A multi-segment injection line and real-time adaptive,model-based controller for vacuum assisted resin transfer molding

To improve process controllability during VARTM, a new resin injection line was developed. The injection line, which consists of multiple segments each independently operated, allows for the delivery of resin to different locations within the mold. Simulation of different injection line configurations for various mold geometries is studied and validated with laboratory size molds used to demonstrate the potential value and benefits of this approach. Computer-based, closed-loop controller strategies have been designed that use point sensor feedback of resin location within the mold during injection to operate the segmented injection line. In addition to a simple closed-loop control algorithm, an adaptive controller was developed that uses a finite element model to simulate filling characteristics in real time and configure the segmented injection line to minimize dry, unfilled regions within the mold. Simulations and experiments with VARTM molds containing large permeability variations revealed that mold filling was significantly improved with closed-loop control compared to the open-loop response with a conventional injection line.     

树脂传递模塑工艺中的非饱和流动过程模拟与实验研究

针对编织类纤维增强体的纤维束之间与纤维束内孔隙的双尺度特点,建立了平纹织物的细观结构模型,并推导了汇函数的数学表达式。建立了局部细观流动特征的非饱和流动控制方程,利用有限元/控制体积方法求解,得到了局部饱和度分布。与实验进行比较,吻合较好。      

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.     

Experimental validation of post-filling flow in vacuum assisted resin transfer molding processes

In Liquid Composite Molding (LCM) processes with compliant tool, such as Vacuum Assisted Resin Transfer Molding Process (VARTM), resin flow continues even after the inlet is closed due to the preform deformation and pressure gradient developed during infusion. The resin flow and thickness changes continue until the resin pressure becomes uniform or the resin gels. This post-filling behavior is important as it will determine the final thickness and fiber volume fraction distribution in the cured composite. In this paper, a previously proposed one dimensional coupled flow and deformation process model has been compared with the experimental data in which the resin pressure and part thickness at various locations during the post-filling stage is recorded. Two different post-infusion scenarios are examined in order to determine their impact on the final part fiber volume fraction and thickness. The effects of different venting arrangements are demonstrated. The model predictions compare favorably with the experimental data, with the minor discrepancies arising due to the variability of material properties.     

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。     

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.     

Optimization of injection flow rate to minimize micro/macro-voids formation in resin transfer molded composites

Resin transfer molding (RTM) has become one of the most widely used processes to manufacture medium size reinforced composite parts. To further enhance the process yield while ensuring the best possible quality of the produced parts, physically based optimization procedures have to be devised. The filling of the mold remains the limiting step of the whole process, and the reduction of the filling time has an important impact on the overall cost reduction. On the other hand, the injection cycle has to be appropriately carried out to ensure a proper fiber impregnation. Indeed, a partial fiber impregnation leads to the creation of micro-scopic and macro-scopic voids.In the present work, based on a double scale flow model and the capillary number Ca, an optimization algorithm is proposed to minimize the micro/macro-voids in RTM composite parts. The optimized injection flow rate ensures an optimum Ca at the flow front during part filling. The implemented algorithm allows the use of various constraints such as maximum capabilities of the injection equipment (i.e., maximum pressure or flow rate at the injection gates) or maximum velocity to avoid fiber washing. Bounded by these constraints, the optimization procedure is devised to handle any injection configuration (i.e., injection gates or vents locations) for two or three-dimensional parts. The numerical model is based on a mixed (FE/CV) formulation that uses non-conforming elements to ensure mass conservation. The proposed algorithm is tested for two and three-dimensional parts while emphasizing the important void reduction that results from the optimized injection cycle.     

