Compaction of fiber reinforcements

In resin transfer molding, dry fiber reinforcements are compacted as the mold closes before injection of a curable resin matrix. This paper presents experimental data of compaction pressure as a function of fiber volume fraction. Data are presented for woven roving mats, random fiber mats, loose fiber rovings for pultrusion, and uniaxial or biaxial roving mats. These data are fit to a mathematical model derived in an Appendix. Experimental data are also given for six combinations of reinforcements. We use the compaction model of each constituent layer to predict the average volume fraction assuming that fiber layers do not interact. However, we see that most combinations of reinforcements have fiber volume fractions greater than expected at pressures under 50 psi, indicating a synergistic packing between the layers of different composition.     

Packing mechanics of fiber reinforcements

Theories of fiber packing, of use in manufacturing composite materials, are developed. The maximum packing fraction of force free fibers is estimated based on a statistical analysis of the distribution of fiber-fiber contact points. The new expressions are more general than previous ones by allowing for a distribution in fiber length and orientation. The forced packing beyond this limit is governed by the bending of fiber segments between contact points. A micromechanical theory is developed for this, again based on the contact point statistics, and equations relating the force response per unit area of a fiber bed to the fiber volume fraction are derived for three basic types of assembly: a general 3D wad, a planar mat of dispersed fibers, and a bundle of almost parallel fibers. Other types of reinforcement structure, such as woven fabrics, and the effect of lubrication are also treated briefly.     

Hot embossing in microfabrication. Part I: Experimental

The relationship among processing conditions, material properties, and part quality in hot embossing was investigated for three optical polymers: polycarbonate (PC), polymethyl methacrylate (PMMA), and polyvinyl butyral (PVB). A series of systematic embossing experiments was conducted using mold inserts having either single or multiple feature depths. The feature dimensions varied from 90 to 3000 μm. The processing conditions studied include embossing pressure, thermal cycles, and heating methods. The displacement profile, replication accuracy and molded‐in stresses were measured experimentally. It was found that for isothermal embossing, both replication accuracy and birefringence pattern depend strongly on the processing conditions. For non‐isothermal embossing, the molded parts showed excellent replication as long as the feature transfer was completed. The flow pattern under isothermal embossing resembles a biaxial extensional flow. Under non‐isothermal embossing, the polymer deformation involves an upward flow along the wall of mold features, followed by downward compression and outward squeezing. Rheological characterization and hot embossing analysis are presented in Part II.     

Moisture adsorption characteristics of a thin bed of polycrystalline potash pellets: Part I. Experimental study

This study is an experimental investigation of heat and moisture transfer within a packed bed of polycrystalline, porous potash pellets. Experiments were first performed to determine the moisture uptake characteristics on individual pellets subject to a step-change in relative humidity. Then a bed of pellets was subjected to flows of humid air on the upper boundary and a cold impermeable surface on the lower boundary resulting in a temperature gradient across the bed. Temperature and moisture content were measured within the bed. It was found that moisture uptake is less for potash pellets and they are less likely to cake than granular and standard potash when subjected to the same ambient air conditions. This is due to the porous nature of the pellet and its nearly uniform size and spherical shape.     

Two-dimensional multilayer coextrusion flow in a flat coat-hanger die. Part I: Modeling

The multilayer stratified flow of several polymers in a flat coat-hanger die was modeled using a finite element method. The problem was formulated using the lubrication approximation theory. A solution procedure in decoupling pressure and streamlines was developed. This new method is very powerful in comparison with more classical approaches, permitting the solution of flow problems involving a great number of layers in a complex industrial geometry. It allows us to obtain, among other things, pressure field, streamlines, residence times, and the values of the interface positions in the whole die.     

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.     

Analysis of coating on glass fiber reinforcements

Several commercially available glass fiber reinforcements were analyzed to determine the amounts (“loadings”) and types of coatings applied by their manufactures. Loadings were determined by solvent extraction and pyrolysis and ranged from 0.54 to 4.22 weight percent with the heaviest amounts on random mats and the least amounts on rovings intended for filament winding and weaving. The major components of the solvent-extracted coatings were identified by infrared (IR) and proton nuclear magnetic resonance (NMR) spectroscopies. All of the coatings analyzed could be sorted into three classes of coatings which matched the matrix types recommended by the glass manufacturers. In each case, the coating consisted primarily of a mixture of two separate materials, designated as the “unreacted resin” and the “lubricant”.     

