Investigation of fiber orientation states in injection‐compression molded short‐fiber‐reinforced thermoplastics

Injection‐compression molding (ICM) has received increased attention because of its advantages over conventional injection molding (CIM). This article aims to investigate the effects of five dominating ICM processing parameters on fiber orientation in short‐fiber‐reinforced polypropylene (SFR‐PP) parts. A five‐layer structure of fiber orientation is found across the thickness under most conditions in ICM parts. This is quite different from the fiber orientation patterns in CIM parts. The fibers orient orderly along the flow direction in the shell region, whereas most fibers arrange randomly in the skin and the core regions. Additionally, the fiber orientation changes in the width direction, with most fibers arranging orderly along the flow direction at positions near the mold cavity wall. The results also show that the compression force, compression distance, and compression speed play important roles in determining the fiber states. Thicker shell regions, in which most fibers orient remarkably along the flow direction, can be obtained under larger compression force or compression speed. Moreover, the delay time has an obvious effect on the fiber orientation at positions far from the gate. However, the effect of compression time is found to be negligible. POLYM. COMPOS., 31:1899–1908, 2010. © 2010 Society of Plastics Engineers.     

短纤维-弹性体复合材料的注射充模I．数学物理模型的建立

研究了短纤维－弹性体复合材料注射充模过程中,流动前锋和上的喷泉效应,固化层及纤维用量与纤维之间相互作用等的影响,建立了薄壁拉伸试样型腔中短纤维－弹性体复合材料注射充模过程及其纤维取向分布的数学物理模型。     

Fiber orientation in simple injection moldings. Part I: Theory and numerical methods

This paper sets out the theory and numerical methods used to simulate filling and fiber orientation is simple injection moldings (a film-gated strip and a center-gated disk). Our simulation applies to these simple geometry problems for the flow of a generalized Newtonian fluid where the velocities can be solved independently of fiber orientation. This simplification is valid when the orientation is so flat that the fibers do not contribute to the gapwise shear stresses. A finite difference solution calculates the temperature and velocity fields along the flow direction and through the thickness of the part, and fiber orientation is then integrated numerically along pathlines. Fiber orientation is three-dimensional, using a second-rank tensor representation of the orientation distribution function. The assumptions used to develop the simulation are not valid near the flow front, where the recirculating fountain flow complicates the problem. We present a numerrical scheme that includes the effect of the fountain flow on temperature and fiber orientation near the flow front. The simulation predicts that the orientation will vary through the thickness of the part, causing the molding to appear layered. The outer “skin” layer is predicted only if the effects of the fountain flow and heat transfer are included in the simulation.     

Numerical analysis of injection molding of glass fiber reinforced thermoplastics. Part 2: Fiber orientation

Numerical predictions of fiber orientation during injection molding of fiber-filled thermoplastics are compared to measurements. The numerical work successfully describes the flow of fiber-filled plastic during injection molding, using finite-difference solutions for the transport equations and marker particles to track the flow front. The flow is modeled as a 2-D, non-isothermal, free-surface flow with a new viscosity model dependent upon temperature, pressure, and fiber concentration. The fiber orientation is based upon solution to the Fokker-Planke equation. The comparison demonstrates fair agreement between predicted fiber orientation and experimental results for slow and fast injection speeds. For the slow speed case at 10 and 20 wt% fibers, the numerical and experimental works show that the fibers are more random at the flow front than at the centerline, and that the fibers become more aligned as they flow from the gate to the midstream region. At fast injection speeds, the agreement between the numerical and experimental works is not as good as at slow injection speed. Possible explanations for the discrepancies are that the flow is assumed to be simple shear when injection molding is known to be a pressure-driven flow, the fibers have an initial orientation for the runner rather than the assumed random orientation, the fibers that were displayed from the camera were more oriented just behind the flow front (owing to the fast injection speed), and the orientation requires more than a 2-D video image to represent a 3-D fiber orientation.     