Monitoring of resin flow in the resin transfer molding (RTM) process using point-voltage sensors

Multiple point-voltage sensors were used to monitor the mold filling stage of the resin transfer molding (RTM) process. Both lineal- and point-voltage sensors are electrical circuits in which the two poles of the sensor are closed when liquid thermoset resin arrives at the sensor location in the mold cavity. The electrical conductance of the liquid resin causes an increase in the output voltage, V_sens of the circuit. Although the gradually varying in situ data of a lineal sensor is more informative than a point-voltage sensor, lineal-voltage sensors might mislead the user if the resin covers the wires at multiple sections, or if the resin covers the wires starting from an unexpected section. Two kinds of sensors were developed: a set of similar, wrapped and compact lineal-voltage sensors acting as point-voltage sensors; and a point-voltage sensor with voltage amplification. Without this amplification, the increase in V_sens might be difficult to detect if the resin system has a low electrical conductivity and there is noise in the DAQ system. The accuracy and reliability of the new sensor system was verified by comparing the in situ sensor data with the visually recorded resin flow.     

新型树脂传递模塑技术

概述了传统树脂传递模塑(RTM)及在其基础上发展起来的新型RTM工艺,包括真空辅助树脂传递模塑(VARTM)、Seemann复合材料树脂浸渍模塑成型工艺(SCRIMP)和树脂膜渗透成型工艺(RFI)的成型原理、优点,并指出目前存在的缺点及解决方法.     

Influence of fabric shear and flow direction on void formation during resin transfer molding

In resin transfer molding, void type defect is one of common process problems, it degenerates the mechanical performances of the final products seriously. Void content prediction has become a research hotspot in RTM, while the void formation when the flow direction and the tow direction are not identical or the fabric is sheared has not been studied to date. In this paper, based on the analysis of the resin flow velocities inside and outside fiber tows, a mathematical model to describe the formation of micro- and meso-scale-voids has been developed. Particular attention has been paid on the influence of flow direction and fabric shear on the impregnation of the unit cell, so their effects on the generation and size of voids have been obtained. Experimental validation has been conducted by measuring the formation and size of voids, a good agreement between the model prediction and experimental results has been found.     

Interlayer toughening of resin transfer molding composites

In previous research it was shown that through using preformed elastomer particle modified tackifier/binder, materials interlayered RTM composite structures could be manufactured. These interlayers resulted in excellent toughness improvements of traditionally brittle RTM materials. In this work, the effects of tackifier application and composition were investigated by modifying spray and powder epoxy tackifiers with polyamide 6 particles. The spray tackifier provided 30% improvements in Mode II interlaminar fracture toughness, and slight increases in the interlaminar shear strength without reducing the thermal properties. The powder tackifier showed a slightly lower performance increase due to a less homogeneous laminate structure.     

Variations in unsaturated flow with flow direction in resin transfer molding: An experimental investigation

The dual-scale nature of fiber preforms due to the presence of large continuous gaps between fiber tows gives rise to the unsaturated flow in resin transfer molding (RTM) process which is characterized by a droop in the injection pressure history due to the delayed absorption of fiber tows (the ‘sink’ effect). In this study, we experimentally investigate the effect of change in flow direction on the unsaturated flow in three anisotropic dual-scale fiber mats. A series of 1-D mold-filling experiments involving a constant flow rate were conducted for a unidirectional woven fiber-mat, a biaxial stitched mat, and a triaxial stitched fiber-mat along with a reference single-scale random mat. In the case of the unidirectional mats, the droop in the inlet-pressure history, signifying the strength of the sink effect, is found to be strongest for flow along the micro-channels aligned with fiber tows. The droop, and hence the sink effect, is observed to weaken progressively for flow-directions at 45° and 90° to this principal direction. In the case of the biaxial and triaxial mats, the situation is more complex due to the multi-layer construction of such mats: maximum droop is found when mats are oriented at a 45° angle with respect to the fiber-mat coordinate, and it weakens in the 0° and 90° directions. The unsaturated flow effect is also quantified by measuring percentage deviation in the area under the experimental curve from that of the predicted curve. A clear correlation between the droop (through the percentage deviation) and the permeability along a flow direction in the unidirectional mats is observable, though such a relationship eludes the triaxial mat. The effect of unsaturated flow on liquid-front progress during the 1-D experiment was also studied. In contrast to the reference single-scale random mat where the observed front progress closely follow the prediction based on the single-scale physics, a small difference was observed between the observed and predicted front progress for the three dual-scale mats considered. However the difference was too small to yield any significant correlation with the flow direction.     

