Liquid Composite Molding Processes Research Articles

Wetting and swelling property modifications of elementary flax fibres and their effects on the Liquid Composite Molding process

Flax fibres were thermally treated, to prove that in the same manufacturing conditions, modifying wetting and swelling properties of fibres changes the flow during LCM processes and thus could enhance the final material health of composite parts. Swelling of untreated and treated fibres was measured with optical and tensiometric methods proving that the thermal treatment increases the dimensional stability of flax fibres. A methodology to minimise the dispersion of contact angle values was also developed, and surface energy components of untreated and treated fibres were then determined in the frame of the Owens and Wendt theory. It was evidenced that the thermal treatment makes fibres less hydrophilic. Composite half plates reinforced with untreated and treated flax fabrics were then simultaneously manufactured by Liquid Composite Molding (LCM) process (resin infusion) and finally, a reduction of porosity was observed in composites reinforced with treated flax fabrics.

Experimental characterization of the pore size distribution in fibrous reinforcements of composite materials

This study uses capillary flow porometry to investigate the porous structure of engineering fabrics used in high-performance polymer composites. This technique consists of applying air pressure to a previously wetted sample to progressively expel the liquid from the pores. A porometry testing device commonly employed to characterize filtration media was used as the principal tool for this investigation. Four types of fibrous fabrics made of glass and carbon fibers with different textile architectures have been experimentally characterized with a through-thickness setup. This allowed obtaining the pore size distribution inside the tested material. In all the cases studied, the porometry technique was able to detect in a reproducible way a bimodal pore size distribution reflecting the presence of both micropores (inside the fiber yarns) and mesopores (between the yarns). Moreover, experimental results indicate that the method can be used to study the influence of the textile pattern on the pore size distribution. Overall, the study shows that capillary flow porometry can give valuable information on the dual scale structure of fibrous reinforcements, which plays a critical role during the impregnation stage of Liquid Composite Molding processes. Because of its simplicity and speed of execution, the proposed approach appears to be a promising way to complement other sophisticated techniques already used for composites such as microscopy and X-ray microtomography.

Wicking Tests for Unidirectional Fabrics: Measurements of Capillary Parameters to Evaluate Capillary Pressure in Liquid Composite Molding Processes

Wicking Tests for Unidirectional Fabrics: Measurements of Capillary Parameters to Evaluate Capillary Pressure in Liquid Composite Molding Processes

Wicking Tests for Unidirectional Fabrics: Measurements of Capillary Parameters to Evaluate Capillary Pressure in Liquid Composite Molding Processes.

During impregnation of a fibrous reinforcement in liquid composite molding (LCM) processes, capillary effects have to be understood in order to identify their influence on void formation in composite parts. Wicking in a fibrous medium described by the Washburn equation was considered equivalent to a flow under the effect of capillary pressure according to the Darcy law. Experimental tests for the characterization of wicking were conducted with both carbon and flax fiber reinforcement. Quasi-unidirectional fabrics were then tested by means of a tensiometer to determine the morphological and wetting parameters along the fiber direction. The procedure was shown to be promising when the morphology of the fabric is unchanged during capillary wicking. In the case of carbon fabrics, the capillary pressure can be calculated. Flax fibers are sensitive to moisture sorption and swell in water. This phenomenon has to be taken into account to assess the wetting parameters. In order to make fibers less sensitive to water sorption, a thermal treatment was carried out on flax reinforcements. This treatment enhances fiber morphological stability and prevents swelling in water. It was shown that treated fabrics have a linear wicking trend similar to those found in carbon fabrics, allowing for the determination of capillary pressure.

Role of fiber distribution and air evacuation time on capillary driven flow into fiber tows

Composite materials are created by reinforcing a matrix material with fibers. In Liquid Composite Molding (LCM) processes, dry fiber preforms are impregnated with resin to form the composite. The fiber preforms usually contain fiber tows and resin impregnation into the tows is driven by the applied pressure gradient and the capillary pressure. The fiber tows are usually less permeable than the regions in between them hence the resin will typically flow around the fiber tows at much faster rate than into them. Due to the dual scale nature of the pore structure, there are often microvoids within fiber tows that result from air and volatiles being trapped. This paper introduces a novel methodology to include both capillary and void pressure in the prediction of tow filling. The methodology allows for addressing the effect of non-uniform fiber volume fraction distributions within the tow on void distribution. The time dependent applied pressure experienced by the tow is also taken into account. The method is used to study the effect of wetting properties and the length of air evacuation time on void entrapment.

