In-plane permeability characterization of the vacuum infusion processes with fiber relaxation

In vacuum infusion processes fiber preforms are placed onto the single molding surface and enveloped with a non-rigid polymer bag which is sealed to the molding surface. The flexible bagging film does deform during the resin infusion process thus changing the compaction of the fabric. However, one can also relax the preform by drawing a partial vacuum in a rigid chamber placed on top of the flexible bag which will increase the permeability of the fabric under the chamber. A numerical model is presented to characterize the change in permeability and describe the mold filling for such processes in which the fabrics undergo controlled relaxation by external stimuli. The predictions from the simplified model agreed reasonably well with the experiments. This characterization and resin flow front prediction with time method should prove useful in processes such as Vacuum Induced Preform Relaxation (VIPR) process which can be used to actively manipulate flow in a vacuum infusion process.     

Application of calcium carbonate in resin transfer molding process: An experimental investigation

The resin transfer molding (RTM) process is used to manufacture advanced composite materials made of continuous glass or carbon fibers embedded in a thermoset polymer matrix. In this process, a fabric preform is prepared, and is then placed into a mold cavity. After the preform is compacted between the mold parts, thermoset polymer is transferred from an injection machine to the mold cavity through injection gate(s). Resin flows through the porous fabric, and eventually flows out through the ventilation port(s). After the resin cure process (cross‐linking of the polymer), the mold is opened and the part is removed. The objective of this study is to verify the application of calcium carbonate mixed in resin in the RTM process. Several rectilinear infiltration experiments were conducted using glass fiber mat molded in a RTM system with cavity dimensions of 320 × 150 × 3.6 mm, room temperature, maximum injection pressure 0.202 bar and different content of CaCO₃ (10 and 40%) and particle size (mesh opening 38 and 75 µm). The results show that the use of filled resin with CaCO₃ influences the preform impregnation during the RTM molding, changing the filling time and flow front position, however it is possible to make composite with a good quality and low cost.     

Variation of part thickness and compaction pressure in vacuum infusion process

In vacuum infusion (VI), it is difficult to manufacture a composite part with small dimensional tolerances, since the thickness of the part changes during resin injection. This change of thickness is due to the effect of varying compaction pressure on the upper mold part, a vacuum bag. In this study, random fabric layers with an embedded core distribution medium is used. The thickness of the composite part and resin pressure are monitored using multiple dial gages and pressure transducers; the results are compared with the model developed by Correia et al. [Correia NC, Robitaille F, Long AC, Rudd CD, Simacek P, Advani SG. Analysis of the vacuum infusion molding process: I. Analytical formulation. Composites Part A: Applied Science and Manufacturing 26, 2005. p. 1645–1656]. To use this model, two material characteristics databases are constructed based on the process parameters: (i) the thickness of a dry/wet fabric preform at different compaction pressures, and (ii) the permeability of the preform at different thicknesses. The dry-compacted preform under vacuum is further compacted due to fiber settling in wet form after resin reaches there; the part thickens afterwards as the resin pressure increases locally. The realistic model solution can be achieved only if the compaction characterization experiments are performed in such a way that the fabric is dry during loading, and wet during unloading, as in the actual resin infusion process. The model results can be used to design the process parameters such as vacuum pressure and locations of injection and ventilation tubes so that the dimensional tolerances can be kept small.     

真空压力下树脂沿织物铺层厚度渗透速率

为了考察树脂膜熔渗(RFI)工艺过程中树脂在高温条件下沿织物铺层厚度方向的不饱和渗透特性, 应用自行设计的测试系统, 考察了液体沿织物铺层厚度方向流动前锋的影响因素, 测试并分析了液体沿玻璃纤维铺层厚度方向渗透速率的主要影响因素及其变化规律。结果表明, 液体沿纤维织物厚度方向流动为宏观上的一维流动。 真空压力增大、 树脂温度升高、 纤维体积分数减小, 均可使液体的渗透速率加快。另外, 对比发现, 70℃ E-51 环氧树脂沿玻璃纤维铺层厚度方向的渗透特性与室温下硅油的渗透特性基本相当。      

