Vacuum Assisted Resin Transfer Molding Process Research Articles

An Enhanced Vacuum-Assisted Resin Transfer Molding Process and Its Pressure Effect on Resin Infusion Behavior and Composite Material Performance.

In this paper, an enhanced VARTM process is proposed and its pressure effect on resin infusion behavior and composite material performance is studied to reveal the control mechanism of the fiber volume fraction and void content. The molding is vacuumized during the resin injection stage while it is pressurized during the mold filling and curing stages via a VARTM pressure control system designed in this paper. Theoretical calculations and simulation methods are used to reveal the resin's in-plane, transverse, and three-dimensional flow patterns in multi-layer media. For typical thin-walled components, the infiltration behavior of resin in isotropic porous media is studied, elucidating the control mechanisms of fiber volume fraction and void content. The experiments demonstrate that the enhanced VARTM process significantly improves mold filling efficiency and composite's performance. Compared to the regular VARTM process, the panel thickness is reduced by 4% from 1.7 mm, the average tensile strength is increased by 7.3% to 760 MPa, the average flexural strength remains at approximately 720 MPa, porosity is decreased from 1.5% to below 1%, and the fiber volume fraction is increased from 55% to 62%.

A study on the optimal design of UAM seats

The objective of this study is to enhance the efficiency of urban air mobility (UAM) air transportation by determining the optimal design, materials, and processes of seven different manufactured fiber-reinforced plastics (FRP), and to confirm their applicability for next-generation UAM aircraft seats. Structural designs for the aircraft seat models were carried out using carbon fiber-reinforced plastic (CFRP), glass fiber-reinforced plastic (GFRP), and GFRP chop materials, employing Autoclave, Hot-press, and vacuum-assisted resin transfer molding (VaRTM) processes, resulting in a total of seven FRP configurations. It was confirmed that CFRP seats were 50% lower weight than aluminum models, while GFRP seats showed a 30% reduction. Although Autoclave processing resulted in the highest tensile strength at 985[Formula: see text]MPa, VaRTM processing also produced strength levels comparable to Autoclave processing. Structural integrity assessments of the seat models, utilizing the Korean aviation standards (KAS), confirmed that the designed seat models exhibited no failure or deformation under the conditions required by the technical standards. This study provides insights into the potential application of the seven types of FRP materials in the design of aircraft seats, offering weight reduction benefits and meeting the structural integrity requirements outlined by KAS.

Surface flame retardancy of clay mineral powder infused onto the top of the layer of vinyl ester/bamboo composites through a modified VARTM process

The mechanical properties of fire-resistant biocomposites are significantly compromised owing to the high amounts of flame retardants (FRs) required for satisfactory results and the limited interfacial interactions between flame retardants (FRs) and polymer/natural fibers. The study addresses these challenges by modifying the vacuum-assisted resin transfer molding (VARTM) process to include two inlets and two outlets, allowing for the simultaneous infusion of an FR-filled resin on the top layer and pure resin in the remaining layers. Vermiculite clay mineral (VCM) was employed as an FR additive in vinyl ester (VE) using mechanical mixing methods such as the three-roll mill and overhead stirring. Bamboo fabric (BF) was used as the reinforcement in fabricating of VC-VE/BF fire-resistant bio-composites. Microscopic and spectral analyses confirmed clay dispersion and interactions within the composites. The FR-loaded side exhibited superior flame resistance (with a peak heat release rate (pHRR) of 467.1 W/g) and a 44% increase in thermal stability compared to the remainder of the composite. The findings indicate improved thermal and mechanical properties. The study presents a novel design for eco-friendly, FR composites.

Evaluation of resin impregnation using self-sensing of carbon fibers

The wettability of fiber-reinforced composites plays a crucial role in both mechanical properties and the efficiency of the manufacturing process. This is particularly significant in large and intricate composite structures where any flaws can have more severe consequences. In this study, the impregnation of epoxy resin into carbon fiber (CF) mats in-situ was monitored by observing the electrical resistance (ER) behaviors of CFs without any other additional sensors. The ERs of CF and CF tows at various points were measured, while epoxy resin was applied to establish fundamental data on ER behavior during the Vacuum Assisted Resin Transfer Molding (VARTM) process. During the VARTM process, the ERs of CF mats in-situ were compared with the behaviors of micro and sub-micro CFs, along with capturing images of the front flow of epoxy resin. The ER data obtained during the VARTM process was visualized through 3D ER mapping, which allowing for real-time monitoring of the flow front. Ultimately, the resin flow front of epoxy resin into CF mats was successfully monitored using the ER behaviors of CFs themselves.

