Simon Bickerton

Modelling and simulation of resin transfer moulding (RTM)—gate control, venting and dry spot prediction

This work presents three simulation features developed in an existing mould filling simulation code for resin transfer moulding to address the processing issues encountered during manufacturing. They are gate control, venting, and dry spot formation. The first feature is achieved using numerical sensors, which simulate the sensors used in practice to control the process. This feature makes it possible to use the developed simulation code to design a control strategy to optimize mould filling time, or to reduce the injection pressure required. The second feature is developed to account for the presence of vents. With vent locations specified, simulation can be used to predict changes in the pressure distribution and mould filling pattern if the resin front is not directly connected to the vent. The third feature provides for the prediction of dry spot formation. The simulation code checks if air is trapped in the mould and keeps track of these trapped air pockets. It computes the location, size and the pressure of all dry spots formed in the filling process. The combination of these features enhances the ability of the code as a design tool. The gate control and dry spot prediction features can be used in conjunction to guide the design of optimal processing conditions. Controlled resin injection as a strategy is an indispensable way to prevent dry spot formation, and reduce operation costs. Similarly, venting and dry spot prediction can be used to detect possible flaws and to correct them before a prototype mould is constructed.

Investigation of draping and its effects on the mold filling process during manufacturing of a compound curved composite part

Resin Transfer Molding (RTM) is a composite material manufacturing process during which resin is injected into a mold cavity filled with a fibrous reinforcing preform. Application of woven and stitched fiber mats in fabricating preforms for RTM is a highly viable means of manufacturing affordable composites. Draping, in this paper, refers to the act of bringing a flat workpiece into contact with an arbitrary tool surface. As a result, the draping of woven and stitched mats tends to cause the mat to deform to the tool geometry. This paper details an experimental study designed to demonstrate the effect of draping preforms on mold filling, and final structure of the composite product. A mold was built providing a conical mold cavity. Flow visualization experiments were performed, as well as shearing angle and preform fiber volume fraction measurements from manufactured cone parts. Experimental results show that draping does significantly change fiber orientation, fiber volume fraction, permeability and hence the injection pressure or flow rate required to fill a mold. This data was compared with numerical prediction codes for preform deformation, permeability calculation, and mold filling. Preform deformation predictions were found to overpredict in areas of high deformation, due to the assumptions made by the numerical algorithm used. Permeability components calculated are reasonable, but are affected by overpredicted preform deformation, and interpolation methods used. Prediction of injection pressure and flow rate histories are good, while predicted flow front shapes fail to capture some experimental features. Difficulties in predicting flow front shapes are attributed to three mechanisms; a decrease in permeability magnitudes due to increased volume fraction, a smaller volume of fluid required due to increased volume fraction, and reorientation of fiber tows changing the direction of principle permeability components.

Characterization and modeling of race-tracking in liquidcomposite molding processes

Abstract Liquid composite molding (LCM) processes require the impregnation of a polymeric resin through a porous preform, usually composed of glass, carbon, or Kevlar fibers. Numerical mold-filling simulations are currently being developed to predict the flow through LCM mold cavities and are a powerful design tool. The accuracy of such simulations are very sensitive to the permeability components, input provided by the user to represent the resistance to flow provided by the porous preform. Air channels can be present within a mold cavity, either unintentionally formed, or intentionally placed to enhance the mold filling process. Such channels provide paths of relatively low flow resistance, and can drastically alter flow front advancement and injection pressures, this effect being commonly referred to as ‘race-tracking. Race-tracking must be considered in numerical mold-filling simulations, one common approach being the application of ‘equivalent permeabilities to finite elements within a model that physically represent the air channels present. A planar rectangular mold cavity is studied here, having a single air cavity of known dimensions running along one side of the mold. This geometry provides the simplest flow configuration, all Darcy velocity components being in-plane. In this paper, the equivalent permeability magnitudes are based upon steady-state, fully developed flow through a rectangular duct. Detailed flow visualization experiments have been completed, recording flow-front advancement and injection-pressure histories, for two different preform types, and a variety of mold cavity thicknesses, preform volume fractions air-channel widths. Significant effort was made to provide exact comparisons of both flow front advancement and injection pressure, between the experiments and simulations based on the equivalent permeability approach. The comparisons are very good, the magnitude of the deviations being close to what should be expected from natural permeability variations from preform to preform. Sources for experimental error have been examined, the major limitation of these experiments being caused by the complex interaction of the acrylic mold, and stiffening bars used. The equivalent permeability approach has been shown to model flow front advancement and injection pressures very well within a Darcys law based mold filling simulation, for the volume fractions studied (0.50 to 0.16). This study has been limited to low volume fractions, and care should be taken when extending the equivalent permeability approach to woven and stitched preform styles. An alternative numerical method was investigated, modeling flow in the preform by using Darcys law, and one-dimensional Stokes flow in the air channel, with a variable source term to model the transverse flow into the preform. The results from the two simulations are in excellent agreement, demonstrating that the equivalent permeability approach employing Darcys law over the entire domain, models the transverse flow into the preform from the air channel accurately.