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.     

Crystalline and amorphous orientations in injection molded polyethylene

The techniques of density, birefringence, and wide X-ray diffraction were employed to characterize the microstructure of injection molded polyethylene parts. Generally, maximum crystallinity (density) occurs at the center of the molding, while the minimum crystallinity occurs near the surface. Higher densities are observed near the gate. Raising the injection temperature tends to cause a marginal increase in the crystallinity throughout the molding. Birefringence measurements suggest that the maximum orientation occurs near the surface and that the relative orientation distribution is independent of the injection temperature. X-ray diffraction indicates that the crystallographic a-axis tends to orient in the flow direction while the b and c axes vary symmetrically about that direction. Increasing the injection temperature creates c-axis orientation near the surface, while towards the core region a-axis orientation is observed. Generally, near the surface it is the amorphous phase that makes the major contribution to the total orientation as measured by birefringence. Increasing the injection temperature tends to decrease the amorphous phase orientation near the surface. The crystalline phase contribution to the total orientation increases as distance from the surface increases, regardless of injection temperature.     

Dynamics of semi-flexible and breakable fibers under Poiseuille flow

In the processing of fiber-reinforced polymer composites, especially in injection molding, the fiber's orientation, length, and distribution vary depending on the location of the channel flow and its properties, which affects the performance of final products. To investigate the intricate behavior of fiber suspensions under Poiseuille flow, we used the hybrid simulation approach, multiparticle collision dynamics–molecular dynamics (MPC-MD), which takes hydrodynamic interactions and fiber properties (strength, flexibility) into account. For non-breakable and rodlike fibers, fibers align well along the flow direction while showing more alignment near the wall. As fiber becomes breakable and/or flexible, the length and orientation of fibers strongly depend on their properties. The interesting phenomenon is specifically seen for breakable and semiflexible fibers, where the orientation of the fiber exhibits non-monotonic behavior depending on the flow rate. This complex behavior highlights the importance of comprehending the dynamics of many types of fibers and necessitates research into the best conditions for injection molding.     

Practical use of the statistically modified laminate model for injection moldings. Part 1: Method and verification

The fibers in injection molded FRP provide the material's strength and stiffness; however, they also supply many of the problems. Preferential orientation of fibers during molding can reduce strength and stiffness below expected values in critical directions, or induce warpage in thin walled sections. Makers of short fiber reinforced injection molded products typically use computer aided engineering packages to optimize product performance and manufacturing variables. However, the reliability of the fiber orientation simulation can be limited, and the method is not easily understood, making an assessment of accuracy for a given situation difficult. In addition, the structural module of flow analysis packages is often a basic package with many features missing. This paper presents a structural analysis system for injection molded parts made of short fiber reinforced plastics. A full-featured commercial structural analysis code is interfaced with a flow analysis program using a practical material model that takes into account the effects of local fiber orientation. The system is completely open to the user, and can be modified as required.     

Effect of packing pressure on fiber orientation in injection molding of fiber‐reinforced thermoplastics

Injection molding of fiber‐reinforced polymeric composites is increasing with demands of geometrically complex products possessing superior mechanical properties of high specific strength, high specific stiffness, and high impact resistance. Complex state of fiber orientation exists in injection molding of short fiber reinforced polymers. The orientation of fibers vary significantly across the thickness of injection‐molded part and can become a key feature of the finished product. Improving the mechanical properties of molded parts by managing the orientation of fibers during the process of injection molding is the basic motivation of this study. As a first step in this direction, the present results reveal the importance of packing pressure in orienting the fibers. In this study, the effects of pressure distribution and viscosity of a compressible polymeric composite melt on the state of fiber orientation after complete filling of a cavity is considered experimentally and compared with the simulation results of Moldflow analysis. POLYM. COMPOS. 28:214–223, 2007. © 2007 Society of Plastics Engineers     

