In situ sensor monitoring and intelligent control of the resin transfer molding process

An intelligent closed-loop expert control system has been developed for automated control of the resin transfer molding process of a graphite fiber preform using an epoxy resin, E905L. The sensor model system has been developed to make intelligent decisions based on the achievement of landmarks in the cure process, such as full preform impregnation, the viscosity, and the degree of cure of the resin rather than time or temperature. In-situ frequency dependent electromagnetic sensor (FDEMS) and the Loos resin transfer model are used to monitor and control the processing properties of the epoxy resin during RTM impregnation and cure of an advanced fiber architecture stitched preform. Once correlated with viscosity (η) and degree of cure (α), the FDEMS sensor monitors and the RTM processing model predicts the reaction advancement of the resin, viscosity and the impregnation of the fabric. This provides a direct means for monitoring, evaluating, and controlling intelligently the progress of the RTM process in situ in the mold throughout the fabrication process and for verification of the quality of the composites.     

Effect of Resin System Parameters on Resin Transfer Molding of Vinyl Ester Based Composites—A Statistically Designed Study

Although the area of composites has advanced significantly over the past three decades, there is still a lack of understanding as to the coupling between materials and processing variables, especially as related to the use of resin systems in emerging processes such as resin transfer molding (RTM). As materials are tailored through the use of additives to resin systems, intricate fiber architectures, and the use of specific processing parameters, the need for a thorough understanding of the effect of minor variations in the materials system and the ultimate need for process control techniques increases. The current investigation is aimed at developing an understanding of variation in performance in three similar vinyl ester resin based composite systems as a function of mold release agent concentration, presence of a wetting/dispersion agent, and preform tool temperature. It is seen that the concentration of mold release agent has a significant influence on performance, which can be correlated with results of dynamic mechanical analysis tests. The importance of using statistically based studies to determine optimum settings and overall variation in performance is emphasized. This approach is favored over the use of an n[sgrave] approach, which gives the user a means of controlling quality based on postproduction control rather than through the selection of settings that are insensitive to variation in the quality of incoming material process changes. The achievement of quality through control of material and process variables is important not only for the cost-efficient production of composite parts but also for the fabrication of large parts such as would be needed for civil infrastructure applications (bridge decks, piers, etc.), where postproduction rejection would result in significant losses.     

Three-dimensional nonisothermal mold filling simulations in resin transfer molding

The manufacture of polymer composites through the process of resin transfer molding (RTM) involves the impregnation of the reactive polymer resing into a mold with preplaced fibrous reinforcements. Determination of RTM processing conditions requires the understanding of various parameters, such as material properties, mold geometry, and mold filling conditions. Modeling of the entire RTM process provides a tool for analyzing the relationship of the important parameters. This study developed a nonisothermal 3-D computer simulation model for the mold filling process of RTM based on the control volume finite element method. The model will be able to simulate mold filing in molds with complicated 3-D geometry. Results of some numerical studies in RTM show the applications of the proposed model.     

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.     

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.     

Multipurpose carbon fiber sensor design for analysis and monitoring of the resin transfer molding of polymer composites

The resin transfer molding (RTM) process is taking an ever‐growing place among the manufacturing technologies of polymer composite parts because of its numerous advantages. However, consistent production of high‐quality parts is difficult to achieve and requires better understanding of the process and good control of the raw materials. Part‐to‐part variations are inevitable as a result of uncarefully controlled molding environments and unidentifiable or undetected disturbances that cannot be completely eliminated or accounted for. Despite of many efforts to understand and model the fundamental physical and chemical behavior of materials during processing, there is no reliable system able of predicting the optimum processing parameters to manufacture high‐quality parts in a productive way. In this context, this work aims to develop systems allowing the monitoring of the whole RTM process (from preforming to resin curing) and that are reliable, cheap, and easy to use on production line. We have chosen to investigate the potential of the electric and dielectric carbon fiber sensors, which have already proved to be suitable for in service damage monitoring and preventive maintenance without any integration issues. However, the development of the continuous electric sensor has been limited by the polarization of the resin under the direct current. The flow front and the cure monitoring of the resin has been achieved with the dielectric sensor, energized by an alternative current preventing the polarization. Additionally, the ability of this carbon fiber sensor to evaluate the thickness of dry reinforcements and to measure online the actual unsaturated permeability of reinforcements has been demonstrated. POLYM. COMPOS., 26:717–730, 2005. © 2005 Society of Plastics Engineers     

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.     

