Moon Koo Kang

Formation of microvoids during resin-transfer molding process

Abstract Voids in a composite part are deleterious because they degrade its strength and modulus. In resin-transfer molding (RTM), voids result mainly from inhomogeneous fiber architecture. Such inhomogeneity leads to non-uniform permeability of the fiber preform, which in turn causes the resin velocity to vary from point to point at a micro scale. The capillary pressure, which also prevails at this length scale, exacerbates the spatial variation of the resin velocity. The combined effect of pressure gradient and capillary pressure can be described by the capillary number. The resulting microscopic perturbations in the resin-flow front allow voids to form. The present paper proposes a mathematical model to describe the mechanisms of void formation. The existing data are used to validate the assumptions introduced. The model is then used to analyze new data from one-dimensional RTM experiments.

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.

A flow-front refinement technique for the numerical simulation of the resin-transfer molding process

Abstract The process of mold filling process during resin-transfer molding has been simulated numerically by using a modified control-volume finite-element method (CVFEM) along with a fixed-grid method to handle problems associated with the moving resin front. The fixed grid was refined in an adaptive manner, by dividing the flow-front elements into two regions using the estimated flow front. Imaginary new nodes were placed at the intersections between the original element border and the temporary flow front. By using the mass conservation of resin, the pressures at these newly added ‘imaginary’ nodes were expressed in terms of the pressure at the old ‘real’ nodes. Through this elimination, the imaginary nodes do not affect the size of the global matrices and thus do not increase the computation time. In order to illustrate the effectiveness of the proposed scheme, a number of mold-filling simulations were performed. The proposed method, referred to as the FINE method, yielded smoother flow fronts and reduced the error in the pressure at the flow front that plagued the conventional fixed-grid methods. The solution accuracy was considerably higher than that of the conventional method for the same number of nodal points and elements, without a significant increase in the computation time.

Analysis of resin transfer moulding process with controlled multiple gates resin injection

Abstract In resin transfer moulding, increase of fibre volume fraction decelerates the resin flow. For fast resin flow without increasing injection pressure, resin can be injected using multiple injection gates. However, with improper injection schemes, the resin front becomes complicated and numerous air bubbles may form where the flow fronts merge. In this study, two different multiple gate injection schemes were investigated. One is to open the gates in a sequential manner as the resin front advances. The other is to control the resin injection pressure as a function of time. Mould filling was simulated numerically using the Control Volume Finite Element Method (CVFEM). Experiments were performed to verify the simulation results. The resin front shapes observed from the experiments were compared with the numerical results. Close agreements were found. The controlled multiple gate injection methods were proven to reduce the injection time by a big margin as well as to prevent the formation of air bubbles due to air entrapment.

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.

Technical Section: A real-time cloth draping simulation algorithm using conjugate harmonic functions

This paper describes a simplified mathematical model and the relevant numerical algorithm that simulates a draped cloth over a virtual human body. The proposed algorithm incorporates an elliptical, or non-consecutive, method to simulate the cloth wrinkles on moving bodies without having to reference the results of past drape simulation time steps. A global-local analysis technique was employed to decompose the drape of a cloth into large-scale deformation and local wrinkles. The large-scale deformation is determined directly by the rotation and translation of body parts to generate a wrinkle-free yet globally deformed shape of the cloth. The local wrinkles are calculated by solving simple elliptical equations based on the orthogonality between conjugate harmonic functions. The large-scale deformation and the local wrinkles are then superposed to simulate the draped cloth. The elliptical equations used to simulate the local wrinkles require no interpolative time frames, even for rapidly moving virtual bodies. Avoiding the incremental approach of time integration used in conventional methods, the proposed method yields markedly enhanced computational efficiency as well as enhanced simulation stability.

A visibility-based automatic path generation method for virtual colonoscopy

In virtual colonoscopy, it is crucial to generate the camera path rapidly and accurately. Most of the existing path generation methods are computationally expensive since they require a lengthy preprocessing step and the 3D positions of all path points should be generated. In this paper, we propose a visibility-based automatic path generation method by emulating the ray propagation through the conduit of the colon. The proposed method does not require any preliminary data preprocessing steps, and it also dramatically reduces the number of points needed to represent the camera path using control points. The result is a perceivable increase in computational efficiency and easier colon navigation with the same level of accuracy.

Fluidic Shadow Dance Using Interactive Fluid Animation

This paper proposes an interactive artwork to visualize the internal conflicts of an individual reacting to environmental impulses. This artwork is implemented using an enhanced particle dynamics simulation method with pre-integrated volume rendering. The computational efficiency is achieved using small number of particles for representing a significant volume. In this artwork, the viewer in front of a video camera touches a pair of columns on which arrays of sensors are located to serve as virtual fluid injectors. The sensor signal and the video image are transmitted to the computer and the video processor, and the computer simulates particle fluids being virtually injected into the shadow of the viewer. The processed shadows and the computer rendered images are then superposed to create a visual illusion that appears as if the fluid is injected into the body and then reacts to diverse body postures accordingly, associating the mental functions of ego.

Wafer level vertical interconnection method for microcolumn array

A concept of miniaturized and arrayed electron beam system using vertical interconnection was proposed and fabrication with interconnection using metal reflow was performed. The interconnection of electron beam array is very important because all single columns should be controlled individually to obtain a uniformity of electron beam array. In addition, the interconnection area should be reduced to make the size of a single column smaller. Therefore, a vertical interconnection method was proposed and the schematic of vertically interconnected monolithic microcolumn is shown. The other advantages of this vertical interconnection method are controllable columns and removal of interference between columns (Han, 2005).

A Study on the Control Strategy to Minimize Voids in Resin Transfer Mold Filling Process

In case of Resin Transfer Molding(RTM) process, “race-track” effects and non-uniform fiber volume fraction may cause undesirable resin flow pattern and thus result in dry spots, which affect the mechanical properties of the finished parts. In this study, a real time RTM control strategy to prevent these unfavorable effects is proposed. The control strategy consists of two “stages” depending on the extent the resin front has reached. Through numerical simulations and experiments, the validity of the proposed scheme is demonstrated. The results show that the proposed scheme is effective in reducing the void formation during RTM mold filling.