An experimental investigation of class A surface finish of composites made by the resin transfer molding process

Achievement of high class surface finish is important to the high volume automotive industry when using the resin transfer molding (RTM) process for exterior body panels. Chemical cure shrinkage of the polyester resins has a direct impact on the surface finish of RTM molded components. Therefore, resins with low profile additives (LPA) are used to reduce cure shrinkage and improve surface quality of the composite parts. However, little is known about the behaviour of low profile resins during RTM manufacturing and their ultimate effects on the surface quality of molded plaques. In this work, the effects of controlled material and processing parameters on the pressure variations, process cycle times and ultimately on the surface quality of RTM molded components were investigated. Taguchi experimental design techniques were employed to design test matrices and an optimization analysis was performed. Test panels were manufactured using a flat plate steel mold mounted on a press. Pressure sensors were inserted in the mold cavity to monitor pressure variations during different stages of cure and at various locations in the mold cavity. It was found that a critical amount of LPA (10%) was required to push the material against the mold cavity and to compensate for the resin cure shrinkage. A significant increase in pressure was observed during the later stages of resin cure due to the LPA expansion. The pressure increase had a significant effect on the surface roughness of the test samples with higher pressures resulting in better surface finish. A cure gradient was observed for low pressure injections which significantly reduced the maximum pressure levels.     

Dielectric cure monitoring for glass/polyester prepreg composites

The on-line cure monitoring of fiber reinforced thermosetting resin matrix composite material has been performed for improving quality and productivity during manufacturing. Since the dissipation factor measured by dielectrometry method is dependent on the degree of cure and temperature of resin, in this study, a new method to obtain the degree of cure during on-line cure monitoring for S-glass/polyester composites without temperature effect was developed by employing a combination function of the temperature and the dissipation factor. The temperature signal was measured with a K-type thermocouple and the dissipation factor signal was measured with an interdigital dielectric sensor during curing process. Then the calculated degree of cure using the measured data from dielectrometry was compared to the measured value from differential scanning calorimetry. The developed on-line cure monitoring method was applied to a 2-step cure cycle for the verification of the developed procedure.     

Cure cycle optimization of composites by dielectric measurements

Cure cycle monitoring by dielectric loss factor was investigated for prepreg composites based on epoxy, epoxy-phenolic, polyimide and phenolic matrices. Dielectric loss measurements were carried out to establish optimal cure conditions for the various polymer composites with respect to testing rates, curing temperature and timing for pressure application. For each polymer composite studied, initial laboratory experiments were carried out in a temperature-programmable oven to determine the onset of polymer flow, the temperature range where resin viscosity is appropriate for pressure application and the duration of the cross-linking reaction. Subsequently, autoclave processing was conducted using similar conditions accompanied by in situ dielectric loss monitoring. Dielectric cure monitoring of room temperature aged, epoxy-based prepreg indicated that curing of aged prepregs is possible provided a flow region exists. Flexural properties of specimens prepared from composite laminates processed under optimized conditions substantiated the adequacy of in situ dielectric loss measurements for cure monitoring of prepreg composites.     

Fabrication,process simulation and testing of a thick CFRP component using the RTM process

The present article introduces the case of a CFRP con-rod beam, and describes many aspects regarding its production with the Resin Transfer Moulding (RTM) process.The objective was to find the best process parameters of the injection and curing stages in order to manufacture the 20 mm thick CFRP part. The results are analysed in terms of the aesthetic aspect, the porosity and the mechanical properties of the final component.For the resin injection stage, results obtained from production experiences are presented, which have been performed with different set-ups, and simulations of the resin flow are used to analyse them. The results show that the resin flow during injection could be rather unpredictable, probably because of the fibre rearrangement and race tracking effects. Improvements in terms of aesthetic aspect and porosity of the part could be achieved by a process which included final compaction of the cavity by means of compressed air.Regarding the curing stage, the article presents the simulation results of a curing cycle, and it’s validation through DSC analysis of specimens obtained from the finished component.Finally, results of tensile mechanical tests are provided, performed on finished components produced by RTM and compared to others produced with the method of hand lay-up of pre-impregnated plies and curing in autoclave (Prepreg + Autoclave). The results confirm that it is possible to achieve components through RTM with comparable mechanical performance to those produced with the Prepreg + Autoclave process.     

