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Liquid composite molding (LCM) is a promising option for low‐cost manufacturing of high‐performance composites compared to traditional prepreg‐autoclave methods. Void formation may be the most significant roadblock to such adaptation of LCM. In this article, the hypothesis that higher wettability, that is, lower contact angles of liquid on solids, would lead to lower void content for LCM is tested. First, a theory that calculates the energy required to form a bubble with varying contact angles is formulated by considering interfacial energy differences of a system with and without it. To experimentally prove this hypothesis, six different carbon fiber reinforcement samples were prepared each with a different fiber surface treatment. The wettability from the surface treatments was evaluated with contact angle measurements based on capillary rise between two fiber yarns. Void formation in situ during infusion was evaluated by a series of 1D infusion experiments using the same six surface modifications. Of the six samples, the reinforcements coated with fluorinated alkane and aminosilane showed the highest wettability and lowest void content, confirming that a lower contact angle can reduce the formation of voids during the infusion process. Highlights: Higher wettability was correlated with less bubble (void) formation.Theoretical model and LCM experimental confirmation.Various surface modifications of carbon fibers tested.Potential application: enhancement of properties from LCM manufactured parts. [ABSTRACT FROM AUTHOR]

AbstractResin Transfer Molding (RTM) is a manufacturing process for fiber reinforced polymer composites where dry fibers are placed inside a mold and resin is injected under pressure. During mold design, filling simulations can study different manufacturing concepts (i.e. placement of injection gates and vents) to guarantee complete filling of the part and avoid air entrapment where flow fronts converge. In this work, a novel software tool LCMsim, which was implemented by the authors, is benchmarked against other tools and real-world flow experiments. Its development was driven by two ideas: Easy-of-use for the mold engineer and maximum flexibility for the researcher. Two experiments were used for validation. In the first, zones with different preform properties were present and in the second, race-tracking was enforced. Flow fronts from LCMsim and experiment agree with 7% error and simulated flow fronts from LCMsim and the commercially available software PAM-RTM agree with 3% error.

Experimental permeability measurements saw a considerable increase in accuracy when recommendations and guidelines were imposed upon the realization of two international benchmarks. Such requirements aid the design stage and experimental validation of a permeameter rig however, systematic errors in the measurements still compromise the comparability of measurements obtained by different radial permeameter rigs. Owing to hurdles and limitations in the data acquisition system, validation of the mold cavity and fluid injection system, optical errors in the visual tracking of a flow front, and uncertainties in the measurement of the fluid viscosity, the measurement’s accuracy is yet lower than the required for a standardized process. In this study, the detailed study and calibration of such parameters was able to identify and minimize error sources that would otherwise result in undetected systematic deviations from the expected results. In conclusion, the verification proposed by the radial benchmark does not guarantee the accuracy of the measurement, as the error in the instruments proposed for the verification is comparable to the requirements themselves. This creates a certain uncertainty in the verification that needs to be tackled with more detailed measurement protocols to ensure not only the compliance with the measurement requirements but also to set the limits of the attainable accuracy. The rig was validated by measuring the permeability of the fabric reinforcements used in the radial benchmark exercise. Due to the scattering in the results reported in the benchmark exercise, 13 out of the 19 reported values were excluded to obtain a good estimation of the expected permeability for each volume fraction. Although the rig complied with all recommendations currently in place, the obtained permeability showed a 20% deviation in the K1 direction, while the K2 was within the expected range for the average value. The observed deviation was later found to be caused by an optical distortion, which affected the measurement of the real-world flow front dimensions. A correction for this deviation needs further systematic investigation, also a possible revision of the future standard since a correction for optical distortions is yet not included in the measurement guidelines.

