In-situ monitoring of the resin transfer molding impregnation and cure process

Resin transfer molding (RTM) of advanced fiber architecture materials promises to be a cost effective process for obtaining composite parts with exceptional strength. However there are a larger number of material processing parameters that must be observed, known, and/or controlled during the resin transfer molding process. These include the viscosity both during impregnation and cure. In-situ sensors which can observe these processing properties within the RTM tool during the fabrication process are essential. This paper will discuss recent work on the use of frequency dependent electromagnetic sensing (FDMS) techniques to monitor these properties in the RTM tool. Our objective is to use these sensing techniques to address problems of RTM scaleup for large complex parts and to develop a closed loop, intelligent, sensor controlled RTM fabrication process.     

Use of the frequency dependence of the impedance to monitor viscosity during cure

The ability to conveniently and continuously measure the processing properties of polymer resins is important both to the resin supplier and to the fabricator. Frequency dependent electromagnetic sensors (FDEMS) provide an in-situ technique for continuous measurement of the resin's rheological changes both in a laboratory press and in manufacturing tools in an autoclave. In this paper the frequency dependence of ?*(w) is used to quantitatively monitor the viscosity for a tetraglycidyl 4,4′-diaminodiphenyl methane (TGDDM) amine epoxy, to quantitatively monitor the viscosity during processing In a styrene-polyester resin, and to monitor the cure process in an autoclave during cure of a high temperature polyimide-graphite prepreg. In addition, the technique is used to measure the viscosity at various ply positions in a thick TGDDM graphite epoxy laminate during processing in an autoclave.     

Curing kinetics and viscosity change of a two‐part epoxy resin during mold filling in resin‐transfer molding process

The curing kinetics and the resulting viscosity change of a two‐part epoxy/amine resin during the mold‐filling process of resin‐transfer molding (RTM) of composites was investigated. The curing kinetics of the epoxy/amine resin was analyzed in both the dynamic and the isothermal modes with differential scanning calorimetry (DSC). The dynamic viscosity of the resin at the same temperature as in the mold‐filling process was measured. The curing kinetics of the resin was described by a modified Kamal kinetic model, accounting for the autocatalytic and the diffusion‐control effect. An empirical model correlated the resin viscosity with temperature and the degree of cure was obtained. Predictions of the rate of reaction and the resulting viscosity change by the modified Kamal model and by the empirical model agreed well with the experimental data, respectively, over the temperature range 50–80°C and up to the degree of cure α = 0.4, which are suitable for the mold‐filling stage in the RTM process. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2139–2148, 2000     

Control of flow in resin transfer molding with real-time preform permeability estimation

Variabilities in the preform structure in situ in the mold are an acknowledged challenge to achieving reliable preform saturation in liquid molding processes. Physical models offer an effective means of deriving real-time process control decisions so as to steer the resin flow in a desired manner, which ensures complete preform saturation. An important parameter influencing the fidelity of the simulations is the preform permeability, which is a strong function of the preform microstructure. A model-based control strategy that incorporates the ability to determine and utilize local permeability information in real-time is of much value, and forms the focus of the paper. An intelligent model-based controller is developed that uses virtual sensing of permeability to derive optimal decisions on controlling the injection pressures at the mold inlet ports in a resin transfer molding (RTM) process. The controller employs an artificial neural network, trained using process simulation data, as an on-line flow simulator, and a simulated annealing algorithm to optimize the injection pressures on-the-fly during the process. Preform permeability is virtually sensed during the process, based on the flow front velocities and the local pressure gradient along the flow front, estimated using a fuzzy logic model. The controller, implemented on an RTM process, is shown to be able to accurately steer the flow fronts through various preform configurations.     

Sensor system for monitoring impregnation and cure during resin transfer molding

In the resin transfer molding (RTM) fabrication of composites, knowledge of the position of a moving resin front during impregnation is important for process optimization. We describe here a simple, inexpensive, multi-point sensor system based on DC conductometry for determination of resin position in an RTM mold. This Resin Position Sensor (RPS) system consists of a matrix of small sensors embedded in the RTM tool, whose combined output can be used to produce a resin flow pattern at any given time after the start of impregnation. As it cures, the resin resistance increases and the sensor can then function as a cure monitor. A large, 24-sensor RTM tool was fabricated for demonstration of the RPS. Flow contour maps generated from sensor data during impregnation of both E-glass and carbon fiber preforms are shown.     

