Vacuum Assisted Resin Transfer Molding Process Research Articles

Occupational Exposures to Styrene Vapor in a Manufacturing Plant for Fiber-Reinforced Composite Wind Turbine Blades

A utility-scale wind turbine blade manufacturing plant requested assistance from the National Institute for Occupational Safety and Health (NIOSH) in controlling worker exposures to styrene at a plant that produced 37 and 42 m long fiber-reinforced wind turbine blades. The plant requested NIOSH assistance because previous air sampling conducted by the company indicated concerns about peak styrene concentrations when workers entered the confined space inside of the wind turbine blade. NIOSH researchers conducted two site visits and collected personal breathing zone and area air samples while workers performed the wind turbine blade manufacturing tasks of vacuum-assisted resin transfer molding (VARTM), gelcoating, glue wiping, and installing the safety platform. All samples were collected during the course of normal employee work activities and analyzed for styrene using NIOSH Method 1501. All sampling was task based since full-shift sampling from a prior Occupational Safety and Health Administration (OSHA) compliance inspection did not show any exposures to styrene above the OSHA permissible exposure limit. During the initial NIOSH site visit, 67 personal breathing zone and 18 area air samples were collected while workers performed tasks of VARTM, gelcoating, glue wipe, and installation of a safety platform. After the initial site visit, the company made changes to the glue wipe task that eliminated the need for workers to enter the confined space inside of the wind turbine blade. During the follow-up site visit, 12 personal breathing zone and 8 area air samples were collected from workers performing the modified glue wipe task. During the initial site visit, the geometric means of the personal breathing zone styrene air samples were 1.8 p.p.m. (n = 21) for workers performing the VARTM task, 68 p.p.m. (n = 5) for workers installing a safety platform, and 340 p.p.m. (n = 14) for workers performing the glue wipe task, where n is the number of workers sampled for a given mean result. Gelcoating workers included job categories of millers, gelcoat machine operators, and gelcoaters. Geometric mean personal breathing zone styrene air samples were 150 p.p.m. (n = 6) for millers, 87 p.p.m. (n = 2) for the gelcoat machine operators, and 66 p.p.m. (n = 19) for gelcoaters. The geometric mean of the personal breathing zone styrene air samples from the glue wipe task measured during the follow-up site visit was 31 p.p.m. (n = 12). The closed molding VARTM process was very effective at controlling worker exposures to styrene. Personal breathing zone styrene air samples were reduced by an order of magnitude after changes were made to the glue wipe task. The company used chemical substitution to eliminate styrene exposure during the installation of the safety platform. Recommendations were provided to reduce styrene concentrations during gelcoating.

Full-field monitoring of resin flow using an area-sensor array in a VaRTM process

Conventional resin flow monitoring relies on point measurements even if using grid sensing, and it is difficult to estimate an accurate flow front or defect figuration. The current work investigates full-field monitoring of resin flow during a vacuum-assisted resin transfer molding (VaRTM) process using an area-sensor array. The squared area sensors are aligned as a matrix on a thin polyimide film without any un-sensing space; thus the film measures the full-field flow monitoring, and does not miss a dry spot that may occur anywhere on the film. To identify the precise flow front and dry spots, the impregnated area is estimated by minimizing the residual sum of squares between the measured and estimated impregnated area ratios. To demonstrate the applicability of the proposed method, the area-sensor array is applied to monitoring a VaRTM resin flow on glass fiber reinforced plastic (GFRP) and foam-cored structures. As a result, the precise figurations of the flow front and the dry spot were successfully estimated in real time.