Rheological behavior and fiber orientation in slit flow of fiber reinforced thermoplastics

The flow behavior and fiber orientation in slit flow of a short fiber reinforced thermoplastic composite melt are investigated. A slit die with adjustable gap and interchangeable entrance geometries was designed and built. The slit die is fed by a single screw extruder. The bulk viscosity is calculated from the axial pressure profiles measured using three flush mounted pressure transducers. The effect of entrance geometry and gap dimensions on the fiber orientation and bulk flow behavior is specifically considered. A skin-core composite fiber orientation is observed in the thickness direction. Fibers are oriented in the flow direction and parallel to the walls in the skin region irrespective of the entrance geometry. Different fiber orientation distributions in the core region can be realized by using different entrance geometries. However, the changes in the core fiber orientation are not fully reflected by the measured viscosities, due to highly oriented skin layer. Exit pressures obtained by extrapolation of linear pressure profiles are found to be all positive, but dependent on the die geometry and entrance conditions, even for the unfilled melts.     

Coupled analysis of injection molding filling and fiber orientation,including in-plane velocity gradient effect

On injection molding of short fiber reinforced plastics, fiber orientation during mold filling is determined by the flow field and the interactions between the fibers. The flow field is, in turn, affected by the orientation of fibers. The Dinh and Armstrong rheological equation of state for semiconcentrated fiber suspensions was incorporated into the coupled analysis of mold filling flow and fiber orientation. The viscous shear stress and extra shear stress due to fibers dominate the momentum balance in the coupled Hele-Shaw flow approximation, but the extra in-plane stretching stress terms could be of the same order as those shear stress terms, for large in-plane stretching of suspensions of large particle number. Therefore, a new pressure equation, governing the mold filling process, was derived, including the stresses due to the in-plane velocity gradients. The mold filling simulation was then performed by solving the new pressure equation and the energy equation via a finite element/finite difference method, as well as evolution equations for the second-order orientation tensor via the fourth-order Runge-Kutta method. The effects of stresses due to the in-plane velocity gradient on pressure, velocity, and fiber orientation fields were investigated in the center-gated radial diverging flow in the cases of both an isothermal Newtonian fluid matrix and a nonisothermal polymeric matrix. In particular, the in-plane velocity gradient effect on the fiber orientation was found to be significant near the gate, and more notably for the case of a nonisothermal polymer matrix.     

Effects of abrupt expansion geometries on flow‐induced fiber orientation and concentration distributions in slit channel flows of fiber suspensions

Flow‐induced fiber orientation and concentration distributions were measured in channel flows of fiber suspension. The test fluids used are a concentrated fiber suspension (CFS), a semidilute one (SDFS), and a dilute one (DFS). The channel has a thin slit geometry with a 1:16 expansion. In the present work, fiber orientation and concentration distributions are quantitatively evaluated by direct observation of fibers even in the CFS flow. It is found that the weak fiber–fiber interaction of the SDFS largely affects the fiber orientation in the flow with a sudden change such as in the expansion flow, while it is ineffective upon the fiber orientation in the flow without a sudden change such as in the far downstream region. Fiber concentration in the CFS has a flat distribution over a channel width in both the entrance region of the expansion and the downstream region. However, fiber concentration distributions in the SDFS and the DFS have a small and a large peak near the sidewall in the entrance region, respectively, due to the fiber‐wall interaction at the channel wall. These peaks, however, disappeared in the far downstream region after the fibers passed through the expansion. POLYM. COMPOS., 26:660–670, 2005. © 2005 Society of Plastics Engineers.     