Correlation analysis of epoxy/amine resin cure,structure and chemorheological behavior in RTM processes

At the reactive mould‐filling stage in resin transfer moulding (RTM) processes, the correlation analysis of epoxy/amine resin cure, structure and chemorheological behavior plays a key role in the optimum control of RTM processes. A new methodology used to simulate the reactive resin flow in RTM processes with edge effect is presented in this article. The recursive approach and the branching theory are used to describe the evolution of molecular structure and resin viscosity, respectively. And then the resin flow process is simulated by means of a semi‐implicit iterative calculation method and the finite volume method. The results reveal the proposed resin cure‐structure‐viscosity model provides excellent agreement with the experimental viscosity data during the RTM filling process. It is also observed that the curing reaction causes the inhomogeneous distribution of resin conversion and resin molecular weight in the mould cavity, which will result in the spatially structural and performance inhomogeneities in the finished products. With the injection temperature or the edge width increasing, the discrepancy of resin conversion and resin molecular weight in the mould cavity is more evident. This study is helpful for understanding the complicated relationship among the processing variables, resin structures, and properties. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Enhancement of resin transfer molding using articulated tooling

Mold articulation is introduced in this concept for resin transfer molding (RTM) to increase mold fill times and potentially allow for the use of high viscosity, hot melt resin systems, or thermoplastics. Following a brief review of conventional RTM and a discussion of the limitations on the factors that control fluid flow through porous media, the articulated concept is described. This is followed by an explanation of the sequence of motion of an articulated segmented mold necessary for consolidation, void removal and accelerated fluid flow through a fibrous preform. An analysis of the process using a fiber preform with orthotropic permeability is outlined from which mold fill time is obtained. This is compared with conventional RTM mold fill times using typical resin properties and fiber volume fractions. For the conservative assumptions used, an improvement by a factor of ten in mold fill time is achieved using the articulated process relative to conventional RTM.     

An RTM densification method of manufacturing carbon-carbon composites using Primaset PT-30 resin

This paper presents a development of carbon-carbon (C-C) composite by resin transfer molding (RTM) process. The RTM was used for both manufacturing of the resin matrix composite part as well as impregnation of the carbonized parts. Materials chosen were heat-treated T300 2-D carbon fabric and Primaset PT-30 cyanate ester. The PT-30 resin has a char yield similar to that of phenolics, very low volatiles, low viscosity at processing temperatures, and no by-products during cure, and hence, an excellent choice for RTM process. The process consists of RTM of the composite part, carbonization, RTM impregnation, and re-carbonization. The last two steps were repeated to achieve the desired density. The measured density and mechanical properties of just two times-densified C-C composite panels were superior to or nearly the same as the data in the literature by other processes. The RTM densification is about twice as fast as the resin solution method and it is environment friendly.     

Resin flow monitoring inside composite laminate during resin film infusion process

In this article, a novel method of measuring resin flow front under vacuum condition is presented. The in situ monitoring system with metal hollow probe based on gas flow balance can be used in resin film infusion (RFI) process, where resin film is used and transverse flow is dominated along thickness direction of fiber preform. The diameter of the probe was chosen to increase the measuring accuracy, and the reliability of the method was evaluated by comparison of visualization experiment. Experimental results demonstrate that the method is suitable for monitoring resin flow in RFI process with and without autoclave, and can obtain the information about resin filling time, nonuniform flow front, and the permeability of fiber preform. Furthermore, by means of the established monitoring system, the influences of pressure and lay‐up sequence of carbon fiber fabric on epoxy resin flow during RFI process were investigated. In addition, resin flow pattern with changing viscosity of epoxy resin was studied. POLYM. COMPOS., 35:681–690, 2014. © 2013 Society of Plastics Engineers     

Resin transfer molding (RTM) process of a high performance epoxy resin. I: Kinetic studies of cure reaction

The cure kinetics of a high performance PR500 epoxy resin in the temperature range of 160–197°C for the resin transfer molding (RTM) process have been investigated. The thermal analysis of the curing kinetics of PR500 resin was carried out by differential scanning calorimetry (DSC), with the ultimate heat of reaction measured in the dynamic mode and the rate of cure reaction and the degree of cure being determined under isothermal conditions. A modified Kamal's kinetic model was adapted to describe the autocatalytic and diffusion‐controlled curing behavior of the resin. A reasonable agreement between the experimental data and the kinetic model has been obtained over the whole processing temperature range, including the mold filling and the final curing stages of the RTM process.     