Cross-sectional monitoring of resin impregnation using an area-sensor array in an RTM process

It is difficult to visualize the flow in the cross-section direction, and most conventional methods for monitoring resin flow are limited to the in-plane direction. This study investigates the monitoring of the cross-section of resin impregnation using an area-sensor array during a resin transfer molding (RTM) process. The area-sensor array is mounted on a thin polyimide film that is integrated with the inter-digital electrode array and associated wiring, and forms the bottom layer of the stacked composite laminates. Each area-sensor is square-shaped and measures the capacitance and electrical resistance of the sensor region. First, we constructed the equivalent electrical circuit model of in-plane and out-of-plane impregnation. Using this model, we proposed a method to identify the flow direction and the ratio of the impregnation thickness by measuring the electrical capacitance and resistance. The validity of the model was confirmed by comparison with the experimental results. To demonstrate the applicability of the proposed method, the area-sensor array was applied to monitoring the resin injection through-thickness to the glass fabric laminates. As a result, the cross-section of the impregnated area could be estimated and the estimated area provided a good match to the actual impregnated area.     

Repair of underground buried pipes with resin transfer molding

Repairing and replacing of worn-out underground pipes, such as sewer pipes, water-supply pipes, gas pipes, and communication cables by excavating not only cause traffic congestion but also produce large amount of waste. Also, the operation requires heavy equipments and longer operating time and high cost.

In this study, the repairing–reinforcing process of underground pipes with glass fiber fabric polymer composites using resin transfer molding (RTM) which overcomes the problems of present trenchless technologies has been developed. The developed process requires shorter operation time and lower cost with smaller and simpler operating equipments than conventional trenchless technologies. For the faultless operation, a simple method to apply pressure and vacuum to the reinforcement was developed. The resin wetting and void removal during RTM process for very large and long-composite buried pipes were experimentally investigated, and the efficient void removal method was suggested. Cure status and resin filling were monitored with a commercial dielectrometry cure monitoring system, LACOMCURE.

From the investigation, it has been found that the developed repair technology with appropriate process parameters and on-line cure monitoring has many advantages over conventional methods.     

Three-dimensional process cycle simulation of composite parts manufactured by resin transfer molding

A process cycle of resin transfer molding (RTM) consists of two sequential stages, i.e. filling and curing stages. These two stages are interrelated in non-isothermal processes so that the curing stage is dominated by the resin flow as well as temperature and conversion distributions during the filling stage. Therefore, it is necessary to take into account both filling and curing stages to analyze the process cycle accurately. In this paper, a full three-dimensional process cycle simulation of RTM is performed. Full three-dimensional analysis is necessary for thick parts or parts having complex shape. A computer code is developed based on the control volume/finite element method (CV/FEM). The resulting computer code can provide information regarding flow progression and pressure field during mold filling; and temperature distribution and degree of cure distribution for a process cycle. The computer code can also be used for process cycle simulation of composite structures with complex geometry and with various molding strategies including switching injection strategy, multiple gate injection strategy and variable mold wall temperature. Numerical examples provided in the present work show the capabilities of the computer code in analyzing the process cycle.     

Cure simulation with resistively in situ heated CFRP molds: Implementation and validation

In composite processing of parts with varying cross-sections, homogeneous cure is sought but poses a significant challenge. Electrically heated molds for resin transfer molding (RTM) processes offer the potential to locally introduce heat and, thus, achieve more homogeneous cure and enhanced part quality. However, low conductivity of CFRP poses a risk of uncontrolled exothermic reactions. To target this potential, an appropriate and efficient numerical method is presented in this study to simulate part cure governed by resistive heated CFRP molds. A numerical control algorithm for 3D finite element cure simulations is developed, which uses the reaction flux of a temperature boundary condition to calculate the arising tool temperature field. The capability of this method to predict non-uniform tool temperatures of self-heated CFRP molds with close to thermocouple accuracy during the cure process is shown by means of numerical verification and experimental validation on a self-heated CFRP plate.     

