Liquid Composite Molding Research Articles

Development of Novel Transportation Shells for the Rapid, Automated Manufacture of Automotive Composite Parts

In this paper the feasibility of a new approach, whereby dry fibre composite preforms of shaped and organised plies are held together by an external polymer shell, is presented for the manufacture of fibre reinforced composite parts at high volumes and low cost. The polymer shell, as a transport vessel, is intended to rapidly provide the composite preform with the required geometric stability and form; so reducing the impact of the time consuming binder activation processes that are currently used in traditional Liquid Composite Moulding (LCM) techniques. Removal of the binder activation process may also improve the final part quality during resin infusion stages, by retaining the preforms’ permeability, plus removing the inclusion of ‘foreign’ material not forming part of either the fibre or matrix systems. This paper presents the design of the new approach and its formulation; the development of understanding via lab-scale test machinery; results in terms of manufacturing capability - such as handling characteristics for pick and place automation, and mechanical performance of the presented LCM structures. Handling performance is particularly positive since better geometric stability and the easy formation of a vacuum seal between the robot head and the part is possible. The paper also presents a further novel development, whereby the transport vessels are retained as an integral element, providing the entire polymer matrix system for the final composite part. This enables further time and cost savings, replacing the need for the expensive LCM machinery that are currently utilised for rapid manufacture of composite parts.

A virtual permeability measurement framework for fiber reinforcements using micro CT generated digital twins

Liquid Composite Molding (LCM) is one of the widely used manufacturing techniques for high performance composite parts. In a simple LCM process, a pressurized liquid resin is injected inside a mold containing fibrous reinforcements. In order to create quality components using LCM processes, reinforcement characterization such as permeability and compaction response need to be performed fast and accurately. In this work, a micro CT based experimental-numerical reinforcement compaction and permeability characterization framework has been proposed which can potentially reduce process characterization cost by lowering material consumption and labor time and effort. The framework is non-destructive in nature and uses a miniaturized compression stage that can be housed in an X-ray computed tomography (XCT) system. This in-situ XCT experimental set-up is used to acquire the stress relaxation curve and XCT images of the reinforcement internal structure at multiple fiber volume fractions which are then converted into digital twins of the reinforcements. The virtual permeability of the reinforcement digital twin is computed through numerical flow simulations using realistic voxel models extracted from the stack of XCT images. The proposed approach has the capability of obtaining a plethora of information about the preform including, compaction response, statistical measurements of the internal preform geometry, and voxel models for numerical simulations. The methodology has been successfully demonstrated using two carbon fiber reinforcements with 3D woven architecture. The virtual permeability predictions were benchmarked against experimentally measured values, and were found to be in excellent agreement with the experimental data.

Mechanical characterisation of the shear, bending and friction behaviour of bindered woven fabrics during the forming process

AbstractA critical process step within the liquid composite moulding constitutes in the preforming of two‐dimensional textile material into a near‐net‐shape and load‐capable fibre structure by stamp forming. This paper presents fundamental material experiments on the shearing behaviour, bending properties and friction behaviour considering the binder quantity and forming temperature. In addition, the influence of the process time on the resulting stability of the preform is examined. The findings indicate that the quantity of the binder as well as the forming temperature and the process time have a great influence on the preform quality.

Mould fill time sensitivity analysis using isothermal mould filling simulations for applications in liquid composite moulding processes

Development of effective injection strategy constituting injection gates and air vents to obtain reduced mould fill time without dry spots are the primary requirements to develop mould system for an application using Liquid Composite Moulding (LCM) processes. Recent days, virtual simulation becomes promising tool in comparison to trial and error experiments due to its costing factor and timely iterations. This work demonstrates the possibility of Ansys Fluent commercial simulator to perform porous flow isothermal mould filling simulations and to quantify the sensitivity of mould fill time and flow front progression with respect to variations in process and raw material parameters. A rectangular mould of dimension 1 m × 0.5 m with an injection strategy of single point gate with 10 mm diameter and single point air vent of diameter 10 mm was used as a domain for isothermal mould filling. The sensitivity of mould fill time and flow front pattern were evaluated by varying inlet velocity, resin viscosity and orthotropic reinforcement mat permeability. From the results, it was found that the mould fill time decreases with increasing velocity, lesser viscosities and more porous preforms. Also, in all case studies, the mould fill pattern ends at the air vent position ensuring there are no dry spots formed.

