低压片状模塑料纤维的取向

低压片状模塑料(LPMC)模压流动成型后玻纤在平面内发生取向,导致模压件呈各向异性。文中从成型后制品上取样烧尽树脂,由扫描仪获取纤维数值图像,用Photoshop软件将图像反相、增强,再利用MATLAB软件确定纤维取向分布。并讨论了纤维取向的影响因素、控制方法及对复合材料力学性能的影响。研究表明,LPMC片材的黏度、加料面积及闭模速度是影响制品纤维分布的主要因素,增大片材黏度、增大加料面积及适当提高闭模速度都可以提高制品纤维分布的均匀性。LPMC制品的弯曲强度几乎不随试样的取样位置不同而改变,但在纤维平行取向方向上具有比垂直取向方向上更高的强度和模量。     

纤维参数对纤维取向及塑件变形影响模拟研究

采用Moldflow软件对短玻纤增强聚丙烯复合材料注塑成型矩形平板塑件的成型过程进行模拟,重点研究纤维含量(A)、纤维长径比(B)和纤维间相互作用系数(Ci)对平均纤维取向(D)和制品变形(E)的影响,进一步探究短纤维增强聚合物注塑成型的特点.研究表明:A、B对D及E的影响较复杂,且存在一个最佳值;随着B的增大,D先增大后减小,再增大;而E随B的增大呈先增大后减小的趋势(B=1除外);随着Ci的增大,D呈减小,E呈先减小后增大的趋势.收缩是引起塑件变形的主要因素.塑件变形在三维空间的Z方向的变形量最大,在熔体流动(X方向)及垂直流动方向(Y方向)的变形量均相对较小.     

RSC模型中相互作用系数和标量因子对短纤维取向的影响

为预测注塑成型中短纤维取向分布,提出了在模拟分析中采用RSC纤维取向预测模型对纤维取向分布进行预测的方法。基于RSC取向模型,通过第二顺序张量aij对三维取向分辨率进行描述,采用Orthotropic闭合近似模型对RSC模型进行求解。采用MoldFlow软件对RSC模型的纤维取向分布进行了模拟,分析了相互作用系数Ci及标量因子k对纤维取向的影响。并获得了纤维取向随空间、时间的取向规律——沿宽度方向,两端面的取向要大于中心层,并且端面在开始阶段就能达到很高的纤维取向;沿流动方向,在浇口及充填末端,纤维取向都呈下降趋势,而中间部分则有所增加。     

注射成型热塑性聚氨酯制件的取向形态演变和力学性能

通过光学双折射、小角X射线散射(SAXS)和广角X射线散射(WAXS)等研究热塑性聚氨酯(TPU)在注射成型过程中熔体流动场对其取向形态演变、残余应力和力学性能的影响.双折射结果表明,沿熔体流动路径上制件残余应力降低.由 SAXS和 WAXS结果可知,TPU分子链中的硬段沿熔体流动方向取向,而团聚成的硬域垂直于熔体流动方向取向.TPU拉伸性能沿熔体流动方向和垂直于熔体流动方向呈现出明显的差异性,制件中间区域的拉伸强度最高,这是因为硬段和硬域在制件中间区域取向度最大,硬段和硬域的取向有助于提升拉伸强度.     

纤维含量对注塑制品残余应力影响的数值模拟

采用短纤维增强尼龙6复合材料,对带有半圆形缺口平板制件的注塑成型过程进行了模拟。以制件半圆形缺口附件的5个单元为研究对象,分别获得了不同位置上纤维取向和制件脱模时应力沿厚度方向上的变化,深入讨论了模具外形、纤维含量变化对纤维取向以及残余应力的影响。研究结果表明,随着纤维含量的增加,纤维间相互作用系数Ci减小,取向程度升高,皮层取向厚度减小,芯层取向厚度增加;纤维含量越高,制品冷却时沿厚度方向的压力幅值越大,残余应力沿厚度分布幅值也越大;制件形状的改变,影响了熔体流经此处的流动状态,同时也影响了缺口附件纤维取向以及残余应力。     