Effects of sizings on microscopic flow in resin transfer molding

Sizings significantly affect the processing and final performance characteristics of resin transfer molded (RTM) parts. Manufacturers often use sizings to enhance compatability between the resin and reinforcement and to assist processing. In particular, such coatings can affect the microscopic flow characteristics of resins during the infusion stages of the RTM process. To understand the mechanisms by which sizings affect preform wet-out and the extent of such effects, the influence of fiber sizings and fiber loadings on the flow of a vinyl-ester resin system, through axially aligned carbon fibers, have been measured. The results and analysis of the experiments reported herein form the basis for a phenomenological model that describes the effect of sizings on micro-flow in RTM. Furthermore, the work provides insight into the relationships among micro-flow, macro-flow, and preform infiltration. It is seen that sizings dramatically influence micro-impregnation and, for the flow configuration studied, radial micro-flow into fiber bundles was the rate limiting process for complete fiber bundle infusion.     

Flow channels/fiber impregnation studies for the process modeling/analysis of complex engineering structures manufactured by resin transfer molding

The success of resin transfer molding (RTM) depends upon the complete wetting of the fiber preform. Effective mold designs and process modifications facilitating the improved impregnation of the preform have direct impact on the successful manufacturing of parts. Race tracking caused by variations in permeabilities around bends, corners in liquid composite molding (LCM) processes such as RTM have been traditionally considered undesirable, while related processes such as vacuum assisted RTM (VARTM) and injection molding have employed flow channels to improve the resin distribution. In this paper, studies on the effect of flow channels are explored for RTM through process simulation studies involving flow analysis of resin, when channels are involved. The flow in channels has been modeled and characterized based on equivalent permeabilities. The flow in the channels is taken to be Darcian as in the fiber preform, and process modeling and simulation tools for RTM have been employed to study the flow and pressure behavior when channels are involved. Simulation studies based on a flat plate indicated that the pressures in the mold are reduced with channels, and have been compared with experimental results and equivalent permeability models. Experimental comparisons validate the reduction in pressures with channels and validate the use of equivalent permeability models. Numerical simulation studies show the positive effect of the channels to improve flow impregnation and reduce the mold pressures. Studies also include geometrically complex parts to demonstrate the positive advantages of flow channels in RTM.     

Processing effects in production of composite prepreg by hot melt impregnation

The hot melt impregnation process for producing composite prepreg has been studied. The role of the exit die is highlighted by operating without impregnation bars. Experimental results show that when a fiber two is pulled through a resin bath and then through a wedge shaped die, the total resin mass fraction and the extent of resin impregnation in the two increase with the processing viscosity. The penetration of resin into a fiber bundle is greater when the resin viscosity is higher. This trend is unchanged over a range of two speeds up to the breaking point. A theoretical model is developed to describe the effect of processing conditions and die geometry on the degree of impregnation. Calculations with this model indicate that for a given die geometry, the degree of impregnation increases from 58% to 90% as the ratio of the clearance between the two and the die wall, to the total die gap is decreased from 0.15 to 0.05. Physical arguments elated to the effective viscosity of the prepreg show that the clearance ratio is independent of the two speed, but decreases as the ratio of the effective shear viscosity of the prepareg to the resin viscosity increases. This provides a connection between the experimental results obtained with varying resin viscosity and the computational results obtained with varying clearance values at the die inlet.     

Processing of highly elastomeric toughened cyanate esters through a modified resin transfer molding technique

Model cyanate ester resins containing different quantities of epoxy functional butadiene-acrylonitrile rubber (ETBN) to improve the fracture performance were developed as matrices for composites. With the elastomeric modification, the resin systems exhibited rheological characteristics inappropriate for laminate fabrication by conventional resin transfer molding (RTM). To fabricate the carbon fiber based laminates in one liquid molding operation successfully, a process named bleed resin transfer molding (BRTM) was established. The BRTM process combines features of RTM and resin film infusion processes (RFI) and was therefore appropriate for processing high viscosity matrix resins. A novel catalyst was selected for the cyanate ester resin that provided enough latency for the impregnation steps in the BRTM process. Through the use of thermal analytical tools, a high degree of phase separation and conversion was obtained. The conversion and the glass transition temperature were found not to decrease with increasing elastomer content, which is in contradiction to most toughening modifications. Mode I and Mode II interlaminar fracture toughness were found to increase significantly with increasing elastomer content. In Mode I, an increase of up to 140% was observed. Collectively, this work showed that through the use of the BRTM technique, matrices with toughness improvements usually only achieved by prepreg systems can be processed in an RTM-like manner.     

