Simulation and experimental validation of flow flooding chamber method of resin delivery in liquid composite molding

A new method of resin delivery, which we refer to as the flow flooding chamber (FFC), is investigated to improve infusion time and reduce material waste associated with the Vacuum Assisted Resin Transfer Molding (VARTM) process. The FFC method uses a rigid chamber that rests on top of the bagging material and a vacuum drawn inside the chamber stretches the bag to take the shape of the chamber above the fiber preform. Resin is then drawn into this chamber unimpeded, and once the chamber is full of resin, the release of the vacuum results in application of atmospheric pressure on top of the bag that drives the resin into the fiber preform. The distribution media and other subsequent materials for its removal are not needed in this modified VARTM process. This process is mathematically modeled using a two event model that couples them by using the output conditions from the first event to the input conditions of the second event. The model is implemented in a numerical simulation so one can track the movement of the resin into the chamber and the preform. Experiments using the FFC process are conducted in complex geometries containing inserts and the flow fronts and fill times are recorded. The results compare very well with the predictions validating the assumptions made in the model to describe the flow.     

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.     

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.     

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.     

基于混合网格方法的VARTM工艺充模仿真与实验验证

针对VARTM工艺的特点,建立了充模过程树脂流动和预成型体变形行为数学模型。提出了基于混合网格方法的VARTM充模仿真算法,在该算法中,模具型腔几何模型进行二维或三维网格划分,在每个真空袋表面单元上增加一个一维附属单元,用于在仿真过程中实时地吸收或挤出因真空袋变形而产生的局部树脂体积变化,形成混合网格仿真模型;求解过程中,对树脂流动和预成型体变形分别进行求解后,基于上述混合网格模型进行两者耦合操作,实现了仿真精度和速度的统一。搭建了VARTM充模实验平台,进行了一维充模实验,通过仿真结果与实验测量结果对比,验证了本文算法的正确性。最后,通过三维仿真算例,验证了算法对三维复杂结构和顺序浇口策略仿真的可行性。     

Flow modeling of the VARTM process including progressive saturation effects

Modeling of vacuum based liquid composite molding methods (e.g., VARTM) relies on good understanding of closely coupled phenomena. The resin flow depends on the preform permeability, which in turn depends on the local fluid pressure; the preform compaction behavior, and the membrane stresses in the vacuum bag. It has also been shown that for many preforms there is a significant unsaturated region behind the flow front, and that the flow in this region affects the overall flow behavior of the process. Studies of preform compaction have shown that the preform stiffness, as well as being non-linear and exhibiting significant hysteresis, is dependant on the fluid saturation. For this reason most researchers model the preform compaction based on the pressure-compaction behavior of saturated preforms during unloading. This assumption leads to an effective discontinuity in preform thickness at the flow front, which is not observed in actual experiments. In this paper an improved compaction model incorporating the saturation dependence of the compaction pressure in the partially saturated region, is used in a one-dimensional model of the VARTM process. The model gives physically more realistic results for the thickness in the flow front region, and an improved model for the consolidation of the preform at the end of infusion.     

Mechanical properties of composite structures fabricated with the vacuum induced preform relaxation process

A new technique called vacuum induced preform relaxation (VIPR) can be used to improve the processing of composite parts manufactured using vacuum resin infusion methods. The VIPR process is a method for manipulating and guiding the resin filling pattern during a vacuum assisted resin transfer molding (VARTM) manufacturing process with a relatively small external vacuum chamber. This VIPR chamber can be sealed against the flexible molding surface of a VARTM mold and used to create vacuum above the preform. This causes the compressive forces compacting the fabric to decrease allowing the resin to flow faster in the effected region under the chamber. Thus the chamber can influence the resin flow pattern as well as avoid the formation of voids due to merging flow fronts. When the regulated vacuum in the chamber is applied it temporarily decreases the fiber volume fraction of the preform. It is important to investigate if this relaxation has a permanent adverse effect on the mechanical properties of the composite. The results of these tests strongly suggest that the use of the VIPR process does not compromise the mechanical properties of composite structures.     