短纤维增强聚丙烯水辅注射成型短纤维取向的数值模拟

水辅注射成型(WAIM)不同于传统注射成型(CIM),其短纤维取向分布相对复杂。文中以中空直管为例,对溢流法WAIM短纤维取向进行3D数值模拟,并通过模拟结果与扫描电镜照片比对来验证短纤维取向预测模型的有效性。提取断面不同厚度处的取向张量值,分析了溢流法WAIM中短纤维取向机理。模拟结果表明,残余壁厚中短纤维取向分布具有明显的外层-壳层-内层结构特点,外层和壳层中短纤维以沿着熔体流动方向取向分布为主,垂直流动方向次之,厚度方向最弱,靠近水道的残余壁厚内层受到高压水柱影响短纤维趋于自由取向分布;对不同部位断面进行分析,由于存在不同应力场和速度场,短纤维在3个方向的取向张量沿轴向有一定变化,同时溢流口模具结构对其存在影响。     

不同流道截面型腔的水穿透行为

纤维取向分布直接影响水辅注塑成型制品的使用性能,如冲击强度、屈服强度及拉伸强度等。多样化的流道截面型腔用于满足水辅制品在不同场合中的应用,不同的流道截面型腔势必会影响水辅制品中纤维取向分布。文中旨在研究圆形、上圆下方形及方形的截面流道型腔中短玻纤增强聚丙烯复合材料的水辅助注射成型过程。结果发现,随着熔体温度的升高、注水压力的增大及注水延迟时间的缩短,3种流道截面型腔制品的中间端处残余壁厚减薄及短玻纤沿聚合物熔体流动方向的取向度提高,且在相同加工变量下,圆形截面流道型腔制品的中间端处残余壁厚最薄及短玻纤沿聚合物熔体流动方向的取向度最高,其次是上圆下方形,最后是方形。综合制品的中间端1处及2处残余壁厚可知,聚合物熔体温度在210～230℃、注水压力7～10 MPa及注水延迟时间1～5 s时,上圆下方形截面型腔制品的中间端处残余壁厚及短玻纤沿聚合物熔体流动方向的取向度更趋近于圆形。     

聚合物注塑成型充模阶段流动取向分子机理研究

以聚碳酸酯(PC)为研究材料,基于分子动力学方法,对三维周期性边界条件下PC流动取向过程中分子链团迁移性能和形态演化进行了研究,与实验结果对比得到充模流动阶段聚合物分子链团的流动取向机制。分子链团均方位移表明流动取向过程实质上是分子链团的空间构象协调变形的结果,虽然随着剪切速率的增大,分子链团均方位移迅速增大、非键合能急剧减小,但分子链基本分子结构却仍然保持不变。同时,在取向过程中无规缠绕的分子链团一方面由于剪切拉伸作用沿流动方向排布,另一方面分子链团之间因发生解缠结而趋于相互分散。注塑实验结果表明,制品双折射率随注射速率的增加而增大,并沿流动方向递减,且取向方向上的拉伸性能随取向度的增加也有所提高,这与模拟结果中聚合物分子链取向机理一致。     

注射成型时纤维取向的计算机模拟

基于Ｈｅｌｅ－Ｓｈｏｗ流动，采用Ｄｉｎｈ－Ａｒｍｓｔｒｏｎｇ本构模型，本文研究了一种数值方法用来模拟中等浓度短纤维增强塑料在注塑成型过程中纤维的取向状态。纤维的取向状态由取向椭球的投影表示，而取向椭球由计算二阶取向张量得到。本文推导了注射成型过程中纤维取向 概率分布函数，并将模拟的结果与以前研究者所做的实验进行了比较。     