Fabric structure and mold curvature effects on preform permeability and mold filling in the RTM process. Part I. Experiments

Abstract Liquid composite molding (LCM) processes require the impregnation of a polymeric resin through a porous preform, being composed of glass, carbon, or kevlar fibers. The successful manufacture of composite parts through these methods is dependent on the successful filling of the mold cavity, expelling all air from within. Numerical simulations are being developed to model this process, and are powerful mold design tools. The accuracy of such simulations is strongly dependent on the specification of the preform permeability tensor throughout the mold cavity, being a numerical description of the resistance to resin flow. Changes in preform architecture can easily modify local permeability, and hence the mold filling. Corners in mold cavities are a potential sites for preform deformation, and are the focus of this study. A family of five molds have been studied, containing corner radii from 0.06 to 8.0xa0in. A detailed flow visualization study has been completed to investigate the influence of mold corner radii on flow front progression and injection pressure. While flow front progression was not significantly affected by different corner radii, injection pressures were found to be greater for the molds with smaller corner radii. Actual composite parts manufactured in the same molds have revealed that the molds are not of constant cavity thickness, as was the original goal. To determine the importance of corner radii on mold filling, it will be necessary to separate the effects of in-plane preform compression from any effect due to corners. To accomplish this goal, the experimental data presented here is being compared with detailed numerical studies, which will be presented in part II of this work.

Fabric structure and mold curvature effects on preform permeability and mold filling in the RTM process. Part II. Predictions and comparisons with experiments

Abstract In Part I, an experimental study was completed in a series of five molds, each having corners of different radii (from 0.06 to 8.0xa0in.). The primary goal of this work has been to determine whether corners in LCM molds significantly affect the filling process, by altering the structure of the preform locally in such regions. Consistent trends were found for each series of experiments completed in the mold, for which the same preform type, and number of layers were used. For constant flow rate injection, the required injection pressures to fill the two molds with tighter radii was significantly increased as compared to the other molds. Composite parts were manufactured in these molds, measurements made on these parts revealing design flaws, the cavity thicknesses not being equal in all sections of the molds. Several numerical simulations are presented in this paper, the goal being to separate any effects due to the varying thicknesses from effects due to the corners present. Careful simulations have been completed, taking into account the actual thicknesses in each mold, and the resulting preform volume fraction. Experimentally measured permeability data was employed, and the predicted injection pressures match very well for all four molds studied, seeming to indicate that the corners present have not affected the filling. A simple model for preform compression in corners has been developed, which predicts local permeability modifications due to in-plane compression of the fabric layers. These predictions have been employed in conjunction with an existing tool to analytically predict the permeability components of a preform in a flat cavity. This code requires from the user a geometrical model of a preform unit cell, this data being measured from samples cut from the parts manufactured. The resulting predictions for injection pressure are good for an entirely predictive approach, underpredicting experimental values by only 30–60%. Sensitivity analyses have demonstrated the strong relationship between permeability and the details of the preform cell. Two numerical studies were completed to determine how sensitive the injection pressure curves are to reduced permeabilities in the corner regions. For the two injection schemes having two different gate locations, pressures were not significantly affected, while the permeabilities in this region were reduced up to 100 times. Though the molds used were not ideal for isolating effects on mold filling due to corner radii, the evidence presented does not show the existence of any strong behavior related to mold radii.

Design and application of actively controlled injection schemes for resin-transfer molding

Liquid composite molding (LCM) processes provide the potential for producing complex net-shape components with relatively simple hardware requirements. The physical complexity of these processes has motivated the development of numerical process simulations, addressing preform manufacture, mold filling, and resin cure. While these simulations have increased the scope of parts successfully manufactured, it is argued here that in order to bring these processes to their full potential, intelligent application of active-control technology should be applied. In this paper we investigate active control of resin transfer molding (RTM), focusing on mold filling. A fully automated flow-visualization testbed has been developed, integrating a multiple line injection system, flow-front sensors, and automated vent assemblies. A mold-filling case study is presented here, demonstrating a design methodology for actively controlled injection schemes. The resulting injection scheme is implemented experimentally, and is shown to eliminate dryspots in all cases. As increasingly complicated parts are addressed, it is imperative that well-defined design methodologies are developed, that can be easily applied to the widest variety of components.

Experimental investigation and flow visualization of the resin-transfer mold-filling process in a non-planar geometry

Abstract Resin-transfer molding (RTM) is a manufacturing process for composite materials in which resin is injected into a mold cavity filled with a fibrous preform. Complete saturation of the preform during RTM is necessary for successful manufacturing and performance of the composite part. Unsuccessful filling of the mold may lead to the formation of macroscopic and microscopic voids, both defects significantly affecting the quality of the finished product. Recently much effort has been directed towards modeling and simulation of the RTM process. Such simulations usually model the filling of RTM molds as flow through porous media employing Darcys law. An experimental study has been completed, the goal of which was to develop a library of experimental flow visualization and pressure measurement data. A mold was constructed, providing a moderately complex three-dimensional mold cavity. The cavity shape is a thin-shelled, five-sided box, having a base and four sides. The mold was built to provide flexibility, including multiple injection, pressure measurement, and venting sites. A wide variety of experiments were completed, varying a number of important processing parameters. All experiments were isothermal, both mold and fluid being held at room temperature. Several examples of the experiments completed are described, and organized into two case studies. The first case study demonstrates how three experiments were used in an initial investigation into the ‘racetracking’ phenomenon. The second study investigates the effect of preform fiber volume fraction on fluid injection pressure. Numerical simulations of the experiments have been completed which demonstrate how existing numerical simulation codes can be validated against the experimental data.

Real-time sensing and control of resin flow in liquid injection molding processes

This paper presents results from an experimental investigation into real-time sensing and control of resin flow in an resin transfer molding (RTM) process. The objective of the research was to develop intelligent process control methodologies using in-situ sensors and process models, for real-time control of the RTM process. The real time control of the RTM process enables an increase in throughput, high yields, low defects, and consistent repeatability of quality between the parts. We concentrated on controlling the resin flow using multiple injection ports and vent locations. The results from the preliminary investigations indicate the feasibility of implementing real-time control on the RTM production floor.