In-mould gel-coating for polymer composites

Surface coatings (gel-coats) are often used on commercial composite mouldings for cosmetic and/or durability reasons. They have traditionally been prepared in open moulds with styrene vapour allowed to escape to the workspace and environment. This paper considers the development of in-mould gel-coating processes. A Double Glass Plate Mould (DGPM) was used to prepare flat composite test panels. Laminates were manufactured by liquid composite moulding processes. Conventional hand painted gel-coat, innovative In-Mould Gel-Coating with a trilayer separator fabric (IMGC) or In-Mould Surfacing with a silicone shim (IMS) were studied. The surface quality of the final products was measured using a Wave-Scan device while the adhesion of the gel-coat was characterised by pull-off tests. The new processes offer reasonable properties in a cleaner, more controlled process.

Mechanical, physical and tribological characterization of nano-cellulose fibers reinforced bio-epoxy composites: An attempt to fabricate and scale the ‘Green’ composite

The development of bio-based composites is essential in order to protect the environment while enhancing energy efficiencies. In the present investigation, the plant-derived cellulose nano-fibers (CNFs)/bio-based epoxy composites were manufactured using the Liquid Composite Molding (LCM) process. More specifically, the CNFs with and without chemical modification were utilized in the composites. The curing kinetics of the prepared composites was studied using both the isothermal and dynamic Differential Scanning Calorimetry (DSC) methods. The microstructure as well as the mechanical and tribological properties were investigated on the cured composites in order to understand the structure-property correlations of the composites. The results indicated that the manufactured composites showed improved mechanical and tribological properties when compared to the pure epoxy samples. Furthermore, the chemically modified CNFs reinforced composites outperformed the untreated composites. The surface modification of the fibers improved the curing of the resin by reducing the activation energy, and led to an improvement in the mechanical properties. The CNFs/bio-based epoxy composites form uniform tribo-layer during sliding which minimizes the direct contact between surfaces, thus reducing both the friction and wear of the composites.

Towards void formation and permeability predictions in LCM processes: A computational bifluid–solid mechanics framework dealing with capillarity and wetting issues

A bifluid–solid contact model involving surface tension and wetting effects is developed within a finite element framework, in order to provide an accurate characterization of the fluids and fibrous behaviours during Liquid Composite Molding processes. This model is based on a Eulerian approach of two immiscible fluid (resin/air) domains with boundary conditions which prescribe wetting phenomena at fluid/fiber interfaces. The fluid interface is described by the Level Set method, on which capillary force is considered. Numerical simulations of a drop evolution with wetting effects are used to illustrate this challenging physical problem.

A Review of Non-saturated Resin Flow in Liquid Composite Moulding processes

The fundamentals of flow in the liquid composite moulding (LCM) processes are reviewed with an emphasis on multiphase flow effects considering the dual-scale thin porous media constituted by the textiles, and the dynamic wetting effects observed with viscous fluids. The main physical phenomena governing multiphase fluid flow in porous media are presented, using a continuum mechanics approach. Strategies towards solving these equations are then reviewed. Two main approaches are currently developed in the LCM literature. One is a direct solution of the multiphase flow equations, which requires dual knowledge of how permeability varies with saturation, and how saturation varies with pressure. The other is an indirect method based on solution of saturated flow with the introduction of sink terms, representing the dual scale of the fibrous reinforcements, which requires knowledge of the flow kinetics within the tows. Even though a complete and accurate tool to simulate multiphase LCM flow is still far from being available, several recent results have contributed to improving our physical understanding of LCM processes leading to process kinetics optimization and possibly porosity reduction in the final part.

On recycled carbon fibre composites manufactured through a liquid composite moulding process

The recovery of carbon fibres from waste and end-of-life carbon fibre reinforced plastic materials is both economically lucrative and environmentally necessary. Here, we characterise the physical and mechanical properties of recycled carbon fibre reinforced plastics (rCFRPs) composed of random and oriented non-woven recycled carbon fibre mats that were impregnated with liquid epoxy matrices using a vacuum-infusion set-up. The low areal density and poor compactability of the non-woven mats implied that press-moulding upon impregnation was essential to control laminate thickness and improve fibre content; this may limit the applications of the resulting rCFRPs. Moreover, the press consolidation process is thought to degrade fibre length, and is a likely cause for the lower-than-expected tensile properties of the rCFRPs. Expectedly, the oriented rCFRPs exhibited better tensile and compressive properties than the random rCFRPs. Notably, while the tensile strength of the rCFRPs was only up to 2.5 times better than the matrix, the tensile modulus was 4–10 times enhanced. Through a comparative literature survey, we found that the liquid composite moulded rCFRPs were outperformed by rCFRPs fabricated through other manufacturing processes (e.g. prepregging), particularly those employing high compaction pressures, and utilising long fibres recovered through pyrolysis and chemical processes, rather than the fluidised-bed process.

