RTM工艺注模过程边缘效应模拟分析

RTM工艺需将纤维预制体预置到模具中,由于纤维预制体结构不均匀性和模具形状、尺寸等影响,极易产生边缘效应。边缘效应会严重影响树脂流场发展和压力场分布。本文作者采用等效渗透系数方法模拟边缘效应,得到了其影响下的树脂流动前峰曲线和压力场。研究表明:一方面边缘效应可能导致不期望的树脂流场发展而形成工艺缺陷——干斑;另一方面可以利用边缘效应提高工艺效率:常流率注射时减小合模压力和注射压力,常压力注射时可以减少注模时间。      

基于孔隙控制的车身结构树脂传递模塑成型工艺设计

以典型车身结构B柱为研究对象,结合实验与仿真分析研究其树脂传递模塑(RTM)工艺的优化设计方法。研究了通过注射方式的优化控制树脂流动前沿,从而达到降低制件孔隙率和保证制件质量的目的。首先通过自制的变厚度渗透率测试模具获取所选用织物的渗透率,之后通过真空辅助RTM实验与对应模拟仿真进行对比分析来验证所采用仿真方法与渗透率数据的可靠性。最后结合充模周期与孔隙率控制理论对RTM工艺注射口分布及注射方式进行优化设计。结果表明,针对所选定车身结构,优化速率注射方式所获得的制件孔隙率最低,但充模周期较长,而基于双点注射的恒流量注射方式能较好地兼顾充模周期与制件孔隙率的要求。     

Flow sensing and control strategies to address race-tracking disturbances in resin transfer molding. Part I: design and algorithm development

In the resin transfer molding process for advanced polymer composites manufacturing, the fiber preform is placed in the mold cavity and a thermoset resin is injected into the mold to impregnate the stationary preform. The resin displaces the air in the mold through openings called vents. Once the resin emerges out of the vents, the injection is discontinued. The near net-shaped composite part can be demolded after the resin cures. Ideally, the vents should be placed at the locations where the resin arrives last to ensure the complete saturation of the preform. However, the racetracking phenomenon, in which the resin flows faster along the minuscule channels induced by imperfect fits between the preform edges and the mold walls, can dramatically change the resin infiltration process. The ramifications of racetracking are that the resin may arrive at the vents before completely impregnating the preform and create undesired dry spots, which are fiber regions devoid of resin. The racetracking strength is not repeatable and may vary from one injection trial to next. Hence, the online strategic flow control can be useful in improving the processing reliability and the parts quality by re-directing the flow to arrive last at the vents. In this article, an online strategic flow control system consisting of a flow sensing network and a flow actuation network is proposed. A flow pattern recognition technique, which is based on the dimensionless time vector collected by the flow-sensing network, is developed in order to perform the online flow characterization effectively. Flow simulations are utilized to off-line design the flow control system. An evaluation function is formulated to optimize the flow sensing network design. A multi-tier genetic algorithm is implemented to optimize the locations of vents and gates along with the necessary control actions. A numerical case study for testing the computer-generated flow control solutions is presented. It was found that there was significant improvement in the success rate (fewer dry spot regions) due to the use of the strategic flow control and the automated design approach.     

发射箱箱盖树脂传递模塑工艺有限元模拟分析

目的为了减少人力和物力资源的浪费,节约产品成本,通过有限元分析技术确定箱盖树脂传递模塑工艺(RTM)的工艺参数。方法对箱盖RTM工艺树脂流动过程进行数值模拟,对比箱盖结构在线注射-点出射和点注射-点出射工艺下,当注模压力为0.2,0.4 MPa时对应的树脂流动前锋位置和压力分布结果。结果综合成本和效率两方面因素,选择0.4 MPa作为RTM的注模压力。采用线注射-点出射工艺得到稳定的树脂流动前锋形状,能够有效预防干斑的产生。结论分析结果与实验具有较好的吻合性,能够有效指导RTM工艺中模具的设计,可降低"试错法"带来的高昂成本和低效率,对提高复合材料构件的制造水平具有明显积极作用。     

Flow analysis during compression of partially impregnated fiber preform under controlled force

Compression resin transfer molding (CRTM) is an alternative solution to conventional resin transfer molding processes. It offers the capability to produce net shape composites with fast cycle times making it conducive for high volume production. The resin flow during this process can be separated into three phases: (i) metered amount of resin injection into a partially closed mold containing dry fiber preform, (ii) closure of the mold until it is in contact with the fiber preform displacing all the resin into the preform and (iii) further mold closure to the desired thickness of the part compacting the preform and redistributing the resin. Understanding the flow behavior in every phase is imperative for predictive process modeling that guarantees full preform saturation within a given time and under specified force constraints.     

Transient gas flow technique for inspection of fiber preforms in resin transfer molding

A transient gas flow method was developed to determine the quality of fibrous preforms in resin transfer molding (RTM) prior to resin injection. The method aims at detecting defects resulting from preform misplacement in the mold, accidental inclusions, preform density variations, race tracking, shearing, etc. Unlike the previously developed method based on steady-state gas flow, the new method allows for the acquisition of continuous time-varying pressure data from multiple ports during a single test. The validity of the method was confirmed by one-dimensional flow experiments.     