Analysis of dynamic flows through porous media. Part I: Comparison between saturated and unsaturated flows in fibrous reinforcements

This article presents a general approach to model flows through unsaturated porous media as they occur in Liquid Composite Molding (LCM). Saturated and unsaturated flows will be studied here both from the experimental and theoretical points of view. It is indeed important to distinguish between these two flow behaviors in order to understand the interactions between the three phases that coexist in a fibrous reinforcement: the solid and fluid phases on one hand, and the air content on the other. The experimental work presented here includes the study of permanent and transient flow regimes, both for saturated and unsaturated porous media. The dynamic effects that occur during fluid injection through fibrous reinforcements highlight the double scale, structure of their pore volume. The ratio between saturated and unsaturated permeabilities appears to be connected to the degree of saturation and to the porosity of the part. Given the importance of permeability as a key input parameter in process simulation, this article proposes to introduce the degree of saturation in the equations that govern the flow in order to increase the accuracy of numerical predictions. This will not only provide a better understanding of the underlying physical phenomena during the fluid impregnation of a fibrous preform, but will also ultimately allow the study of air entrapment mechanisms that govern the quality of composite parts.     

Measurement of in-plane permeability of anisotropic fiber reinforcements

Three of the most common methods (two with parallel flow and one with radial flow) for determination of the in-plane permeability tensor are studied both theoretically and experimentally. An error analysis shows that the difference between the methods is negligible if the error levels are equal. However, the radial flow method is found to be susceptible to large errors from mold deflection in an experimental comparison between the methods. Additional experiments with the radial flow method in a stiffer mold show that the method gives the same values for the permeability tensor as the other two methods. A new method with multiple cavities in parallel is proposed that combines the simplicity of the radial flow method with the stiff mold of the parallel flow method. Only mass and time need to be measured in one experiment and it eliminates the need to measure fluid viscosity, temperature, and injection pressure. The method depends on the availability of a reference material with known permeability.     

Optimization of polyester sheet molding compound. Part I: Experimental study

A series of differential scanning calorimetry (DSC) and molding experiments were carried out to measure the effect of curing agents, namely initiators and inhibitor, on the SMC reaction. Results showed that the induction time, the reaction rate, and the limiting conversion of sheet molding compounds can be modified through the change of curing agents. The SMC resin with a higher concentration of low temperature initiator and molded at higher temperature may cure in a shorter period of time and reach a higher conversion. The shortened scorch time and shelf life can be balanced by adding small amount of inhibitor. Surface quality of molded SMC parts measured by solvent extraction process showed that limiting conversion is an important factor in SMC molding.     

Continuous emulsion polymerization of vinyl acetate. Part I: Experimental studies

An interesting feature of commercial continuous emulsion polymerization reactors is the important phenomenon of sustained oscillations. Laboratory investigations of emulsion polymerization in a continuous stirred tank reactor (CSTR) have shown that the conversion, number of polymer particles and other related properties often oscillate widely with time. These oscillations can lead to emulsifier levels too small to adequately cover polymer particles resulting in excessive agglomeration and reactor fouling. Herein is reported an extensive experimental study of vinyl acetate emulsion polymerization in a CSTR. The effects of initiator and emulsifier concentrations, mean residence time and rate of agitation on the production rate and size distribution of polyvinyl acetate latices were investigated. Domains of reactor operation which give rise to sustained oscillations and massive agglomeration of the latex are mapped which give rise to sustained oscillations and massive agglomeration of the latex are mapped out. The effect of different start-up policies on reactor transients were also investigated.     

Compaction of textile reinforcements for composites manufacturing. III: Reorganization of the fiber network

The objectives of this series of papers are to describe the mechanical behavior of textile reinforcements under normal load and to quantify the effects of diverse processing parameters on that behavior. In the first and second papers of the series, experimental compaction and relaxation results were reported; general trends were identified and the effects of changes in the processing parameters were analyzed. In this paper, the results of sequences of successive compaction cycles applied to dry textiles and to textiles saturated in distilled H₂O and silicone oil are presented. The reinforcements investigated are produced by assembling tows or rovings following different patterns; it is shown that the resulting heterogeneity, or regular variation of the local fiber volume fraction, can be associated to some particular elements of the mechanical behavior of the reinforcements. The reorganization of the fiber network and the effect of friction at the fiber contacts are demonstrated. Different stages in the reorganization process are identified; each stage is controlled by different parameters and corresponds to a precise behavior. Successive compaction cycles applied to a preform can reduce the void content of the final part.     

