Experimental investigation of flow-induced microvoids during impregnation of unidirectional stitched fiberglass mat

The complexity of the resin injection step in liquid composite molding (LCM) processes, such as resin transfer molding (RTM) and structural reaction injection molding (SRIM), often results in flow-induced defects such as poor fiber wetting and void formation. These defects have a deleterious effect on the mechanical properties of composites. In this work, high resolution video-assisted microscopy was used for in situ observation of flow-induced interstitial voids or microvoids formed inside the fiber tows during mold filling. Flow visualization experiments were carried out with different liquids to better understand the microscale flow behavior that led to the formation of microvoids for flow both along and normal to the fiber tows. Microvoid formation was correlated to the modified capillary number, Ca#* = μν/γcos(θ). The study revealed that for axial flow, microvoids were formed at Ca#* > 10⁻³. For transverse flow, microvoids were formed at an even lower capillary number. ∼ 10⁻⁴. Once formed, microvoids were difficult to purge and remained trapped even after bleeding the liquid at much higher flow rates than those at which they were formed.     

Thermoplastic composites reinforced with long fiber thermotropic liquid crystalline polymers for fused deposition modeling

This work is concerned with preliminary studies on developing thermoplastic composites for use in fused deposition modeling (FDM). Polypropylene (PP) strands reinforced with thermotropic liquid crystalline polymer (TLCP) fibrils were generated in a novel dual extruder process. The process allowed the reinforcement of PP with a melting point (T_m) of 165°C with continuous fibrils of a high melting (283°C) TLCP (Vectra A950). The strands were then re-extruded in a capillary rheometer forming monofilaments to simulate piston actuated FDM. The effects of the thermal and deformation histories on the mechanical properties of the re-extruded strands were evaluated. It was found that tensile properties of the strands improved with draw ratio and that the maximum modulus of the composite strands was similar to that predicted by composite theory. Strands were consolidated uniaxially via compression molding at temperatures just above the melting point of the matrix to determine the effect of thermal history. This resulted in a ∼20% reduction in tensile modulus relative to the modulus of the strands. Monofilaments were extruded from a capillary rheometer in which long fiber strands were used as feedstock to study the effects of deformation history on the tensile properties. It was found that the tensile properties of the monofilaments were dependent on capillary diameter, capillary L/D, and apparent shear rate due to fibril alignment.     

环氧片状模塑料在模压过程中的纤维分布

纤维的流动分布对环氧片状模塑料(ESMC)制品的各项性能有十分重要的影响。以纤维含量作为性能指标,对原材料组分、片材黏度、铺层方式、模压工艺参数等因素与纤维流动分布的关系进行系统研究。研究结果表明,当玻璃纤维含量为24%、长度为24 mm、填料粒径为45μm、片材黏度为5×103Pa·s、铺料面积为60%、压机闭模速度为1 mm/s时,ESMC模压制品纤维分布的均匀性较好。     

Influence of glass fiber content on the morphology and mechanical properties in injection molded polypropylene composites

Propropylene was reinforced with various concentrations of glass fiber (0, 10, 20, 28, and 35 percent by weight) and standard test samples were prepared by injection molding under identical conditions. The influence of fiber concentration on the morphology and mechanical properties was investigated using polarizing microscope, scanning electron microscope, Instron universal testing machine, etc. Tensile strength, tensile modulus, and flexural strength were found to increase with glass fiber content, while impact strength was reduced. These observed changes in mechanical properties were explained in terms of alterations in skin-core morphology, spherulite size, transcrystallinity, etc. The efficiency of reinforcement factor (K) was calculated for all the composites. It was found that K decreases with the increase of fiber concentration. This study attempts to correlate fiber concentration and polymer morphology with mechanical properties and the overall efficiency of reinforcement. This would help in optimizing the fiber content to prepare molded composites with desired mechanical properties.     