基于图像处理的纤维分布与取向分布对水泥基材料弯曲性能的影响

聚乙烯醇(PVA)纤维增强水泥基材料的弯曲性能与纤维在水泥基体内的分布和取向分布相关。采用抛光断面后涂荧光粉的显微成像法,基于图像处理程序对PVA纤维在水泥基材料中的分布和取向分布进行量化测定,对不同基体结构特征影响纤维分布的机理进行了讨论。结合弯曲试验结果,研究了纤维分布和取向分布对材料弯曲性能的影响。纤维分布测定结果表明,均匀的基体结构特征利于纤维的分布,同时对于材料组分和加工制作过程完全相同的试件,纤维分布系数越大,试件的弯曲强度与韧性越大;纤维取向分布测定结果表明,乱向分布的纤维当其长度方向与抛光断面方向的角度接近90°分布概率越大,试件的弯曲韧性也越大。     

考虑纤维方向分布的玻纤增强PP复合材料拉伸性能

纤维方向及其分布对玻纤增强PP复合材料的力学特性具有至为关键的影响。提出了一种快速获取纤维数量及每根纤维方向的方法。通过引入方向张量, 利用Moldflow软件进行玻纤增强PP树脂注塑成型模拟获得纤维方向的平均分布, 结合显微方法观察判断特定点的纤维沿厚度方向的分层情况及定量判断纤维方向的分布。对轿车玻璃纤维增强注塑仪表板的纤维方向相对一致处取与纤维方向呈0°、45°、90°的样条, 通过拉伸实验测得拉伸模量, 利用所提出的方法研究了仪表板内玻纤方向的分布及其对拉伸模量的影响。研究结果表明: 玻纤增强注塑仪表板的力学性能是各向异性的, 其沿厚度方向纤维按方向大致可分为三层。     

Prediction of fiber orientation structure for injection molded short fiber composites

Structure of fiber orientation in injection molded short fiber composites is predicted by the numerical analysis. To analyze the packing stage as well as the filling stage, a compressible generalized Hele-Shaw model is adopted. A numerical scheme free from coordinate transformation is developed for three-dimensional shell-like geometry. Flow-induced fiber orientation can be predicted by solving evolution equations for the orientation tensor with a suitable closure approximation. Fibers are mainly oriented toward the flow direction near the top cavity wall due to high shear rates, while they are randomly oriented near the centerline of cavity where low shear rates prevail. Thus, the molded parts show the skin-core structure of orientation. Structure of fiber orientation continues to change during the packing stage due to additional velocity gradients – which are likely to align fibers more towards the flow direction. Electronic Publication     

Flow-induced fiber orientation in gas-assisted injection molded part

Polyamide 66 with 33 wt.% glass fiber (DuPont, Zytel 70G33) was molded by gas-assisted injection molding (GAIM). Scanning electron microscope (SEM) micrographs indicated that fibers orientated notably in the core layer and slightly in the region near the mold wall, but aligned disorderly in the region near the gas channel. However, fibers orientated remarkably in the center of the thickness of the GAIM part, which was greatly different from the fiber orientation behavior in the samples molded by the conventional injection molding (CIM) and the water-assisted injection molding (WAIM) as reported in the literatures. Combining with a previous simulation dealing with gas penetration, the mechanisms for fiber orientation in the GAIM part are also discussed.     

Electrical properties of hybrid carbon black/carbon fiber polypropylene composites

The electrical conductivity and morphology of injection molded polypropylene based composites containing two conductive fillers, carbon black (CB) and carbon fibers (CF) were studied. Injection moldings containing both, CB and CF, where the content of each filler was above its own percolation threshold, resulted in similar or lower values of overall composite volume resistivity compared with the resistivity of systems filled only with CB at the corresponding content. However, the resistivity of two-filler systems is always higher than the resistivity of systems filled only with CF at the corresponding content. The morphology and fiber length analysis of the injection molded composites are quite intriguing. Fiber orientation in the injection molded two-filler systems was found to be almost perpendicular to the melt flow direction, with no significant skin-core fiber orientation patterns, contrary to the typically observed fiber orientation in injection molded fiber filled composites. Moreover, the CF breakage in the presence of the CB was found more intense than when just CF is used, resulting in shorter fibers with narrower length distributions. This unexpected fiber behavior is responsible for the unexpected electrical behavior. However, the coexistence of CB and CF electrically conductive networks, supporting each other, was confirmed, in spite of the mechanical disturbances caused by the presence of fibrilar and particulate fillers.     