Glass fiber-filled thermoplastics. II. Cavity filling and fiber orientation in injection molding

Mold filling of a rectangular cavity of three different thick nesses fed from a reservoir is studied for unfilled and glass fiber-filled polypropylene and polystyrene. The shapes of flow fronts studied by short-shots are affected predominantly by the thickness of the cavity with other parameters playing a less important role. Pressure drop versus volumetric flow rate inside the thinnest cavity is studied experimentally and predictions are made from a computer simulation of mold filling. The orientation of fibers in the cavity is examined using a reflect-type microscope and the orientation is found to depend on cavity thickness, melt temperature, fiber content, and to a lesser extent, on volumetric flow rate. In the thinnest cavity, where the flow is quasi-unidirectional, the fibers remain in the plane of flow oriented either along the flow direction or perpendicular to it, except in the region near the flow front, where they follow a “fountain” flow behavior.     

Numerical simulation of fiber orientation in injection molding of short-fiber-reinforced thermoplastics

The present study develops a numerical simulation program to predict the transient behavior of fiber orientations together with a mold filling simulation for short-fiber-reinforced thermoplastics in arbitrary three-dimensional injection mold cavities. The Dinh-Armstrong model including an additional stress due to the existence of fibers is incorporated into the Hele-Shaw equation to result in a new pressure equation governing the filling process. The mold filling simulation is performed by solving the new pressure equation and energy equation via a finite element/finite difference method as well as evolution equations for the second-order orientation tensor via the fourth-order Runge-Kutta method. The fiber orientation tensor is determined at every layer of each element across the thickness of molded parts with appropriate tensor transformations for arbitrary three-dimensional cavity space.     

Effects of molding conditions on the electromagnetic interference performance of conductive ABS parts

Polymers filled with conducting fibers to prevent electromagnetic interference (EMI) performance have recently received great attention due to the requirements of 3C (computer, communication, and consumer electronics) products. In the present article, the effect of fiber content and processing parameters, including melt temperature, mold temperature, and injection velocity, on the electromagnetic interference shielding effectiveness (SE) in injection molded ABS polymer composites filled with conductive stainless steel fiber (SSF) was investigated. The influence of fiber orientation and distribution resulting from fiber content and molding conditions on EMI performance was also examined. It was found from measured results that fiber content plays a significant role in influencing part EMI SE performance. SE value can reach the highest values of approximately 40 dB and 60 dB at 1000 MHz frequency for fiber content 7 wt % and 14 wt %, respectively, under the best choice of molding conditions. Higher melt and mold temperature would increase shielding effectiveness due to a more uniform and random fiber orientation. However, higher injection velocity leading to highly‐orientated and less uniform distribution of fiber reduces shielding effectiveness. Among all molding parameters, melt temperature affects SE performance most significantly. Its influence slightly decreases as fiber content increases. Injection speed plays a secondary importance in affecting SE values, and its influence increases as fiber content increases. Upon examination of fiber distribution via optical microscope and subsequent image analysis, it was found that the fiber becomes more densely and random distributed toward the last melt‐filled region, whereas fiber exhibits less concentration around the middle way of the flow path. This can be attributed to the combined effects of fountain flow, frozen layer thickness, and gapwise melt front velocity. The results indicate that molding conditions, instead of fiber content alone, are very important on the SE performance for injection molded SSF filled ABS composites. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1072–1080, 2005     

短纤维增强熔体三维充模模拟及制品性能预测

基于气-液-固三相模型,给出了适用于三维流场的纤维质心虚拟速度、纤维平动与取向、动量交换源项的求解公式,建立了描述短纤维增强聚合物熔体充模过程的三维模型。采用同位网格有限体积法和Level Set界面追踪技术,实现了充模过程的三维动态模拟。并且,根据模拟计算出的平均取向角,提出了三维取向短纤维增强复合材料力学性能参数计算的一种简化模型。数值结果表明:三维模拟技术可有效反映注塑成型充模的流动过程和喷泉效应;纤维取向分析可量化显示纤维在型腔中的表层-芯层结构取向;弹性模量计算结果与实验结果吻合较好。     

Effects of fiber orientation on strain concentrations in glass reinforced polyamides

Orientation of reinforcement fibers in injection molded parts is a key factor in determining their strength and stiffness: therefore stress-strain analyses based on isotropic material models produce only rough results. We present a flow/strain analysis methodology that accounts for the actual anisotropic material properties and fiber orientation. Material properties are determined by experiment, fiber orientation is inferred from flow simulation results (velocity vectors). Stress/strain fields are calculated by means of finite element analysis. Results show that for notched parts molded from short glass reinforced polyamide resin, there is a significant dependence of the strain concentration on the local fiber orientation resulting from different injection molding conditions.     