XCT-scan assisted flow path analysis and permeability prediction of a 3D woven fabric

Compaction response and resin flow through a fabric are key characterization parameters in Liquid Composite Molding (LCM) processes. In this study, we present a detailed flow path analysis of a complex 3D woven angle interlock fabric using an in-situ X-ray micro computed tomography (XCT) experimental set-up. The set-up is used to obtain the reinforcement compaction response and three principle permeability values at different thicknesses in a single experiment. The compaction provides real-time features of inter-tow gaps, as well as elongation/spreading of weft and warp tows associated with a through-thickness deformation of the binder yarn. The reduction of the pore network during compaction is analyzed through pore size distribution using a granulometry and percolation path analysis. The resin flow path and reinforcement permeability in principle directions are predicted through vigorous digital flow simulations using several computational unit cells extracted from the reconstructed 3D models from XCT scans. The resin flow paths and principle permeability values show a very strong dependence on the solid volume fraction of the scanned images. A relationship between the solid volume fraction and permeability is developed using a modified Kozeny-Carman relation. The predicted permeability values were found to be in good agreement with traditional benchmark experimental data, highlighting the utility of this novel technique.

Flow of quasi-spherical nanoparticles in liquid composite molding processes. Part II: Modeling and simulation

This paper focuses on the modeling and simulation of the flow and filtration behavior of quasi-spherical nanoparticles (NPs) during liquid composite molding (LCM), as a succession to the experimental studies in Part I. The influence of the bimodal dispersion size characteristic of NPs and the dual-scale characteristic of the preform porous structure on the viscosity of the suspension, the permeability of the preform, and the impregnation behavior is investigated. The modeling and simulation helps not only to predict the impregnation behavior of the NP-modified matrix systems, but also to inversely study the critical particle size that may lead to cake filtration.

Flow of quasi-spherical nanoparticles in liquid composite molding processes. Part I: Influence of particle size and fiber distance distribution

This paper studies the flow and filtration of two quasi-spherical NPs (silica and boehmite) during liquid composite molding (LCM) processes. The nanosuspensions are prepared differently by “top-down” and “bottom-up” approaches, and the particle size is analyzed before and after injection process. In parallel, the fiber distance distribution is systematically characterized using improved image processing techniques and algorithms. It is found that nanosuspensions that are prepared by “top-down” approach may show a bimodal particle distribution, and that the filtration of quasi-spherical NPs is closely related to the bimodal distribution characteristics of the particle size and the dual-scale characteristic of the preform. Furthermore, a threshold fiber volume fraction (Vf) of 55% is identified, at which the investigated preform shows a critical phase transition from dual-scale to single micro-scale porous structure. It is shown that above the threshold Vf even a small fraction of coarse particles can strongly hinder the impregnation process.

Numerical simulation of double-bag progressive compression method of resin delivery in liquid composite moulding

ABSTRACTAn improved technique of resin delivery, called double-bag progressive compression method (DBPCM), has been developed to manufacture composite parts associated with the vacuum infusion (VI) process. During resin infusion, both bags are pulled upward by the vacuum and the flow resistance offered by the preform is reduced. After the completion of resin infusion, the vacuum on the segmented bags are progressively released to atmospheric pressure allowing the inflated bags to squeeze the surplus resin from the saturated preform. During progressive compression, the dry preform is kept loose and resin easily flows into the preform interstices. Heated air can be utilised at the appropriate compression segment. After the entire preform is wetted, the post-filling stage is carried out to obtain a uniform thickness of the component. This paper describes the numerical results of DBPCM complete filling process in finite difference analysis. The typical VI process is also modelled for comparison purpose.

Experimental study on the influence of cyclic compaction on the fiber-bed permeability, quasi-static and dynamic compaction responses

Fiber-reinforcements’ permeability and compressibility are crucial parameters in Liquid Composite Molding Processes, as they represent the main textile properties that influence the behavior of the injected resin. Before designing LCM molds and optimizing manufacturing processes, permeability and compaction characterizations are performed; however, the results obtained often show considerable deviations. One reason for this behavior is the variable compaction histories of fiber-beds, which can be altered during storage or handling. This work presents a novel method to normalize compaction histories through cyclic compaction, thus increasing the reproducibility of compaction and through-thickness permeability responses. It also shows how fiber-bed’s viscoelasticity can be reduced with minor influence on permeability. It is greatly beneficial in developing processing routes for vacuum assisted processes, and for future compaction or permeability benchmarks, in defining optimal characterization methods. It provides also solid basis for the comparison and validation of compaction and through-thickness permeability models with reduced measurement errors.