硅橡胶热膨胀工艺预浸料铺层内树脂压力变化规律

采用硅橡胶热膨胀工艺制备了碳纤维/双马树脂复合材料层板, 通过自行设计的成型模具及树脂压力在线测试系统测试并分析了成型过程中热胀压力和预浸料铺层内树脂压力的变化规律, 考察了工艺间隙和温度分布的影响, 并通过显微观察分析了不同工艺条件下层板的密实状况。结果表明: 采用树脂压力在线测试系统, 可实现热膨胀工艺预浸料铺层内树脂压力的测试; 工艺间隙和硅橡胶内的温度分布对树脂压力及硅橡胶的热胀压力有重要影响, 在零吸胶工艺条件下, 当工艺间隙设计合理时, 凝胶前热胀压力和树脂压力的变化趋势及变化程度基本一致, 固化层板纤维密实并且厚度均匀; 通过增加恒温平台减小硅橡胶内部温差, 可使热胀压力的增加速度减小。      

A numerical simulation of the resin film infusion process

A numerical analysis was conducted for the resin film infusion (RFI) process using semi-cured thermosetting resin films. Mathematical models were developed for the compression of fiber and the viscosity of resin. The force balance between the fiber preform and the resin was considered to account for the deformation of fiber preform and the swell of fiber during the infusion. In an effort to locate the optimal process conditions such as the mold temperature, the fiber volume fraction, and the infusion pressure, a parametric study was carried out for the progression of resin and the infusion time for different process conditions. The numerical code developed in this study was found to be useful in determining the maximum height of vertical sections that can be infused by squeezing the liquefied resin film from the base panel.     

Comparison of Tensile Strength of Composite Material Elements with Drilled and Molded-in Holes

Holes are generally obtained through drilling operations; this causes a property decrease for polymer composites reinforced by fibers, brought about by damage due to fiber continuity interruption, and to delamination between the laminate layers. In this study, specimens with circular holes, both drilled and molded-in, obtained in different ways, are tested in order to investigate on whether it is possible to avoid the decrease in mechanical properties of components with holes. In particular, a number of laminates were manufactured by RIFT (Resin Infusion under Flexible Tool), a closed mold process capable of obtaining large and complex forms, impregnating, under vacuum, a dry preform placed on the rigid mold. At specific points of these laminates, molded-in holes are generated during the resin infusion phase, operating in two different ways: displacing or cutting the fibers in the dry preform. Tensile tests were carried out in order to compare the mechanical properties of elements in composite materials which have molded-in holes generated during the impregnation process, with the properties of those with holes produced after the resin cure by drill operations.     

Measurement of transverse permeability using gaseous and liquid flow

Accurate measurement of transverse permeability is important for processes such as resin film infusion and vacuum-assisted resin transfer molding. In these liquid composite molding processes the out-of-plane flow is dominant and thus the transverse permeability is needed for flow prediction. This paper introduces an apparatus to measure saturated permeability for fibrous preforms using both gaseous and liquid flow. The setup creates a uniform one-dimensional flow through-the-thickness of the reinforcement by integrating a high permeability layer on the mold surfaces. A wide range of permeability as a function of fiber volume fraction can be measured in one experiment while applying a known load under a hydraulic testing machine. The system has been designed using process simulation. The measurements using the gaseous medium are comparable to the saturated fluid flow results. The measurement system can also be used to measure changes in dry fabric permeability prior to infusion due to debulking or application of binders on the fabric surface.     

The effect of fabric and fiber tow shear on dual scale flow and fiber bundle saturation during liquid molding of textile composites

In Liquid Composite Molding (LCM) processes, a fibrous reinforcement preform is placed or draped over a mold surface, the mold is closed and a resin is either injected under pressure or infused under vacuum to cover all the spaces in between the fibers of the preform to create a composite part. LCM is used in a variety of manufacturing applications, from the aerospace to the medical industries. In this manufacturing process, the properties of the fibrous reinforcement inside the closed mold is of great concern. Preform structure, volume fraction, and permeability all influence the processing characteristics and final part integrity. When preform fabrics are draped over a mold surface, the geometry and characteristics of both the bulk fabric and fiber tow bundles change as the fabric shears to conform to the mold curvature. Numerical simulations can predict resin flow in dual scale fabrics in which one can separately track the filling of the fiber tows in addition to flow of resin within the bulk fabric. The effect of the deformation of the bulk fabric due to draping over the tool surface has been previously addressed by accounting for the change in fiber volume fraction and permeability during the filling of a mold. In this work, we investigate the effect of shearing of the fiber tows in addition to bulk deformation during the dual scale filling. We model the influence of change in fiber tow characteristics due to draping and deformation on mold filling and compare it with the results when the fiber tow deformation effect is ignored. Model experiments are designed and conducted with a dual scale fabric to characterize the change in permeability of fiber tow with deformation angle. Simulations which account for dual scale shear demonstrate that the tow saturation rate is affected, requiring longer fill times, or higher pressures to completely saturate fiber tows in areas of a mold with high local shear. This should prove useful in design of components for applications in which it is imperative to ensure that there are no unfilled fiber tows in the final fabricated component.     

