Analysis of resin injection molding in molds with preplaced fiber mats. I: Permeability and compressibility measurements

This work presents the characterization of fibrous reinforcement mats in resin injection molding. The fiber mat characterization involved determining the mat permeability and compressibility. Mold filling experiments were conducted using two or more different fiber types in the mat stack, which created transverse porosity, permeability, and compressibility variations. The effect of these variations was studied by taking flow pressure measurements and observing the progress of the flow front of a non-reactive fluid filling a clear acrylic mold that contained the reinforcement mat stack.     

Physical and numerical modeling of mold filling in resin transfer molding

Finite element modeling and experimental investigation of mold filling in resin transfer molding (RTM) have been performed. Flow experiments in the molds were performed to investigate resin flow behavior into molds of rectangular and irregular shapes. Silicone fluids with viscosity of 50 and 100 centistoke as well as EPON 826 epoxy resin were used in the mold filling experiments. The reinforcements consisted of several layers of woven fiberglass and carbon fiber mats. The effects of injection pressure, fluid viscosity, type of reinforcement, and mold geometry on mold filling times were investigated. Fiber mat permeability was determined experimentally for the five-harness and eight-harness woven mats. Resin flow through fiber mats was modeled as flow through porous media. Pressure distributions inside both types of molds were also determined numerically. In the case of resin flow into rectangular molds, numerical results agreed well with experimental measurements. Comparison between the experimental and numerical results of the resin front position indicated the importance of edge effects in resin flow behavior in small cavities with larger boundary areas. Reducing the resistance to resin flow at the edge region in the numerical model allowed for good agreement between the numerical simulation and the physical observations of the resin front position and mold filling time.     

Liquid flow in molds with prelocated fiber mats

The mechanism associated with mold filling in the manufacture of structural RIM (SRIM) and resin transfer molding (RTM) composites is studied by means of flow visualization and pressure drop measurements. To facilitate this study, an acrylic mold with a variable cavity was constructed and the flow patterns of nonreactive fluid flowing through various layers, types, and combinations of preplaced glass fiber reinforcement mats were photographed for both evacuated and nonevacuated molds. The pressure drops in the flow through a single type of reinforcement (e.g., a continuous strand random fiber mat) and also a combination of reinforcement types (e.g., a stitched bidirectional mat in combination with a random fiber mat) were recorded at various flow rates to simulate high-speed feeding processes (e.g., SRIM) and low-speed feeding processes (e.g., RTM). By changing the amount of reinforcement placed into the mold, the permeabilities of the different types and combinations of glass fiber mats were obtained as a function of porosity. It is shown that partially evacuating the mold cavity decreases the size of bubbles or voids in the liquid, but ultimately increases the maximum pressure during filling. The results also show that glass fiber mats exhibit anisotropic permeabilities with the thickness permeability, K_z, being extremely important and often the determining factor in the pressure generated in the mold during filling.     

Trans-plane fluid permeability measurement and its applications in liquid composite molding

This work discusses tow independent methods to measure and analyze the trans-plane fjuid permeability of various fiber reinforcements. In the unidirectional flow method, the measured injection pressure and flow rate, together with a one-dimensional Darcy's law were used to calculate the trans-plane permeability of fiber mats was independent of flow rate only at low injection pressure. Flow-induced fiber mat permeability change occurred when the injection pressure exceeded the clamping pressure. Measured permeability in conjunction with a three-dimensional mold filling computer program was used to simulate the effect of stacking sequence for a combination of different fiber mats on the mold filling pattern. Finally, a method is proposed to simplify the simulation of a three-dimensional flow through the fiber perform.     

