Relation Of Permeability And Textile Preform Structure In Liquid Composites Moulding Process

As polymer composites in liquid composites moulding in the excellent performance, processing and other aspects of the advantages, composites have attracted greater attention more and more in recent years. For any type of fiber-reinforced composites in composites moulding process, the most important requirement is that the fiber and resin distributed evenly in final products, making the resin impregnated reinforcing materials completely while not affect the good placement of preforms. The research presented here aims to increase the fundamental knowledge on a crucial part in liquid composites process technology:the permeability and impregnation behavior of textile reinforcements, including the influence and relationship between resin flow and fabric structure parameters, design and preparation of preforms structure, and modeling of permeability and textile preform structure, and systematic simulation molding filling behavior process and so on.(1) Permeability testing mechanism and permeability measurement developmentDescribed the mechanism of permeability test,2D permeability test used the plane penetration model, and 3D permeability testing by the penetration of semi-ellipsoidal model. Indirect monitoring permeability measurement work cells are constructed, based on analogue linear sensor penetration testing techniques in 2D and, based on ultrasound technology in 3D, and record the output signal with the LabView program, which allows to obtain the desired flow front position at any instant of time and to calculate the permeability and other parameters of different textile preforms. Also the testing process and data analysis and post-processing curve are introduced. Finally, the characteristics of the testing liquid are introduced.(2) Effect of textile preform structure on permeability and resin flow behaviorBy systematic analysis influence and relationship between resin flow and fabric structure parameters, a number of impact factors of preforms on permeability have obtained.Different type fabric structures have different effect on permeability, and plain weave fabrics have higher two principal permeability tensor K1 and K2, and K1 and K2 in satin fabric are closer, but K1 and K2 in twill fabric shown significant difference as there are some certain flow permeate direction in twill fabrics, while the flow front contour lines near circle in plain and satin weaves. Crimp angle of yarn will cause different shape and size of flow channels in preforms, which ultimately affect the flow behavior and permeability. Fabrics with lower linear density yarn showed a larger crimp bending angle, while with the performance of higher crimp shrinkage rate and lower tension in yarn tows. Any changes of yarn fineness will cause changes in structure and size of internal voids, while the principal permeability Kl and K2 will decrease with the increase of yarn fineness. Consolidation of different fiber sizing and fluid interaction, there will be a different level of surface tension, capillary force and thus produce different affect on flow behavior and permeability. The melted thermoplastic adhesive after reaction in preforms will format more distribution of film groups on the fiber surface adhesion, and thus resin flow permeate better faster and easier than before reaction. Stitch mode and overlay placements in the form will cause permeability changes in fabric preforms. Different angle between the flow channel by stitch penetrated and fiber direction, also make permeability and anisotropy to significantly different. Relatively thick layer, lower layers numbers, the total effective size of each channel between layers will also increase, flow permeation will increase. Improved injection pressure can be increased flow speed. But if the pressure is too high the liquid from the injection gate into the mold cavity will form compression and make permeability decrease.For the 3D permeability in thickness direction in carbon fiber fabrics, there are significant different permeability in twill, biaxial NCFO/90, unidirectional NCFO preforms. Resin flow in unidirectional fabric is easy along plane fabric, but flow difficultly in transverse and thickness direction. Especially permeation in thickness direction is very low, and lower by 1-2 orders of magnitude compared to the other two structure fabrics. In compacted fabric structure, the biaxial fabric has higher permeability in thickness direction than twill fabric. For different glass fiber fabrics, plain weave has higher 3D permeability in thickness than twill. Lower linear yarn density can improve the 3D flow permeability in thickness. The yarn linear density of warp and weft one or two become thinner will help the permeation of resin flow.(3) Design and preparation of textile preformFor thermoset resin, because of its low viscosity, good fluidity in liquid molding process, the resin matrix can permeate and impregnate into the reinforcing fiber preforms or fiber porous media by controlling the processing technique. But for thermoplastic resin, because of its higher molecular weight, it has a high viscosity under melting which will lead to impregnate difficultly. So there is the biggest problem for a thermoplastic resin poor impregnation of the reinforcing fibers, not conducive to the distribution of reinforcing fibers and matrix resin impregnation.Co-wrap spun yarns for reinforcing and thermoplastics offer a potential for low-cost manufacturing of complex shaped and uniformity of two-phase and good drapable composite parts due to reduce impregnation times and resin flow distance and increase the permeability and impregnation during processing. The empirical model by response surface methodology was established to optimum combination of hollow spindle twist and hollow spindle rotational speed in order to produce uniform co-wrapped yarns and preforms. To get evenly distribution of fibers and thermoplastic resin in preforms, good compactness of