Pavel Simacek

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.

A numerical model to predict fiber tow saturation during liquid composite molding

Abstract A dual scale porous medium contains two distinct scales of pores. We will consider the case in which small scale pores are ordered within well-defined sub-regions. Typical examples of such media are textile preforms used in various composite-manufacturing processes. Rigorously, the phenomenon can be modeled by using Darcys law to describe flow through porous medium and mass conservation for the flow within the larger pores. The smaller pores can be included within these equations as a sink term. This approach, though straightforward, poses implementation difficulties. In this paper, we suggest an alternative way to model this concept. We use the standard finite element/control volume approach and model the “internal” sink term by appending extra one-dimensional elements to control volumes associated with the control volumes of discretized part geometry. This approach offers two advantages over previously attempted schemes: (1) the problem to be solved remains linear and flow can be calculated explicitly within the time domain and (2) existing simulation packages for RTM filling simulation will be able to incorporate saturation effects to simulate the flow in dual scale media. To illustrate this point we present the implementation of the developed algorithm within the frameworks of an existing simulation package LIMS. The implementation is capable of providing saturation data during filling of arbitrarily shaped part and can capture the influence of saturation on filling pressure or flow-rate. This should prove useful in determining the time needed to completely saturate the fiber tows, which is crucial for part performance and also allow us to explain the variation in injection pressure and difficulties in “assigning” single scale permeability for such porous medium.

An approach to couple mold design and on-line control to manufacture complex composite parts by resin transfer molding

During impregnation of the resin into a closed mold containing the preform in the resin transfer molding (RTM) process, increased yield of successful parts can be achieved if one could account for the inherent disturbances such as race-tracking and preform variability. One way to address this is to use sensors and actuators to control the resin flow dynamics during the impregnation process to counteract the disturbances. In this paper, we use a mold filling simulation tool to develop a design and control methodology that, with the help of sensors and actuators, identifies the flow disturbance and redirects the resin flow to successfully complete the mold filling process without any voids. The methodology is implemented and experimentally validated for a mold geometry that contains complex features such as tapered regions, rib structures, and thick regions. The flow modeling for features such as ribs and tapered sections are validated independently before integrating them into the mold geometry. The approach encompasses creation of software tools that find the position of the sensors in the mold to identify anticipated disturbances and suggest flow control actions for additional actuators at auxiliary locations to redirect the flow. Laboratory hardware is selected and integrated to automate the filling process. The effectiveness of the methodology is demonstrated by conducting experiments that, with feedback from the sensors, can automate and actively control the flow of the resin to consistently impregnate all the fibers completely despite disturbances in the process.

Use of Resin Transfer Molding Simulation to Predict Flow, Saturation, and Compaction in the VARTM Process

Vacuum Assisted Resin Transfer Molding (VARTM) and Resin Transfer Molding (RTM) are among the most significant and widely used Liquid Composite manufacturing processes. In RTM preformed-reinforcement materials are placed in a mold cavity, which is subsequently closed and infused with resin. RTM numerical simulations have been developed and used for a number of years for gate assessment and optimization purposes. Available simulation packages are capable of describing/predicting flow patterns and fill times in geometrically complex parts manufactured by the resin transfer molding process. Unlike RTM, the VARTM process uses only one sided molds (tool surfaces) where performs are placed and enclosed by a sealed vacuum bag. To improve the delivery of the resin, a distribution media is sometimes used to cover the preform during the injection process. Attempts to extend the usability of the existing RTM algorithms and software packages to the VARTM domain have been made but there are some fundamental differences between the two processes. Most significant of these are 1) the thickness variations in VARTM due to changes in compaction force during resin flow 2) fiber tow saturation, which may be significant in the VARTM process. This paper presents examples on how existing RTM filling simulation codes can be adapted and used to predict flow, thickness of the preform during the filling stage and permeability changes during the VARTM filling process. The results are compared with results obtained from an analytic model as well as with limited experimental results. The similarities and differences between the modeling of RTM and VARTM process are highlighted.Copyright

Notes on the Modeling of Preform Compaction: I -Micromechanics at the Fiber Bundle Level

Resin Transfer Molding (RTM) can be described as a two step process: fiber preforming followed by resin infusion and cure. In the first stage, the fabric form making up the preform element is placed in the tool and is compacted due to the pressure from tool closure. During this process, the microstructure, and the resultant properties, change considerably since compaction flattents the weft yarn bundles from the initial configuration into flat ellipsoids of higher aspect ratio, while simultaneously reorienting and flattening the bundles in the warp direction. The movement also results in nesting and inter-layer packing yielding a higher localized fiber volume fraction. An aspect unique to RTM and other allied resin infusion processes is the compaction of fibers in the dry (unlubricated) form. This gives rise to rather unique behavior at the fiber tow level, which in turn affects the compaction of the fabric form itself. It is thus essential that any model constructed for the specific purpose of investigating the compaction stage in RTM (or allied processes that involve the compaction of a dry preform) be able to reflect that the tow is dry, usually untwisted and when compressed in bulk it has frictional resistance to the shear mode of deformation. In this paper we focus on the possible tow behavior, rather than the behavior of the fabric structure, with the motivation being that of developing a simple, yet comprehensive model at that level which can then be intregrated into a fabric level model at a later stage.