RTM工艺中玻纤增强材料渗透率的测量与分析

通过对树脂传递模塑成型工艺中广泛使用的纤维增强材料——玻纤连续毡渗透率的测量和分析，建立了该增强材料在模具中的纤维体积含量与渗透率之间的关系，考察了纤维增强材料的渗透率与注模时间的关系，分析对比了纤维增强材料的结构型式对注模时间和纤维浸透性的影响。结果表明；随着纤维体积含量的增大，渗透率迅速下降，对于玻纤连续毡，其渗透率ｋ与纤维体积含量ｖｆ的关系可以表示为一个多项式；在恒定的压力下，渗透率大，注模需要的时间越短，体积含量相同的玻纤连续毡和玻纤方格布比较，玻纤连续毡的渗透率约大一倍，而所需注模时间约为玻纤方格布的１／２；玻纤方格布中存在渗透率相差特别大的两大区域是造成其浸透性差的主要原因。     

Fiber mat deformation in liquid composite molding. I: Experimental analysis

When a resin in injected into the mold in liquid composite molding, the preplaced fiber mat may deform near the inlet gate because of the high momentum carried by the injected fluid. A fiber free region near the gate followed by the fiber mat deformation may emerge. This phenomenon is most likely to occur when the stacked fiber mats have low permeability and the resin has high viscosity. A set of mold filling experiments were carried out using an instrumented metal mold and a small transparent mold to investigate the fiber mat deformation during mold filling. Experimental results showed that the fiber mat deformation was limited to a small region near the gate and that deformation greatly reduced the molding pressure. A forced fiber mat deformation employing a modified gate design was proposed to facilitate mold filling in liquid composite molding.     

热塑性树脂浸渍黄麻纤维毡的研究

研究了热塑性树脂PP(聚丙烯)浸渍黄麻纤维毡的速率及相关因素,以解决天然纤维毡增强热塑性复合材料中连续熔融浸渍,考察了浸渍速率、温度等参数对其加工成型及力学性能的影响.用改装的毛细管流变仪作为实验装置,以一维Darcy定律处理实验数据,研究了压力对毡体空隙率以及熔体粘度对浸渍速率的影响.结果显示,相同的压力下,浸渍速率和熔体粘度成反比,麻纤维毡的压缩空隙率都要高于玻璃纤维毡.通过对纤维毡体结构、可压缩性、纤维直径以及毡体渗透率进行对比,进一步讨论了纤维毡浸渍速率的影响因素.表明麻纤维平均直径远大于玻璃纤维,纤维堆叠产生的空隙明显大于玻璃纤维且在麻纤维毡中不存在玻璃纤维毡中的束内浸渍,麻毡的浸渍速率约为玻纤毡的3.5倍,平均渗透率K约为玻纤毡的14倍.运用毛细管模型计算了两种毡体的Kozeny常数,其值分别为2950和442.     

The effects of preforming induced variable permeabilities on the rtm molding flow

Forming bidirectional woven fabric into complex shapes consisting of double curvature surfaces leads to rearrangement of the network structure of fiber mats. In this study, the geometric configuration of the fabrics after deformation is modeled using a modified pin-jointed net model. Deformation of the fabric can result in the local change of the permeability in the reinforcements. In the flow simulation, the prediction of the filling flow highly depends on the correct values of permeabilities. The primary goal of this study is to investigate the effects on filling flow phenomena caused by the deformation of the fiber mats. A general integrated procedure of computer simulation with variable permeabilities for the filling process of arbitrary mold shape is also presented.     

Effect of fiber‐mat anisotropy on 1D mold filling in LCM: A numerical investigation