A Prediction Method for In-Plane Permeability and Manufacturing Applications in the VARTM Process

VARTM (Vacuum Assisted Resin Transfer Molding) is a popular method for manufacturing large-scaled, single-sided mold composite structures, such as wind turbine blades and yachts. Simulation to find the proper infusion scenario before manufacturing is essential to avoid dry spots as well as incomplete saturation and various fiber weaves with different permeability affect numerical simulation tremendously. This study focused on deriving the in-plane permeability prediction method for FRP (Fiber Reinforced Plastics) laminates in the VARTM process by experimental measurements and numerical analysis. The method provided an efficient way to determine the permeability of laminates without conducting lots of experiments in the future. In-plane permeability imported into the software, RTM-Worx, to simulate resin flowing pattern before the infusion experiments of a 3D ship hull with two different infusion scenarios. The close agreement between experiments and simulations proved the correctness and applicability of the prediction method for the in-plane permeability.

Fatigue damage development in new fibre metal laminates made by the VARTM process

This paper investigates the tensile and fatigue properties of a newly developed fibre metal laminate (FML) manufactured using the vacuum assisted resin transfer moulding (VARTM) method. This manufacturing method allows the glass fibre reinforced epoxy and 2024-T3 aluminium FML to be prepared at lower cost than conventionally manufactured FMLs. However, in order for the resin to infiltrate the FML, the metal sheets need to be perforated. These perforation holes act as crack initiators and reduce the FML's performance. Tension and fatigue test results of three different designs are reported and compared to mechanical property predictions. Additionally, single sheet Al alloy specimens were tested in order to analyse the influence of the drilling method.

Glass Fiber Permeability Using the VARTM Process

This study investigated a flow rate control system for the vacuum-assisted resin transfer molding (VARTM) process. Using Darcy’s equation, the permeability of multiaxial glass fiber composites is predicted and experimentally confirmed. The resin velocity vector is inversely proportional to the fiber mat length and resin viscosity, but proportional to the fiber mat permeability. In this study, the permeability of the preform and viscosity of the epoxy resin were measured using multiaxial glass fiber by VARTM. The permeability and time for impregnation differed according to the fiber direction in the VARTM process. The results indicated the need for further study of reinforcement fiber permeability in VARTM processing design.

519 VaRTM法による粒状充填材-植物由来樹脂基複合材料の成形・評価(高分子/高分子基複合材料(1),ものづくりにおける基礎研究と先端技術の融合)

This study dealt with molding of natural composites using granular materials and naturally derived resin using vacuum-assisted resin transfer molding (VaRTM). The VaRTM process is a variation of vacuum-infusion RTM in which one of the solid tool faces is replaced by a flexible polymeric film. The VARTM process is a very clean and economical manufacturing method and suitable for the production of the large-size structure. Bamboo tips were used as reinforcements. Bamboo culm only and mixture of culm and leaf were crushed finely or roughly, which result in the four types of reinforcements. Bending, vibration and heat transfer characteristics were measured and the possibility of the application to wall material was examined.

Process Development Issues of Glass—Carbon Hybrid-reinforced Polymer Composite Wind Turbine Blades

Large structures made with composites (e.g., wind turbine rotor blades, yachts, and airplane fuselages) employ fabrics with small tow sizes especially when fabricated by vacuum-assisted resin transfer molding (VARTM). Simulation tools are frequently used to aid the manufacturing process development for large structures. This article details the development of a low cost VARTM manufacturing process for a 3.2 m long aeroelastically tailored rotor blade that has been completed without an infusion modeling tool. This robust VARTM-based process has been developed for very large tow sized fabrics with very poor permeability. Changes in the manufacturing process are made based on a parametric matrix until the desired part quality is achieved. Principal in-plane permeability is measured using 1D channel flow method for various fibers used in this study. Details of the methods employed to assemble the final blade are presented. The final process results in the fabrication of six rotor blades each with different lay up and fiber constituents. Problems and solutions related to migrating from a small-scale VARTM process to larger structures are detailed. Final blade quality is determined by full-scale mechanical testing under static and fatigue loading conditions. All the manufactured blades fail at least 160% above the proof load indicating compliance with the design quality criteria.

