Fiber Orientation In Injection Research Articles

Fiber Orientation and Concentration in an Injection-Molded Ethylene-Propylene Copolymer Reinforced by Hemp.

This paper characterizes and analyzes the microstructures of injection-molded polypropylene parts reinforced with 20 wt% of hemp fibers in order to understand the process induced variations in thermomechanical properties. In-thickness fiber orientation and fiber content were determined by X-ray tomography along the flow. The fiber content along the flow path was also determined by direct fiber content measurements after matrix dissolution, showing an increase of 2%/100 mm for a 2.2 mm-thick plate due to fiber migration during the filling stage. A typical shell/core structure for fiber orientation in injection molding was observed, but with a very clear transition between the layer solidified under high shear rates and the core in which the fiber content was reduced by more than 50%. The orientation of hemp fibers is lower than the one of glass fibers, especially in thickness direction. However, the overall fiber orientation in the injection direction induces significant anisotropic thermomechanical properties, which cannot be explained by simple micromechanical models that consider isotropic mechanical properties for hemp fibers. These phenomena must be taken into account in process simulation codes for injection molding to better predict thermomechanical properties as well as part shrinkage and warpage to design molds.

Particle-level simulation for the prediction of short fiber orientation in injection molding

Particle-level simulation is used to predict the motion of short glass fibers in a center-gated disk during injection molding to evaluate its applicability to the prediction of the fiber orientation distribution. The interaction of the flexible fibers is included in the simulation, and the experimentally obtained fiber length distribution and inlet orientation are used as initial conditions. In the sprue, the aligned orientation at the inlet is predicted to become random when the fibers reach the bottom because of the diverging flow. In the disk, the fibers are mainly oriented perpendicular to the flow, and the orientation profile along the thickness is found to change from asymmetric to symmetric about the channel center with increasing radial position. These predicted orientations closely match the experimental results obtained by X-ray micro-computed tomography. It is also demonstrated that the particle-level simulation successfully reproduces the dependence of the fiber orientation on the fiber length.

The effect of fiber concentration on fiber orientation in injection molded film gated rectangular plates

Fiber orientation in film gated injection‐molded rectangular plates was characterized using micro computed tomography. Polyamide 12 materials with 15, 30, and 50 wt% glass fibers were used in this study. For all three materials, the typical core‐shell layer structure of fiber orientation was observed, but several features of the fiber orientation were highly dependent on the fiber content. Increasing fiber content resulted in increasing core layer thickness and a higher degree of orientation in the shell layer. These effects may be associated with plug flow induced by extra stress from the fibers, which changes the melt rheology compared to nonfilled material. The Dinh–Armstrong model for the fiber‐induced extra stress could not explain the fiber content dependencies. This appears to be attributed to the uncertainty in determining the particle number Np for the high fiber concentrations present in the samples. Analyses of fiber orientation as a function of fiber length revealed a dependence on fiber length in qualitative agreement with Jeffery's model for fiber dynamics. POLYM. COMPOS., 40:615–629, 2019. © 2017 Society of Plastics Engineers

Analysis of Short Glass Fiber Orientation in Injection Moulded Components

Fiber reinforced polymer composites are common materials used to produce parts for structural applications. The attractiveness of these materials is a result of the combined advantages of the fiber reinforcing effect and the high production rates achievable by technologies such as injection molding. The fiber orientation in short fiber reinforced thermoplastics depends on injection moulding technology parameters. The aim of this paper is to propose possibilities for comparing fiber orientation of the real sample and the result from simulation software.

Simulation of Injection Molding Using a Model with Delayed Fiber Orientation

Abstract Delay in fiber orientation evolution based on the Folgar-Tucker model with a slip parameter correction has been proposed as a simple alternative to improve predictions of fiber orientation in injection molded parts. Predictions based on this model and model parameters fitted to data from simple shear experiments were compared with experimentally determined fiber orientation in a center-gated disk. Three methods of fitting to simple shear data were assessed to obtain the isotropic diffusivity and the slip parameter. The model parameters and orientation data evaluated at the entry, lubrication and near-end-of fill regions in a center-gated disk for 30 wt.% short glass fiber-filled polybutylene terephtalate (PBT) were obtained from earlier efforts in our laboratory. Simulation results based on the Folgar-Tucker model with the slip correction using customarily assumed inlet orientation being random and experimentally measured at the gate disagreed with measured orientation values at certain positions along the disk. However, improvement in the prediction of orientation due the slip correction was found at the core and transition layers in the lubrication region. The use of inlet conditions washes out quickly in the absence of the slip correction and induced a general reduction of orientation towards the center of the sample causing underestimation of orientation at the entry and lubrication region. Model predictions combining the slip correction and experimentally determined orientation at the gate are in agreement with the experimental data for the core layers near the end-of-fill region.