The 1D flow experiment is one of the most common methods to measure the saturated and unsaturated permeabilities as well as to study the unsaturated flow in liquid composite molding (LCM) processes used for manufacturing polymer composites. The effective permeability along a flow direction in an anisotropic fiber mat, which is a function of the principal components of the permeability tensor and the angle between the principal and flow directions, is based on the assumption of uniform 1D flow in the mat. In the present paper, the validity of such unidirectional flow assumption is shown to depend on three factors, i.e., the fiber‐mat aspect ratio, the anisotropy ratio (the ratio of the major and minor principal in‐plane permeabilities), and the angle of the principal permeability direction. A mold‐filling simulation for hard‐mold LCM processes based on the control volume/finite element formulation is used to investigate how these three factors affect the 1D flow. A new algorithm for applying the uniform inlet‐pressure condition prevalent in the 1D flow mold under the constant‐flow‐rate injection is developed to match the actual conditions in the mold. Our numerical results show that the three aforementioned factors have significant influence on the inlet‐pressure history as well as on the mold‐filling pattern. Maximum deviations from the unidirectional flow predictions in the inlet‐pressure history as well as the flow‐front progress is seen when the principal permeability direction is at an angle of 45° with respect to the flow direction; these deviations worsen with a decrease in the mat aspect ratio and with an increase in the anisotropy ratio. However, the inlet‐pressure history remains linear and the progress of the average flow‐front remains predictable by the 1D flow theory. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Experimental analysis and simulation of flow through multi-layer fiber reinforcements in liquid composite molding

Liquid composite molding (LCM) is a process in which a reactive fluid is injected into a closed mold cavity with preplaced reinforcement. Combined layers of different permeabilities are often used in LCM, which creates through thickness and inplane porosity and permeability variations. These inhomogeneities may influence the flow front profile in the thickness direction. To investigate the effect of the through thickness inhomogeneities, mold filling experiments were performed using preforms containing layers of two different fiber architectures. Aqueous corn syrup solutions were injected into a tempered glass mold containing the reinforcement stack. The progress of the flow front at various locations within the reinforcement was measured by an electrical conductivity technique based on the insertion of small wires between the reinforcement layers. Experimental data reveal the details of the flow front shape as the fluid penetrates the preform. Using these data, a model is proposed to calculate the overall in-plane permeability of the preform. Numerical simulations of the flow front progression performed with the computer software RTMFLOT developed in our laboratory are compared to the experimental flow front for various stacking arrangements. Results show good agreement between simulations and experiments and demonstrate the capability of the software to simulate multi-layer flow process.     

Measurement of effective permeability of reinforcement mats using sensitivity analysis

A methodology using sensitivity analysis is proposed to measure the effective permeability which includes the interaction of the resin and the reinforcement. Initially, mold‐filling experiments were performed at isothermal conditions on the test specimen and the positions of the flow front were tracked with time using a flow visualization method. Following this, mold‐filling experiments were simulated using a commercial software to obtain the positions of the flow front with time at the process conditions used for experiments. Several iterations were performed using different trial values of the permeability until the experimentally tracked and simulated positions of the flow front with time were matched. Finally, the value of the permeability thus obtained was validated by comparing the positions obtained by performing the experiments at different process conditions with the positions obtained by simulating the experiments. In this study, woven roving and chopped strand mats of E‐class glass fiber and unsaturated polyester resin were used for the experiments. From the results, it was found that the measured permeabilities were consistent with varying process conditions. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Optimization of filling process in RTM using a genetic algorithm and experimental design method

In the resin transfer molding (RTM) process, preplaced fiber mat is set up in a mold and thermoset resin is injected into the mold. An important issue in RTM processing is minimizing the cycle time without sacrificing part quality or increasing the cost. In this study, a numerical simulation and optimization process for the filling stage was conducted in order to determine the optimum gate locations. The control volume finite element method (CVFEM), modeled as a 2‐dimensional flow, was used in this numerical analysis along with the coordinate transformation method to analyze a complex 3‐dimensional structure. Experiments were performed to monitor the flow front to validate the simulation results. The results of the numerical simulation corresponded with that of the experimental quite well for every single, simultaneous, and sequential injection procedure. The optimization analysis of the sequential injection procedure was performed to minimize fill time. The complex geometry of an automobile bumper core was chosen. A genetic algorithm was used to determine the optimum gate locations in the 3‐step sequential injection case. Taguchi's experimental design method was also used for determining the pressure contribution of each gate. These results could provide the information on the optimum gate locations and injection pressure in each injection step and predict the filling time and flow front.     

