Flow front measurements and model validation in the vacuum assisted resin transfer molding process

Through‐thickness measurements were recorded to experimentally investigate the through thickness flow and to validate a closed form solution of the resin flow during the vacuum assisted resin transfer molding process (VARFM). During the VART'M process, a highly permeable distribution medium is incorporated into the preform as a surface layer and resin is inftised Into the mold, under vacuum. During Infusion, the resin flaws preferentially across the surface and simultaneously through the thickness of the preform, giving rise to a three dimensional‐flow front. The time to fill the mold and the shape of the flow front, which plays a key role in dry spot formation, are critical for the optimal manufacture of large composite parts. An analytical model predicts the flow times and flow front shapes as a function of the properties of the preform, distribution media and resin. It was found that the flow front profile reaches a parabolic steady state shape and the length of the region saturated by resin is proportional to the square root of the time elapsed. Experimental measurements of the flow front in the process were carried out using embedded sensors to detect the flow of resin through the thickness of the preform layer and the progression of flow along the length of the part. The time to fill the part, the length of flow front and its shapes show good agreement between experiments and the analytical model. The experimental study demonstrates the need for control and optimization of resin injection during the manufacture of large parts by VARTM.     

泡沫夹芯结构板泡沫壁流道内的流动特性

沟槽型真空辅助树脂传递模塑成型工艺（VARTM）是一种新型的泡沫夹芯结构板成型方法,利用实验探明了泡沫夹芯结构板芯材上不可渗泡沫壁流道内的流动行为。实验结果表明,液体在泡沫壁流道的流动能力大幅降低,只有光滑壁流道的60%左右,泡沫壁流道的粗糙内表面是造成这种现象的主要原因。提出了相应的压力驱动流动方程,并采用等效渗透率来表征液体在泡沫壁流道内的流动能力,得到了考虑粗糙表面影响的等效渗透率计算公式,提出了一个正确计算不可渗泡沫壁流道内流动的处理方法。     

Modeling of resin infusion in vacuum assisted resin transfer molding

Resin infusion was modeled and analytic solutions were obtained for vacuum assisted resin transfer molding (VARTM). Compaction behavior of the fiber preform was examined experimentally and the influence of compressibility of the preform on the resin infusion was investigated mathematically. Flow front advancement through the preform was predicted by the analytic model proposed in the present study. The model provided pressure and thickness distributions of the region impregnated by the resin. For verification of the analytic solutions, a resin infusion experiment and a mold filling simulation for VARTM were performed and compared with the analytic ones. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers.     

Vacuum‐assisted resin transfer molding using tackified fiber preforms

A low cost composite fabrication process—tackified SCRIMP—is described for fabricating aerospace‐grade composites based on tackification and vacuum‐assisted resin transfer molding (VARTM). Tackification based on a commercial tackifier (FT 500 from 3M) was used to make the net‐shape fiber preform. It was found that tackifier concentration and application conditions play important roles in governing the moldability of tackified fiber preforms. An epoxy resin (PR 500 from 3M) was used in the VARTM process‐SCRIMP at high temperatures. Experimental results show that composites with high fiber content (> 60% by volume) can be manufactured at low cost using tackification. Effects of tackification methods on composite dimension control, void content and mechanical properties were investigated and compared in both RTM and SCRIMP.     

A prediction method on in‐plane permeability of mat/roving fibers laminates in vacuum assisted resin transfer molding

Vacuum assisted resin transfer molding (VARTM) is one of the promising manufacturing techniques for large‐scaled composite components with complex geometry, such as yachts or fishing vessels. To reduce the failure risk of production, numerical simulation of resin infusion process before manufacture is helpful. In general, basic characteristics of perform, such as permeability, need to be measured by experiments in practice. However, this experimental approach sometimes may be costly because specific types of fibers as well as preform with different layer numbers need individual experiments. This study first introduces the experimental procedure of measuring the permeability of reinforcements via Darcy's Law. On the basis of experimental observation of permeability of different layer order, we assumed that the change of the permeability in different experiments is mainly affected by the space provided by the fiber. Accordingly, an efficient prediction method based on the idea of “total porous space of the reinforcement” is proposed. It is shown that this method can give reference between prediction and experiments of the mat/roving fiber preform. Though the resin flowing is complex, this prediction gives a simple, macroscopic reference way for the injection characteristic of large‐sized ships, and consequently facilitates the numerical design work of composite structures manufactured by VARTM technique. POLYM. COMPOS., 27:665–670, 2006. © 2006 Society of Plastics Engineers     

