Lih-Wu Hourng

Numerical and Experimental Study on the Edge Effect of Resin Transfer Molding

The correlation of the porosity and the permeability, based on the experimental data, for the TGFM-300 P/E random fiber mat is proposed in the present work. The edge effect is simulated by the creeping motion of the resin with permeable boundary condition along the edge of the fiber mats. Results show that the edge effect is very sensitive to the shape of the fiber mat. Numerical predictions agree with experimental results, when the fiber mat is well shaped. The edge effect is negligible as the dimension of the fiber mat is slightly larger than that of the mold cavity. The present paper combines the body-fitted FDM and the FEM to deduce an auto-meshed and auto-node-element relating numerical scheme. This scheme is convenient and effective in solving the moving boundary problems with irregularly geometric boundaries, such as the filling process of RTM. The difference between the elliptic type body-fitted FDM and the FEM are compared and discussed in detail.

Effect of process variables on the quality of compression resin transfer molding

Compression resin transfer molding (CRTM), combining resin transfer molding (RTM) and compression molding, have been developed to fabricate fiber reinforced plastic (FRP) components with large dimensions or high fiber volume content. Although a lot of literature on CRTM is available, relatively little useful technical information is present regarding the proper choice of molding conditions for component optimization. The objective of this research is to investigate the effects of process variables, including injection pressure, mold opening distance, resin temperature, compression pressure, pre-heated mold temperature, and cure temperature, on the quality of CRTM products. The influence of these process variables on the part quality are investigated by applying Taguchi’s method. The ultimate stress measured by tensile test serves as an indicator of the part quality. Experimental result show that the compression pressure and the resin temperature are significant variables for improvements in the mechanical properties of the part, while the effect of pre-heated mold temperature on the mechanical properties appears to be trivial.

Numerical Simulation for the Transverse Impregnation in Resin Transfer Molding

A model has been developed for the transverse impregnation of the resin into a continuous unidirectional packed fiber bundles. The model takes into account two types of flow, namely the macro-flow around the fiber bundles and the micro-flow around the fibers in the bundles, occurring simultaneously. Both flows are described by Darcys law. Results show that an elliptic type void is formed within the fiber bundle under certain circumstances. The void will soon shrink to nothing soon if vacuum assistance is applied in the mold, while it is mobilized and compressed if vacuum assistance is not applied in the mold. By the similarity analysis, we also show that the initial size of the trapping air depends merely on the ratio of permeability between the micro-flow and macro-flow, but not on the injection pressure and the resin viscosity. The numerical simulation is based on the body-fitted finite element method.

Numerical simulation of resin injection molding in molds with preplaced fiber mats

The non-isothermal flow-field of a mold filling process in resin injection molding with preplaced fiber mats is numerically simulated in the present article. A body- fitted coordinate transformation is applied such that the irregular, moving flow front can be fitted exactly as a boundary, and the error induced from the treatment of boundary con ditions is thus reduced. The effects of several parameters, such as types of mats, porosity, mold wall temperature and flow rate of resin at the inlet, on the filling process is deeply investigated.

Numerical Study on the Capillary Effect of Resin Transfer Molding

A model for the impregnation of resin into a unidirectional packed fiber mats by the dynamics of capillary penetration is developed at a low flow rate where resin flow is parallel to fiber axis. The model takes into account two types of flow, the macroflow around the fiber bundles and the microflow around the fibers in the bundles, occurring simultaneously. Both follows are described by Darcys law. The large difference in the position of flow front between these two types of flow leads to the potential of void formation in resin transfer molding. The capillary force is considered along the flow front. It can be evaluated as a function of liquid contact angle, interfacial tension, the porosity, and the velocity of flow front. The simulation is based on the body-fitted finite element method. Result shows that the void formation is affected not only by the properties of fiber mats, but also by the injection condition. And the capillary effect is limited as the capillary number is less than 0(10-2).

Investigation of wick debinding in metal injection molding: numerical simulations by the random walk approach and experiments

Abstract Metal powder injection molding is an important net-shape manufacturing process. Wick debinding is described as fluid transport through a wick bed by capillary action and can shorten the debinding cycle time. The porosity really varies locally, but little research has investigated how the porosity varying with the control volume affects the wick debinding process, as we do in this paper. The present numerical simulation shows the walking flow edges behave randomly, the contours of the wetting wick are irregular and the total debinding time is linearly dependent on the radius of the compact. In addition, an experiment has successfully been developed that can observe the phenomenon of the walking flow edges, measure the total debinding time, and calculate the percentages of residual binder and the pore space filled. The numerical results agree well with the experimental results.

