Prediction of Fill Time in Compression Resin Transfer Molding of Composite Structures

Baskaran M., Aretxabaleta L., Mateos M., Aurrekoetxea J.

The effects of gap thickness, injection flow rate, overflow percentage, initial fiber content, and resin viscosity on the resin distribution at the end of the injection stage of compression resin transfer molding have been studied. Experimental results have shown that in all the cases, a certain amount of resin penetrates the preform, contrary to the extended hypothesis that the resin flows exclusively through the gap. Furthermore, resin distribution is not homogeneous and the penetration depth varies depending on the distance from the injection point, being deeper near the inlet point. Simulation results based on the quantitative analysis of the amount of resin penetrating into the preform demonstrates that the most influent parameters are the initial fiber content, the gap percentage and the overflow percentage. In fact, the ratio between the permeability of the gap and the preform (r) governs the tendency of the resin to flow through the gap or to penetrate into the preform. Increasing r reduces the amount of resin which penetrates into the preform, but there is an upper asymptotic limit from which no differences are noted. Finally, simulation results have also demonstrated that the preform compression can be neglected during the injection stage, as the pressure into the preform is low. POLYM. COMPOS., 2017. © 2017 Society of Plastics Engineers

Yang B., Jin T., Li J., Bi F.

Compression resin transfer molding (CRTM) is an effective process for the manufacturing of composite parts with large size and high fiber content, while the existence of open gap, the dynamically changing dimensions of cavity geometry and the deformation of preform during filling process bring great difficulties to the three-dimensional simulation of resin flow in CRTM. In order to develop a convenient and efficient three-dimensional simulation approach for CRTM filling process, a unified mathematical model for resin flow in both open gap and preform is established instead of considering the gap as high permeability preform, then the analysis of the clamping force and stress distribution are presented. In order to avoid direct solving the coupled equations of resin flow and cavity deformation, volume of fluid (VOF) multiphase flow technology and dynamic mesh model are applied to track the resin flow front and update the cavity geometry during filling simulation, respectively. The master–slave element method is used to modify the amount of resin release and ensure the resin mass conservation. The validity of the numerical approach is verified by comparison with analytical and experimental results, three-dimensional simulation examples are also presented.

Yang B., Jin T., Li J., Bi F.

Compression resin transfer molding (CRTM) is an effective process for the manufacturing of composite parts with large size and high fiber content. The analysis of the resin flow and stress distributions can only be performed by directly solving the coupled flow/deformation equations, but it is difficult to handle the complicated preform deformation models and geometry models; therefore, the simulation precision and application range are extremely limited. In this paper, an alternative approach is introduced to overcome the above problems, in which the preform deformation and the accompanying resin release during the secondary compaction phase are calculated in an additional element associated with each unit of the discretized model geometry instead of solving the coupled governing equations directly, so the complex compaction models can be adopted. Three simulation examples are presented to demonstrate the accuracy and capability of the above numerical approach on velocity-controlled, force-controlled 3D CRTM processes.

Walbran W.A., Bickerton S., Kelly P.A.

Chang C.

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.

Mamoune A., Saouab A., Ouahbi T., Park C.H.

The optimization of the couplings between LCM processes and structural performances by repeated numerical simulations is expensive in terms of the computational cost. In order to overcome this difficulty, we propose a semi-analytical modeling of RTM and CRTM processes. Various processing routes of CRTM are considered according to the combinations of different stages of resin injection and reinforcement compression. This article is composed of two parts. First, we present analytical and semi-analytical models concerning the CRTM process with an imposed compression speed, in order to explain the methodology of resolution for the other CRTM manufacturing routes by an imposed compression force. The problems related to different constraints imposed on the CRTM process (imposed fiber volume fraction, feasibility criteria of process, and mold strength) are analyzed and formulated, so that they can be taken into account a numerical optimization. In the second part, a new numerical modeling of CRTM process with reinforcement compression under a force was developed and a comparison with semi-analytical model carried out. Recent work in the literature [Merotte J, Simacek P and Advani SG. Flow analysis during compression of partially impregnated fiber perform under controlled force. Compos Sci Technol 2010; 70: 725–733.] about the analysis of resin flow during compression of partially impregnated fiber preform under controlled force, allowed us to explore another alternative of comparison. Finally, this approach is used to control the RTM and CRTM processes and it is applied to a series of parametric studies, considering the effects of part size and fiber volume fraction on the mold filling time. This study not only contributes to the optimization of the couplings between manufacturing processes and structure, but also provides a tool for the control of the CRTM and RTM processes.

Advani S.G., Sozer E.M.

Merotte J., Simacek P., Advani S.G.

Resin flow during Compression Resin Transfer Molding (CRTM) can be best described and analyzed in three phases. In the first phase, a gap is created by holding the upper mold platen parallel to the preform surface at a fixed distance from it. The desired amount of resin injected into the gap quickly flows primarily over the preform. The second phase initiates when the injection is discontinued and the upper mold platen moves down squeezing the resin into the deforming preform until the mold surface comes in contact with the preform. Further mold closure during the final phase will compact the preform to the desired thickness and redistribute the resin to fill all empty spaces. This paper describes the second phase of the infusion. We assume that at the end of phase one; there is a uniform resin layer that covers the entire preform surface. This constrains the resin to flow in through the thickness direction during the second phase. We model this through the thickness flow as the load on the upper mold forces the resin into the preform, simultaneously compacting the preform. The constitutive equations describing the compaction of the fabric as well as its permeability are included in the analysis. A numerical solution predicting the flow front progression and the deformation is developed and experimentally verified. Non-dimensional analysis is carried out and the role of important non-dimensional parameters is investigated to identify their correlations for process optimization.