Development of a measuring system for on-line in situ monitoring of composite materials manufacturing

A unique portable measuring system using an impedance spectroscopy method with a self-adapting frequency of measurement is introduced. The system is intended for the on-line in situ monitoring of composite materials curing under industrial conditions. The capabilities of the developed system are demonstrated through the results obtained from on-line in situ measurements of unreinforced thermosetting resin, as well as of composites under real manufacturing conditions. Observations are supported by the results of other established methods for determining the degree of curing: temperature-modulated differential scanning calorimetry (MDSC), Fourier transform infrared spectroscopy (FT-IR) and broadband dielectric spectroscopy (BDS). Compressive and bending tests were also carried out on manufactured composites removed at different stages of the post-curing phase. Due to the self-adapting frequency, the system has enhanced sensitivity in the post-cure phase when the diffusion-controlled reactions proceed and, therefore, is suitable also for the analysis of hard post-cure samples.     

The effect of thermal mismatch on Z-pinned laminated composite structures

Z-pinning is a method of improving the through-thickness properties of composite laminates by inserting a solid pin through the laminate prior to curing. The thermal expansion mismatch between the Z-pin and base laminate produces large residual stresses during the cure cycle. Finite element modelling has shown that these stresses are greater than the failure stress of standard resin systems indicating the resin around the Z-pin should fail. This was confirmed through microscopy, which showed cracking around the perimeter of the Z-pin. Changing the material properties and dimension of the model to represent different Z-pinning situations could not significantly reduce the residual stresses, indicating cracking should occur in all Z-pinned laminates. These results show that probably all published Z-pinning properties have been obtained from laminates that would have exhibited cracking, indicating that the improved through-thickness properties are due more to mechanical interlocking than bonding. Questions are raised about the suitability of using Z-pinned laminates in specific applications, and the effects of increased moisture ingress and long term durability.     

The effect of process parameters on volatile release for a benzoxazine–epoxy RTM resin

Volatile release during cure is a potential cause of void formation during the resin transfer molding of complex thermosetting resins. In this study, a blended benzoxazine–epoxy resin system is analyzed to determine the rate at which volatiles are evolved, as well as the dependence of that rate on process parameters. The evolution of thermophysical and thermochemical resin properties is characterized using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The identity and rate of evolution of the gaseous byproducts released during cure are determined at ambient pressure using a Fourier transform infrared spectrometer (FTIR) linked to a reaction cell. The results show that gas release during cure can be reduced but not eliminated by degassing at elevated temperature. Furthermore, the results indicate that the nature and rate of volatile release can be modified by judicious selection of cure cycle, as shown by a preliminary analysis of manufactured neat resin panels.     

RTM工艺中树脂固化温度与介电性能

RTM为闭模成型工艺, 难以观察到模腔内树脂的充模与固化反应过程。利用介电分析仪对RTM工艺进行在线监测, 测得了树脂固化过程中不同模具温度下树脂反应的温度及离子导电率、 不同纤维含量条件下的介电常数以及不同纤维织物方式下的离子导电率。实验结果表明: 固化反应放热使复合材料局部温度升高, 并形成复杂的温度梯度分布; 在较高温度和较低纤维含量条件下, 离子导电率变化较快, 对树脂固化反应的影响较大; 不同织物方式的玻璃纤维对离子导电率也有一定的影响, 玻璃纤维复合毡比方格布影响大。      

Eliminating volatile-induced surface porosity during resin transfer molding of a benzoxazine/epoxy blend

We studied the mechanism of volatile-induced surface porosity formation during the resin transfer molding (RTM) of aerospace composites using a blended benzoxazine/epoxy resin, and identified reduction strategies based on material and processing parameters. First, the influence of viscosity and pressure on resin volatilization were determined. Then, in situ data was collected during molding using a lab-scale RTM system for different cure cycles and catalyst concentrations. Finally, the surface quality of molded samples was evaluated. The results show that surface porosity occurs when cure shrinkage causes a sufficient decrease in cavity pressure prior to resin vitrification. The combination of thermal gradients and rapid gelation can generate large spatial variations in viscosity, rendering the coldest regions of a mold susceptible to porosity formation. However, material and cure cycle modifications can alter the resin cure kinetics, making it possible to delay the pressure drop until higher viscosities are attained to minimize porosity formation.     