Mold filling analysis in vacuum-assisted resin transfer molding. Part I: SCRIMP based on a high-permeable medium

Mold filling in SCRIMP based on a high-permeable medium is complicated because of the considerable difference in the permeabilities of the fiber reinforcement, the peel ply and the high permeable medium. The objectives of this paper are to understand the flow mechanism through flow visualization experiments and to present models that can be used to predict the filling time and flow pattern. Permeabilities of a stitched fiber mat, a high-permeable medium and a peel ply were measured. Flow visualization of SCRIMP mold filling was carried out under various molding conditions. It was found that although the resin flowed faster in the high-permeable medium than in the fiber reinforcement, the flow front lead-lag was not very large and it remained nearly constant through the entire mold filling process. A three-dimensional Control Volume/Finite Element Method (CV/FEM) was adopted to solve the flow governing equations, i.e. the Darcy's law, and the influences of the flow properties of the high-permeable medium, the fiber reinforcement and the peel ply on filling time were investigated. Based on experimental observations and CV/FEM simulation, a simplified leakage flow model is also presented. The comparison of experimental and simulation results show good agreement.     

Impregnation of thermoplastic resin in jute fiber mat

Impregnation rate of thermoplastic resin (polypropylene) in jute fiber mat and influence of relative factors on impregnation were studied, aiming to develop the continuous melt impregnation technique and to investigate the effect of impregnation rate and temperature on processing conditions and mechanical properties of natural fiber mat-reinforced thermoplastics. Influence of pressure on porosity of fiber mat and effect of melt viscosity on impregnation rate were also investigated. The modified capillary rheometer was used as apparatus and experimental data were analyzed based on the one-dimension Darcy’s law. Results showed that at a given pressure, the impregnation rate is inversely proportional to melt viscosity and jute fiber mat has higher porosity than glass fiber mat. The architecture, compressibility, permeability and fiber diameter of jute fiber mat were compared with those of glass fiber mat and their effects on impregnation were discussed further. It could be seen that the average diameter of jute fiber is much bigger; the porosity of jute fiber mat is significantly higher and inner bundle impregnation does not exist in jute fiber mat. Therefore, it is not difficult to understand why the impregnation rate in jute fiber mat is 3.5 times higher and permeability is 14 times greater. Kozeny constants of jute and glass fiber mats calculated based on the capillary model are 2950 and 442, respectively.     

Analysis of resin transfer/compression molding process

Resin transfer/compression molding (RT/CM) is a two-step process in which resin injection is followed by mold closing. This process can enhance the resin flow speed and the fiber volume fraction, as well as reducing the mold filling time. In this study, a simulation program for the mold filling process during RT/CM was developed using the modified control volume finite element method (CVFEM) along with the fixed grid method. The developed numerical code can predict the resin flow, temperature, pressure, and degree of cure distribution during RT/CM. The compression force required for squeezing the impregnated preform can also be calculated. Experiments were performed for a complicated three-dimensional shell to verify the feasibility of the RT/CM process and the numerical scheme. The compression force and the compression speed were measured. A close agreement was found between the experimental data and the numerical results. The resin front location obtained from a short shot experiment was compared with the numerical prediction. Again, a close agreement was observed. In order to demonstrate the effectiveness of the numerical code, simulations were performed for more complicated process conditions with anisotropic permeability of the preform at higher fiber volume fractions.     

Characterization of dual‐scale fiber mats for unsaturated flow in liquid composite molding

The drooping inlet‐pressure history during 1D filling experiments has been used in the past to detect the onset of the unsaturated flow in certain stitched mats, called dual‐scale mats, during mold‐filling in liquid composite molding. In this study, one such triaxial stitched mat was tested for unsaturated flow and manifests the characteristic inlet‐pressure droop that increases with fiber mat compression. A correlation between the previously proposed dimensionless numbers pore volume ratio (γ) and sink effect index (ψ) and the droop in the inlet pressure history was also sought. By studying the micrographs of the composite samples, γ and ψ were measured for the biaxial and triaxial stitched mats at various states of compression. The observation that the droop in the inlet‐pressure profiles increase with an increase in γ and ψ establishes the two dimensionless numbers, along with the presence of straight channels aligned with the flow direction, as good predictors of the unsaturated flow in the dual‐scale fiber mats. POLYM. COMPOS. 26:756–769, 2005. © 2005 Society of Plastics Engineers     