短纤维—热塑性聚氨酯弹性体注射充模过程的研究进展

介绍了短纤维－热塑性聚氨酯弹性体（ＳＦ－ＴＰＵ）注射充模过程国内外的研究进展,ＳＦ－ＴＰＵ注射充模过程与通常高聚物有许多相似之处,更有其不同特点。国内外的研究者们对其注射充填过程中的传热、流动、纤维取向与加工条件、模腔几何参数之间的关系进行了研究,但纤维与流体的相互作用、前锋流对纤维取向的影响等,还须进一步研究。     

Use of wet‐laid techniques to form flax‐polypropylene nonwovens as base substrates for eco‐friendly composites by using hot‐press molding

The wet‐laid process with flax (base) and polypropylene (binder) fibers has been used to obtain nonwovens for further processing by hot‐press molding. Mechanical characterization of nonwovens has revealed that slight anisotropy is obtained with the wet‐laid process as better tensile strength is obtained in the preferential deposition direction. The thermo‐bonding process provides good cohesion to nonwovens, which is critical for further handling/shaping by hot‐press molding. Flax:PP composites have been processed by stacking eight individual flax:PP nonwoven sheets and applying moderate temperature and pressure. As the amount of binder fiber is relatively low (<30 wt%) if compared with similar systems processed by extrusion and injection molding, it is possible to obtain eco‐friendly composites as the total content on natural fiber (flax) is higher than 70 wt%. Mechanical characterization of hot‐pressed flax:PP composites has revealed high dependency of tensile and flexural strength on the total amount of binder fiber as this component is responsible for flax fiber embedment which is a critical parameter to ensure good fiber–matrix interaction. Combination of wet‐laid techniques with hot‐press molding processes is interesting from both technical and environmental points of view as high natural fiber content composites with balanced properties can be obtained. POLYM. COMPOS., 2012. © 2011 Society of Plastics Engineers     

Interface and mechanical properties of poly(methyl methacrylate)–fiber composites

Long‐fiber pellets were made by an in situ pultrusion process. Fiber‐reinforced composites were prepared by an injection‐molding process and an extrusion/injection‐molding method with pellets, respectively. SEM observations showed that the strong interface was maintained during the injection process for low shearing forces, although polymer adhesion to the fiber surface was completely delaminated in the process of extrusion/injection molding for very high shearing forces. Enhanced adhesion of composites promoted substantial improvement of mechanical properties compared to those with poor adhesion. However, the enhanced adhesion between the fiber and the matrix also sacrificed the impact resistance properties. Longer fibers substantially enhanced the properties of composites. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2478–2483, 2004     

Mold filling and curing analysis in liquid composite molding

Non-isothermal mold filing and curing experiments of liquid composite molding were carried out in this work. To compare the experimental results with a previously developed numerical simulation model, measurements of volumetric heat transfer coefficient between the resin and the fiber, and characterization of resin kinetics and rheological changes were also conducted. Combined with the previously measured fiber perform permeability, the numerical model provided a good prediction of temperature profiles during molding for a polyurethane/glass fiber composite.     

An evaluation of the curing characteristics of thermosetting epoxy carbon fiber sheet molding compounds and validation through computer simulations and molding trials

Sheet molding compounds (SMC) are ready-to-mold thermoset composite materials reinforced with discontinuous fibers, usually compression molded. Finite element (FE) based compression molding tools can be employed to optimize this process; FE tools require to define material models using raw material data measured through different characterization techniques. In this study, the cure kinetics of an epoxy-based carbon fiber SMC has been characterized by means of differential scanning calorimetry (DSC) and moving die rheometer (MDR) techniques. Based on these datasets, Claxton-Liska and Kamal-Souror models have been set and the compression molding of a validation plate was performed, both experimentally and virtually. The results indicate that, even if both characterization techniques are valid for SMC curing characterization, MDR technique enables the characterization of the material at real molding temperatures and the model based on MDR leads to more accurate results.     

Micro scale flow behavior and void formation mechanism during impregnation through a unidirectional stitched fiberglass mat

Liquid composite molding (LCM) processes such as resin transfer molding (RTM) and structural reaction injection molding (SRIM) have been perceived as high potential processes for the near-net-shape manufacturing of composite parts. This paper addresses two major issues in LCM technology: fiber wetting and void formation during mold filling. Flow visualization experiments were carried out to develop a better understanding of the flow induced voids. The formation and elimination of voids were studied using several liquids and a unidirectional stitched fiberglass mat. Void formation was correlated to capillary number and liquid-fiber-air contact angle.     

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.     

聚丙烯/玻璃纤维复合材料注塑工艺条件研究

基于自制的聚丙烯/玻璃纤维(PP/GF)复合材料,研究了复合材料不同配方与注塑工艺之间的关系.结果表明,注射成型加工时,PP/GF复合材料的配方组成不同,对注塑工艺条件和制品的性能都有较大的影响,因此应根据PP/GF复合材料的不同配方采用不同的注塑工艺条件.加有改性剂或偶联剂的PWGF复合材料注塑时的塑化温度、注塑压力、保压压力和保压时间及注射成型后的冷却时间均高于未加改性剂的PP/GF复合材料.     