Room temperature‐curable VARTM epoxy resins: Promising alternative to vinyl ester resins

AbstractThe objective of this investigation is to characterize various room temperature (RT)‐curable epoxies for vacuum‐assisted resin transfer molding (VARTM) of large structure manufacturing. Six epoxy candidate resins: X‐40, 780‐33, 780‐35, 8601/8602, 8602, 8603, and two vinyl ester resins (VE), 411‐350 and 411‐510A, are physically and thermochemically characterized. All the resins are cured at RT with extended period of time. The degree of cure for 24‐h RT‐cured samples ranges from 70 to 85% for epoxies and is comparable with the baseline VE systems (75%). After 1 year at RT, the degree of cure increases from 90 to 98%. Most of the epoxies show a single transition in dynamic mechanical analysis and differential scanning calorimetry. However, two heterogeneous transitions are observed for the VE systems. The glass transition temperature increases monotonically with exposure time, except X‐40, that rapidly achieves a plateau and remains constant. The degree of cure for the majority of the systems increases logarithmically with RT curing time with excellent fitting (R2 varies from 0.92 to 1). Consistent with the increase in degree of cure, the storage modulus increases and (tan δ)max decreases with time of exposure. A negative correlation between the curing temperature range and the total heat of reaction is observed among the epoxy systems. However, the VE systems show the reverse trend. RT curing epoxy resin (X‐40) shows promising overall result to VE system and can be a viable alternative to VE for VARTM processing. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Modeling VARTM Processes with Hybrid Media Incorporating Gravity Effects

Vacuum-assisted resin transfer molding (VARTM) processes are increasingly used in manufacturing scale-up composite applications. Accordingly, gravity can significantly influence the flow behavior in tall-structure composite manufacturing processes. The present study developed a closed-form analytic solution incorporating gravity effects with the equivalent parameter approach in order to predict the resin flow behavior in a tall structure resin infusion. A hybrid model was used that consists of thin distribution media and a fibrous preform. An analytic solution was developed and validated with experiments as well as numerical methods in terms of the resin flow front shape, lag length, and the flow front location with time for the horizontal, upward, and downward infusion cases. The flow front locations were monitored using time domain reflectometry sensors embedded in the fabric layers. The lag length was constant in the horizontal infusion case, but decreased and increased in the upward and downward infusion cases, respectively, as resin progressed. The downward infusion case showed the fastest fill time, which corresponds to the previous results using a homogenous model. However, the flow became unstable at a certain location where the local flow front speeds increased suddenly along the distribution media. These phenomena were not observed in a model with homogenous media but were observed in a model with hybrid media, only in the downward infusion cases. The analytic solution also identified the stable flow conditions with the mold angles. The results obtained in the present study can be used to estimate stable process conditions in designing VARTM processes for manufacturing large-scale tall composite structures.

Effect of Fiber Sizing — Test Fluid Interaction on the Unsaturated and Saturated Flow in the VARTM Process

The focus of this experimental investigation is the sizing-test fluid interaction reported to result in variable flow behavior in and permeability values for porous media. The flow behavior is studied for Vacuum Assisted Resin Transfer Molding (VARTM). Three types of sizing and three test fluids were chosen for the experiments. Two carbon stitch and two glass stitch bonded fabrics of different tow sizes were used for flow characterization. The present study uses two fluids with the same viscosity for each fabric to determine fluid-sizing interaction. The results show that the permeability values are the same with the fluids of same viscosity. The saturated flow behavior due to the heterogeneous nature of the media is different from the one observed in RTM processes. For VARTM processes the dual scale flow characteristics is not observed for a variety of tow sizes.