Improved genetic algorithm for 2D resin flow model optimization in VARTM process

In this study, a combination of block-centered grid modeling and an enhanced genetic algorithm (GA) is introduced with the aim of optimizing the random permeability field within the Vacuum Assisted Resin Transfer Molding (VARTM) infusion model to enhance the accuracy of predicted resin flow distribution. Within the established 2D-VARTM model, random permeability values in the x and y directions are assigned to each grid. The model is then solved using the central difference method in conjunction with the upstream weighting method to predict the resin flow distribution. Subsequently, an improved GA based on heuristic mutation strategies was designed and validated. This algorithm employs the discrepancy between model predictions and actual sampling results as its fitness function and integrates heuristic strategies for iterative optimization. Simulation results revealed a significant improvement in the predictive accuracy of the model, with a jump from an initial 87.49%–97.19%. In practical applications, the predictive accuracy of the model reached 95.25%. This research offers an effective optimization approach for VARTM models and underscores the potential applicability of the enhanced GA in related fields.

Bamboo fiber‐reinforced epoxy composites fabricated by vacuum‐assisted resin transfer molding (VARTM): Effect of molding sequence and fiber content

AbstractFabricating bamboo fiber‐reinforced epoxy composites (BFREs) using the vacuum‐assisted resin transfer molding (VARTM) process has many advantages, such as low cost, high product quality, controllability, and broad applicability. However, the immature fabrication process seriously affects product performance and hinders the full exploitation of the advantages of the VARTM process. The effects on the physical‐mechanical and thermal properties of BFREs by molding sequence and fiber content were investigated in this study. The molding sequence of fiber compression followed by resin injection was beneficial to ensure hermeticity, thus reducing the voids in the BFREs. The mechanical properties of BFREs gradually increased with increasing fiber content. With 50 wt% fiber content, the bending strength and modulus, shear strength, and impact toughness of BFREs reached 71.27 MPa, 6.78 GPa, 11.68 MPa, and 5.45 kJ/m2, respectively. Although the thermal stability of the BFREs was slightly lower than that of pure epoxy, the bamboo fibers' high rigidity evidentially increased the epoxy's dynamic mechanical properties. The life cycle assessment indicated that the BFREs effectively reduced the environmental impacts and production energy consumption compared to the glass fiber correspondent. The VARTM‐fabricated BFREs have great potential for construction, transportation, household items, packaging, sports equipment, and leisure goods applications.Highlights Fiber compression followed by resin injection could ensure great hermeticity. Micro‐CT accurately measured the void characteristics of the composites. Voids had detrimental effects on the properties of the composites. Dense bamboo fiber preforms enhanced the mechanical properties of epoxy. Bamboo fibers reinforcing epoxy reduced the impact on the environment.

Mechanical, Thermal Properties and Void Characteristics of Bamboo Fiber-Reinforced Epoxy Resin Composites Prepared by Vacuum-Assisted Resin Transfer Molding Process

ABSTRACT This study aimed to examine the effect of epoxy resin (EP) systems on the impregnation of bamboo fibers (BFs) by EP and the properties of BF/EP composites prepared by vacuum-assisted resin transfer molding (VARTM) process. BF preforms obtained by solution suspension method were used as reinforcement, and epoxy/anhydride system (EP-1), epoxy/amine system (EP-2) and two-component epoxy system (EP-3) were employed as matrix; on this basis, BF/EP-1, BF/EP-2 and BF/EP-3 composites were prepared by VARTM. The physical, mechanical and thermal properties and void characteristics of BF/EP composites were characterized. The results showed that EP-1 with a viscosity of 265 mPa·s at 23°C and an operating time of room temperature without curing was beneficial for the EP-1 to impregnate BFs adequately and uniformly. The incorporation of BFs into EP-1 significantly improved the flexural, shear and impact properties, thermal properties and interface properties of EP-1. Moreover, the properties of BF/EP-1 composites were better than those of BF/EP-2 and BF/EP-3 composites. Three-dimensional X-ray microscopy scans revealed that the volume and distribution of voids in the BF/EP composites differed significantly depending on the EP systems, and the percentage of void volume in the BF/EP-1 composites was only 0.14% in 3D reconstruction models.