Prediction of fiber orientation in a rotating compressing and expanding mold

In the rotating/compressing/expanding mold (RCEM), one mold wall can expand, compress, and rotate during injection molding, thus offering opportunities to control the thermomechanical history of a polymer and its microstructure. A computer simulation of flow and fiber orientation in RCEM was developed. The predictive model extends the generalized Hele‐Shaw formulation to account for compression/expansion and rotation of the mold wall, and uses the Folgar–Tucker model for fiber orientation predictions. A 20% GF polypropylene was molded under various molding conditions. The predicted fiber orientation distributions were compared with experiments. The model compares favorably with experiments, provided that the fiber orientation equation is modified by a strain‐reduction factor that slows the transient development of fiber alignment. The effect of fountain flow on orientation must also be included to correctly predict fiber orientation near the mold walls, mainly for the case of stationary and linear motions of the mold surface. Compression or expansion of the mold has only a small effect on fiber orientation, but rotation of the mold dramatically changes the orientation, causing fibers to align in the tangential direction across the entire thickness of the molding. This rotation action perturbs the fountain flow and becomes the dominant factor affecting fiber alignment across the entire cavity thickness. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers.     

Optimization of fiber prediction model coefficients in injection molding simulation based on micro computed tomography

Fiber‐reinforced thermoplastic for low weight application become increasingly important for many industrial branches. During the injection molding of short fiber‐reinforced thermoplastic parts the fibers become orientated. This orientation is determined on the one hand by the geometry of the part, and on the other hand by the injection molding parameters, and influence the mechanical behavior of the part. The determination of the fiber properties that is, the orientation distribution of the fibers, is therefore of considerable interest. Since a more accurate fiber orientation prediction of the injection molding simulation will lead to a more precise structural simulation the objective of the present work is to achieve a preferably accurate orientation distribution. To describe the orientation distribution of the fibers, the fiber orientation tensor defined by Advani and Tucker (Advani and Tucker, Journal of Rheology, 31, 751 (1987)) was used. To determine the entries of this tensor micro computed tomography scans (μCT‐scans) of an injection‐molded plate, as well as an injection‐molded specimen with different cross section and shape were performed. Injection molding simulation using Autodesk Moldflow Insight were carried out. The residual strain closure (RSC) model was the underlying model to depict the fiber orientation distribution, or rather the orientation tensors. The two model parameters, the fiber interaction coefficient Ci and the scalar factor κ , were adapted by an optimization procedure, in such a way that the orientation distributions of the simulations fit the results of the μCT‐analysis at its best. POLYM. ENG. SCI., 59:E152–E160, 2019. © 2018 Society of Plastics Engineers     

Effects of preferential encapsulation of glass fiber on the properties of ternary GF/PA/PP blends

Long fiber molding materials are expected to play an important role in the near future. This paper describes a series of experiments performed to examine properties of ternary blends containing glass fiber (GF), polyamide (PA), and polypropylene (PP). The continuous glass fiber was impregnated with one of the blend constituent polymers by our specially designed impregnation apparatus and cut into chips of 6 mm length. These chips and the other polymer were used to produce various testing specimens in a twin screw extruder or in injection molding machine. The results indicated that the effect of fiber addition on the mechanical and rheological properties is clearly dependent on the order of impregnation process. In the blends containing the GF/PA + PP, the GFs are preferentially encapsulated with PA, and therefore the mechanical properties are superior to the blends with the GF/PP + PA in which the PP phase is located surrounding the GFs. This improved wetting of fibers by sequential impregnation not only resulted in better properties but also protected the fibers from shear action of the screw, thereby allowing significant increase in average fiber length to be achieved in the injection molding process.     