树脂传递模塑成型复合材料机翼干斑成因及控制方法

基于树脂传递模塑成型(RTM)工艺制作了整体成型的复合材料机翼,并针对机翼出现的干斑缺陷,采用PAM-RTM软件对干斑的成因进行分析,一方面由于上下翼面和前缘交接区域的纤维体积分数较高,导致该区域的树脂渗透率低于其他区域,从而出现包络现象,形成干斑;另一方面由于边缘效应的影响,使机翼内产生树脂快速流动通道,造成气泡包裹...     

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.     

Non-isothermal injection molding with resin cure and preform deformability

In this paper, a non-isothermal model to simulate some injection molding processes used to fabricate composite materials is deduced. The model allows the solid constituent in both the dry and the wet region to deform during infiltration. The dry porous material is assumed to behave elastically, while the mixture of resin and preform is assumed to behave as a standard linear solid. The model also takes into account the fact that the liquid undergoes an exothermic cross-linking reaction during infiltration and eventually gels stopping the infiltration process. Focusing then on one-dimensional problems it is shown that the integration of the mechanical problem in the uninfiltrated region can be reduced to the integration of an ordinary differential equation defining either the space-independent volume ratio or the location of the infiltration front, depending on whether the flow is driven by a given infiltration velocity or by a given inlet pressure. The remaining system of partial differential equations in the two interfaced and time-dependent domains is then posed with the proper interface and boundary conditions. After writing the problem in a Lagrangian formulation fixed on the solid constituent, domain decomposition techniques are used for the simulation.     

Analysis of vacuum bag resin transfer molding process

An analytical model is developed to analyze the resin flow through a deformable fiber preform during vacuum bag resin transfer molding (VBRTM) process. The force balance between the resin and the fiber preform is used to account for the swelling of fiber preform inside a flexible vacuum bag. Mold filling through multiple resin inlets is analyzed under different vacuum conditions. The formation of dry spots is demonstrated in the presence of residual air. Molding of a three-dimensional ship hull with lateral and longitudinal stiffeners is simulated to demonstrate the applicability of the model.     

Constraints on monitoring resin flow in the resin transfer molding (RTM) process by using thermocouple sensors

In this study, a thermocouple sensor system was used to monitor the resin transfer molding (RTM) process. These sensors are low-cost and durable; and they do not disturb the resin flow. They can be used if the inlet resin is either hotter or colder than the mold walls. In experiments of this study, much of the hot resin’s internal energy was transferred to cold mold walls by conduction, when the mold parts were made of a material with high thermal conductivity, such as aluminum. A mathematical model based on 1D flow and 2D unsteady energy conservation was developed to investigate the heat transfer between resin and mold walls. The numerical solution of this model is in qualitative agreement with the results of our experiments. The thermocouple sensor system developed is more useful with the following process parameters: low thermal conductivity of mold material, high resin flow rate, high temperature difference between inlet resin and initial mold walls, and high specific heat of resin. However, for the typical use of RTM materials and typical injection parameters, thermocouples should not be preferred over other sensor types and should be used with caution due to the shortcomings investigated in this study.     

Numerical simulation of molding-defect formation during resin transfer molding

The present study investigated a numerical simulation of molding-defect formation during resin transfer molding using boundary element method and line dynamics. The proposed method enables to simulate small molding defects by increasing the node for required position during time evolution; thereby, the method computes high-resolution flow front without being affected by the initial mesh geometry. The method was applied to the radial injection RTM with single inlet, and it was confirmed by comparison with theoretical value based on Darcy’s law that the flow advancement was computed with high accuracy. In addition, the method was also applied to the flow advancement for inclusion problem with cylinder, and four-point injection problem. The simulated flow behavior, void formation, and shrinkage agreed with the results in references. Finally, the method was compared with experiments using two-point injection problem. The computed configuration of the flow front and weld line agreed well with the experimental results.