Characterization of a peek composite segmental bone replacement implant

A composite material of polyetheretherketone and short, chopped E-glass fibers was used to produce a segmental bone replacement implant. Problems with current metallic implants include stress-shielding of the surrounding bone and subsequent loosening of the implant. A better match between the bulk material properties of the implant and the bone it replaces can decrease the occurrence of these problems. Composite materials were chosen because their properties can be tailored to match the requirements. Material selection was accomplished with the aid of modeling software, which predicted the composite properties based on its composition and fiber directional parameters. Prototype parts were completed through a series of in-house molding and machining processes. Sections complete with an embedded metallic porous surface were tested to measure the strength of the attachment of the surface. The molded parts were characterized both destructively and nondestructively. The results of tensile tests performed on molded parts were comparable to those using commercially supplied samples. The fiber orientation was measured to verify the random positioning of fibers throughout the part, as assumed in the initial material selection. Ultrasonic C-scanned images confirmed that the molded parts had a very low density of air pockets or voids.     

Numerical simulation of the variation of fiber orientation distribution during flow molding of Ultra High Performance Cementitious Composites (UHPCC)

In this paper, the variation of the fiber orientation distribution along the flow of fresh UHPCC was studied. In order to describe the rotational motion of a single fiber, Jeffery’s equation was adopted, in which the interaction among fibers is neglected. Two cases of flow patterns were considered: shear flow and radial flow. Starting with a three-dimensional random distribution of fibers, the fiber orientation distribution along the flow distance was simulated. These results reveal that fibers gradually become more parallel (in the case of shear flow) and perpendicular (in the case of radial flow) to the flow direction as the flow distance increases. This approach will be useful to predict flow-dependent tensile behavior considering the change of fiber orientation distribution.     

Design methodology for the molding of short-fiber thermoset composites

The thermoelastic properties of a molded component are a strong function of the fiber orientation, which in turn is governed by the mold geometry and the processing parameters. A methodology is presented to relate mold geometry and the molding parameters to the desired thermoelastic properties of a molded section. Such a methodology enables the designer to satisfy stiffness constraints by controlling the microstructure through the selection of appropriate process conditions and rheological properties of the molding compound. The fluid flow is modelled as a laminate of two Newtonian fluids, and a non-isothermal constitutive relationship for thermoset molding compounds is proposed as the dominant mechanisms governing the thickness of the surface layer of aligned fibers. As a consequence of the resulting laminated microstructure, the inplane and flexural properties are not equivalent.     

Fiber Reinforcement in Injection Molded Nylon 6/6 Spur Gears

Injection molded polymer composite gears are being used in many power and or motion transmission applications. In order to widen the utilization of reinforced polymers for precision motion transmission and noise less applications, the accuracy of molded gears should be increased. Since the injection molded gear accuracy is significantly influenced by the material shrinkage behaviour, there is a need to understand the influence of fiber orientation and gate location on part shrinkage behaviour and hence the gear accuracy. Unreinforced and 20% short glass fiber reinforced Nylon 6/6 spur gears were injection molded in the laboratory and computer aided simulations of gear manufacturing was also carried out. Results of the mold flow simulation of gear manufacturing were correlated with the actual fiber orientation and measured major geometrical parameters of the molded gears. Actual orientation of the fibers near the tooth profile, weld line region and injection points of molded gears were observed using optical microscope and correlated with predicted fiber orientation.     

短纤维增强塑料注射成型中三维纤维取向的数值预测

在建立聚合物熔体在型腔中充填流动以及短纤维取向的数学模型的基础上，对平面薄壁型腔内纤维取向的预测算法加以推广，提出适合于具有任意几何形状的三维薄壁型腔内纤维取向的数值预测技术，并且给出一个熔体充填三维薄壁壳体状型腔的算例，预测的结果与由流动引起的纤维取向定性规律相符合。