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成型玻璃纤维增强阴离子聚合尼龙6复合材料及其性能

针对己内酰胺阴离子聚合反应特性,通过自行设计的适合该反应体系的树脂注射成型机和搭建的热塑性树脂传递模塑(T-RTM)成型实验平台,在恒压注射条件下,成功地制备了玻璃纤维(GF)体积分数高达48vol％的GF增强阴离子聚合尼龙6(APA-6)复合材料.研究了不同进料方式和压力、模具温度和纤维含量等工艺条件对GF/APA-...     

三维夹芯层连织物复合材料真空辅助成型工艺影响因素

三维夹芯层连织物复合材料是由纤维连续织造呈空芯结构的织物作为增强体制备而成的新型轻质复合材料, 本文中以三维夹芯层连织物复合材料为研究对象, 发展了适用于酚醛树脂的真空辅助成型工艺方法, 重点考察了影响复合材料制造质量的关键因素。结果表明, 芯柱高度对织物的抗压缩能力与厚度回复率影响显著, 热处理有利于织物的厚度回复; 注射过程中, 树脂沿织物平面纬向渗透速率大于经向, 使用高渗透率介质层、 降低树脂黏度有助于提高树脂在层连织物中的分布均匀性。      

Simulating tape resin infiltration during thermoset pultrusion process

The thermoset tape pultrusion is a widely adopted manufacturing process to produce long, constant cross-section composite structural parts. For high volume production, low cost can be achieved by maximizing the production rate which is a function of the material and process parameters, more specifically the rate of resin infiltration and resin cure. During resin infiltration, the resin saturates the dry reinforcement either under positive pressure in the pressure chamber, or, by the action of capillary and surface forces, within the resin bath. In either case, the saturation must be completed as the tape is squeezed into the final cross-sectional form at the entrance of the heated mold where the resin will be cured to form the composite part.This paper models the resin infiltration process during pultrusion, by modifying the pre-existing simulation tool for liquid molding processes. The formulated capability can be used not only to optimize the impregnation dynamics within the pressure chamber, but can also be used to predict the required forces for the selected pulling rate. The proposed model does allow one to handle a variety of tape cross-sections, not just rectangular prisms.     

Compaction of e-glass fabric preforms in the Vacuum Infusion Process,A: Characterization experiments

An experimental procedure was designed to realistically characterize the compaction behavior of e-glass fabric preforms during initial application of vacuum and mold filling stages of Vacuum Infusion (VI). To mimic VI, the loading (compaction) was done on a dry preform, and the unloading (decompaction) was done after the preform was saturated with resin. When fabrics were wetted at constant full compaction pressure, a significant decrease in thickness was observed for the random fabric, but not for woven and biaxial fabrics. The rate of change of thickness, ?h/?t had different signs and order of magnitudes when various constant compaction pressures were applied during fiber relaxation stage. Thus, previous compaction-mold filling models based on static relationship between thickness and compaction pressure do not appropriately simulate the compaction physics of VI. Time-dependent database of this study is a useful and straightforward tool to model VI, as demonstrated in Part B of this study.     

浸渍过程中树脂流体在纤维集合体内的流动行为

建立了浸渍过程中树脂基体在纤维集合体内流动的统计力学模型。将树脂流体的流动过程视为纤维/树脂系统降低能量达到平衡的过程,从微观角度研究树脂流体在纤维集合体内的流动行为。在建模中,不仅考虑了代表系统内能的Hamilton函数和界面张力及驱动流体的压力对系统所作的功,还考虑了流动过程中纤维集合体对流体的摩擦阻力所作的功,完整地反映了在流体的流动过程中系统能量的变化。模拟了水在聚酯纤维非织造织物内及不饱和聚酯树脂在玻纤机织布内的平面径向流动,并设计了相应的实验,以检验模型对不同流动性能流体和不同结构纤维集合体的适应情况。实验与模拟结果良好的一致性表明,所建立的模拟能正确地反映流体在纤维集合体内的流动特征。     

Resin impregnation during composites manufacturing: Theory and experimentation

An experimental investigation into the area of resin impregnation during the manufacturing of composite materials is undertaken. The study is specifically directed at furthering the degree of understanding of the resin transfer molding and resin film stacking manufacturing processes. An additional goal of the present work is the testing of a previously developed numerical model for simulating such processes. The experimentation performed consists of the fully monitored impregnation of resin into a thin mold containing a three-dimensional reinforcement fiber composite preform material. Experimental results for two cases involving highly anisotopic resin impregnation are compared to corresponding numerical results. Reasonable agreement between the two sets of results is found to exist and suggestions are made as to future work which could further increase the technical understanding of such processes.     