Spatial distribution of molecular orientation in injection molded iPP: influence of processing conditions

In this paper, the influence of processing conditions on the spatial distribution of the molecular orientation was determined within the depth of the thickness of injection molded isotactic polypropylene (iPP) plates. Small 35 μm-thick slices were microtomed from the surface to the core of 1 and 3 mm-thick plates. The orientation functions along the three crystallographic axes were determined on the slices from IR dichroism measurements and WAXS pole figures. It was found that the orientation of the amorphous phase was low and the crystalline orientation had a maximum in the shearing layer, which was solidified during the filling stage. The plate thickness seemed to govern the global level of orientation, while the injection speed determined the thickness of the shearing layer without changing the maximum of orientation. Changing the mold temperature from 20 to 40 °C did not modify the molecular orientation. A specific bimodal crystalline orientation was found in the shearing layer. This crystalline structure continued in the post-filling layer, but the local symmetry axes tilted towards the core.     

Development of Micro-Debond Methods for Thermoplastics Including Applications to Liquid Crystalline Polymers

Micro-bead and related debond techniques were used to study adhesion of liquid crystalline copolyesters (LCPs) and other semi-crystalline thermoplastic polymers to glass fibers. For polymers with poor flow even at high temperatures, symmetric beads on fibers were difficult to prepare so an alternative sample preparation method was developed where glass fibers were inserted into thin sections of molten polymer. Glass fibers of widely-varying diameters were used in order to extend the dynamic range of the debond techniques in terms of debonding area, showing a significant improvement in precision over that demonstrated previously with micro debond techniques. The fibers were freshly prepared in our laboratory and silane coated when necessary, which allowed us to minimize fiber surface heterogeneity effects which are believed to influence strongly debond test results. It was found that chemical bonding of the LCPs was quite favorable as was indicated by fracture surface analysis and by comparison with the shear strength of the neat resins. The apparent poor interphase strength in fiber-reinforced LCP composites is proposed to be due to orientation of the LCP molecules near the fiber interface leading to a cohesively weak layer of LCP near the interface. Reactive silane coupling agents lead to no improvement in interface strength as compared with bare glass because chemical reaction occurs on both surfaces. This results in very strong interfaces leading to polymer cohesive failure near the interface of all thermoplastics studied here     

Prediction of fiber orientation in the thickness plane during flow molding of short fiber composites

Fiber orientations caused by the flow in the thickness plane during injection molding of short fiber reinforced polymer composites has been simulated. The Lagrangian scheme was employed for the finite element analysis. Flow fields were solved by using a penalty method with Uzawa's scheme and orientation fields were also solved by using the second order orientation tensor. A generalized Newtonian fluid whose rheological behavior is independent of fiber orientation was assumed. Automatic mesh generation using an elliptic grid generator was developed for quadrilateral elements. Mold filling and orientation analyses were performed for a cavity of rectangular cross section. To determine the orientation state in other cross-sectional geometries, numerical analyses were also performed for two different typical cross sections. As the result, orientation of short fibers in the flow field was analyzed qualitatively and quantitatively. According to the state of short fiber orientation in the thickness plane, the orientation field can be classified into three regions in the flow direction and three layers in the thickness direction. Orientation of short fibers was mainly influenced by elongational and shear flows. It was observed that critical values are present for upper limits of orientation. Effects of initial orientation at the inlet on the orientation field were examined.