Effect of Fiber Sizing - Test Fluid Interaction on the Unsaturated and Saturated Flow in the Vartm Process

The focus of this experimental investigation is the sizing-test fluid interaction reported to result in variable flow behavior in and permeability values for porous media. The flow behavior is studied for Vacuum Assisted Resin Transfer Molding (VARTM). Three types of sizing and three test fluids were chosen for the experiments. Two carbon stitch and two glass stitch bonded fabrics of different tow sizes were used for flow characterization. The present study uses two fluids with the same viscosity for each fabric to determine fluid-sizing interaction. The results show that the permeability values are the same with the fluids of same viscosity. The saturated flow behavior due to the heterogeneous nature of the media is different from the one observed in RTM processes. For VARTM processes the dual scale flow characteristics is not observed for a variety of tow sizes.

<p>Bio-based Nanocomposites: An Alternative to Traditional Composites</p>

Polymer matrix composites (PMC), often referred to as fiber reinforced plastics (FRP), consist of fiber reinforcement (E-glass, S2-glass, aramid, carbon, or natural fibers) and polymer matrix/resin (polyester, vinyl ester, polyurethane, phenolic, and epoxies). Eglass/polyester and E-glass/vinyl ester composites are extensively used in the marine, sports, transportation, military, and construction industries. These industries primarily use low-cost open molding processes, such as manual/spray lay-up. Polyester and vinyl ester resin systems produce styrene emissions. Because of the stringent EPA regulations on styrene emissions, composite manufacturers are interested in using low-cost closed molding processes, such as vacuum-assisted resin transfer molding (VARTM) and styrene-free resin systems such as non-foam and full-density polyurethanes (PUR). Polyurethanes are polymers created by addition of polyisocyanates and polyols. The polyol component in polyurerhane can be produced from soybean oil. This study demonstrates that with the proper addition of nanoparticles, mechanical properties of soy-based polyurethane can be enhanced. These nanomodified soy-based polyurethane/glass composites manufactured by using the low-cost VARTM process provide alternatives to traditional glass/polyester and glass/vinyl ester composites. These composites will be more environmental friendly for two reasons: (a) Polyurethane does not produce styrene emission, thereby, resulting in a safer work place and (b) Polyol is made from a renewable resource (soybean oil).

Strain monitoring in glass fiber reinforced composites embedded with carbon nanopaper sheet using Fiber Bragg Grating (FBG) sensors

A novel approach has been developed to fabricate multifunctional nanocomposites using carbon nanopaper sheets. In this study, vapor grown carbon nanofibers are pre-formed as carbon nanopaper sheet. The carbon nanopaper sheet had a porous structure with highly entangled carbon nanofibers. The as-prepared carbon nanopaper sheet was integrated into the laminates through vacuum-assisted resin transfer molding (VARTM) process. To study the integrity of the hybrid nanocomposites, Fiber Bragg Grating (FBG) sensors were embedded into laminated composites. Two types of composite specimens were fabricated to study the characteristics of strain distribution between different layers. The inter-layer strains of the laminates with carbon nanopaper sheet under static bending loads were measured, indicating that carbon nanopaper sheet was fully integrated to the laminates. A tensile test of the laminates with carbon nanopaper sheet as surface layer was conducted, indicating a good bonding between carbon nanopaper sheet and baseline laminates. In addition, scanning electron microscopy (SEM) was used to investigate the impregnation of carbon nanopaper by the resin during the VARTM process. The SEM images show that a complete penetration of the resin through carbon nanopaper was achieved. Therefore, there was a good structural integrity of carbon nanopaper sheet with the laminates.

Numerical investigation on flow through porous media in the post‐infusion process

AbstractDuring the vacuum‐assisted resin transfer molding (VARTM) processing, the post‐infusion behavior after complete wet‐out and before gelation of the resin is critical for the development of the thickness and fiber volume fraction distribution in the cured composite part. The pressure gradient developed during infusion results in a thickness gradient due to the flexible nature of the bagging approach. After full infusion, the resin typically bleeds into a vacuum trap, allowing redistribution of pressure and preform thickness. In this study, a non‐rigid control volume is used to formulate a set of governing equations for analysis of the post‐infusion process. The model is used to investigate the effects of processing parameters and different processing scenarios on resin flow, resin pressure, and thickness variation of the composite laminate. This work provides a tool for optimization of the VARTM process to reduce final part variability. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers

Experimental Investigation of the Controlled Atmospheric Pressure Resin Infusion (CAPRI) Process

Controlled atmospheric pressure resin infusion (CAPRI) is a variation of the vacuum-assisted resin transfer molding (VARTM) process. The CAPRI process increases the fiber volume fraction of the preform prior to infusion via debulking and applies a reduced pressure gradient during infusion to minimize thickness gradients during processing. This study experimentally investigates the effect of debulking and reduced pressure gradient on the incoming material parameters, process behavior and final dimensional tolerances. The effect of debulking on fabric permeability and compaction behavior has been investigated and shows a significant impact on the infusion time and final fiber volume fraction. Several E-glass plain weave preforms have been infused and flow, pressure and thickness data has been recorded and compared to traditional VARTM processing. A previously developed model uses the experimentally obtained permeability data and good agreement of the flow behavior is observed, the CAPRI process decreases thickness gradients to less than 1% while increasing fiber volume fraction by 5% in the composite part.

Influence of alcohol pre‐infusion on the quality of VARTM composites

AbstractIn the vacuum assisted resin transfer molding (VARTM) process, part‐to‐part variations such as the uncertainty in the permeability and race tracking phenomenon make it difficult to achieve consistent mold filling and ensure part quality of composites. Alcohol pre‐infusion was presented in this study as a novel real‐time monitoring and control approach for the flow process in the VARTM process, alcohol test fluid is infused before the actual resin infusion to locate the potential dry spots without using the large quantity of sensors. Then corresponding process control strategy is designed, such as opening the auxiliary gate at specific moment on those predicted dry spot locations to compensate flow defects. Moreover, alcohol can be easily removed by heat without changing the local permeability. The influence of alcohol pre‐infusion on the quality of VARTM composites were investigated in this study. The mechanical tests were conducted to verify that the alcohol pre‐infusion approach has no significant effect on composite properties because alcohol can be removed from fiber by heat and air flow. Specifically, DMA, TGA, and FTIR spectrum proved that negligible difference existed on the resin–fiber interface between the composites with or without alcohol pre‐infusion. Finally, the microscopy results revealed a similar failure path in a resin matrix. TMA results also demonstrated similar dimension stability. This alcohol pre‐infusion approach was effective when compared with computer simulation and could eliminate the occurrence of dry spots and voids without using sensors or data‐acquisition system. The control schemes were shown in a case study to be capable of compensating the flow defects and achieving desired fill patterns in the face of permeability uncertainty. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers

A modified rule of mixture for the vacuum-assisted resin transfer moulding process simulation

The vacuum-assisted resin transfer moulding process (VARTM) offers many advantages over the traditional resin transfer moulding such as lower tooling cost, room temperature processing. Computer simulation has become a powerful tool for liquid composite moulding process design and optimisation. In the VARTM process simulation, due to the presence of HPM and flow channels, the conventional simulation will require 3-D geometric model and mesh generation. The extensive computation load and complexity of pre-processing limit the usage of simulation in the process design and optimisation. In this paper, a two-step method for improving the efficiency of VARTM process simulation is presented. First, a design of experiment approach was coupled with 2-D CVFEM (control volume finite element method) simulation to calculate the equivalent permeability and porosity for various process variable combinations. Based on the data from DOE, a modified rule of mixture for calculating the equivalent permeability and porosity was developed, with which the mould filling can be simulated in 2-D. Second, the algorithm for the through-thickness flow front construction was developed. The presented method was validated by comparing the result against that from the 3-D simulation. It shows that the accuracy is within 5% while the computation time saving is over 99%.