Hybrid carbon fiber composite lattice truss structures

Carbon fiber reinforced polymer (CFRP) composite sandwich panels with hybrid foam filled CFRP pyramidal lattice cores have been assembled from linear carbon fiber braids and Divinycell H250 polymer foam trapezoids. These have been stitched to 3D woven carbon fiber face sheets and infused with an epoxy resin using a vacuum assisted resin transfer molding process. Sandwich panels with carbon fiber composite truss volumes of 1.5–17.5% of the core volume have been fabricated, and the through-thickness compressive strength and modulus measured, and compared with micromechanical models that establish the relationships between the mechanical properties of the core, its topology and the mechanical properties of the truss and foam. The through thickness modulus and strength of the hybrid cores is found to increase with increasing truss core volume fraction. However, the lattice strength saturates at high CFRP truss volume fraction as the proportion of the truss material contained in the nodes increases. The use of linear carbon fiber braids is shown to facilitate the simpler fabrication of hybrid CFRP structures compared to previously described approaches. Their specific strength, moduli and energy absorption is found to be comparable to those made by alternative approaches.     

Cure shrinkage characterization and modeling of a polyester resin containing low profile additives

Resin Transfer Moulding (RTM) has great potential as an efficient and economical process for fabricating large and complicated composite structural components. The low capital investment cost required and process versatility in component integration and assembly make RTM very attractive for high volume automotive applications. One of the challenges facing the automotive field is the resulting surface finish of manufactured components. The shrinkage associated with the curing of thermoset resins contributes to the poor surface quality. Low profile additives (LPA) are added to the resin to compensate for the cure shrinkage; however their effects on the thermal, rheological and morphological properties of polyester resins are not well understood. In this paper, the effect of LPA on cure kinetics, cure shrinkage and viscosity of a polyester resin is studied through differential scanning calorimetry (DSC) and special rheological techniques. Models are developed to predict cure shrinkage, LPA expansion, cure kinetics and viscosity variations of the resin as a function of processing temperature. Finally, morphological changes in the resin with and without LPA, during isothermal cure, are studied with hot stage optical microscopy. The results show that the LPA content in the range tested had no significant effect on the cure kinetics. However, higher LPA content reduced cure rate and cure shrinkage. A minimum of 10% LPA was required to compensate for cure shrinkage. Shrinkage behavior of all formulations was similar until a degree-of-cure of 0.5. However, resin formulations with higher LPA content showed expansion at later stages during curing.     

单根光纤光栅监测复合材料固化工艺过程多目标参量技术的研究

采用单根布拉格光纤光栅传感器实现了对复合材料工艺过程不同阶段、不同目标的监测。在成型阶段监测树脂的温度(粘度)发展历程；在施加成型压力时监测树脂内部压力的变化；在固化与降温阶段监测复合材料层板内部的应力变化。试验结果表明，利用-根布拉格光纤光栅传感器同时实现对复合材料固化工艺全过程的主要控制参量的监测是完全可行的。     

基于折射率变化的固化在线监测研究

先进复合材料的固化成型工艺十分复杂,目前在很大程度上仍依赖于经验工艺。为了实现复合材料固化工艺的透明化、科学化及智能化,提出了将光纤折射率传感器用于复合材料制造过程,研制了一种折射率光纤传感器,同时将光源、探测器、信号处理器等元器件集成化为一个便携式的光纤传感仪以利于生产现场使用。利用该传感器对常温固化树脂的固化过程进行了测量,将其与同等条件下迈克尔逊干涉仪对固化过程的监测结果进行了比较,二者具有良好的一致性。在实验室与生产现场两种条件下将光纤折射率传感器埋设于预浸料中对其固化工艺过程进行了监测。结果表明,该技术不仅可以用于新型树脂固化工艺制度的确定,也可用于复合材料固化工艺过程的在线监测。     

