnetic sensing (FDEMS) has been developed for in situ monitoring of the RTM process. Flow visualization tests were performed to obtain data which can be used to verify the sensor measurements and the model predictions. Results of the tests showed that FDEMS can accurately detect the position of the resin flow-front during mold filling, and that the model predicted flow-front patterns agreed well with the measured flow-front patterns