Simple stitch-assisted carbon fiber-aluminum laminates via out-of-autoclave process

Fiber–metal laminates (FMLs) are hybrid materials that provide the advantages of both metal and fiber-reinforced polymer composite (FRP) such as improved damage tolerance and impact strength, particularly in aerospace and automotive applications. However, the production of FMLs remains expensive because the manufacturing process is performed in an autoclave. In addition, delamination failure due to low interfacial adhesive strength between the metal and fiber layer is a problem that must be solved to improve the mechanical properties of FMLs. In this study, simple stitch-assisted carbon fiber-aluminum laminates were developed using vacuum-assisted resin transfer molding (VARTM). The FMLs produced through vacuum-assisted resin transfer molding were stitched with aramid fibers through holes on Al sheets for the resin flow associated with the VARTM. The mechanical properties of the FMLs were investigated by impact and compression after impact tests as well as double cantilever beam and flexural tests. The stitching effectively improved the interfacial strength and toughness owing to the anchoring between the aluminum and carbon fiber layers, thus preventing delamination crack propagation. As a result, the stitched FML with significantly improved interfacial adhesive and toughness was developed by the VARTM process, which will accelerate the application of FML to various lightweight structures.

The effect of stacking sequence and fiber orientation on tensile and flexural strength of fiber reinforced composite fabricated by VARTM process

In this study, Carbon, Glass, and Aramid fiber reinforced composite and their hybridized forms were fabricated using five different stacking sequences of the fabrics. Using the Vacuum Assisted Resin Transfer Molding (VARTM) procedure, epoxy resin was injected into these fabrics and allowed to cure at room temperature. From these five stacking sequences, a standard specimen with four different orientations viz. 0/90°, 15/75°, 30/60°, 45/-45° orientations were obtained using the Abrasive Water Jet Machining(AWJM) Process. The influence of stacking order and fiber orientation on tensile and flexural properties of composite was investigated. From the result of tensile testing, the highest and lowest tensile strength values were observed for neat carbon fiber reinforced composite at 0/90° orientation and at 45/-45° orientation respectively. The highest flexural strength was achieved in a hybrid combination of two layers of carbon, glass and aramid fabric for 0/90° whereas the lowest flexural strength was found in glass reinforced composite for the 45/-45° orientation.

VARTM process of composites using double-bag air cushion method

This article introduced the double-bag air cushion method (DBACM) as an economical and efficient manufacturing process of polymer composites associated with the vacuum-assisted resin transfer molding (VARTM) process. The method utilizes an improved double-bag along with several air cushions to create distribution channels for enhancing the resin flow during infusion. Experimental evaluation on resin infusion behavior and mechanical properties, including ultimate strength, void content, surface finish, and thickness uniformity, is performed for composites reinforced with both jute fibers and glass fibers. Results showed that the DBACM reduces the infusion time by more than 26.0% as compared with traditional VARTM. The difference in ultimate strength of the part between the DBACM and the VARTM is about 2.9%–3.9%. The void content of the E-glass composites is 1.3%–1.5% due to the few fissure-like microvoids, whereas the void content of the jute composites is high, about 2.4%–4.0%, because of the sparse spherical macrovoids. Additionally, there are no major differences in surface finish and thickness uniformity between the parts fabricated by DBACM and by conventional VARTM.