Numerical Characterization of Permeability Tensor of Fiber Preforms for Liquid Composite Molding Applications

Composite materials are of paramount importance in aeronautics, aerospace and automotive applications. Consisting of fibers and matrix keeping the fibers together, composite materials offer high strength and low weight. In one of the most common composite manufacturing method, Liquid Composite Molding (LCM) process, reinforcing glass, carbon or Kevlar fiber preforms are placed in a mold cavity and a liquid resin is introduced to cover the remaining empty space to form a composite by curing the resin. The primary objective in LCM processes is the successful filling of the preform with resin system. In order to obtain a desired composite product, the process needs to be modeled to have an insight about the filling and curing stages of the LCM process. The fiber preform permeability plays a key role in the filling pattern of the mold, which dictates if there will be any voids in the composites. Thus, to achieve a successful model of the LCM process, one needs to characterize the permeability tensor to quantify the resistance to resin flow through fibers. Besides various experimental and numerical techniques to characterize the permeability, this study offers a new numerical methodology to characterize the permeability of a unit cell of a fiber system. The study involves the characterization of the permeability of a unit cell under various shear strains and project a permeability map from the unit cell characterizations on the macro scale including the drape effect.

Simulation of liquid composite moulding using a finite volume scheme and the level-set method

In this paper, a 2D numerical framework for the simulation of flows in porous media that are characterised by a sharp transition between the saturated and unsaturated zone is presented. Using a finite volume scheme and the level-set method, the framework is derived based on a conventional, implicit solution of Darcys law for the pressure field, while the level-set function is advected explicitly locally at the flow front. With the main application to liquid composite moulding (LCM) in mind, the numerical framework is verified against analytical solutions, experiments, and other benchmark cases for a variety of situations that occur in this composite manufacturing process. The cases include local changes in permeability related to edge-effects, merging of flow fronts, and continuous manufacturing processes. It is highlighted that the numerical framework achieves convergence with a spatial accuracy between 1st and 2nd-order; the level-set operations only take about 20% of the total CPU time; and the propagation of the resin front can be achieved with CFL-sized time steps without overfilling cells and without applying any artificial smoothing.

Investigation on the Influence of Pressure Terms in a Volume-Averaged Energy Balance in the Modelling of Liquid Composite Moulding Processes

The growing use of composite materials has generated interest in improving and optimising composite manufacturing processes such as Liquid Composite Moulding (LCM). In LCM, dry preforms are placed in a mould and impregnated with the matrix material. The efficiency of filling the moulds can be improved by using Computational Fluid Dynamics (CFD) filling simulations during the design of the mould. As part of an on-going effort to develop a CFD tool for the simulation of LCM processes, a volume averaged energy balance equation has been derived and implemented in a custom OpenFOAM solver. The energy balance is implemented in a custom OpenFOAM solver with and without the pressure terms for comparison with results from RTM experiments. It is found that the pressure terms do not significantly influence the results for LCM processes.

A Novel Simulative-Experimental Approach to Determine the Permeability of Technical Textiles