Mold-filling simulations for the injection molding of continuous fiber-reinforced polymer

Injection molding can be used to fabricate fiber-reinforced polymer composites by impregnating a continuous fabric mat preplaced in a mold cavity with a polymer resin. The mold-filling time is dependent on the flow and heat transfer behavior in the mold. A model is proposed that considers the non-Newtonian How through the porous fabric mat and the heat transfer between mold, fabric mat, and flowing fluid. The model was simulated for the mold filling of a carbon fiber mat with a pseudoplastic polymer solution. The results from the simulation provide Information for optimizing mold-filling parameters through proper selection of inlet fluid pressure, heat source temperature, and type of polymer-solvent system.     

Reaction injection molding process of glass fiber reinforced polyurethane composites

Structural reaction injection molding (SRIM) was used to produce polyurethane composites containing random continuous glass fiber mats. A long rectangular mold was used to carry out the SRIM experiments. 4,4′‐diphenylmethane diisocyanate and poly(propylene oxide) triol were used to formulate a thermoset polyurethane system. Dibutyltin dilaurate was used as a catalyst. A second order Arrhenius equation described the PU polymerization kinetic data obtained from the adiabatic temperature rise measurement. A viscosity as a function of temperature and conversion was developed using rheometer data. The pressure rise at the gate was measured during filling. The flow behavior within the mold was described by Darcy's law and the Kozeny's equation. The temperature profile within the mold measured by thermocouples during filling and curing coincided fairly well with the simulation results. The thermal transient problem at the wall was solved using the overall heat transfer coefficient, and it was analyzed as a function of Biot number. The dimensional stability of the fiber reinforced PU parts was excellent compared to the pure PU parts.     

Mold filling analysis in vacuum-assisted resin transfer molding. Part II: SCRIMP based on grooves

Compared with SCRIMP based on the high-permeable medium, SCRIMP based on grooves has the advantage of a much higher mold filling rate. This paper analyzes the influences of various molding conditions on mold filling and presents models that can be used to predict the filling time and flow pattern in SCRIMP based on grooves. Mold filling experiments were carried out to investigate the effect of various factors such as the size of the groove, groove spacing, number of fiber layers and resin viscosity on mold filling. A leakage flow model was developed to simplify the simulation of the mold filling process in SCRIMP based on grooves. An “equivalent permeability” was introduced to represent the average flow capacity in the grooves. Compared with the Control Volume/Finite Element Method (CV/FEM) model, the leakage flow model greatly reduced computation time and yet provided simulation results that were in good agreement with experimental observations.     

Edge effect in RTM processes under constant pressure injection conditions

In resin transfer molding processes, the edge effect caused by the nonuniformity of permeability between fiber preform and edge channel may disrupt resin flow patterns and often results in the incomplete wetting of fiber preform, the formation of dry spots, and other defects in final composite materials. So a numerical simulation algorithm is developed to analyze the complex mold‐filling process with edge effect. The newly modified governing equations involving the effect of mold cavity thickness on flow patterns and the volume‐averaging momentum equations containing viscous and inertia terms are adopted to describe the fluid flow in the edge area and in the fiber preform, respectively. The volume of fluid (VOF) method is applied to tracking the free interface between the two types of fluids, namely the resin and the air. Under constant pressure injection conditions, the effects of transverse permeability, edge channel width, and mold cavity thickness on flow patterns are analyzed. The results demonstrate that the transverse flow is not only affected by the transverse permeability and the edge channel width but also by the mold cavity thickness. The simulated results are in agreement with the experimental results. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010