Mechanical properties of Assam’s bamboo-epoxy composite laminates – An experimental investigation

The dominance of contemporary composite materials has reached heights as well worth weight-saving materials. The current policies for manufacturing cost-effective and sustainable composites lead toward utilizing natural materials in polymeric matrixes. Bamboo is one of the rapidly growing crops which is as stronger as timber and can be utilized in developing semi-replaceable industrial products. Lack of adequate standardization and codes restricts its direct utilization for commercial use. The current study aims to investigate the mechanical properties of composite laminates from high-performance bamboo strips collected from the North-Eastern (NE) region of India. In this study, the alkali-treated bamboo samples were infiltrated with epoxy resin using VARTM (vacuum-assisted resin transfer molding) process. The Fourier transform infrared spectroscopic (FTIR) and Field Emission Scanning Electron Microscopy (FESEM) analysis is carried out to have a better understanding of the chemical composition, and surface morphology. It has been confirmed from the obtained results of FTIR that lignin content is greatly reduced from the delignified samples. In addition, the FESEM images have depicted an outsize layer of epoxy along the fiber cross-section and some pores have been observed as unfilled. On the other hand, these composites are also subjected to tensile, compressive, flexural, and impact tests. The composites achieved a maximum tensile strength and stiffness of 259 MPa and 16.8 MPa which is 253 % and 384 % higher than neat epoxy. However, the strength of the alkali-treated samples is found to be decreased which may be due to the loose fiber bundles and holes formed during the chemical treatment. Improvements in dimensional stability against moisture are achieved for the composites with treated fibers which can be related to better fiber-matrix interlocking. The investigation of the mechanical strength of these highly-cellulosic fiber composites is conducted through various experiments and the results are validated with theoretical equations. It is found that the theoretical values are in good agreement with the experimental data with an error < 15 %. These composites demonstrate large-scale production of lightweight, environmentally friendly, and sustainable future materials with high strength and stiffness. • Laminates with three species of bamboo from the North-Eastern region are developed. • A significant increment of 253 % in strength of composite laminates is observed. • Both longitudinal and transverse material properties are evaluated. • Experimental results differ by < 15 % error from theoretical. • Simultaneously, an increase in bending and toughness is observed.

Low carbon footprint preparation of MXene incorporated lignocellulosic fibers for high thermal conductivity applications

New wood-based composite materials with thermal conductivity are greatly desired in the fields of packaging materials for electronic components. In this study, a new multifunctional composite material (M@FC) is prepared by simply blending clay-like Ti3C2Tx MXene and delignified wood fibers together, and then followed by an infusing epoxy resin with environmentally friendly vacuum assisted resin transfer molding (VARTM) process. The resulting M@FC (0.92 W m−1 K−1) possesses superior thermal conductivity as compared to natural wood (0.099 W m−1 K−1) and most polymers. Furthermore, after the VARTM process, the structure of the M@FC is tighter, and thus showing excellent mechanical properties (tensile strength of 93.0 MPa and flexural strength of 172.7 MPa). In addition, good water resistance and excellent flame retardant property are observed for M@FC. The improvement of thermal conductivity provides the possibility for its application for packaging materials in electronic components. This study using waste wood as the important component provides a new idea for carbon cycling and recycling of natural resources.

Enhancing buckling capacity of angle steel using unbonded CFRP laminates processed by Vacuum-assisted Resin Transfer Molding (VaRTM)

The carbon fiber reinforced polymer (CFRP) adhesive-bonding technique has been increasingly developed for strengthening steel structures. However, the application of this strengthening method is still considered ineffective since it requires complicated and burdensome steel surface treatments. The bonding strength of CFRP on steel, on which this method highly depends, is also prone to performance decreases due to inevitable environmental exposures. Alternatively, unbonded CFRP strengthening is proposed in this study. A total of twelve specimens are prepared and tested to evaluate the possibility of application of this strengthening method in enhancing buckling performance of angle steels. All CFRPs on the strengthened specimens are created through a process of Vacuum-assisted Resin Transfer Molding (VaRTM). It is concluded from the investigation results that the unbonded CFRP is effective in strengthening angle steels, where the buckling strength enhancements by 8.5%-54.3%. The strength increase is clearly affected by CFRP length, number of CFRP layers, and steel’s slenderness ratio. A change in the curvature of the specimens due to the presence of unbonded CFRPs can also be observed. The VaRTM process produces higher fiber content of laminates. Analytical studies carried out at the end of this paper provide a reasonable solution for strength design and a good correspond for post-buckling prediction.