A closed form solution to describe infusion of resin under vacuum in deformable fibrous porous media

In composites manufacturing, when the preform is compacted and the resin is drawn under vacuum, the resin flow can be modelled using Darcys Law in deformable fibrous porous media. The compaction of the preform and the resin pressure are coupled. The governing equations for resin flow and compaction are presented. An analytical solution for one-dimensional flow in the axial direction and compaction in the thickness direction is derived that includes the local deformations of the porous medium and compaction effects on permeability. The coupling between flow and compaction is introduced with arbitrary constitutive laws that describe permeability of the material as a function of fibre volume fraction and a non-linear elastic material equation for compaction. The important implications of the results to composites manufacturing are discussed.

Modeling Flow in Compression Resin Transfer Molding for Manufacturing of Complex Lightweight High-Performance Automotive Parts

Lightweight vehicles for energy savings encourages the use of composites in the new generation of vehicles. The compression resin transfer molding process (CRTM) is a novel variation of liquid composite molding (LCM) which offers fast manufacturing cycle for net-shape complex parts with excellent performance, ideal for the automotive industry. The process combines features of resin transfer molding (RTM) and compression molding. The process stages are identified and compared to other LCM processes to take advantage of existing simulation tools. A numerical model that simulates the resin flow in this process is proposed. Several first-order analyses are developed to estimate important process parameters to simplify modeling. Finally, this approach is used to model and simulate the process and is applied to a complex automotive part (the Automotive Composites Consortium B-pillar) with qualitative experimental validation.

Notes on the Modeling of Preform Compaction: II-Effect of Sizing on Bundle Level Micromechanics:

Composite materials processed through the general class of Liquid Composite Molding schemes must essentially go through a two step process-fiber preforming, followed by resin infusion. Although the preform itself gives the basic skeleton of the structure, the compaction stage results in significant changes to the microstructure, resulting in flattening of weft yam bundles and simultaneous reorientation and rearrangement of those in the warp direction. The movement also results in nesting and inter-layer packing resulting from significant inter-bundle movement. Sizings are generically used to assist in wet-out and to reduce fiber damage during preforming and compaction. However, they also absorb volume in a dry preform, and indicate response similar to a two-phase system consisting of a softer concentric shell surrounding a stiffer fiber, thereby causing changes in response of the preform and composite. In this paper we focus on the effect of sizing on behavior at the bundle level, accounting for compaction effects including those under shear deformation.

An analytic solution for the temperature distribution in flow through porous media in narrow gap II. Radial injection

Abstract The paper examines the temperature distribution during radial non-isothermal saturated flow through the porous media, with orientation towards the composite materials processing. The problem is simplified and scaled suitably for the proposed application. Closed form series solution of the simplified equation is obtained. The need to model the heat dispersion as a flow-velocity dependent phenomenon is demonstrated. Temperature profiles in the thin mold are shown to be well approximated by the first series term except near the inlet.

Determination of the Thermal Dispersion Coefficient During Radial Filling of a Porous Medium

Resin Transfer Molding is one of the Liquid Composite Molding processes in which a thermoset resin is infiltrated into a fibrous porous media in a closed mold. To reduce the curing time of the resin, the mold may be heated, influencing other filling parameters such as the resin viscosity. Analysis of the non-isothermal effects during filling will help to understand the manufacturing process. One of the issues of non-isothermal filling in porous media is the variation of the velocity profile at the micro scale level, which as it is averaged, cannot be included in the convective term To account for it, the thermal conductivity tensor is modified and a thermal dispersion coefficient K d is introduced to model the micro convection effects. In this paper we explore the temperature profile under non-isothermal conditions for radial injection during Resin Transfer Molding in order to determine the thermal dispersion coefficient. An approximate solution is derived from the series solution and validated with a numerical method. Experiments using carbon fibers and polyester resin were conducted. The triermial dispersion coefficient is determined by comparing experimental results with the steady state analytical solution. The comparison between radial and linear injection results shows that the same degree of dispersion is present in isotropic fibrous porous media.