Statistical characterization of fiber permeability for composite manufacturing

Resin transfer molding (RTM) is considered one of the most promising composite fabrication techniques because of its relatively low equipment and tooling costs, short cycle times, and excellent design flexibility. Yet several technical issues impede wider application for this process. One of those issues is the measurement and characterization of fiber preform permeability, which plays a key role in process design and control. In this study, an automatic permeability measurement apparatus with automatic data acquisition and processing capabilities has been developed to improve measurement accuracy and reliability. Four major factors and their effects on permeability for a knitted and a woven fabric performs are investigated. Design of experiments, ANOVA analysis, and distribution characterization techniques are used to analyze the permeability‐influencing factors and statistical properties of fiber preform permeability. It is found that the fiber preform permeability values vary significantly for different mold shapes and fiber handling conditions. It is also revealed that permeability values for both fabrics follow normal distributions, even though their means and variances are different. These results can be used to better understand the behavior of the RTM process and to improve process design and control.     

Optimization method to select temperature based on chemorheological and exothermal reaction of RTM

Heating mold and resin have been widely used in resin transfer molding (RTM) to improve injection and manufacturing efficiency. The unreasonable mold/resin temperatures sometimes lead to excessive viscosity of resin and premature curing, which will result in failure of the filling process. Selection of optimal mold and resin temperature has become a source of concern in the polymer industry. This article presents an optimization method to select mold and injection resin temperatures by using numerical simulation based on chemorheological and exothermal reaction of the RTM process. The results show that the optimization method has high computational efficiency for three-dimensional parts with different shapes. The selected mold/resin temperature ensures the smooth filling process, which provides a powerful tool for parameter design in polymer industry. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48245.     

Primary/Secondary Recycling of Preform Materials: Use in Rtm

The increasing societal awareness of, and preoccupation with recycling of materials poses a technological problem for composites, in terms of both recycling of products and reuse of waste constituent materials. Although economical, the resin transfer molding (RTM) process generates substantial amounts of waste fabric that is traditionally discarded. In this paper the reuse of scrap fabric through the fabrication of preform material and its subsequent processing through RTM is discussed. Performance levels achieved through the use of glass and hybrid carbon-glass secondary level recyclate preforms are reported and are shown to have considerable potential for further development. Dynamic mechanical analysis is also used to assess the viscoelastic behavior of the specimens.     

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.     

In situ polymerization and nano‐templating phenomenon in nylon fiber/PMMA composite laminates

Supercritical Carbon Dioxide (SC CO₂) is used as a reaction/processing medium in the fabrication of fiber‐reinforced composite materials. SC CO₂ allows resin (reactive monomer), to penetrate inside the fibers themselves, partitioning into the amorphous regions of the fiber. The crystal structure then templates polymerization of matrix within the fiber. This process produces a composite that exhibits ultralong‐range order from the nanoscale reinforcement of crystals to the macroscale fiber reinforcement of matrix. In addition, SC CO₂ lowers resin viscosity and aids in wetting out Nylon 6,6 fiber reinforcement in a process similar to reaction injection molding (RIM) or resin transfer molding (RTM). This article will discuss the fabrication technique in detail, including process parameters and the structure of resulting composites and morphology of modified fibers. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1600–1607, 2003     

In situ polymerization of thermoplastic composites based on cyclic oligomers

The high melt viscosity of thermoplastics is the main issue when producing continuously reinforced thermoplastic composites. For this reason, production methods for thermoplastic and thermoset composites differ substantially. Lowering the viscosity of thermoplastics to a value below 1 Pa.s enables the use of thermoset production methods such as resin transfer molding (RTM). In order to achieve these low viscosities, a low viscous mixture of prepolymers and catalyst can be infused into a mold where the polymerization reaction takes place. Only a limited number of polymerization reactions are compatible with a closed mold process. These polymerization reactions proceed rapidly compared to the curing reaction of thermosets used in RTM. Therefore, the processing window is narrow, and managing the processing parameters is crucial. This paper describes the production and properties of a glass fiber reinforced polyester produced from cyclic oligoesters. POLYM. COMPOS., 26:60–65, 2005. © 2004 Society of Plastics Engineers     