Using a Fiber Bragg Grating Sensor to Measure Residual Strain in the Vacuum-Assisted Resin Transfer Molding Process

Vinyl ester (VE) resin has strong environmental tolerance and is the matrix commonly used in the composite materials of fiber-reinforced plastics (FRP). VE resin is often combined with glass fiber in different maritime structures, such as wind turbine blades, spinner cases, and nacelle cases. However, VE resin exhibits exothermic reactions and shrinkage during curing, which often generates residual strain in large structures and those with a high stacking number. This study explored the exothermic reaction and shrinkage of VE resin and glass fiber during the vacuum-assisted resin transfer molding process, as measured using a fiber Bragg grating sensor. The experiment results verified the relationship between the stacking number and residual strain shrinkage. In addition, the symmetric laminate method was used to prevent the bending–twisting coupling effect and subsequent warping deformation of the FRP laminated plate during curing. The experiment results also verified that the bottom layers of the FRP laminated plates produced using VE resin were closer to the mold, and exhibited more shrinkage as the stacking number increased. In addition, this study discovered that during the experiment, the symmetry layer of the FRP laminated plate had a higher exothermic temperature than the bottom layer as a result of the symmetry layer’s ineffective heat dissipation. Therefore, the curing shrinkage of the symmetry layer resin was measured. The experiment results indicated that if the stacking number was between 10 and 30, the residual strain shrinkage of the symmetry layer was greater than that of the surface layer. However, because of the symmetric laminate, the residual strain of the symmetry layer did not increase when the temperature increased. Therefore, the greatest residual strain occurred at the surface of the bottom layer of the laminated plate with a stacking number of 40.

Lateral Compression Properties of Special-Shaped 3D Tubular Woven Composites

In order to avoid the instability of composite materials laminated tube connection between layer and layer, with green basalt fiber as raw material, through the reasonable design, the special-shaped 3D (three-dimensional) tubular woven fabrics with two different thicknesses and shapes were fabricated on a semi-automatic sample loom with low cost. The special-shaped 3D tubular woven composites were fabricated by VARTM (vacuum assisted resin transfer molding) process. Load-displacement scatter plots and fitting curves and energy-displacement scatter plots and fitting curves were obtained by lateral compression tests. The results showed that the same shape,the load and energy absorption values increased with thickness and compression property was better. For the special-shaped 3D tubular woven composites with the same thickness, the load and energy absorption values of the circular tube was larger than the square tube. The correlation coefficients of load-displacement and energy-displacement polynomial fitting formulas calculated by the least square method in the Origin 8.5 software were all close to 1, indicated that the fitting effect was good. This method provided a direction for the research of special-shaped 3D tubular woven composites.

Analysis on fracture behaviour of stitched foam sandwich composites using interlaminar tension test

Sandwich composites are susceptible to interfacial delamination, owing to the mismatches in the material properties between the face sheets and core. Previous studies have shown that stitching can improve the performance of sandwich composites. In this study, an analysis approach was developed to investigate the fracture behaviour of stitched foam sandwich composites. The stitched foam sandwich composites were manufactured by a vacuum-assisted resin transfer moulding process. Interlaminar tension tests revealed the effects of the linear thread density on the failure mechanisms of the stitched foam sandwich composites. Asymmetric double cantilever beam tests were performed to investigate the influences of the stitch thread reinforcement on the fracture behaviour. An analytical approach combining extended finite element method and nonlinear spring elements was proposed to predict the failure behaviour of the stitched sandwich composites. Experiment and simulation approaches were employed to investigate the influences of the stitch parameters (stitch pitch and linear thread density) on the ultimate load and energy absorption. The results show that stitched method can significantly enhance the mechanical properties of sandwich composites. The energy absorption and ultimate load values of the specimens tend to increase with an increase in the linear thread density or a decrease in the stitch pitch.

NUMERICAL ANALYSIS OF INFUSION STRATEGIES IN VACUUM ASSISTED RESIN TRANSFER MOLDING (VARTM) PROCESS

The Liquid composite Molding (LCM) process, such as Vacuum Assisted Resin Transfer Molding (VARTM), offers a fast and high-quality production of composites laminates. In the VARTM process, the simulation tool is found beneficial to predict and solve composite manufacturing issues. The part quality is dependent on the resin mold filling stage in the VARTM process. The infiltration of resin into a porous fibrous medium is taken place during the resin mold filling stage. The permeability has a crucial role during the resin mold filling stage. In this study, simulation of resin infusion through multiple injection gates is discussed. The various infusion schemes are simulated to identify defect-free composite manufacturing. The simulation approach is applied to five different stacking sequences of reinforcements. In this transient simulation study, permeability and resin viscosity is essential inputs for the resin flow. The simulation approach found that a gating scheme plays a vital role in mold filling time and defect-free composite fabrication. It is found that the line gating system can be useful for fast mold filling over the point gating system.

Vacuum assisted resin transfer moulding process monitoring by means of distributed fibre-optic sensors: a numerical and experimental study

A novel composite manufacturing process monitoring application using fibre-optic (FO) sensors is reported for vacuum-assisted resin transfer moulding (VaRTM) with a rigid-closed mould. A fully distributed Rayleigh-Backscattering based load-monitoring approach is demonstrated by numerical modelling and experimental application. Lateral fabric compression is reliably tracked throughout the entire VaRTM process, giving live insights on local pressure distribution, fabric stack compaction and fibre-volume fraction. These parameters have a great influence on the quality of fibre-reinforced composites and real-time tracking of them will significantly improve the quality of the manufactured part, while reducing the number of scraps and destructive testing. The final fibre volume fraction of a medium-sized plate manufactured using industrial VaRTM equipment was successfully predicted based on embedded FO sensor readings only. With a low bend-loss single mode optical fibre sensor being designed for process survivability and forming an integral part of the composite component, it enables a true entire life-cycle monitoring.

In-situ resin flow monitoring in VaRTM process by using optical frequency domain reflectometry and long-gauge FBG sensors

The vacuum-assisted resin transfer molding (VaRTM) process has attracted increased attention because of its ability to provide fast production times for large structures while maintaining a high quality. However, the occurrence of process-induced defects during the VaRTM process significantly affects the mechanical properties of the composites, resulting in a decrease in the reliability of the process. Therefore, flow monitoring should be performed during the manufacturing process to eliminate process-induced defects, such as dry spots and unimpregnated areas in the preform.Herein, we report a facile method for monitoring resin flow using a distributed strain sensing method during the VaRTM process. For the distributed strain sensing method, optical frequency domain reflectometry (OFDR) and a long-gauge fiber Bragg grating (FBG) sensor were used simultaneously. To analyze the change in the sensor signal according to the resin impregnation state, the strain along the in-plane direction of the preform during the VaRTM process was investigated through a numerical simulation and compared with the strain change measured by the OFDR system. The strain rate from the FBG sensor showed a positive value in the impregnated region but a negative value in the unimpregnated region, which means that the point at which the reversal of the strain rate occurs is the same as the position of the flow front. Thus, not only the flow front over the entire gauge length but also the approximate position and size of the unimpregnated area were accurately monitored.

Dolomite dust filled glass fiber reinforced epoxy composite: Influence of fabrication techniques on physicomechanical and erosion wear properties

AbstractThis article investigates the effect of fabrication techniques on the physical, mechanical, and erosion wear properties of glass fiber reinforced dolomite dust‐filled epoxy composites. The composites were fabricated by hand layup and vacuum‐assisted resin transfer molding (VRTM) techniques with fixed glass fiber (20 wt%) and varying dolomite dust (0 to 15 wt%) with epoxy resin. In physical properties, the experimental density consistently increased with the increase in the dolomite hand layup and the VRTM process; in opposite the voids content increased in the hand layup process and decreased in the VRTM process. In mechanical properties, the hardness, impact energy, and interlaminar shear strength increases while tensile and flexural strength consistently decreases with dolomite dust content. Using Taguchi's optimization technique, the erosion wear of the fabricated composites was also evaluated at different impingement angles, impact velocities, and erodent sizes. Erosion test results indicate that the VRTM process fabricated composite has minimum erosion wear compared to the hand layup process. The eroded surfaces were studied using a scanning electron microscope to understand the erosion behavior of particulate‐filled fiber reinforced polymer composites.