缝合夹层结构复合材料树脂传递模塑成型工艺充模仿真

对复合材料与金属经缝合连接形成的夹层结构板的树脂传递模塑成型（RTM）工艺进行了充模模拟研究。首先通过实验和数值计算的方法,分别获得缝合夹层结构织物和芯层孔洞的渗透率;随后,建立能够反映缝孔内流动情况的二维和三维简化模型,进行RTM充模仿真,讨论不同工艺参数对成型流动的影响;最后通过成型实验验证工艺的可行性。缝线与孔洞直径之比为0.3~0.8时,孔洞渗透率随缝线直径的增大而减小,预制体织物渗透率与孔洞渗透率相差两个数量级;缝孔内容易产生缺陷,没有缺陷的区域随着注射压力的增加、孔洞密度和芯层厚度的减小而增大,在芯层表面沿每排孔洞单向开槽能够改善树脂在孔洞内的浸润;线注射时,树脂整体流动情况优于点注射,而点注射时,将进胶口设置在一角,能够减少表面干斑。     

Three-Dimensional Flow Simulations for the Resin Transfer Molding Process

In resin transfer molding (RTM) a stack of fiber mats or woven rovings is laid inside the mold cavity. Then the mold is sealed and resin is injected. The computer simulation of the injection phase in resin transfer molding (RTM) can help the mold designer to position properly the injection ports and the air vents, to select an adequate injection presssure and to optimize the cycle time. The purpose of this article is to present a finite element simulation model of the filling process that can be applied to three-dimensional “thin shell” molds. An application to a subway seat is described to illustrate the various stages of the simulation     

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.     

耐350℃ RTM聚酰亚胺树脂及其复合材料性能

以苯乙炔苯酐（4-PEPA）为封端剂,异构联苯四甲酸二酐（α-BPDA）作为二酐单体,通过选择合适的二胺单体及优化配比,研制了耐温等级高于350℃,适用于RTM工艺的聚酰亚胺基体树脂HT-350RTM,选用U3160单向碳纤维织物作为增强体,采用RTM工艺制备了HT-350RTM树脂基复合材料层合板（U3160/HT-350RTM）。结果表明：HT-350RTM树脂最低黏度可达390 mPa·s,在280℃下保持黏度低于1 Pa·s的时间大于2 h,能够满足RTM工艺的要求。经过高温固化后,HT-350RTM树脂的玻璃化转变温度为392℃,热分解温度（分解5%）高达537℃。采用RTM工艺制备的U3160/HT-350RTM复合材料层合板孔隙率仅为0.34%,室温下具有良好的基本力学性能,315℃和350℃下的力学性能保持率均高于60%,能够满足350℃工况下的长期使用要求。     

基于“离位”增韧技术Z向注射RTM成型的浸润研究

针对"离位"增韧技术和Z-RTM成型技术,引入饱和度参数修正Darcy定律,建立描述树脂在纤维预制件中非稳态流动的偏微分方程,研究恒流注射过程中体积流量、树脂黏度和纤维预制件渗透率等工艺参数对非稳态浸润过程注入压力的影响,模拟树脂在层间未增韧和增韧纤维预制件束内和束间的流动。结果表明:数值模拟结果具有可靠性;随着注射时间的增加,纤维预制件内部各点的压力增加;随着体积流量、树脂黏度的增加,注入压力线性增加,而随着纤维渗透率的增加,注入压力减少,符合Darcy定律;实现了树脂在纤维预制件细微观层次浸润的可视化,这种可视化结果为预测树脂在预制件中的宏观流动提供了重要补充,并为实际工艺提供了一定指导作用。     

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.     

Non‐isothermal ‘2‐D flow/3‐D thermal’ developments encompassing process modelling of composites: flow/thermal/cure formulations and validations

In the manufacturing process of large geometrically complex components comprising of fibre‐reinforced composite materials by resin transfer molding (RTM), the process involves injection of resin into a mold cavity filled with porous fibre preforms. The overall success of the RTM manufacturing process depends on the complete impregnation of the fibre mat by the polymer resin, prevention of polymer gelation during filling, and subsequent avoidance of dry spots. Since a cold resin is injected into a hot mold, the associated physics encompasses a moving boundary value problem in conjunction with the multi‐disciplinary study of flow/thermal and cure kinetics inside the mold cavity. Although experimental validations are indispensable, routine manufacture of large complex structural geometries can only be enhanced via computational simulations, thus eliminating costly trial runs and helping the designer in the set‐up of the manufacturing process. This study describes the computational developments towards formulating an effective simulation‐based design methodology using the finite element method. The specific application is for thin shell‐like geometries with the thickness being much smaller than the other dimensions of the part. Due to the highly advective nature of the non‐isothermal conditions involving thermal and polymerization reactions, special computational considerations and stabilization techniques are also proposed. Validations and comparisons with experimental results are presented whenever available. Copyright © 2001 John Wiley & Sons, Ltd.     