Homogenous Modeling of VARTM Processes with Hybrid Layered Media

Numerical methods have been widely used to simulate vacuum assisted resin transfer molding (VARTM) processes to obtain optimum process parameters. Many VARTM parts have very large in-plane dimensions compared to the thickness. However, the incorporation of a highly permeable distribution media on the surface of a fibrous preform results in significant through thickness flow particularly at the flow front where large gradients can exist. In general, flow simulations that can model the distribution media as a discrete layer are needed, which, however, significantly increases the number of elements and thus computational costs in the numerical analyses. The present study introduces a homogenous preform system using equivalent porosity and permeability over the out-of-plane direction that retains the through-thickness flow characteristics in order to reduce complexity in modeling the distribution media in VARTM processes. First, a mass-average approach was developed to reduce the number of dimensions that needs to be modeled (e.g., 3-D is reduced to 2-D in-plane and 2-D cross section is reduced to a 1-D flow problem). Parametric studies were carried out over a reasonable range of the input parameters such as preform thickness, length, porosity, and permeability and showed limitations in using the mass average approach with homogenous models. Second, a reconstruction method was developed to predict accurately the flow pattern in the out-of-plane direction from the homogenous models. The flow pattern was successfully reconstructed using the flow data obtained from the homogenous model with an analytic solution of the flow front shape in the out-of-plane direction. Average flow front locations and the flow front patterns were investigated by numerical analyses for a few flow scenarios, which validated the present methods and quantified error levels in the resin flow pattern and resin fill time. The mass-average and reconstruction approaches successfully estimated the resin flow locations and patterns with respect to time and significantly reduced complexity in modeling complex geometries as well as computational costs.

FTIR and SEM analysis of polyester‐ and epoxy‐based composites manufactured by VARTM process

AbstractPolyester‐ and epoxy‐based composites containing glass and carbon fibers were manufactured using a vacuum‐assisted resin transfer molding (VARTM) process. Fourier transform infrared (FTIR) spectroscopy analyses were conducted to determine the interaction between fibers and matrix material. The results indicate that strong interaction was observed between carbon fiber and epoxy resin. However, weak interactions between remaining fiber‐matrix occur. Scanning electron microscopy (SEM) analysis was also performed to take some information about strength of interaction between fibers and matrix material. From SEM micrographs, it is concluded that the findings in SEM analysis support to that obtained in FTIR analysis. Another aim of the present work was to investigate the influence of matrix on composite properties. Hence, the strengths of composites having same reinforcement but different matrix systems in axial tension and transverse tension were compared. Short beam shear test has been conducted to characterize the interfacial strength in the composites. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Bond Strength of Carbon Fiber Sheet on Concrete Substrate Processed by Vacuum Assisted Resin Transfer Molding

High quality and expedient processing repair methods are necessary to enhance the service life of bridge structures. Deterioration of concrete can occur as a result of structural cracks, corrosion of reinforcement, and freeze–thaw cycles. Cost effective methods with potential for field implementation are necessary to address the issue of the vulnerability of bridge structures and how to repair them. Most infrastructure related applications of fiber-reinforced plastics (FRPs) use traditional hand lay-up technology. The hand lay-up is tedious, labor-intensive and relies upon personnel skill level. An alternative to traditional hand lay-up of FRP for infrastructure applications is Vacuum Assisted Resin Transfer Molding (VARTM). VARTM uses single sided molding technology to infuse resin over fabrics wrapping large structures, such as bridge girders and columns. There is no work currently available in understanding the interface developed, when VARTM processing is adopted to wrap fibers such as carbon and/or glass over concrete structures. This paper investigates the interface formed by carbon fiber processed on to a concrete surface using the VARTM technique. Various surface treatments, including sandblasting, were performed to study the pull-off tensile test to find a potential prepared surface. A single-lap shear test was used to study the bond strength of CFRP fabric/epoxy composite adhered to concrete. Carbon fiber wraps incorporating Sikadur HEX 103C and low viscosity epoxy resin Sikadur 300 were considered in VARTM processing of concrete specimens.