Sizing related kinetic and flow considerations in the resin infusion of composites

Fibres used in preforms of resin transfer moulded (RTM) composites are coated with sizings, binders, and/or finishes that serve multiple purposes, including facilitating handling, protection of the fibres from compaction and process induced damage (including notching), aiding in compatibility and wetting of the fibres by the resin, and overall enhancement of the behavioural response of the composites. In this investigation four different sizings applied to S2 glass fibres are shown to significantly affect two aspects of RTM processing - resin infusion, and cure. In both cases phenomena at the microscopic level are seen to affect response variables at the macroscopic level. On a microscopic level, the behaviour of a thermosetting resin based composite is affected by the formation of interphase regions that greatly affect the cure kinetics and hence the mechanical and physical properties of the composite, which are dependent on the inter-constituent variations in local properties such as modulus and glass transition temperature. Similarly fibre-sizing-resin interactions occurring during the infusion stage affect wet-out and local flow behaviour through the development of stoichiometric imbalances in local regions. It is shown that the molecular interactions between the constituents (as initiated by the sizing) are affected by processing conditions such as temperature and rate of resin flow, and that heat evolution and resin rheology may be affected by the stoichiometric imbalances resulting from interphasial level reactions. This revised version was published online in November 2006 with corrections to the Cover Date.     

Microwave diagnosis of low-density fiberglass composites with resin binder

Dielectric properties of low-density fiberglass composites are studied at microwave frequencies and are related to the resin binder content and state of cure. A completely filled short-circuited waveguide method is used to study the dielectric properties of fresh and slightly cured resin binder (used in the production of low-density fiberglass composite materials) in the frequency range of 4–18 GHz. The preliminary results indicate that the dielectric properties of resin binder may be used to determine its state of cure. The same measurement approach is used to determine the dielectric properties of fiberglass materials containing uncured resin binder, no resin binder, and three different levels of resin binder. It is found that the dielectric properties of these low-permittivity and low-loss materials are very similar. Consequently, to distinguish among real fiberglass products with different resin binder levels, an open-ended rectangular waveguide measurement approach, is used. This technique allows for noncontact and on-line inspection. The standoff distance is utilized as an optimization parameter increasing the sensitivity of detection to slight dielectric property variations. Two microwave images, at 24 and 10 GHz of a panel containing five different fiberglass specimens are presented.     

A grid of dielectric sensors to monitor mold filling and resin cure in resin transfer molding

A grid of 50 dielectric sensors has been embedded in the walls of a mold to monitor resin transfer molding (RTM). The capacitance of each sensor increased as resin occupied the space between sensor plates, and it decreased with curing. Monitoring data can be used for process control to prevent dry spots and to determine when to de-mold the part. In previous studies, Skordos et al. [Skordos AA, Karkanas PI, Partridge IK. A dielectric sensor for measuring flow in resin transfer molding. Meas Sci Technol 2000;11:25–31] used a lineal sensor, Hegg et al. [Hegg MC, Ogale A, Mescher A, Mamishev AV, Minaie B. Remote monitoring of resin transfer molding processes by distributed dielectric sensors. J Compos Mater 2005;39(17)] used three large sensors. As experimentally shown in this study, these lineal or large-plate dielectric sensors may mislead since a sensor measures total fraction of the sensor’s plate area occupied by resin but not the resin’s whereabouts. To avoid ambiguity and yet maintain detailed monitoring, a sensor grid was made at the projections of embedded orthogonal electrodes. The developed sensor operation system eliminated tedious and costly manufacturing of conventionally shielded separate sensors. The success of the developed sensor system was demonstrated in RTM experiments.