High temperature vacuum assisted resin transfer molding of phenylethynyl terminated imide composites

Vacuum Assisted Resin Transfer Molding (VARTM) has proven to be a cost effective process for manufacturing composite structures compared with prepreg/autoclave and traditional resin transfer molding (RTM) processes. However, VARTM has not been accomplished with high temperature resins (such as polyimides) until recently, primarily because no resins had low melt viscosity and long melt stability that are required by VARTM. With the recent invention of phenylethynyl terminated imides (PETIs), high temperature VARTM has been achieved. Two processing methods, in‐plane and through‐thickness resin flow, were proposed and tested. Both methods are capable of fabricating polyimide matrix composites; and the carbon fiber laminates yield good fiber‐resin interfacial bonding and comparable mechanical properties to those laminates fabricated using RTM. POLYM. COMPOS., 2011. © 2010 Society of Plastics Engineers     

Resin infusion of triaxially braided preforms with through‐the‐thickness reinforcement

Vacuum assisted resin transfer molding (VARTM) has shown potential to significantly reduce the manufacturing cost of high‐performance aerospace composite structures. In this investigation, high fiber volume fraction, triaxially braided preforms with through‐the‐thickness stitching were successfully resin infiltrated by the VARTM process. The preforms, resin infiltrated with three different resin systems, produced cured composites that were fully wet‐out and void free. A three‐dimensional finite element model was used to simulate resin infusion into the preforms. The predicted flow patterns agreed well with the flow patterns observed during the infiltration process. The total infiltration times calculated using the model compared well with the measured times.     

Vacuum‐assisted resin transfer molding model

A model of the vacuum‐assisted resin transfer molding (VARTM) process is developed that includes the most important aspects of the processing physics. The model consists of several submodels, such as preform mechanics, Darcy flow, wicking flow, and void formation. The preform mechanics model treats the preform as a linearly elastic, one‐dimensional (1D) solid. However, the key physical process is the lubrication of the preform due to fluid wetting, and this is modeled as a reduction in preform modulus, an easily measurable parameter. Residual stress, three‐dimensional (3D) structural behavior, and nonlinearity are neglected, but can all be included. The fluid flow model of capillary wicking is not tacked onto the Darcy equation as a modified boundary condition, as was previously done. The wicking is treated simply, but more realistically, by performing a force balance on the fluid in a pore. Balancing the capillary pressure and the viscous drag allows the development of a wicking front that precedes the main Darcy flow front to an extent that depends on several easily measurable factors. It is this wicking front that is responsible for the small void formation that reduces the quality of VARTM parts, relative to resin transfer molding (RTM) parts. POLYM. COMPOS. 26:477–485, 2005. © 2005 Society of Plastics Engineers     

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

During 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     

Elevated‐temperature vacuum‐assisted resin transfer molding process for high performance aerospace composites

Vacuum‐assisted resin transfer molding (VARTM) is commonly used for general temperature applications (<150 °C) such as boat hulls and secondary aircraft structures. With growing demands for applications of composites in elevated temperature environments, significant cost savings can be achieved by employing the VARTM process. However, implementation of the VARTM process for fabricating elevated temperature composites presents unique challenges such as high porosity and low fiber volume contents. In the present work, a low cost and reliable VARTM process is developed to manufacture elevated temperature composites for aerospace applications. Modified single vacuum bagging infusion and double vacuum bagging infusion processes were evaluated. Details of the method to obtain high quality composite parts and the challenging issues related to the manufacturing process are presented. Density and fiber volume fraction testing of manufactured panels showed that high quality composite parts with void content less than 1% have been consistently manufactured. A property database of the resin system and the composites was developed. A three‐dimensional mathematical model has also been developed for flow simulation and implemented in the ABAQUS finite element package code to predict the resin flow front during the infusion process and to optimize the flow parameters. The results of the present study indicate that aircraft grade composite parts with high fiber volume fractions can be manufactured using the developed elevated temperature VARTM process. © 2013 Society of Chemical Industry     

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.     

