Chih-Yuan Chang

Simulation of mold filling in simultaneous resin injection/compression molding

In the present article, an open gap is present between the fibrous reinforcements and the upper mold during the filling stage of resin injection/compression molding (I/CM). Thus resin can quickly fill up the gap. After that, the mold platens are brought together and drive the resin through the preform. The resin motion in the gap is simplified using the Hele-Shaw flow model, while Darcy’s law is used to calculate the flow fields in the fiber mats. The numerical simulation is based on the body-fitted finite element method (FEM). Results show that the resin injection time is short and most filling time is elevated to the closing of the cavity in the simultaneous I/CM filling process. Small gap size and high compression speed can be used to achieve the minimum mold filling time. However, the improper process parameters can cause the incomplete filling or reversed flow at the gate. In order to avoid the above conditions, the restrictive conditions of simultaneous I/CM are also discussed. The simultaneous I/CM can reduce either the mold filling time or injection pressure significantly compared to resin transfer molding (RTM).

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 on the Void Distribution in the Fiber Mats During the Filling Stage of RTM

A model based on the multiphase Darcy’s law has been developed to simulate the void distribution and movement in a unidirectional fiber mat by introducing the concept of saturation during the filling stage of RTM. The model takes into account two immiscible fluids, resin and air, and two types of flow, the macro-flow around the fiber bundles and the micro-flow around the fibers in the bundles, flowing simultaneously. Results show that the voids form only in the wicking flow field and the saturation of resin will become equal to 1 everywhere if the resin is continuously filled. The range of void distribution is dependent on the porosity of fiber mats and inlet velocity. By dimensionless analysis, the capillarity is negligible and the saturation distribution is uniform within the fiber mats at high modified Weber numbers. The numerical simulation isbased on the body-fitted finite element method.

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).

Experimental analysis of mold filling in vacuum assisted compression resin transfer molding

Vacuum-assisted compression resin transfer molding, a flexible resin transfer molding process, has been developed to reduce a cycling period in the present study. The vacuum-assisted compression resin transfer molding utilizes an extra elastic film placed between the upper mold and the mold cavity compared with resin transfer molding. Through the stretchable film, the state of the fabric stack is under control. During resin injection, a loose fiber stack is present and then resin is easily introduced into the cavity. Once enough amount of resin is injected, a compression pressure is applied on the film that compacts the preform and drives the resin through the preform. Prior to vacuum-assisted compression resin transfer molding experiment, a compression test is performed to understand the variation of the preform thickness at various loads. Through observing vacuum-assisted compression resin transfer molding experiments, some experimental shortcomings are inevitable including the edge effect and excess of injected resin. More resin leads to a longer injection and compression phase and more wastes. At all events, vacuum-assisted compression resin transfer molding is a feasible process and can be expected to fabricate a better part quality. It also reduces the mold filling time/injection pressure and cleaning mold time compared with resin transfer molding.

Numerical simulation of the pressure infiltration of fibrous preforms during MMC processing

In the pressure infiltration process of metal-matrix composites, molten metal is injected under external pressure into a porous preform of the reinforcing phase and solidified, either during infiltration or after the mold is filled. To simplify the problem, the assumption of isothermally saturated fibrous preform with molten metal is adopted. The flow of molten metal is a moving boundary phenomenon which is similar to a fluid flowing through a porous medium. A numerical technique, body-fitted finite element method (FEM), is used to generate grids inside the physical domain and to calculate the distribution of pressure and other relative properties. Results show that the injection flow rate exponentially decreases with the infiltration time. Increasing injection pressure can effectively reduce the infiltration time. A simple and practical prediction of infiltration time under the various parameters is also provided in this study.

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.

Tow impregnation of unidirectional fibrous preform during resin transfer molding

The flow pattern determines the formation and distribution of the voids, and subsequently the quality of the products during the mold filling of RTM. A model is developed to simulate the interaction between the flow in the gaps around the fiber bundles and the flow inside the fiber bundles. Brinkmans equation is used to calculate the flow fields inside the fiber bundles, while the Stokes approximation is used to simplify the resin flow equation in the gaps. Results show that the mechanism of air entrapment is formed inside the fiber bundles. A boundary layer occurs along the permeable interface of the fiber bundles. The thickness of the boundary layer increases with larger permeability and larger gap size. The thickness of the boundary layer is about O(K½). For flow outside of boundary layer regions the Brinkmans equation can reduce to the Darcys law.

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.