Fiber orientation in injection molding with rotating flow

AbstractThe development of fiber orientation in injection molding was manipulated by a special molding tool, the RCEM mold, which imposes a rotation action by one of the cavity surfaces during the filling stage. Center‐gated disc moldings were produced from glass fiber reinforced polypropylene with different cavity rotation velocities, inducing distinct distributions and levels of fiber orientation. The morphologies of the moldings were characterized by optical and electronic microscopy. The through‐thickness profiles of fiber orientation were assessed by means of the orientation tensor, and the relationship between the processing thermo‐mechanical environment and the fiber orientation was established. At high rotation velocities, the resulting fiber orientation pattern is mainly controlled by the rotational motion, inducing a much more homogeneous through‐the‐thickness fiber orientation distribution, with a preferential alignment on the circumferential direction. POLYM. ENG. SCI., 2008. © 2007 Society of Plastics Engineers

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

AbstractInjection 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

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.

Description and modeling of fiber orientation in injection molding of fiber reinforced thermoplastics

As mechanical properties of short fiber reinforced thermoplastic injected components depend on flow induced fiber orientation, there is considerable interest in validating and improving models which link the flow field and fiber orientations to mechanical properties. The present paper concerns firstly the observation and quantification of fiber orientation in a rectangular plaque with adjustable thickness and molded with 30 and 50 wt% short fiber reinforced polyarylamide. An automated 2D optical technique has been used to determine fiber orientations. A classical skin (with orientation parallel to the flow)–core (with orientation perpendicular) structure is observed for thick plaques (thickness greater than 3 mm) but the core region is fragmentary for thickness less than 1.7 mm. It is shown that the gate design and different levels of fiber interactions, due to different fiber concentrations, are responsible for these observations. Secondly, computer simulations of flow and fiber orientation are shown. The agreement with the actual data is good, except in the case of the core for thin plaques. The limitations that have to be resolved come not only from the standard fiber orientation equations, but also from the flow kinematics computation.

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.

Numerical simulation of three‐dimensional fiber orientation in injection molding including fountain flow effect

AbstractIt is essential to predict the nature of flow field inside mold and flow‐induced variation of fiber orientation for effective design of short fiber reinforced plastic parts. In this investigation, numerical simulations of flow field and three‐dimensional fiber orientation were carried out in special consideration of fountain flow effect. Fiber orientation distribution was described using the second‐order orientation tensor. Fiber interaction was modeled using the interaction coefficient CI. Three closure approximations, hybrid, modified hybrid, and closure equation for CI=0, were selected for determination of the fiber orientation. The fiber orientation routine was incorporated into a previously developed program of injection mold filling (CAMPmold), which was based on the fixed‐grid finite element/finite difference method assuming the Hele‐Shaw flow. For consideration of the fountain flow effect, simplified deformation behavior of fountain flow was employed to obtain the initial condition for fiber orientation in the flow front region. Comparisons with experimental results available in the literature were made for film‐gated strip and centergated disk cavities. It was found that the orientation components near the wall were were accurately predicted by considering the fountain flow effect. Test simulations were also carried out for the filling analysis of a practical part, and it was shown that the currently developed numerical algorithm can be effectively used for the prediction of fiber orientation distribution in complex parts.

Flow‐induced fiber orientation in injection molded fit fiber reinforced polypropylene

AbstractA through experimental study of flax fiber orientation in a plate processed by injection molding is presented. The state of orientation is described by oriental tensors and partly by frequency distribution diagrams. Composite stiffness is predieted by use of a modified classical laminate theory including unidirectional mels and orientation averaging. Comparison of the measured and calculated modulus in tension shows good agreement.

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

AbstractThe 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.

Numerical prediction of fiber orientation in injection molding

AbstractWe develop a numerical method for calculating fiber orientation in the midsurface of a molded part of small thickness. Two‐dimensional fiber orientation is predicted on the basis of either Jeffery's equation or a constitutive equation for the orientation tensor. The calculation is fully transient; it is performed on a time‐dependent flow domain. The method is based on finite elements. Updated finite element meshes are generated at every instant of filling and allow one to perform an accurate calculation of the orientation even along the boundary of the flow domain. The method is applied to several examples in plane and three‐dimensional geometries.

Prediction of fiber orientation in injection molded parts of short‐fiber‐reinforced thermoplastics

AbstractFiber orientation induced by injection mold filling of short‐fiber‐reinforced thermoplastics (FRTP) causes anisotropy in material properties and warps molded parts. Predicting fiber orientation is important for part and mold design to produce sound molded parts. A numerical scheme is presented to predict fiber orientation in three‐dimensional thin‐walled molded parts of FRTP. Folgar and Tucker's orientation equation is used to represent planar orientation behavior of rigid cylindrical fibers in concentrated suspensions. The equation is solved about a distribution function of fiber orientation by using a finite difference method with input of velocity data from a mold filling analysis. The mold filling is assumed to be nonisothermal Hele‐Shaw flow of a non‐Newtonian fluid and analyzed by using a finite element method. To define a degree of fiber orientation, an orientation parameter is calculated from the distribution function against a typical orientation angle. Computed orientation parameters were compared with measured thermal expansion coefficients for molded square plates of glass‐fiber‐reinforced polypropylene. A good correlation was found.

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

AbstractMold 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.