Flow front advancement during composite processing: predictions from numerical filling simulation tools in comparison with real‐world experiments

Liquid composite molding (LCM) techniques are innovative manufacturing processes for processing fiber reinforced polymer parts used e.g. for aerospace structures. Thereby the reinforcing material is placed in a mold and infiltrated with a low viscosity polymer matrix. Increasing production rates as well as part complexity lead to high production risks such as air inclusions or incomplete mold filling. Numerical mold filling simulations are promising tools enabling the composite manufacturing engineer to detect dry spots in the mold and find the optimal positions of the resin entry and ventilation system at an early process development stage. Today, different numerical models and software packages are available for modeling the flow through the reinforcing structure for visualization of the flow behavior. The goal of this study is the systematic comparison of two different software packages, namely PAM‐RTM^® and OpenFOAM. Both software tools are operated as they are commonly foreseen. Real world experiments under real process conditions are the basis for the assessment of the numerical predictions. The resin transfer molding (RTM) experiments are executed in a tool with a transparent upper mold half in order to see the flow front advancement. POLYM. COMPOS., 37:2782–2793, 2016. © 2015 Society of Plastics Engineers     

Modeling nonisothermal impregnation of fibrous media with reactive polymer resin

The manufacture of polymer composites through resin transfer molding (RTM) or structural reaction injection molding (SRIM) involves the impregnation of a fibrous reinforcement in a mold cavity with a reactive polymer resin. The design of RTM and SRIM operations requires an understanding of the various parameters, such as materials properties, mold geometry, and mold filling conditions, that affect the resin impregnation process. Modeling provides a potential tool for analyzing the relationships among the important parameters. The present work provides the physical model and finite element formulations for simulating the mold filling stage. Resin flow through the fibers is modeled using two-dimensional Darcian flow. Simultaneous resin reaction and heat transfer among resin, mold walls, and fibers are considered in the model. The proposed technique emphasizes the use of the least squares finite element method to solve the convection dominated mass and energy equations for the resin. Excellent numerical stability of the proposed technique provides a powerful numerical method for the modeling of polymer processing systems characterized by convection dominated transport equations. Results from example numerical studies for SRIM of polyurethane/glass fiber composites were presented to illustrate the application of the proposed model and numerical scheme.     

Modeling resin transfer molding of polyimide (PMR-15)/fiber composites

Polyimide (PMR-15)/fiber composites are important structural components in many high temperature applications requiring high performance lightweight materials. Most composites are fabricated from “prepregs,” but resin transfer molding (RTM) is an alternative route to the formation of a resin-saturated perform, further processing of which yields the final part. A simple model is proposed for the RTM of “thin” composite parts. Numerical solution of the model is accomplished through integration of the equation s formulated using he finite element method. This approach accounts for the changing shape of the flow front during RTM. The resulting computational schemes are numerically stable and efficient.     

Applications of flow simulation in reactive processing for cases in which the filling stage can be decoupled from heat transfer and chemical reaction

Flow simulation to predict the position of the flow front during SMC and IMC compression molding, RIM, SRIM, and RTM, is very useful in obtaining better molded parts. We have derived criteria to determine when the filling stage for these processes can be decoupled and simulated without regard to heat transfer and chemical reaction. We discuss why a Generalized Hele-Shaw model can be used to predict the flow front. Examples are given of the use of flow simulation in designing and locating SMC charges so as to reduce or eliminate knit lines, in locating IMC injection ports so as to avoid trapping air, in locating RIM injection gates to avoid trapping air and minimize knit-lines, and in locating vents to avoid trapping air in RTM or SRIM molding.     

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.     

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.     

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.