Development of a composite cargo door for an aircraft

Nowadays, aircraft manufacturers are not only looking for ways to reduce the structural weight of their aircraft but they are also searching for structural concepts that will lead to a cost reduction. One way to realize a cost reduction is to design a component with a high level of part integration since this will lead to a reduction in labor intensive trimming and assembly costs. By using composites in combination with new fabrication concepts this part integration becomes feasible. One of these new fabrication concepts is resin transfer moulding (RTM) with pre-pregs. In traditional RTM processes, dry fiber pre-forms are positioned in a mould cavity. After the mould is closed, resin is injected into the mould and the fibers are impregnated. In the RTM process described in this paper, parts of the dry pre-form are replaced by pre-preg. After closure of the mould, the mould is heated and the resin in the pre-preg starts to melt. Then RTM resin is injected into the mould. The pressure of the RTM resin is used to pressurize the pre-preg. The main advantage of this fabrication concept is that sub-preforms can be made very easily in pre-preg that would be very difficult to make with dry fabric due to the lack of tack. Another advantage is that the RTM process time is reduced, because only a small quantity of resin has to be injected. In order to demonstrate the feasibility of this fabrication concept, a hat-stiffened cargo door concept was developed. Two doors were made. The doors were tested by applying a pressure difference to the door of 0.12 MPa. Both doors did not fail during the tests.     

Analysis and minimization of void formation during resin transfer molding process

In the resin transfer molding process, residual air in the pores of fiber preform results in dry spots and microvoids in the finished product. The dry spots are usually formed due to irregular permeability of fiber mat and improper injection locations. The microvoids result from non-uniform microarchitecture of the fiber preform, and they are transported through the gap between fiber tows during infiltration of the resin. In this study, a real-time simulation/control method was proposed to actively control the formation and the transport of air voids during the mold filling. The flow equations were solved in real time to predict the change of the flow front shape. The flow front was detected by optical sensors and the control actions were taken based on the sensor signals. Through this automated simulation/control scheme, a real-time control of resin flow could effectively avoid the dry spots and minimize the formation of microvoids by modulating the injection pressure.     

视窗化RTM工艺充模过程模拟仿真技术研究

根据RTM工艺树脂流动充模模型,研究和开发了基于FEM/CV算法的RTM工艺复杂渗流充模过程数值模拟软件平台-BHRTM-2。BHRTM-2在视窗系统下运行,带有FEM网格捕捉器窗口可直观方便地设置注射口、溢料口和工艺参数,操作简单,能够模拟复杂边界制件的树脂流动充模过程、显示充模过程中任意时刻模腔内压力的分布场、流动前峰和预测充模时间及可能的干斑缺陷位置,为RTM工艺设计与优化提供了有效技术手段。文中对BHRTM-2的模拟结果的正确性和可靠性进行了理论与实验验证,并给出了具体算例。      

Dry fiber automated placement of carbon fibrous preforms

The superior material properties of carbon fiber-reinforced composites make them especially attractive for applications in aeronautics and aerospace industries. Cost reduction and time saving are continuously driving industry, leading to new industrial challenges which include manufacturing composite structures with optimal mechanical performances using the potential of advanced processes using robotics.To produce complex part shapes, technologies implying fabric draping in a mold imply large waste amount, fabric structure variability and uncertainties concerning local fiber volume fraction amount and thus final mechanical properties. To overcome such issues and comply with cost and time efficiency, automated dry fiber placement for preform manufacturing is proposed. This approach allows to integrate many functions in a complex part thank to the ability of the robot to steer fiber tows at specific locations. The final composite part is obtained by injecting the produced preform with resin using RTM (Resin Transfer Molding) or infusion process.The presented project aims to define the influence of the process driving parameters during fiber placement on the final preform properties range. Preforms were produced using a lab-scale automated placement demonstrator. Three preforms configurations were tested to highlight the influence of the preform structure on permeability and mechanical parameters through characterization of the compression behavior and permeability of the produced preforms. Choice of configuration will affect mechanical properties on the manufactured preforms, whereas creation of open channels to enhance the flow propagation during manufacturing does not necessarily increase the preform permeability.