Preparation and mechanical characterization of chicken feather/PLA composites

Green composites, a bio‐based polymer matrix is reinforced by natural fibers, are special class of bio‐composites. Interest about green composites is continuously growing because they are environment‐friendly. This study describes the preparation and mechanical characterization of green composites using polylactic acid (PLA) matrix including chicken feather fiber (CFF) as reinforcement. Extrusion and an injection molding process were used to prepare CFF/PLA composites at a controlled temperature range. CFF/PLA composites with fiber mass content of 2%, 5%, and 10% were manufactured. The effects of fiber concentration and fiber length on mechanical properties of CFF/PLA composites have been studied. Mechanical properties of composites were investigated by tensile, compression, bending, hardness, and Izod impact testing. The results of experiments indicated that Young's modulus, compressive strength, flexural modulus, and hardness of the PLA reinforced CFF composites are higher but tensile strength, elongation at break, bending strength and impact strength of them are lower than pure PLA. The results indicate that these types of composites can be used for various applications. POLYM. COMPOS., 38:837–845, 2017. © 2015 Society of Plastics Engineers     

Preparation of novel reactive plastisol based on poly(vinyl chloride) and multifunctional acryl esters

Poly(vinyl chloride) (PVC) powder was mixed with various polyfunctional acryl monomers as plasticizers to prepare acrylate‐modified plastisols. This class of plastisol was hardened into the B stage without reaction and then cured into a harder material through crosslinking of the acrylate. The best formulation was attained after evaluation of various acrylates, PVC grades, and peroxides. Several difunctional acrylates with solubility parameters similar to that of PVC could be most conveniently used as the plasticizer. A sheet molding compound was obtained by the combination of the resultant plastisols with glass fiber by compression molding using conventional machines. It was cured into a PVC‐based fiber reinforced plastic with high performance. This class of acrylate‐modified plastisol is called reactive plastisol. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1794–1801, 2000     

Electrical Conduction of a Single Electrospun ZnO Nanofiber

The dc electrical characterization of single electrospun ZnO nanofibers (NFs) calcined at various temperatures was investigated. The linearity of Ti/Au ohmic contacts was confirmed for a wide range of voltage values. Parameters such as dc fibers resistivity in an atmosphere of dry air and nitrogen were determined. The studies have shown that the entire volume of fiber is depleted of charge carriers. The I–V dependence of fibers and contacts showed a linear behavior for a wide range of temperatures. Temperature dependence of conductivity was evaluated. Arrhenius plots revealed that the electrical conduction is mainly thermally assisted in the extended states. The activation energy was found to be strongly dependent on the grain size, which in turn depends on the fiber annealing temperature. This could be caused by segregation of point defect in nanocrystalline ZnO and changes in carrier concentration. To explain this effect, the authors proposed a model of donor depletion in grain based on the Mott–Schottky approximation. To the best of our knowledge, we report the first systematic study related to electrical characterization of single electrospun ZnO NFs, calcined at various temperatures, which allows for estimation of resistivity and activation energies in dry air and nitrogen atmosphere.     

Morphology,mechanical performance,and nanoindentation behavior of injection molded PC/ABS‐MWCNT nanocomposites

In this work, nanocomposites of polycarbonate/acrylonitrile‐butadiene‐styrene (PC/ABS) with various loads of multiwall carbon nanotubes (MWCNT) are investigated. Material is previously formed by masterbatch dilution approach and further processed by injection molding at various velocities. Microscopic characterization of nanocomposites morphology reveals stronger dependence of MWCNT dispersion on processing parameters at higher nanofiller load. Dispersion of carbon nanotubes at various distances from the injection gate is studied by Raman spectroscopy showing lower deviation at elevated injection velocity. Nanoindentation results that are in agreement with uniaxial tensile testing show a slight decrease of nanocomposites' mechanical performance at 3.0 wt % MWCNT in samples injected at reduced velocity. This is explained by the increase of agglomeration behavior at these conditions. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42014.     

Injection molding of glass fiber reinforced phenolic composites. 2: Study of the injection molding process

This study of injection molding of glass fiber reinforced phenolic molding compounds examines fiber breakage and fiber orientation with key material and processing variables, such as injection speed, fiber volume fraction, and the extent of resin pre-cure. The fiber orientation, forming discrete skin-core arrangements, is related to the divergent gate to mold geometrical transition, the extent of pre-cure and injection speed functions of the melt viscosity. Transient modifications to the melt viscosity during mold filling produce variations in skin/core structure along the flow path, which are correlated to the mechanical properties of injection moldings. The melting characteristics of the phenolic resin during plasticization impose a severe environment of mechanical attrition on the glass fibers, which is sequentially monitored along the screw, and during subsequent flow through runners and gates of various sizes. Differences found between the processing characteristics of thermosets and thermoplastics raise questions concerning the applicability of thermoplastic injection molding concepts for thermosets.