纤维编织增强酚醛树脂基热防护材料研究进展

介绍了二维织物、2.5维织物、三维织物等纤维预制件的结构特性,分析总结了各种纤维预制件的发展及研究状况,综述了三维编织增强酚醛树脂基热防护材料及其树脂传递模塑成型工艺研究进展。总结了目前研究中存在的问题,并对未来的研究趋势进行了展望。指出三维编织复合材料是不分层的整体结构,其比强度、比模量高,力学性能和功能性优异,开展编织复合材料力学性能有限元分析、结构与功能一体化设计、低成本制造工艺等研究是十分迫切的。在此基础上,开展多种编织工艺、多种纤维混合编织也是新的研究方向,特殊形状的一次性编织复合材料的力学性能研究有待进一步深入。     

Optimization method to reduce three-dimensional thermal gradients in resin transfer molding

Presently, the mold and resin are heated to promote resin flow and shorten curing period in order to improve manufacturing efficiency of resin transfer molding (RTM). This nonisothermal manufacturing process easily generates three-dimensional thermal gradients in the direction of resin flow and thickness of composite part. However, the existing heating systems only consider the thermal gradients along thickness direction. The thermal gradients in direction of resin flow cannot be reduced which will lead to residual stress even deformation and cracking in composite part. This article aims at reducing the three-dimensional thermal gradients in the direction of resin flow and thickness of composite part. Based on the theory of energy and fluid flow, an optimization method of heating system design by using numerical simulation is proposed. The results show this method reduces the three-dimensional thermal gradients effectively in composite part manufactured by RTM process. This study can provide powerful tools for heating system design to manufacture composites products in polymer industry. © 2020 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48948.     

Use of eco‐friendly epoxy resins from renewable resources as potential substitutes of petrochemical epoxy resins for ambient cured composites with flax reinforcements

In the last years, some high renewable content epoxy resins, derived from vegetable oils, have been developed at industrial level and are now commercially available; these can compete with petroleum‐based resins as thermoset matrices for composite materials. Nevertheless, due to the relatively high cost in comparison to petroleum‐based resins, their use is still restricted to applications with relatively low volume consumption such as model making, tuning components, nautical parts, special effects, outdoor sculptures, etc. in which, the use of composite laminates with carbon, aramid and, mainly, glass fibers is generalized by using hand layup and vacuum assisted resin transfer molding (VARTM) techniques due to low manufacturing costs and easy implementation. In this work, we study the behavior of two high renewable content epoxy resins derived from vegetable oils as potential substitutes of petroleum‐based epoxies in composite laminates with flax reinforcements by using the VARTM technique. The curing behavior of the different epoxy resins is compared in terms of the gel point and exothermicity profile by differential scanning calorimetry (DSC). In addition, overall performance of flax‐epoxy composites is compared with standardized mechanical (tensile, flexural and impact) and thermal (Vicat softening temperature, heat deflection temperature, thermo‐mechanical analysis) tests. The curing DSC profiles of the two eco‐friendly epoxy resins are similar to a conventional epoxy resin. They can be easily handled and processed by conventional VARTM process thus leading to composite laminates with flax with balanced mechanical and thermal properties, similar or even higher to a multipurpose epoxy resin. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Flow enhancement in liquid composite molding processes by online microwave resin preheating

Liquid composite molding techniques are increasingly applied for the manufacturing of fiber-reinforced plastic components for civil, aerospace, and automotive applications. Being the preform impregnation a key step during the process, resin viscosity should meet the precise requirements. Opportune resin preheating increases its fluency, thus enhancing the impregnation and saturation flow through the fabric and reducing the mold filling time. This paper explores the application of microwave technology for resin preheating. The integration of an online microwave preheating system within a demonstrative resin infusion facility is described and the effects of preheating on the infusion time are discussed. Parallel-plate dielectric sensors were embedded into the mold to track the unheated and preheated resin flow through the fiber preform. The obtained results highlighted the effectiveness of online microwave heating to reduce the time required for the impregnation of the dry fiber reinforcement.     