Since years, fiber reinforced polymer composite (FRPC) parts made by Liquid composite molding (LCM) make up a significant share of the composite market. In LCM dry reinforcing structure gets impregnated with a resin system. Permeability, a material parameter with key influence on all LCM processes, quantifies the conductance of technical textiles for resin flow. Today, when a numerical filling simulation is applied for process design, large experimental test programs are required for characterization of permeability, as the permeability has to be measured for all textiles processed and also the dependency e.g. on the fiber volume content has to be considered. Together with Math2Market GmbH (M2M), the IVW currently develops a novel simulative-experimental approach (SEA), using experimental tests to calibrate a simulation model for replacing a significant amount of the experimental tests through “virtual” measurements. In a first step the functionality of the simulation and the most appropriate methods for textile modelling were investigated. For this, three routes were followed: At first a micro-computer tomograph (μCT)-scan of a glass fiber non-crimp fabric was fed into GeoDict, the material simulation software developed by M2M. Second, a digital model (DM) of the textile was created by computer modelling of basic structure and subsequent virtual compaction. μCT-model and DM were then used for computational fluid flow simulation which gives the direction-dependent permeability as an output. The DM calibrated by experiments represents the SEA and results in the digital twin. Third route was the experimental permeability measurement to generate reference values. Comparing the results of all three routes allows statements about the functionality of the simulation and accuracy of modelling. The rather deficient correlation between the results of experiments and μCT-model based simulations revealed that segmentation is a remarkable source of error despite the use of recognized methods. Different modeling approaches were followed to build up the digital twin. The best results were achieved with models undergoing a virtual compaction step, which takes various imperfections such as yarn deformation and varying nesting behavior into account. With this 2.25 - 6.75% deviation from the experimental results at an average standard deviation of 21.9 - 61.2% were achieved. Hence, the digital twin shows a better correlation than the μCT-model and high potential for substitution of experiments. Even better results are expected when in a next step a local anisotropic permeability will be allocated to the yarns.

Textile-Integrated Elastomer Surface for Fiber Reinforced Composites

Elastomer layers offer a wide range of surface functionalization options for fiber-reinforced polymer composites (FRPC), e.g. erosion protection or increased impact resistance. Goal of this study was to investigate if it is possible to prepare a textile-based semi-finished product with elastomeric surface, which can easily be used as outermost layer in different liquid composite molding (LCM) processes. For this purpose, different types of elastomer were pressed and vulcanized onto a biaxial glass fiber fabric. Target of this procedure was to reach partial immersion of the elastomer into the textile with remaining dry textile areas. The dry areas of the textile can later be impregnated with a thermoset resin system. The strategy is to have the transition region between elastomer and thermoset within one textile layer and to give a robust and easy to handle semi-finished-product in order to achieve a maximum bonding strength of the elastomer surface to the final composite part. It could be shown by micrographs and computer tomography that the elastomer only penetrates the textile at its boundary. A remarkable microimpregnation of individual filaments within the rovings does not take place. Concerning the manufacturing, since water evaporates during vulcanization, a sufficient process pressure must be maintained throughout the entire vulcanization process to ensure a pore-free elastomer. Peel-off tests similar to DIN EN 28510-1 on the finished composite showed a failure in the laminate and not in the boundary layer between laminate and elastomer, so that the desired high joint strength could be demonstrated.

Assessment of capillary phenomena in liquid composite molding

The interplay between capillary and viscous forces plays a significant role for optimal part quality in fiber-reinforced composites manufactured by Liquid Composite Molding (LCM); these are however difficult to analyze during flow. Two approaches are proposed in this study: (i) a woven glass fabric’s surface was modified by corona treatment to improve its wetting characteristics without altering its permeability, (ii) flow of a UV-curable resin was stopped during LCM by fast radiation cure. Capillary rise experiments and unsaturated/saturated permeability measurements confirmed the wetting improvement by corona treatment. Flow front morphology analysis of cured pristine and treated fabrics, processed under various pressure differentials, revealed the enhancement of dual-scale flow at capillary-dominated regime and improvement of intra-tow flow in viscous-dominated regime. Furthermore, the obtained results provide insight into the so-called optimal capillary number to attain the minimal void content in composites and its alteration by surface treatment of fiber reinforcements.

Longitudinal and transverse flows in fiber tows: Evaluation of theoretical permeability models through numerical predictions and experimental measurements

Tow permeability is a very important input in the dual-scale flow and void formation studies in Liquid Composite Molding processes for manufacturing polymer composites. In this comparative study aimed at checking the accuracy of various theoretical models for permeability, we evaluated the permeability of unidirectional fibrous media at tow scale for flow transverse and parallel to the fiber directions. The falling-head parameter was adapted to measure the permeability along and across the fibers within tows for three different cases of fiber volume fractions (fvf). Unit cells corresponding to the measured fvf and fiber diameters were generated using GeoDict. Two separate simulations, one using GeoDict to solve Stokes flow and the other using COMSOL to solve Whitaker’s closure-formulation, were done. Eighteen different analytical models were evaluated.

A review on numerical optimization in liquid composite moulding processes

Abstract Liquid Composite Moulding (LCM) processes became popular for producing near-net shape structural composite parts over traditional methods. However, some critical issues arise during making of the composite part using LCM processes. This article presents the literature review on prevention methodologies of process induced defects in LCM which arises mainly in the mold filling and curing stage. Different numerical methods are developed for reducing the flow induced defects like macro-voids, dry spot and micro-voids formation and cure induced defects like matrix micro-cracks and geometrical distortions. Currently, researchers are attracting towards heuristics techniques such as Genetic Algorithm (GA), especially for the development of Multi-objective optimization (MOO) problems, to optimize multiple conflicting objectives simultaneously. The lack of connection between the selection of optimization technique and process induced defects are highlighted in the future work.

Production of Hybrid Tubular Metal-Fiber-Preforms: Material Characterization of Braided Hoses with a Binder

Hybrid shafts or rods, where the area of load introduction is metallic (e.g. steel or aluminium) and the area of load transfer is made of fibre reinforced plastics (FRP), are an established concept for lightweight parts. Besides the monolithic FRP and the metallic areas, the overlap area of both materials is particularly important. Such parts can beneficially be produced by the use of liquid composite moulding (LCM), where the bonding process takes place during the resin curing. This is called intrinsic hybridization. Beforehand it is crucial to produce a near-net-shape preform in which the metallic end fittings for the load introduction are already integrated. To manufacture such parts constantly with a high quality, a process model of the joining by draping the braided preform is necessary.In this paper an approach for the production of hybrid preforms made of braided hoses and metallic fittings is presented in order to develop a process model. The process starts with a cylindrical multi-layer preform made of braided hoses, in which the layers are bonded by a thermoplastic binder powder. The decisive process step is the draping of the preform onto the metallic fitting. For this forming step, the material characterization of the hybrid preform plays an important role. Several material tests to determine the textile parameters of the preform are therefore evaluated and performed. Finally, the results of these tests are presented and discussed.

Rheokinetic study of acrylic reactive mixtures dedicated to fast processing of fiber‐reinforced thermoplastic composites

ABSTRACTThis study focuses on the characterization of acrylic reactive mixtures designed for liquid composite molding (LCM) and yielding thermoplastic matrices for composite applications. Conversion degree, complex viscosity, and cure exothermicity are tracked during the radical polymerization. Results show that these systems suffer from Trommsdorff's effect, that is, the polymerization is highly exothermic. The rheological study also highlights a limit of injectability for LCM processes around a conversion degree of 10%. The vitrification curve, which correlates conversion degree to glass‐transition temperature, has been established for methyl methacrylate/poly(methyl methacrylate) mixtures. It constitutes a useful tool to identify the thermal conditions required to reach a full monomer conversion and thus, to manage the residual monomer content. The use of such acrylic‐based reactive solutions for fast LCM processing appears appropriate as fully polymerized matrices were obtained in less than 5 min at 90 °C. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47391.

Contribution for analyzing the intra-ply yarn sliding mechanism in preforming of the woven fabric

Manufacturing complex composite part via Liquid Composite Molding (LCM) processes involves preforming dry textile perform before injecting the liquid resin. The defect that may be encountered within the textile structure during preforming step decreases the expected mechanical properties of the composite part. The intra-ply yarn sliding is a defect frequently observed during preforming of a woven preform but its mechanism is far from being fully understood. In this paper, a contribution for analyzing the mechanism of this defect and the influencing process parameters is presented. This analysis is based on an experimental study performed for one ply carbon fabric using a hemispheric punch. The effect of the ply orientation and blank holder force on the fabric behavior during preforming is evaluated regarding the required preforming force and yarn slippage. For the performed tests, it was obvious the impact of these two factors on the fabric behavior. The slippage phenomenon occurred in zones with low shear angles, for specific ply orientation. The fabric behavior has been analyzed by considering the evolution of the yarn tension which is related to the contact shear stress induced by sliding the fabric across the die and blank holder during preforming. Using an analytical model for the yarn tension in friction sliding, the influencing ply geometry and process parameters have been identified. Based on this analysis a solution relative the ply geometry to reduce the yarn tension is examined.