真空辅助成型工艺中预成型体的厚度变化与过流控制

采用无接触式电涡流位移传感系统, 对真空辅助成型工艺中预成型体的厚度变化进行了实时监测。揭示了该成型工艺过程中预成型体的厚度变化规律, 并考察了树脂过流控制时间对制件厚度与纤维体积含量的影响。结果表明, 在整个工艺过程中预成型体的厚度变化可分为三个阶段: 在树脂浸入后, 预成型体厚度迅速增加; 在树脂过流控制阶段, 预成型体厚度变化较小且保持在较高水平; 在树脂管关闭后, 预成型体厚度迅速下降并逐渐趋于稳定。制件厚度与树脂过流控制时间的变化关系类似于正弦曲线, 在树脂过流控制时间约为10 min时, 纤维体积分数最低, 较无过流控制降低1.7%; 在树脂过流控制时间约为40 min时, 纤维体积分数最高, 较无过流控制提高1.6%。     

In situ measurement and monitoring of whole-field permeability profile of fiber preform for liquid composite molding processes

For liquid composite molding (LCM) processes, such as resin transfer molding (RTM), the quality of final parts is heavily dependent on the uniformity of the fiber preform. However, the conventional permeability measurement method, which uses liquid (oil or resin) as its working fluid, only measures the average preform permeability in an off-line mode. This method cannot be used to create an in situ permeability profile because of fiber pollution. Further, the conventional method cannot be used to reveal preform's local permeability variations. This paper introduces a new permeability characterization method that uses gas flow to detect and measure preform permeability variations in a closed mold assembly before resin injection. This method is based upon two research findings: (1) resin permeability is highly correlated with air permeability for the same fiber preform with well-controlled gas flow, and (2) the whole-field air permeability profile of a preform can be obtained through measuring the pressure field of gas flow.In this study, first the validity of the gas-assisted, in situ permeability measurement technique was established. Then the technique was demonstrated as effective by qualitatively detecting non-uniformities and permeability variations in fiber performs. Finally, a two-dimensional flow model, based on the finite difference scheme, was developed to quantitatively estimate the whole-field preform permeability profile using predetermined pressure distribution. The efficacy of the new method was illustrated through experimental results.     

VARI工艺成型纤维增强树脂复合材料层合板厚度和纤维体积分数的影响因素

分析了影响真空辅助成型技术（VARI）工艺成型复合材料的纤维体积分数和厚度均匀性的关键因素,即VARI成型工艺的树脂流动控制形式、纤维预制体状态、织物状态、树脂黏度,通过试验分析了各因素对VARI成型复合材料厚度和纤维体积分数的影响。试验结果表明,采用HFVI（high fiber-volume vacuum infusion）工艺、BA9914树脂及真空处理后的U3160单向机织物成型的纤维增强树脂复合材料层合板,其纤维体积分数和厚度均匀性能够接近预浸料/热压罐成型的复合材料制件的水平。     

Investigation on the spring-in phenomenon of carbon nanofiber-glass fiber/polyester composites manufactured with vacuum assisted resin transfer molding

Process-induced residual stress arises in polymer composites as a result of mismatched resin contraction and fiber contraction during the cure stage. When a curved shell-like composite part is de-molded, the residual stress causes the spring-in phenomenon, in which the enclosed angle of the part becomes smaller than the angle of its mold. In this paper, a new approach is presented to control and reduce the spring-in angle by infusing a small amount of carbon nanofibers (CNFs) together with liquid resin into the glass fiber preform using vacuum assisted resin transfer molding (VARTM) process. The experimental results showed that the spring-in angles of the L-shaped composite specimens were effectively restrained by the CNFs. An analytical model and a 3-D FEA model were developed to predict the spring-in phenomenon and to understand the role of CNFs in reducing the spring-in angle. The models agreed with the experimental results reasonably well. Furthermore, the analytical model explains how the CNF-enhanced dimensional tolerance control is accomplished through the reductions in the matrix’s equivalent coefficient of thermal expansion and linear crosslinking shrinkage.     

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.     

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.     

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。     

Processing for Sandwich Composite Based on Natural Fiber Facesheet and Nomex Honeycomb Core

The sandwich composite based on ramie fiber facesheet and Nomex honeycomb core was produced.The thermal properties of resin and fiber were measured by the differential scanning calorimetry (DSC),Thermogravimetrie analysis (TGA) and melt viscosity spectrometer.Influence of vacuum bag process and autoclave process on large curvature deformation capability and flatwise tension strengths of sandwich composites were studied.The results showed that honeycomb core of 48kg/m3 had very good deformation capability and the flatwise tension strength of sandwich composite by autoclave process is 1.07MPa vs 0.76MPa by vacuum bag process.     