Numerical simulation of capillary-induced flow in a powder-embedded porous matrix

Abstract Capillary-induced fluid motions in an isotropic powder-embedded porous matrix are studied by the Monte-Carlo simulation method. The concept of random walk and a two-block hexagonal network model are employed to accomplish the simulation procedures. The pressure field, intrinsically representing the internal collision effects among fluid particles, is correlated by the Laplace equation and can facilitate the determination of the pressure-orienteddisplacing strength, Γ, of the random walk. Also, a set of normalized random numbers on the interval (0, 1) is used to modify the displacing strength for ‘chance’ variation. Simulated conditions are selected to coincide with the practical capillary-wick debinding process in metal powder injection molding. The numerical results agree well with the theoretical and experimental ones in the literature. This shows that capillary fingering in the wicking material tends to lift the potential of the local suction malfunction of the leading front. Methods that shorten the length of the liquid column in the wick will reduce the debinding time.

Analysis of Flow Phenomena during the Filling Stage of CTM

Resin transfer molding (RTM) is one of the most popular polymer composites manufacturing processes because it provides the advantages of low injection pressure, fast cyclic periods, and the ability to mold parts with highly complex shapes. However, the mold-filling process may need a long time for some cases such as large parts or parts with low permeability of the reinforcement. A flexible RTM process, compression transfer molding (CTM), is developed to reduce the mold-filling time. The mold-filling process of CTM can be divided into resin-injection and mold-closing periods. Flow visualization experiments are carried out to understand the flow behavior of each period in the CTM mold-filling process. As the closing action of the mold proceeds before the resin-injection period ends, experiments are also performed to investigate the effect of simultaneous procedures on the total mold-filling time.

The Influence of Capillary Flow on Edge Effect in Resin Transfer Moldinga

A model has been developed for analyzing the capillary flow along the edge of the unidirectional fiber mats. The model is based on the existence of two flow regions of resin, namely, the porous media region formed by fiber mats and the fiber free gap region between the wall and the fiber mats. The lubrication approximation is used to simplify the resin flow equation in the gap, while Darcys law is used to calculate the flow fields in the fiber mats. Results show that 2KD 5 is the critical gap size for the consideration of the edge effect. Capillarity cannot be neglected for such a little gap as the injection pressure is low. Relationship analogous to the Washburn one is used to simplify the capillary flow. Results show that the mechanism of formation of dry spots is eliminated in the case with low injection pressure. Based on the experimental data for the TGFM-300 P/E random fiber mat, the correlation between the quasi-steady capillary pressure and the other properties of fibrous network is proposed by Buckingham Xr theorem. The body-fitted finite element method is used in the numerical simulation.

A theoretical and experimental study on subcooled flow boiling at high heat flux

In a subcooled flow boiling system at high heat flux, the major heat transfer mechanism places emphasis on a very thin liquid layer, known as the ‘sublayer’ which is trapped between the heated surface and the vapor blankets. Base on the convective boiling heat transfer dominated by the heat conduction through the liquid sublayer, a theoretical model for subcooled flow boiling heat transfer has been developed. To provide useful data in the simulation of Light Water Reactors (LWRs) conditions, heat transfer experiments for up-flow boiling water through a vertical tube at the pressure ranging from 6.9 to 15.5 MPa have been conducted. The experimental results are compared with the predictions of the present model and other five famous correlations. For the LWRs subcooled flow boiling, the comparison reveals that the present model show the best agreement with the measured data.ZusammenfassungIn einem Fluidsystem, das bei hohem Wärmefluß den Effekt des unterkühlten Siedens zeigt, findet der wesentliche Wärmetransportmechanismus in einer sehr dünnen Schicht statt, die zwischen der beheizten Oberfläche und den Dampfpolstern liegt und als „Unterschicht“ bekannt ist. Basierend auf den Gesetzmäßigkeiten des konvektiven Siedens unter dominierendem Einfluß der Wärmeleitung durch die Flüssigkeits-Unterschicht wurde ein theoretisches Modell zu Beschreibung des Wärmeübergangs bei unterkühltem Sieden entwickelt. Um nützliche Daten für die Simulation der in Leichtwasserreaktoren (LWR) herrschenden Bedingungen zu gewinnen, erfolgten die Experimente bei Aufwärtsströmung siedenden Wassers in einem senkrechten Rohr im Druckbereich 6,9 bis 15,5 MPa. Diese experimentellen Ergebnisse werden mit Vorausberechnungen nach dem erstellten Theoriemodell, sowie jenen nach fünf der bekanntesten Korrelationen verglichen. Für unterkühltes Sieden in Leichtwasserreaktoren zeigte sich hierbei, daß die Experimente am besten durch das neuentwickelte Modell wiedergegeben werden.