Merotte J., Simacek P., Advani S.G.

Compression resin transfer molding (CRTM) is an alternative solution to conventional resin transfer molding processes. It offers the capability to produce net shape composites with fast cycle times making it conducive for high volume production. The resin flow during this process can be separated into three phases: (i) metered amount of resin injection into a partially closed mold containing dry fiber preform, (ii) closure of the mold until it is in contact with the fiber preform displacing all the resin into the preform and (iii) further mold closure to the desired thickness of the part compacting the preform and redistributing the resin. Understanding the flow behavior in every phase is imperative for predictive process modeling that guarantees full preform saturation within a given time and under specified force constraints. In this paper, the last phase of the resin flow during CRTM process is simplified and modeled as a one dimensional flow to obtain estimates for process time if the applied force is known. The constitutive equations describing the material used are discussed. Due to the non-linear nature of the formulated governing equations, a numerical solution is developed and the flow front progression and the change in preform thickness are experimentally verified. Finally, a non-dimensional analysis identifying the parameters involved in the process is presented. The impact of the main process parameter is then studied and conclusions regarding process feasibility and optimization are formulated.

Walbran W.A., Bickerton S., Kelly P.A.

Mould tools used for LCM processes such as Resin Transfer Moulding (RTM) and Injection/Compression Moulding (I/CM) must withstand local forces due to compaction of the fibre reinforcement, and due to resin pressure generated within the laminate. A series of RTM and I/CM experiments have been carried out, with the focus placed on measurement of normal stress distributions exerted on the mould surface. In addition, total mould clamping force and injection gate pressure histories have been recorded. I/CM experiments using force-controlled secondary compaction were also undertaken, and compared to the velocity-controlled cases. Observed fluid pressure fields showed good agreement with theory, namely a logarithmic distribution during fluid injection and a quadratic distribution during the compression driven filling phase of I/CM. Significant spatial variation in normal stress due to reinforcement compaction was observed. The influence of the fluid pressure on the total stress experienced by the mould was observed to be a function of both the fibre volume fraction of the part and the applied injection pressure, the latter being more pronounced at lower part volume fractions.

Bhat P., Merotte J., Simacek P., Advani S.G.

Compression resin transfer molding process (CRTM) combines features of compression molding with traditional Resin Transfer Molding (RTM). The CRTM process is described in three stages, with resin being injected into the gap in Stage I, closing of the gap in Stage II and actual compression of preform and re-distribution of the resin in Stage III. To fabricate a void free part, one has to understand the resin flow during these stages. Governing equations for each stage are formulated, relevant non-dimensional parameters are derived and brief description of the numerical model is presented. Assembling the process parameters into non-dimensional groups significantly reduces the number of variables to be explored in the process. The paper analyzes the impact of the dimensional less parameters on impregnation time with the numerical process model. The results show that the impact of many parameters on process speeds is negligible, identifying two non-dimensional parameters that influence the resin injection.

Voller V.R.

Simacek P., Advani S.G., Iobst S.A.

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.

Buntain M.J., Bickerton S.

The term liquid composite molding (LCM) encompasses a growing list of processes, including resin transfer molding (RTM), injection/compression molding (I/CM), and resin infusion (a.k.a. VARTM). All LCM techniques involve compressive deformation of the fiber reinforcement prior to, and in many cases during mold filling. Forces acting on molds are primarily due to the requirement to compact the reinforcement, and pressure generated due to resin flow through these fibrous structures. An experimental study of the forces exerted on a mold during the RTM and I/CM processes is presented here. Two reinforcing materials have been considered, exhibiting significantly different resistance to compaction. The evolution of mold clamping force has been shown to be strongly influenced by the complex, non-elastic compaction behaviour of fiber reinforcements. The important effects include stress relaxation, an apparent lubrication by the injected fluid, and permanent deformation. Efforts to simulate the experiments will be presented in Part B of this study.

Bickerton S., Buntain M.J.

Liquid composite molding (LCM) processes generate forces on tooling due to internal resin pressure fields and the resistance to compaction offered by fiber reinforcements. In Part A of this work the authors have presented a detailed study on the evolution of total clamping force during resin transfer molding (RTM) and injection/compression molding (I/CM) cycles. The influence of the complex compaction response of two different reinforcements was demonstrated, important effects including stress relaxation, an apparent lubrication by the injected fluid, and permanent deformation. In the current paper attempts are made to model clamping force evolution utilizing elastic reinforcement compaction models. The predictions are shown to have significant qualitative errors if a single elastic model is applied, particularly if forces due to reinforcement compaction dominate those due to fluid pressure. By using a combination of elastic models significant qualitative and quantitative improvements were made to the predictions. It is concluded that careful characterization of both reinforcement permeability and compaction response are required for an accurate LCM tooling force analysis.

WADA A., WASEDA K., YAMAMOTO H., Fujii Y.