yarn, different flax fiber content and ideal mechanical properties, the main spinning parameters (hollow spindle twist and hollow spindle rotational speed) are analyzed and optimized.The combining form and evenly distribution of reinforcing fibers and resin matrix, and the effect of fabric preform structure parameters on mechanical properties are systematic analyzed. Co-wrapped yarn woven fabric preforms and composites have higher tensile properties and peeling performance than co-weave fabric preforms, due to better impregnation and better interface bonding. The changes of weft weaving density have greater impact composites than warp density. An integrated facilitative weaving and the tensile properties of fabric preforms to a collaborative role could be set at a smaller warp density range and a moderate level of weft density. The fabric structure (include plain, twill, and basket weave fabric) and yarn tensile orientation (in 0°,90°,45°), have greatly influence on the tensile strength and elongation of preforms, due to different fabric structure, different numbers of interwoven yarn in warp and weft, the varying crook degrees of crimp, and the angle of woven yarn. As the structure of three types of fabrics, the largest tensile strength is along the warp direction (0°), and the minimum is direction of 45°. But the elongation in direction of 45°is the largest. Considering the tensile, peel strength and interface effect of impregnation, the material of the have better performance at a moderate level of mold thickness, they shown easy impregnation and better bonding between layers, and the corresponding smaller fluctuate differences of peel peaks. Finally, to improve the impregnation and bonding interface during processing, different pre-treatments on flax/PP are investigated, and the microstructure and mechanical properties of thermoplastic composites show improved wettability of impregnation and good adhesion and bonding interface between flax fiber and PP matrix in composites after alkali treatment and coupling agent pre-tremens.(4) Permeability modeling and analyzingThe 2D,3D permeability models and fluid flow behavior are modeling and analyzing including thermoset and thermoplastic resin flow permeability behavior, different forms of fiber assembly and preform structure on the permeability and so on. The resin flow process in various preforms or fiber assembly, dynamic flow front, pressure and temperature distribution and filling time are investigated, and the preform structure parameters (including the fiber arrangement, fabric structure parameters, layer interaction effects, etc.) impact on the relationship between flow behavior and permeability.For 2D plane permeability in anisotropic media, the Cartesian coordinate system is transformed to the elliptic coordinate system to simplify modeling. For heavy size components of permeate processing, 3D permeability model in anisotropic media has been build, used point injection flow source and semi-ellipsoidal diffusion model. Layer interaction effect model analysis the relations of not same mobility, hysteresis of permeability between adjacent layers. The smaller of permeability in thickness and the greater differences of permeability in flow plane, the more obvious effect of this hysteresis lag. The more difference of permeability between the upper and lower two adjacent fabric layers, regardless of higher or lower of permeability in thickness direction, will result in poor flow simultaneous impregnation.A radial impregnation model for thermoplastic preforms has been established. The permeability model is transformed to the local state of impregnation model to study, avoiding long distance flow permeate similar in RTM, the modeling and analysis focus on each yarn bundle preforms and permeate process until fully impregnation. The model describes the relationship of impregnation degree and impregnation process parameters include the impregnation pressure, temperature (changed caused by viscosity), fiber bundle size, time, and some interacting factors.(5) Simulation of resin flow and liquid composites processFrom the preform parameters and injection molding parameters, analysis of the permeate flow filling form and interaction effect facters, simulation the flow behavior and resin molding filling process in various parameters.Preform parameters, including the principle permeability, anisotropy, orientation angle have a greater impact on the flow behavior and molding filling time. When principle permeability is large, the resin flows impregnate faster, and filling time is shorter. The difference in permeability will cause anisotropy different, resulting in great differences in flow front counter and the roundness of circle and ellipticity, which affect the filling time. The orientation angle will change the orientation of flow front counters, the greater orientation angle, the shorter the filling time at same conditions. Injection molding parameters (pressure, injection gate and flow.channel distribution) have significant impact on the flow behavior and molding filling time. At the appropriate pressure range filling time can be reduced by increasing the injection pressure. When used one injection gate, resin flow slower in the center injection, and fill time is long in unilateral side injection especially when the channel length is short, but fill time is short in the annular channel with radial injection. The filling ways have more significant effect on filling time than the numbers of injection gate. When setting different numbers of injection gates in annular channel, the filling time is slightly reduced, but the effect is not significant while injection gate increase. Whether the flow channel within mold or not, it will have quite different effect on filling time and flow behavior, a number of flow channels can divide the entire flow field into more smaller sub-regional field, and each radial impregnation will diffuse to center respectively, which will accelerate flow filling velocity and reduce filling time.