关于玻纤增强基材发展的讨论

1前言 玻璃纤维70%以上用于增强基材(塑料增强),与之相对应多数的玻璃纤维生产企业也都生产增强基材.因此不断研究玻纤增强基材的发展,就等于研究了玻璃纤维的主要市场,有利于企业的健康发展.     

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.     

Bulk polymerization in tubular reactors I. Experimental observations on fouling

Conditions under which fouling (polymer deposition) occurs in tubular polymerization reactors were studied using bulk methyl methacrylate polymerizations at 70°. It was demonstrated that reactor orientation and flow direction have a significant effect on fouling behaviour. Natural convection becomes increasingly important as concentration gradients in the reactor increase. Using the optimum reactor configuration determined in the first part of the study, a feasible operating region for the reactor was established, thereby permitting selection of conditions which will prevent reactor fouling.     

Compressibility and relaxation of fiber reinforcements during composite processing

In many reinforced composite manufacturing processes it is necessary to compact the fiber materials to obtain the desired fiber/resin ratio in the finished part. Detailed knowledge of applied surface force versus material fiber volume is particularly important in processes such as pultrusion, resin transfer molding, and compression molding. The force required to compact a stack of reinforcing material is strongly dependent on the type of fiber used and its material form. Complicated interactions are possible, particularly when mixtures of unidirectional, oriented cloth and random fiber mats are used. This paper will present results of an experimental and analytical investigation of the response of various dry reinforcing materials subjected to compressive forces applied normal to their principle plane. Experiments were conducted by applying up to 8.6 MPa normal force to thick stacks of E-glass, graphite cloth, mat and unidirectional material and combinations of two different fiber orientation. Pressure versus fiber volume data were generated for both individual materials and various combinations. Experimental results were compared to analytical predictions. Data showed that the force versus deformation is very strongly dependent on the details of the fiber form or forms being used. There is structural relaxation during fiber compression. Relaxation is very related to fiber orientation, span length, and fiber breakage during compaction. Relaxation behavior decreases with fiber alignment. Random mats and 0/90 cloth show much more relaxation than unidirectional fibers. Data of relaxation is very well fitted with a Maxwell-Wiechert viscoelastic model.     

Model resin permeation of fiber reinforcements after shear deformation

In liquid composite molding processes such as resin transfer molding and structural reaction injection molding, fiber reinforcements are formed with automated processes to conform to the complex shape of the mold cavity. Deformation of the fiber reinforcement during the forming operation can be characterized by factors such as the local surface curvature of the mold and the type of reinforcement. For bidirectional fiber fabrics, simple shear is the major deformation mode in the forming process. Deformation of the fiber reinforcement after being formed to the mold cavity shape results in variations of local fiber content. In addition, the network structure of the fiber reinforcement is also rearranged. This may cause some significant effects on the fiber permeability and result in a mold filling pattern quite different from that expected. Therefore, a good understanding and measurement of the permeabilities for the deformed fiber reinforcements is of great importance. In the flow simulation of the filling process, the success of the prediction depends greatly on the correct values of in-plane permeabilities. A change of the in-plane permeability of the fabric after shear deformation must be well understood before an accurate flow simulation can be obtained. This article investigates the permeability of fiber reinforcements in relation to different shear angles. Several flow experiments were conducted on bidirectional woven roving fabrics at different shear angles. Two relevant factors—the ratio of principal permeabilities and the direction of principal axes with respect to the orientation of the fabric—are studied to investigate their variations with respect to shear deformation of the fiber reinforcements. It is found that the angle shift of the principal axes increases with the shear angle. At the same time, the in-plane permeability ration may decrease with the shear angle.     

Compaction of textile reinforcements for composites manufacturing. I: Review of experimental results

As new developments are brought to the group of manufacturing processes for composite parts known as liquid composites molding (LCM), the compaction behavior of the textile reinforcements is increasingly seen as an important parameter of the definition of these processes. The evolution of the permeability tensor of the reinforcements with time, the general kinetics of the manufacturing operations, and the modelization of these processes depend on a large extent on the compaction behavior of the reinforcements used, especially in flexible-wall RTM and autoclave molding. Also, more research efforts are devoted toward the development of a complete analytical model of the properties of heterogeneous textile reinforcements. In this paper the published experimental data related to the compaction and relaxation of random mats and woven reinforcements are gathered. Observed parameters are defined, which allow numerical comparisons of the experimental curves to be made, as well as the identification of general trends seen with most tested reinforcements. The effects of various processing parameters are identified, and relations to published analytical models of the mechanical properties of fibrous assemblies are discussed.