Characterization of 3D woven reinforcements for liquid composite molding processes

Compaction and permeability characterization of fibrous reinforcements is fundamental to liquid composite molding (LCM) processes, given that these fabric properties determine the part thickness, fiber volume content and mold filling time and patterns. In this study, the permeability of three different 3D woven carbon fiber reinforcements (orthogonal, angle interlock and layer-to-layer) was studied, each having a different weave style and z-binder pattern. For all reinforcements, single-cycle and multiple cycle compaction experiments were conducted on dry and saturated samples. The orthogonal preforms were more difficult to compact to the target fiber volume fraction of 0.65, with peak stresses reaching up to 2.3 MPa. Cyclic compaction tests were conducted to highlight the importance of permanent deformation in the reinforcements, when higher fiber volume content is desired via manufacturing using an LCM process. Unsaturated in-plane radial and saturated through-thickness permeability data were obtained at several fiber volume fractions. The orthogonal and layer-to-layer fabrics exhibited greater levels of anisotropy in the plane of flow. The rate of change of in-plane permeability with increasing fiber volume content was lower compared to through-thickness values. The effect of cyclic compaction on permeability was greater in the orthogonal and angle interlock reinforcements. Micrographs of infused samples showed significant permanent deformation of the z-binder yarns, an effect that reduced permeability at high fiber volume contents.

Characterisation of the draping behaviour of unidirectional non-crimp fabrics (UD-NCF)

Thin composites shell structures manufactured from stitched unidirectional non-crimp fabrics (UD-NCF) in a liquid composite moulding process provides high lightweight design capabilities. However, draping behaviour of UD-NCF has been investigated only sparsely, in contrast to research on woven fabrics or biaxial non-crimp fabrics. Hence, this contribution focuses on fundamental investigations of the draping behaviour of UD-NCF. Within this investigation picture frame tests and uniaxial bias extension tests are performed to examine the in-plane shear behaviour of UD-NCF. Furthermore, a new method is presented to examine ambivalent tensile behaviour of UD-NCF transverse to the carbon fibre roving orientation. In particular, the influence of thin glass fibres on transverse tensile behaviour of UF-NCFs is investigated using a new clamping mechanism in tensile testing. Finally, hemisphere tests are performed to observe the forming behaviour of UD-NCF in a realistic forming process and to evaluate the proposed material characterisation methods regarding its suitability for UD-NCFs.

Mechanical characterization of scalable cellulose nano-fiber based composites made using liquid composite molding process

Plant derived cellulose nano-fibers (CNF) are a material with remarkable mechanical properties compared to other natural fibers. However, efforts to produce nano-composites on a large scale using CNF have yet to be investigated. In this study, scalable CNF nano-composites were made from isotropically porous CNF preforms using a freeze drying process. An improvised Liquid Composite Molding (LCM) process was used to make the nano-composites using a high bio-content ‘green’ epoxy resin. Formation of the freeze dried CNF preforms' porous network highly affects the mechanical properties of the composite, therefore mechanical testing was performed to characterize the effects of pore structure on global mechanical properties. The level of cure was investigated by comparing DSC results and the effect of curing on the composites was studied by tensile and dynamic mechanical analysis tests. The efficacy of silylation on the CNF preforms was analyzed with Water Contact Angle (WCA) measurements where the treatment led to hydrophobicity and hence better wettability by the non-polar resin. The causes of the failure in the composites were investigated using SEM analysis of the fractured surfaces. In general, silylation improved the infusion of resin into CNF preforms and resulted in better mechanical properties.

Mechanical and Tribological Characterization of Composite Laminates Manufactured by Liquid Composite Molding Processes

The aim of the present work is to investigate the influence of the Vacuum Assisted Resin Transfer Molding process steps on the impregnation quality of the laminates as well as on mechanical and tribological properties of the processed material. Composite laminates were realized using epoxy resin reinforced with carbon (CF) or glass continuous (GF) fibers. Two different textile architectures, namely non-crimp fabrics (UD) and woven-mat (0/90), were used and various processing conditions were employed. Optical observations revealed an unexpected trend relatively to the intra and inter bundle voids concentration with respect to the impregnation velocity, especially using UD-CF and UD-GF reinforcements and low impregnation rate. Tensile and three points bending tests highlighted the strong impact of fiber material and architecture on mechanical properties, whereas the presence of voids played a slightly influence on the fiber dominated characteristics analyzed. Tribological outcomes evidenced a reduction of the friction coefficient when the resin is reinforced by carbon or glass fibers as well as when the sliding direction of the counterbody is oriented parallel to the fiber direction.

Evaluation of nanoalumina and nanosilica particle toughened high glass-transition temperature epoxy for liquid composite molding processes

In this study, nanoalumina (Al2O3) and nanosilica (SiO2) particles are evaluated as tougheners for a high glass-transition temperature (Tg) epoxy system in correlation with liquid composite molding (LCM) processability. The aim of this paper is to directly compare the effectiveness of nanoalumina and nanosilica of the same nominal particle size as epoxy tougheners on the same neat resin system. The epoxy resin system used in this study was Dow D.E.R. 330 amine cured epoxy with a Tg of 150℃. Both particle types are observed to be Tg neutral and increase fracture toughness of the base epoxy system. Between the two particle types, nanoalumina is found to be more effective than nanosilica in terms of achievable fracture toughness at a given particle loading. As resin viscosity increases with particle addition, the addition of fewer particles with the use of nanoalumina is also beneficial to LCM processing where a lower viscosity is preferable.

Fast Mold Filling Simulation Based on the GeodesicDistance Calculation Algorithm for Liquid CompositeMolding Processes

Fast Mold Filling Simulation Based on the GeodesicDistance Calculation Algorithm for Liquid CompositeMolding Processes

Simulation of coupling filtration and flow in a dual scale fibrous media

In this article, numerical simulation of suspension (particles filled-resin) flow through a fibrous media taking into account dual scale porosity in LCM (Liquid Composite Molding) processes is presented. During the flow, a strong interaction between the particle motion and the fluid flow takes place at the porous media wall (the fiber bundle surface). In this study, the Stokes–Darcy coupling is used to describe the resin flow at mesoscopic scale to treat the particles in suspension. A “fluid” model to describe the suspension flow, a “filtration” model to describe the particle capture and a “solid” model dedicated to the modeling of mass particles dynamics was used. The “solid” model is also operated to identify the particles retention.For validation, the numerical results of proposed model were compared with the experimental results from the literature and found in good agreement. Then, other numerical results studying the suspension’s rheological behavior are presented.

Bisphenol A based polyester binder as an effective interlaminar toughener

Bisphenol A based thermoplastic polyesters are commonly used in the industry as binders, or tackifiers, to produce cost-saving preforms in Liquid Composite Moulding processes such as Vacuum Assisted Resin Transfer Moulding (VARTM). However, it is often reported that the presence of these polyesters has a detrimental effect on the mechanical properties of the resulting composite laminates. In contrast, this study shows that interlaminar toughness can be increased without negatively affecting other properties by coating the reinforcing plies with a bisphenol A based thermoplastic polyester if some precautions are taken in mind.The polyester was added to an epoxy resin in order to study its effect on the thermophysical properties and fracture toughness of the bulk epoxy. The polyester molecules acted as a plasticizer for the epoxy resin when the polyester was added in low amounts. This increased the bulk fracture toughness of the epoxy resin by 30%. Polyester modified glass/epoxy laminates were produced and tested for Mode I interlaminar fracture toughness and flexural properties. The increased toughness of the epoxy matrix led to a 60% increased Mode I interlaminar fracture toughness of the laminates, without negatively affecting flexural stiffness and strength of the laminates.

Characterization of textile permeability as a function of fiber volume content with a single unidirectional injection experiment

Textile permeability is a fundamental property to describe preform impregnation in Liquid Composite Molding (LCM) processes. It depends on textile architecture and fiber volume content (FVC). Conventional methods to measure in-plane permeability are based on radial or unidirectional injection experiments performed at fixed FVC. A complete characterization involves a series of tests and requires several material samples. This study presents a novel approach to characterize permeability as a function of FVC through a unique unidirectional injection experiment with a preform containing different FVC sections. The same experimental set-up as in conventional unidirectional unsaturated permeability measurements is used with a second pressure transducer embedded in the mold in addition to the one located at the inlet gate. A fast algorithm is developed to exploit the data from the two sensors and automatically derive the permeability distribution without any need of visual flow front observations. The methodology is validated with a random fiber mat and a woven fabric. Results show that accurate permeability characterization can be achieved for both kinds of textiles.

Influence of preforming technology on the out-of-plane impregnation behavior of textiles

Various Liquid Composite Molding processes provide out-of-plane impregnation. Thereby, the emerging pressure distribution leads to a hydrodynamic compaction of the fiber structure. A prediction of the behavior of the fiber structure under these conditions is highly complex, due to strong interdependencies between pressure distribution, compaction behavior, and impregnation behavior. In this study possibilities to influence the hydrodynamic behavior of textiles by preforming technology are investigated, by comparing an untreated glass fiber woven textile with sewed, bindered, sheared, and pre-compacted samples of the same material. A novel measurement apparatus, reproducing the conditions during out-of-plane impregnation, is applied for this purpose. It enables out-of-plane permeability determination simultaneous to online compaction monitoring. Sewing accelerates out-of-plane impregnation, while inactivated binder can preserve the available permeability at high injection pressures. The increasing superficial density and the geometrical changes caused by shearing decrease permeability.