Effects of resin storage aging on rheological property and consolidation of composite laminates

The ability to predict the viscosity of thermoset resin is important to understand the manufacturing process of composites and optimize the processing parameters. During resin or prepreg storage course, the cure reaction may happen and the degree of cure increases gradually. The storage aging effect reduces the fluidity of resin, and hence alters the processability of resin. In this article, the rheological properties of an epoxy resin and a bismaleimide resin used in composite autoclave process were measured and a viscosity model was established, which can predict the viscosity progression during cure for different aging degree of resin. Moreover, a computer simulation method was used to study the effects of aging degree on the composite consolidation and the processing operations. It is found that the viscosity model of aged resin can be obtained by modified dual Arrhenius model of fresh resin with the dynamic rheological measurement. The resin aging strongly alters the flowability, so influences composite consolidation. According to the simulated results, the processing parameters need to be adjusted to achieve cured composites with appropriate fiber content. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

RTM用环氧树脂体系的浸润性及化学流变特性研究

对树脂传递模塑（RTM）用环氧树脂体系对纤维的浸润性能进行了研究,同时在基本的黏度实验基础上,采用双阿仑尼乌斯方程研究了其化学流变特性。结果表明,纤维经丙酮处理后浸润性能提高,RTM环氧树脂浸润纤维变得容易;并通过双阿仑尼乌斯方程建立了RTM环氧树脂体系的化学流变模型,该模型揭示了RTM环氧树脂在35~75 ℃的温度范围内黏度低于800 mPa·s,且低黏度保持时间大于20 min,满足复合材料RTM成型的基本工艺要求。     

Properties of poly(butylene terephthatlate) polymerized from cyclic oligomers and its composites

The high viscosity of thermoplastic matrices hampers fiber impregnation. This problem can be overcome by using low viscous polymeric precursors such as cyclic butylene terephthalate (CBT^® resins), which polymerize to form a thermoplastic matrix. This allows thermoset production techniques, like resin transfer molding (RTM), to be used for the production of textile reinforced thermoplastics. Due to the processing route and more specifically the time-temperature profile, inherent to the RTM process, the crystallites of the matrix consist out of well-defined, thick and well-oriented crystal lamellae. Together with a high overall degree of crystallinity and a low density of tie molecules, these large and perfect crystals cause polymer brittleness. Matrix brittleness lowers the transverse strength of unidirectional composites to below the matrix strength, but leaves the mechanical properties in the fiber direction unaffected. Although not a valid option for the RTM production route, crystallization from a truly random melt and at a sufficiently high cooling rate would substantially improve the ductility.     

Particle distribution from in‐plane resin flow in a resin transfer molding process

In liquid composite molding (LCM) processes such as resin transfer molding (RTM), particle distribution can be problematic as the particle fillers can be filtered by the reinforcement fibers during the resin infusion process. In this paper, the filtration of alumina and silica nanoparticles in the production of aramid fiber epoxy composites is characterized. The laminates are produced by in‐plane RTM and the effects of selected process variables on the laminate particle distribution are investigated. The objective is to evaluate the assumption that nanoparticles due to their small physical size inherently do not filter in resin infusion processes. The nanosilica particles are found to effectively not filter, while the nanoalumina particles are much more sensitive to filtration as they formed micro‐scale agglomerates as small as a few microns in size prior to injection. The filtration behavior follows a simple theoretical model for micro‐scale particle filtration, already existing in the literature. For the filtration sensitive particles, it was found that the filtration is influenced by the preform fiber volume content. Other common process variables such as resin viscosity, particle concentration in the injected resin, and saturated resin flow time (resin overflow volume) are found to be filtration independent and do not change the filtration behavior. POLYM. ENG. SCI., 59:22–34, 2019. © 2018 Society of Plastics Engineers     

A simplified chemorheological model of viscosity evolution for solvent containing resol resin in RTM process

The solvent content‐dependent chemorheology of the solvent containing resol resin for resin transfer molding (RTM) was investigated. The curing behavior of the resol resin was studied by in situ Fourier transform infrared spectroscopy together with rheology tests. The chemorheological behavior of resol resins with a series of solvent contents was measured under isothermal conditions. The four parameters of empirical dual‐Arrhenius equation regarding isothermal resin viscosity and reaction rate constant were found to be functions of the solvent content. A simplified chemorheological model involving only three parameters of curing temperature, time, and solvent content was first established to facilely describe the viscosity during precuring process. The simulated viscosity results during isothermal curing process agreed well with the experimental data which shows the simplified chemorheological model can be utilized to describe the viscosity evolution and offer guidance for optimizing the injection process and improving the design flexibility of RTM process. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45282.