RTM快速成型环氧树脂时间-温度-转变图的构建及其碳纤维/环氧树脂复合材料评价

针对自行研制的树脂传递模塑工艺（RTM）快速成型环氧树脂,利用唯象动力学模型、DiBenedetto方程和凝胶模型研究了树脂体系的等温及非等温固化动力学,构建了时间-温度-转变（TTT）的关系图,表明树脂体系兼具较长的适用期与快速固化特性。以此设计RTM快速成型工艺,考察了树脂体系对碳纤维织物的浸润流动行为,并评价了快速成型碳纤维/环氧树脂复合材料的界面力学性能与微观形貌。结果表明,注射温度下树脂体系的浸润填充性良好,RTM快速成型碳纤维/环氧树脂复合材料的力学性能和内部成型质量较好。      

Simultaneous gate and vent location optimization in liquid composite molding processes

In liquid composite molding processes the resin is injected into the mold cavity, which contains pre-placed reinforcement fabrics, through openings known as gates while the displaced air leaves the mold through openings called as vents. Gate and vent locations determine process outputs such as fill time, pressure requirements and whether the fabrics will be saturated entirely, a requirement for the success of the mold filling operation. Disturbances such as racetracking, in which the resin flows faster along the edges of the mold, further complicate the gate and vent selection process. In this study, a cascaded optimization algorithm, which is created by integration of branch and bound search and map-based exhaustive search, is proposed for simultaneous gate and vent location optimization in the presence of racetracking. Three case studies are presented to demonstrate usefulness of this methodology and the results are validated in a Virtual Manufacturing Environment.     

Analysis of non-isothermal mold filling process in resin transfer molding (RTM) and structural reaction injection molding (SRIM)

In this paper, we present a modeling and numerical simulation of a mold filling process in resin transfer molding/structural reaction injection molding utilizing the homogenization method. Conventionally, most of the mold filling analyses have been based on a macroscopic flow model utilizing Darcy's law. While Darcy's law is successful in describing the averaged flow field within the mold cavity packed with a porous fiber preform, it requires experiments to obtain the permeability tensor and is limited to the case of porous fiber preform-it can not be used to model the resin flow through a double porous fiber preform. In the current approach, the actual flow field is considered, to which the homogenization method is applied to obtain the averaged flow model. The advantages of the current approach are: parameters such as the permeability and effective heat conductivity of the impregnanted fiber preform can be calculated; the actual flow field as well as averaged flow field can be obtained; and the resin flow through a double porous fiber preform can be modelled. In the presentation, we first derive the averaged flow model for the resin flow through a porous fiber preform and compare it with that of other methods. Next, we extend the result to the case of double porous fiber preform. An averaged flow model for the resin flow through a double porous fiber preform is derived, and a simulation program is developed which is capable of predicting the flow pattern and temperature distribution in the mold filling process. Finally, an example of a three dimensional part is provided.     

Vent Location Optimization Using Map-Based Exhaustive Search in Liquid Composite Molding Processes

In liquid composite molding (LCM) processes, the resin is injected into the mold cavity, which contains preplaced reinforcement fabrics, through openings known as gates, while the displaced air leaves the mold through openings called vents. Under nominal conditions, the last points to fill are chosen as vent locations. However, due to imperfect preform cutting and placement, gaps and channels may form along the edges and curvatures in a mold, offering a path with less resistance for resin flow. The faster advance of resin through these gaps and channels, a common disturbance known as racetracking, will cause the last filled regions to vary, which complicates the vent selection process. In this study, probabilistic racetracking modeling is used to capture last-filled region distribution over the mold geometry. Success criteria for mold filling are defined in terms of dry spot tolerances, and vent fitness maps, which display potential vent locations, are created. Next, exhaustive search algorithm is coupled with vent fitness maps to determine optimal vent configurations. The map-based exhaustive search is demonstrated on three geometries and results are compared with existing combinatorial search results. The performance of the optimal vent configurations is evaluated in a virtual manufacturing environment. Sensitivity analyses are conducted to determine the influence of optimization parameters on the results.     

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。      

Resin flow analysis with fiber preform deformation in through thickness direction during Compression Resin Transfer Molding

Resin flow during Compression Resin Transfer Molding (CRTM) can be best described and analyzed in three phases. In the first phase, a gap is created by holding the upper mold platen parallel to the preform surface at a fixed distance from it. The desired amount of resin injected into the gap quickly flows primarily over the preform. The second phase initiates when the injection is discontinued and the upper mold platen moves down squeezing the resin into the deforming preform until the mold surface comes in contact with the preform. Further mold closure during the final phase will compact the preform to the desired thickness and redistribute the resin to fill all empty spaces. This paper describes the second phase of the infusion. We assume that at the end of phase one; there is a uniform resin layer that covers the entire preform surface. This constrains the resin to flow in through the thickness direction during the second phase. We model this through the thickness flow as the load on the upper mold forces the resin into the preform, simultaneously compacting the preform. The constitutive equations describing the compaction of the fabric as well as its permeability are included in the analysis. A numerical solution predicting the flow front progression and the deformation is developed and experimentally verified. Non-dimensional analysis is carried out and the role of important non-dimensional parameters is investigated to identify their correlations for process optimization.