With the rapid development of high-performance fibers such as carbon, enhanced glass fibers in structural applications, the use of fiber-reinforced composite (FRC) materials has also increased in many areas. Liquid composite molding (LCM) is a widely used manufacturing process in composite manufacturing; however, the rapid impregnation of resin in the reinforcing fibers during processing poses a significant issue. The optimization of resin impregnation is related to tow deformations in the reinforcing fibers. The present study therefore focuses on this tow deformation. The permeability behaviors in double-scale porous media were observed under different flow rates and viscosity conditions to examine the overall tendencies of structural changes in the reinforcement. The permeability results showed hysteresis with increasing and decreasing flow rate conditions of 50–800 mm3/s, indicating structural changes in the reinforcement. The tow behaviors of the double-scale porous media with respect to the thickness and flow rate were investigated in terms of the representative indices of the minor axis (tow thickness) and major axis. The minor axis and major axis of the tow showed decreasing and increasing trends of 2–5% and 2%, respectively, with minimum and maximum values at different positions along the reinforcement, affected by the different hydrodynamic entry lengths. Finally, the deformed tow behavior was observed microscopically to examine the behavior of the tow at different flow rates. [ABSTRACT FROM AUTHOR]

Process monitoring of resin permeation in fiber preform and fabric thickness variation in liquid composite molding (LCM) is important to ensure the quality of the composite parts. However, existing technologies cannot monitor both of them by a single sensor and may disrupt resin flow patterns or fiber deformation inside the part due to their large thickness or rigidity. To achieve accurate monitoring of the signals within the part during the LCM process, a thin and flexible Pt-coated film capacitive sensor was designed to minimize the effect of sensor on flow behaviour. The accuracy of the embedded sensor was verified by the consistent resin flow front and the negligible fiber deformation around the sensor. Moreover, the flow front, thickness variation and curing inside composites preform in LCM can be captured based on the variation of the capacitance curve and its second derivative. These results demonstrated that this multifunctional sensor offers a new solution to obtain signals accurately in the part in LCM. [ABSTRACT FROM AUTHOR]

With the rapid development of high-performance fibers such as carbon, enhanced glass fibers in structural applications, the use of fiber-reinforced composite (FRC) materials has also increased in many areas. Liquid composite molding (LCM) is a widely used manufacturing process in composite manufacturing; however, the rapid impregnation of resin in the reinforcing fibers during processing poses a significant issue. The optimization of resin impregnation is related to tow deformations in the reinforcing fibers. The present study therefore focuses on this tow deformation. The permeability behaviors in double-scale porous media were observed under different flow rates and viscosity conditions to examine the overall tendencies of structural changes in the reinforcement. The permeability results showed hysteresis with increasing and decreasing flow rate conditions of 50–800 mm3/s, indicating structural changes in the reinforcement. The tow behaviors of the double-scale porous media with respect to the thickness and flow rate were investigated in terms of the representative indices of the minor axis (tow thickness) and major axis. The minor axis and major axis of the tow showed decreasing and increasing trends of 2–5% and 2%, respectively, with minimum and maximum values at different positions along the reinforcement, affected by the different hydrodynamic entry lengths. Finally, the deformed tow behavior was observed microscopically to examine the behavior of the tow at different flow rates.

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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. [ABSTRACT FROM AUTHOR]

Flexible Injection is a new processing technique for the manufacture of advanced composites made of continuous reinforcing fibers and thermosetting polymer matrix. The primary objective of this new process is to provide faster and less expensive manufacturing than traditional methods like autoclave processing or Resin Transfer Molding. The present paper investigates the effectiveness of the proposed method to produce composite structures possessing sharp corners. A specially devised setup is used to manufacture out of glass fibers and vinyl ester resin a series of Z-shaped parts with small radii. The quality of the fabricated parts is assessed to detect possible defects induced by the manufacturing process. Each stage of the production cycle is analyzed thoroughly to develop a simplified finite element model reproducing the fiber bed behavior during processing. Parametric studies are conducted to evaluate the impact of processing conditions on the quality of final products. The combination of numerical simulations and experimental observations demonstrates clearly the importance of the preforming stage. At the same time, it provides useful insights on the physical phenomena occurring during Flexible Injection. [ABSTRACT FROM AUTHOR]

Abstract: Numerous filler particles are being developed and applied to improve the properties of conventional composite materials or to develop composite materials with additional functionalities. When filler particles are added to the polymer matrix of a fiber-reinforced composite material, the particles dispersed in the polymer resin can be filtered between fiber strands during the manufacturing process, which leads to a nonuniform particle distribution and material defects. Therefore, understanding and controlling such filtration phenomena has become a critical issue. In the present study, a new microscopic methodology for measuring the distribution of filler particles in fully cured composite parts using an electron probe microanalyzer (EPMA) is proposed. The distributions of spherical titanium dioxide particles and carbon nanotubes conjugated with silver nanoparticles as tracers were visualized by elemental mapping analysis. Furthermore, the concentrations were separately measured in the intratow and intertow regions by quantitative analysis. [Copyright &y& Elsevier]

In the present work, the effects of fiber structure and fluid absorption on the compaction behavior of jute woven fabrics and sisal mats were analyzed and compared with the response of glass fiber mats. It was found that the fiber content that can be achieved with a certain compaction pressure is lower in the case of natural fiber preforms. In addition, due to the hollow structure of these natural fibers, jute and sisal preforms suffered larger permanent deformation than glass fiber preforms after the compressive loading cycle. In addition, it was found that fluid absorption reduced the compaction pressure in natural reinforcements due to fiber softening. These phenomena were not observed in glass fiber mats. [ABSTRACT FROM PUBLISHER]

The aim of this work is to characterize the compaction behavior of jute woven fabric preforms. The maximum compaction pressure, permanent deformation, and stress relaxation of the preforms were found to be dependent on the final fiber volume fraction and the compaction speed. Higher compaction speeds led to higher compaction pressures, stress relaxation, and permanent deformation. On the other hand, as the fiber content was raised, the maximum compaction pressure and the permanent deformation increased, while the stress relaxation decreased. In addition, it was found that the structure of natural fibers affected the compaction behavior of the preforms. Each fiber is composed of several hollow elementary fibers, which collapsed due to the compressive loading. Furthermore it was found that fluid absorption reduced the compaction pressure in natural fiber preforms due to fiber softening. [ABSTRACT FROM AUTHOR]

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

Computer simulation has become an efficient and cost-effective tool for the liquid composite molding (LCM) processes, including the RTM, VARTM, and resin infusion, compared to trial-and-error. The purpose of simulation is to accurately reflect the real situation. Since the simulation is predominantly based on the control volume finite element method (CVFEM), the boundary conditions such as gate and vent definition need to be properly defined. In this paper, the errors of the CVFEM based mold filling simulation were analyzed. The error sources were identified as the mesh near gates and vents, and gate definition. By conducting 3-D and 2-D simulation case studies, the influence of the gate definition on the simulation result was studied. Because many composite parts are shells and can be regarded as surfaces. 2-D simulation is favorable because of the better efficiency and simpler pre-processing. For this purpose, a method for mold filling simulation incorporating the gate size effect the effective gate method (EGM) - was developed. This method was validated through case studies. It shows that the EGM provides an effective and efficient approach to the CVFEM mold filling simulation incorporating the effect of injection gate size. [ABSTRACT FROM AUTHOR]

In liquid composite molding (LCM) processes, the resin is injected into the mold cavity, which contains preplaced reinforcement fabrics, through openings known as gates, while the displaced air leaves the mold through openings called vents. Under nominal conditions, the last points to fill are chosen as vent locations. However, due to imperfect preform cutting and placement, gaps and channels may form along the edges and curvatures in a mold, offering a path with less resistance for resin flow. The faster advance of resin through these gaps and channels, a common disturbance known as racetracking, will cause the last filled regions to vary, which complicates the vent selection process. In this study, probabilistic racetracking modeling is used to capture last-filled region distribution over the mold geometry. Success criteria for mold filling are defined in terms of dry spot tolerances, and vent fitness maps, which display potential vent locations, are created. Next, exhaustive search algorithm is coupled with vent fitness maps to determine optimal vent configurations. The map-based exhaustive search is demonstrated on three geometries and results are compared with existing combinatorial search results. The performance of the optimal vent configurations is evaluated in a virtual manufacturing environment. Sensitivity analyses are conducted to determine the influence of optimization parameters on the results. [ABSTRACT FROM AUTHOR]

In liquid composite molding technologies such as RTM, this study of the inlet-pressure history for the constant flow-rate 1-D flow experiment reveals that the measured pressure profile, which droop downwards as in earlier studies on the unsaturated flow, is at a variance with the linear pressure profile predicted by the physics used for state-of-the-art LCM mold filling simulations. The droop along with the error in the inlet-pressure predictions increase with an increase in the fiber-mat compression. The effect of fiber-mat architecture on the droop in the inlet-pressure profiles is studied and significant droops are observed for various stitched mats as compared to the woven mats. This study repudiates the long-held view that mere presence of cylindrical or elliptical tows in the woven or stitched fiber-mats automatically leads to the unsaturated flow characteristic of dual-scale porous media; rather the presence of continuous uninterrupted macrochannels along the flow direction for preferential channel-flow is found to be necessary for the appearance of a drooping inlet-pressure history characteristic of the unsaturated flow. [ABSTRACT FROM AUTHOR]

In this study, optically transparent glass fiber-reinforced polymers (tGFRPs) were produced using a thermoset matrix and an E-glass fabric. In situ polymerization was combined with liquid composite molding (LCM) techniques both in a resin transfer molding (RTM) mold and a lite-RTM (L-RTM) setup between two glass plates. The RTM specimens were used for mechanical characterization while the L-RTM samples were used for transmittance measurements. Optimization in terms of the number of glass fabric layers, the overall degree of transparency of the composite, and the mechanical properties was carried out and allowed for the realization of high mechanical strength and high-transparency tGFRPs. An outstanding degree of infiltration was achieved maintaining up to 75% transmittance even when using 29 layers of E-glass fabric, corresponding to 50 v.% fiber, using an L-RTM setup. RTM specimens with 44 v.% fiber yielded a tensile strength of 435.2 ± 17.6 MPa, and an E-Modulus of 24.3 ± 0.7 GPa. [ABSTRACT FROM AUTHOR]

Three types of dry spot changes, which have been described in Part I, are modeled in this study. Darcy's law and ideal gas law are used to model the Type I and Type II dry spot changes, while a simplified form of the two phase Darcy's law is used to model the Type III dry spot change. A computer code based on the control volume finite element method is developed to simulate the dry spot formation and changes. In order to save computation time, a coarse mesh rebalancing method is implemented in the code. Compared to the experimental results, this flow model predicts the Type I and Type II dry spot changes and the packing and bleeding process very well. It shows the right trend of Type III dry spot changes when compared to some preliminary experimental results. [ABSTRACT FROM PUBLISHER]

Dry spot formation and changes in liquid composite molding (LCM) were investigated by flow visualization experiments. The dry spot size can be reduced by three mechanisms: (1) it can be compressed by the hydrostatic pressure from the surrounding saturated region; (2) it can be reduced by bleeding trapped air into the surrounding saturated region if there is a pressure gradient in the saturated region; and (3) it may gradually disappear by the wicking flow due to the capillary pressure difference between the saturated region and the dry fiber region. Experiments of packing and bleeding, a technique used in industry to eliminate dry spots, were also carried out and the results are explained based on the three types of dry spot changes. [ABSTRACT FROM PUBLISHER]

In Liquid Composite Molding (LCM) processes, a fabric reinforcement is placed in a closed cavity and resin is injected into the mold. Almost all reinforcements are dual scale containing fiber tows, which fill at a different rate than the region in between the fiber tows. Simulation of LCM processes can help identify regions that fail to fill. However, the presence of dual scale flow is usually neglected. Here this phenomenon is modeled using Liquid Injection Molding Simulation (LIMS) software in which a complex network of one-dimensional elements is created. This allowed us to simulate transverse and longitudinal flow through the fiber tows taking into consideration the orientation and architecture of the reinforcement and, additionally, adding capillary effects to the model. A sensitivity study has been performed to investigate the effects of properties in non-dimensional form, allowing for comparison with experiments that were conducted to validate the model by visualization of the flow front position and dual scale area dimensions. [Display omitted] • Dual scale flow simulation through complex one-dimensional elements network. • Simulation of anisotropic flow inside fiber tows with low computational effort. • Validation through experiments visualizing flow front and dual scale area. • Parametric study with non-dimensional properties showing model capability. • Study of tow compaction variability using experimental data. [ABSTRACT FROM AUTHOR]