液体模塑成型工艺用纤维织物厚度方向饱和渗透率的预测模型液体模塑成型工艺用纤维织物厚度方向饱和渗透率的预测模型

树脂在复合材料预成型体厚度方向的渗透能力对复合材料液体模塑成型工艺（LCM）的成功实施至关重要。本文采用连续加载的方式,研究了玻璃纤维增强树脂基复合材料液体成型过程中多轴向无屈曲织物（NCF）和斜纹织物（WF）的压缩响应行为,并建立描述该行为的数学模型。采用自制测试装置对预成型体在重力等不同注射压力驱动下的厚度方向渗透率进行测试,考察了预成型体纤维体积分数、测试流体注射压力等对预成型体厚度方向渗透率K_z的影响。基于预成型体压缩响应数学模型和厚度方向渗透率与注射压力的关系,对Kozeny-Carman公式进行修正,提出了变注射压力条件下的厚度方向渗透率预测模型。结果表明:预成型体厚度方向渗透率随着纤维体积分数的增大而减小,与Kozeny-Carman方程结果相符合。当纤维体积分数为0.42≤V_f≤0.58时,注射压力对厚度方向渗透率影响较大,实验结果验证了本文提出的预测模型;当纤维体积分数V_f≥0.58时,注射压力对厚度方向渗透率影响较小,厚度方向渗透率趋于恒定。      

Simulation of void formation in liquid composite molding processes

Air entrapment within and between fiber tows during preform permeation in liquid composite molding (LCM) processes leads to undesirable quality in the resulting composite material with defects such as discontinuous material properties, failure zones, and visual flaws. Essential to designing processing conditions for void-free filling is the development of an accurate prediction of local air entrapment locations as the resin permeates the preform. To this end, the study presents a numerical simulation of the infiltrating dual-scale resin flow through the actual architecture of plain weave fibrous preforms accounting for the capillary effects within the fiber bundles. The numerical simulations consider two-dimensional cross sections and full three-dimensional representations of the preform to investigate the relative size and location of entrapped voids for a wide range of flow, preform geometry, and resin material properties. Based on the studies, a generalized paradigm is presented for predicting the void content as a function of the Capillary and Reynolds numbers governing the materials and processing. Optimum conditions for minimizing air entrapment during processing are also presented and discussed.     

Application of natural fiber reinforced composites to trenchless rehabilitation of underground pipes

Although various trenchless technologies for repairing worn-out underground pipes have been developed and tried, they have not been widely used due to their drawbacks such as high cost, inconvenience of operation, and long construction time. The new rehabilitation process with glass fabric preform based on the vacuum assisted resin transfer molding (VARTM) could reduce cost and construction time much, but the entrapped air during resin transfer process often produced interior defects in the repaired composite.Since the glass fabric preform was not suitable for drain pipes or water pipes because of the health hazard of small broken glass fibers when the underground pipe is worn out, in this work, natural fibers were used for the trenchless rehabilitation of underground pipes. The permeability and strength of natural fiber reinforced composites were measured. The developed process with the natural fiber composites was found to satisfy both the required structural strength for rehabilitation and shorter processing time without interior defects.     

Post-filling flow in vacuum assisted resin transfer molding processes: Theoretical analysis

For rigid mold filling processes such as resin transfer molding, the resin flow stops when the preform is fully saturated with the resin. However, in vacuum assisted resin transfer molding process (VARTM), due to preform deformation the resin flow continues after the filling stage is complete as it does take a finite time for the pressure field to become uniform during this post-filling period. In this paper, the post-filling flow in the VARTM process with and without the membrane is examined. The governing equations for post-filling flow, in which the preform is allowed to deform, are developed with simplifying assumptions. A one-dimensional flow and deformation coupled process model is developed to simulate the time dependent pressure distribution during the post-filling stage. The model is implemented using finite differences, both in time and space, and utilizes the explicit time integration which is found to be conditionally stable. The change in pressure inside the mold during the post-filling stage is predicted for three different injection scenarios. The influence of the pressure distribution at the end of filling on the dwell time for the pressure to equilibrate and on the final thickness of the part is discussed. The effects of change in preform permeability and compliance on the dwell time and thickness are demonstrated and the extension of the model to more complex geometries and systems is outlined.     

A nonlinear compaction model for fibrous preforms

Based on the mechanics of porous media and physical insight gained from experimental observation, a model for predicting the nonlinear compaction of fibrous preforms in the resin transfer molding process is developed. A key physical constant — namely, preform bulk compressibility — is proposed to establish the relationship between the applied pressure and the preform bulk volume. The preform bulk compressibility is a function of fiber volume fraction and five parameters — the initial fiber volume fraction, the final (maximum attainable) fiber volume fraction, the initial pore volume compressibility, the fiber compressibility, and an empirical index. Results of compaction experiments on plain-woven fabric preforms and unidirectional non-woven materials support the validity of the model. Excellent agreement between theory and experiments has been obtained. The present model provides for fibrous preforms a nonlinear constitutive law whose coefficients can be physically interpreted.