Fluid mechanics analysis of a two-dimensional pultrusion die inlet

Fluid mechanics plays an important role in many manufacturing processes including the pultrusion of composite materials. The analysis of fluid mechanics problems generally involves determination of quantities such as pressure and velocity. During the pultrusion process, the short, tapered inlet region of the pultrusion die experiences a significant amount of fluid resin pressure rise. The quality of a pultruded product can be affected by the amount of pressure rise in the pultrusion die inlet. Void formation can be suppressed and good fiber “wet out” achieved by a sufficiently high pressure rise in the pultrusion die inlet region. In this study the change in fluid resin pressure rise as a function of die entrance geometry is investigated by developing a finite element model based on the assumptions of Darcy's law for flow in porous media. The momentum equations are combined with the continuity equation to save computational time and memory. A Galerkin weighted residual based finite element method is developed to solve the resulting equation. This model is capable of predicting the pressure rise in the tapered inlet region of the pultrusion die as well as the straight portion of the die. By varying the size of the preform plates the thickness of the fiber/resin matrix approaching the die inlet can be varied. The finite element model predicts the impact of changing the preform plate size on the fluid resin pressure rise in the pultrusion die. The effect of varying the wedge angle for a linearly tapered die inlet region is also studied using this model. The results in this work can be useful for designing a pultrusion die system.     

真空辅助树脂转移模塑工艺研究进展

综述了真空辅助树脂转移模塑(VARTM)工艺的树脂体系、工艺过程研究与模拟等的研究进展.表明环氧树脂是VARTM成型工艺的重要树脂基体,双马树脂作为一个重要的基体材料,目前基本处在研究阶段;工艺过程的试验研究和数值模拟发展迅速,国外借助先进技术手段,采用数值模拟方法等对工艺过程进行优化和控制的研究日趋活跃;在航空、船舶...     

A dual-scale analysis of macroscopic resin flow in vacuum assisted resin transfer molding

In Vacuum Assisted Resin Transfer Molding (VARTM) where a sacrificial medium is used to facilitate the resin flow, the velocity of the resin varies drastically between inside the sacrificial medium and inside the fiber preform. Although the thickness-to-length ratio of a VARTM product is usually small, a 3-D analysis is required for analyzing the lead-lag flow in the two different media. The problem associated with the full 3-D analysis is the CPU time. A full 3-D numerical mesh comprising a large number of nodes requires a CPU time impractical on most computer platforms. In this study, a dual-scale analysis technique was proposed. First, the flow analysis for the entire calculation domain was conducted in 2.5-D. Using the results of the 2.5-D calculation, the 3-D analysis was performed for a small area of special concern. In some numerical examples, the local 3-D analysis could discover an eccentric flow pattern as well as the lead-lag flow that would inevitably be neglected in 2.5-D simulations. The global-local analysis technique practiced in this study can be used to analyze the intricate flow of resin through non-uniform media in affordable CPU times. Polym. Compos. 25:510–520, 2004. © 2004 Society of Plastics Engineers.     

Edge effect in RTM processes under constant pressure injection conditions

In resin transfer molding processes, the edge effect caused by the nonuniformity of permeability between fiber preform and edge channel may disrupt resin flow patterns and often results in the incomplete wetting of fiber preform, the formation of dry spots, and other defects in final composite materials. So a numerical simulation algorithm is developed to analyze the complex mold‐filling process with edge effect. The newly modified governing equations involving the effect of mold cavity thickness on flow patterns and the volume‐averaging momentum equations containing viscous and inertia terms are adopted to describe the fluid flow in the edge area and in the fiber preform, respectively. The volume of fluid (VOF) method is applied to tracking the free interface between the two types of fluids, namely the resin and the air. Under constant pressure injection conditions, the effects of transverse permeability, edge channel width, and mold cavity thickness on flow patterns are analyzed. The results demonstrate that the transverse flow is not only affected by the transverse permeability and the edge channel width but also by the mold cavity thickness. The simulated results are in agreement with the experimental results. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Modeling and analysis of thickness gradient and variations in vacuum‐assisted resin transfer molding process

As vacuum‐assisted resin transfer molding (VARTM) is being increasingly used in aerospace applications, the thickness gradient and variation issues are gaining more attention. Typically, thickness gradient and variations result from the infusion pressure gradient during the process and material variations. Pressure gradient is the driving force for resin flow and the main source of thickness variation. After infusion, an amount of pressure gradient is frozen into the preform, which primarily contributes to the thickness variation. This study investigates the mechanism of the thickness variation dynamic change during the infusion and relaxing/curing processes. A numerical model was developed to track the thickness change of the bagging film free surface. A time‐dependent permeability model as a function of compaction pressure was incorporated into an existing resin transfer molding (RTM) code for obtaining the initial conditions for relaxing/curing process. Control volume (CV) and volume of fluid (VOF) methods were combined to solve the free surface problem. Experiments were conducted to verify the simulation results. The proposed model was illustrated with a relatively complex part. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers