Injectin molding of short fiber reinforced thermoplastics in a center‐gated mold

We have studied injection molding of a rectangular box using short fiber reinforced polypropylene. The fiber orientation distribution and the local stiffness properties in radial and transverse directions have been measured in the bottom plane. The deduced orientation tensors are compared with predictions of a commercial computer code. Discrepancies are related to approximations made in the calculations and effects not accounted for by the present modeling approach. In the calculations the fiber interaction coefficient was varied seeking to fit experiments. We comment on the out‐of‐plane components of the orientation tensor, the relative thickness of skin and core layers, and the radial dependence of the fiber orientation in each layer. Values of the components of the 4^th‐order orientation tensor calculated from the measured orientation distribution are used to compare different closure approximations referred in the literature. Anisotropy in the stiffness properties, calculated form the measured fiber orientation, agree well with measurements.     

Orientation structure in transverse sections of carbon fibers from dehydrated polyvinyl alcohol

Transverse sections of carbon fibers prepared from two different lots of dehydrated polyvinyl alcohol fibers, with and without graphitizing treatment, were observed by both polarized light and scanning electron microscopy. Several different types of transverse layer plane preferred orientation were found in the graphitized fibers: layers aligned parallel with the section major axis in the central zone of elliptical cross sections; parallel with the fiber surface in concave regions of an irregular cross section shape; and parallel with the fiber surface in a highly crystallized fiber with an irregular cross section shape. These graphitic layer plane configurations are considered in connection with the lamellar structure in crystalline regions of the dehydrated polyvinyl alcohol fibers.     

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.     

Influence of out‐of‐plane fiber orientation on reaction‐to‐fire properties of carbon fiber reinforced polymer matrix composites

This work investigates the influence of the out‐of‐plane orientation of carbon fibers on the reaction‐to‐fire characteristics of polymer matrix composites. A deep insight into combustion processes is gained, which is necessary to fully understand and assess advantages of composites with out‐of‐plane fiber angles. Epoxy‐based Hexply 8552/IM7 specimens with primarily low fiber angles between 0° and 15° are characterized by cone calorimetry. Heat release during fire is greatly affected by the out‐of‐plane fiber angle because of the thermal boundaries created by the fibers. The advancement of the pyrolysis front during fire was determined from peak heat release rates and validated by temperature measurements along the back surface of the panels, representing a novel method of determining position‐dependent pyrolysis migration velocity. These measurements show a transverse shift in pyrolysis front velocity for increasing out‐of‐plane fiber angles. Pyrolysis pathways between the fiber boundaries facilitate faster combustion through the composite thickness, especially for increasing angles from 0° to 15°. It was determined that under the chosen conditions, the pyrolysis front advances approximately 4 times faster when propagating parallel to the fibers than perpendicular.     

A phenomenological study of the mechanical properties of long‐fiber filled injection‐molded thermoplastic composites

Tensile and flexural tests on specimens cut from rectangular injection‐molded plaques show that long‐fiber filled thermoplastic composites are complex, non‐homogeneous, anistropic material systems. Like all fiber‐filled materials, they exhibit through‐thickness nonhomogeneity as indicated by differences between tensile and flexural properties. The in‐plane orientation of fibers in through‐thickness layers causes the material to have in‐plane anisotropic properties. However, these long‐fiber filled materials exhibit an unexpectedly large level of in‐plane nonhomogeneity. Also, the effective mechanical properties of these materials are strongly thickness dependent. The thinnest plaques exhibit the largest differences between the flow and cross‐flow tensile properties. These differences decrease with increasing thickness. A methodology for part design with this class of materials is discussed.     

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

It 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 C_I. Three closure approximations, hybrid, modified hybrid, and closure equation for C_I=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.     

The effect of fiber orientation on the toughening of short fiber‐reinforced polymers

The effect of fiber orientation on the toughening of polymers by short glass fibers generally below their critical length was investigated using specimens with either well‐aligned or randomly oriented fibers. The fibers were aligned by an electric field in a photopolymerizable monomer, which was polymerized while the field was still being applied. These materials were fractured with the aligned fibers in three orientations with respect to the crack plane and propagation direction. Specimens with fibers aligned normal to the fracture plane were the most tough, those with randomly oriented fibers were less tough, and those with fibers aligned within the fracture plane were the least tough. The fracture behaviors compared favorably with predictions based on observed processes accounting for fiber orientation. The processes considered were fiber pull‐out (including snubbing), fiber breakage, fiber–matrix debonding, and localized matrix‐yielding adjacent to fibers bridging the fracture plane. Fibers not quite perpendicular to the fracture plane provided the greatest toughening; these fibers pulled out completely and gave a significant contribution from snubbing. Fibers at higher angles provided less toughening, involving nearly equal contributions from pull‐out, breakage, and debonding. Fibers within the fracture plane provided the least toughening, involving debonding alone. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2740–2751, 2003     

Mechanical properties of injection‐molded short‐fiber thermoplastic composites. Part 1: The elastic moduli and strengths of glass‐filled poly(butylene terephthalate)

The effects of processing and part geometry on the local mechanical properties of injection‐molded, 30 wt% short‐fiber‐reinforced filled poly(butylene terephthalate) (PBT) are characterized by mechanical tests on specimens cut from rectangular plaques of different thicknesses injection molded at several different processing conditions. Stiffness data from tensile tests at 12.7‐mm intervals on 12.7‐mm‐wide strips cut from injection‐molded plaques—both along the flow and cross‐flow directions—and flexural tests on these strips show consistency of plaque‐to‐plaque local properties. Also, in addition to the well‐known anisotropic properties caused by flow‐induced fiber orientation, injection‐molded short fiber composites exhibit in‐plane and through‐thickness nonhomogeneity—as indicated by in‐plane property variations, by differences between tensile and flexural properties, and by the flexural strength being significantly higher than the tensile strength. The sensitivity of these mechanical properties to process conditions and plaque geometry have also been determined: the flow‐direction tensile modulus increases with fill time, the differences between flow and cross‐flow properties decrease with increasing thickness, and both the flow and cross‐flow flexural moduli decrease with increasing plaque thickness. While the flexural modulus is comparable to the tensile modulus, the flexural strength is significantly higher than the tensile strength. POLYM. COMPOS., 26:428–447, 2005. © 2005 Society of Plastics Engineers     

Characterization of birefringent distribution of high‐modulus PET fibers by senarmont compensation method

The birefringent distribution of poly(ethylene terephthalate) (PET) fibers has been measured by means of Senarmont's compensation in this article. The difference from the traditional method is that two new approaches (i.e., measuring the shift of either straight fringes on a fiber cylinder or arched fringes on a fiber wedge) are established to figure out the cross‐birefringence at different thicknesses of the fiber. Further, the fiber radial birefringent distribution is estimated by both using a series of theoretical models and their algorithms and by measuring with the fiber‐wedge method combined with deferential calculation. There exists high coincidence between theoretical and measured values and the PET fiber is a skin‐core structure of macromolecular orientation. Meanwhile, the experimental results show that (1) the double‐angle measurement set a fiber at both the 0° and the 90° positions is necessary for high accuracy; (2) the setting of each optical element and the fiber is vitally important to obtain the exactly compensated values; and (3) the bulk birefringence is correlated with the measured initial modulus of the PET fibers. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 598–608, 2004     

A simplified in‐plane permeability model for textile fabrics

In this paper, a simplified in‐plane permeability model for textile fabrics is developed. The model is based on a rectangular unit cell geometry, the one‐dimensional Stokes equation for flow in the channels or gaps between fiber tows, and the one‐dimensional Brinkman equation for flow in fiber tows. Three different textile fabrics are considered in the model: plain woven, 4‐harness, and bidirectional stitched fiberglass mats. The model incorporates the effect of porosity changes on permeability of fiber preforms under compression, which usually occurs in the molding process. To verify the validity of the model, the theoretical values are compared with a set of permeability measurements. Good agreement is found between the model prediction and the permeability measurements in the porosity ranges of ϕ ≤ 0.59 for plain woven fiber mats, ϕ ≤ 0.60 for 4‐harness fiber mats and ϕ ≤ 0.62 for bidirectional stitched fiber mats.     

Studies on high‐speed melt spinning of noncircular cross‐section fibers. III. Modeling of melt spinning process incorporating change in cross‐sectional shape

A numerical analysis program for high‐speed melt spinning of flat and hollow fibers was developed. Change in cross‐sectional shape along the spin line was incorporated adopting a formulation in which energy reduction caused by the reduction of surface area was assumed to be equal to the energy dissipation by viscous flow in the plane perpendicular to the fiber axis. In the case of flat fiber spinning, the development of temperature distribution in the cross section was considered. It was found that the empirical equations for air friction and cooling of the spin line of circular fibers can be applied for the flat fiber spin line if the geometrical mean of long‐axis and short‐axis lengths was adopted, instead of fiber diameter, as the characteristic length for Reynolds number and Nusselt number. Three features expected through the high‐speed spinning of noncircular cross‐section fibers could be reproduced: (1) although cooling of the flat fiber spin line was enhanced, calculated tension at the position of solidification was not affected much by the difference in cross‐sectional shape; (2) change in cross‐sectional shape proceeded steeply near the spinneret; and (3) temperature at the edge became significantly lower than that at the center in the cross section of flat fibers. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1589–1600, 2001     

Orientation‐dependent mechanical and thermal properties of plasma‐sprayed ceramics

Due to droplet‐based assembly, microstructure anisotropy is expected in atmospheric plasma‐sprayed coatings (APS), with lamellar separations and interfaces having critical effects on properties. Quantitative determination of these anisotropic properties is difficult due to geometric test constraints. This has been overcome in the literature through a variety of indirect, local, or modeled evaluation, however direct measurement on like‐dimensioned coatings is not available. In this work, 25‐mm thick ceramic coating variants, deposited at two different feed rates, were obtained from industry and macroscopic mechanical and thermal properties were evaluated in both in‐plane and out‐of‐plane orientations using identical specimen geometries. As expected, and confirming select past work, coating anisotropy has a direct influence on measured properties. The response of each property is microstructure‐dependent, highlighting the specific interaction: for instance, the fracture toughness is 120% higher in the through‐thickness orientation versus in‐plane after thermal aging, while the thermal conductivity was 24% lower in the through‐thickness. The former benefits from the lamellar interfaces that provide obstacles to crack propagation while the latter sees these interfaces as efficient phonon scatters. The results provide insights for design through robust property measurements and into operational mechanisms in this class of highly defected ceramics.     

Improved through‐plane electrical conductivity in a carbon‐filled thermoplastic via foaming

Flow‐induced orientation of the conductive fillers in injection molding creates parts with anisotropic electrical conductivity where through‐plane conductivity is several orders of magnitude lower than in‐plane conductivity. This article provides insight into a novel processing method using a chemical blowing agent to manipulate carbon fiber (CF) orientation within a polymer matrix during injection molding. The study used a fractional factorial experimental design to identify the important processing factors for improving the through‐plane electrical conductivity of plates molded from a carbon‐filled cyclic olefin copolymer (COC) containing 10 vol% CF and 2 vol% carbon black. The molded COC plates were analyzed for fiber orientation, morphology, and electrical conductivity. With increasing porosity in the molded foam part, it was found that greater out‐of‐plane fiber orientation and higher electrical conductivity could be achieved. Maximum conductivity and fiber reorientation in the through‐plane direction occurred at lower injection flow rate and higher melt temperature. These process conditions correspond with foam flow during filling of the mold cavity, indicating the importance of shear stress on the effectiveness of a fiber being rotated out‐of‐plane during injection molding. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Influence of molecular orientation direction on the in‐plane thermal conductivity of polymer/hexagonal boron nitride composites

The effect of the molecular orientation direction of a polymer matrix on the in‐plane thermal conductivity (TC) of injection‐molded polymer/hexagonal boron nitride (h‐BN) composites is investigated. In this system, the h‐BN platelets align in the in‐plane direction owing to injection shear flow. Three molecular orientations (perpendicular, random, and parallel to the h‐BN plane) are achieved using liquid crystalline polyesters and the in‐plane TCs are compared. Although a parallel orientation of the polymer chains provides the highest TC of the matrix in the injection direction, the TC of the composites is the lowest of the three systems for this orientation. The highest in‐plane TC is found in the perpendicularly oriented system, irrespective of the in‐plane direction. These results reveal that perpendicularly oriented molecular chains serve as effective heat paths between h‐BN platelets that are arranged one above the other, and consequently, a continuous thermal network is created in the in‐plane direction. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39768.     

Fiber orientation control of short‐fiber reinforced thermoplastics by ram extrusion

In this study we examine the fiber orientation distribution, fiber length and Young's modulus of extruded short‐fiber reinforced thermoplastics such as polypropylene. Axial orientation distributions are presented to illustrate the influence of extrusion ratio on the orientation state of the fibrous phase. Fibers are markedly aligned parallel to the extrusion direction with increasing extrusion ratio. The orientation state of extruded fiber‐reinforced thermoplastics (FRTP) is almost uniform throughout the section. The control of fiber orientation can be easily achieved by means of ram extrusion. Experimental results are also presented for Young's modulus of extruded FRTP in the extrusion direction. Young's modulus follows a linear trend with increasing extrusion ratio because the degree of the molecular orientation and the fiber orientation increases. The model proposed by Cox, and Fukuda and Kawada describes the effect of fiber length and orientation on Young's modulus. The value of the orientation coefficient is calculated by assuming a rectangular orientation distribution and calculating the fiber distribution limit angle given by orientation parameters. By comparing the predicted Young's modulus with experimental results, the validity of the model is elucidated. The mean fiber length linearly decreases with increasing extrusion ratio because of fiber breakage due to plastic deformation. There is a small effect on Young's modulus due to fiber breakage by ram extrusion.     

Interrogation of “surface,” “skin,” and “core” orientation in thermotropic liquid‐crystalline copolyester moldings by near‐edge X‐ray absorption fine structure and wide‐angle X‐ray scattering

Injection molding thermotropic liquid‐crystalline polymers (TLCPs) usually results in the fabrication of molded articles that possess complex states of orientation that vary greatly as a function of thickness. “Skin‐core” morphologies are often observed in TLCP moldings. Given that both “core” and “skin” orientation states may often differ both in magnitude and direction, deconvolution of these complex orientation states requires a method to separately characterize molecular orientation in the surface region. A combination of two‐dimensional wide‐angle X‐ray scattering (WAXS) in transmission and near‐edge X‐ray absorption fine structure (NEXAFS) spectroscopy is used to probe the molecular orientation in injection molded plaques fabricated from a 4,4′‐dihydroxy‐α‐methylstilbene (DHαMS)‐based thermotropic liquid crystalline copolyester. Partial electron yield (PEY) mode NEXAFS is a noninvasive ex situ characterization tool with exquisite surface sensitivity that samples to a depth of 2 nm. The effects of plaque geometry and injection molding processing conditions on surface orientation in the regions on‐ and off‐ axis to the centerline of injection molded plaques are presented and discussed. Quantitative comparisons are made between orientation parameters obtained by NEXAFS and those from 2D WAXS in transmission, which are dominated by the microstructure in the skin and core regions. Some qualitative comparisons are also made with 2D WAXS results from the literature. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Determination of 3‐D Alumina Grain Orientation,Size, Shape,and Growth Kinetics from 2‐D Data in Nextel™ 610 Fibers

Nextel^? 610 alumina fibers were heat‐treated at 1100°C–1500°C for 1–100 h in air. Grain size distributions (GSDs) and grain orientation distributions (ODs) with respect to the fiber axis were characterized by analysis of TEM images from longitudinal fiber sections. The 2‐D GSDs and ODs were characterized as ellipses. 3‐D GSDs and ODs were calculated by fitting distributions of oriented oblate ellipsoids to 2‐D GSDs and ODs formed by ellipsoid–section‐plane intersections. The standard deviations (SDs) of log‐normal GSDs consistently increased with grain size, which is not diagnostic of normal grain growth. The grain aspect ratio (α) and the tendency of the short grain axis to orient perpendicular to the fiber axis also increased with grain size, resulting in more textured fibers at larger grain sizes. Average 3‐D grain sizes were larger than 2‐D sizes for GSDs with small SDs, but smaller for GSDs with large SDs because of under sampling of small grains. 3‐D grain growth kinetics had the same 815 kJ/mol activation energy as that found by 2‐D analysis, but the grain growth exponent m of 6.0 was larger and the pre‐exponential factor much smaller. Expressions for 3‐D log‐normal GSDs as a function of heat treatment temperature and time were determined. α‐distributions and ODs were determined as a function of grain size. Methods for determining 3‐D GSDs are discussed.     

On the effect of the fiber orientation on the flexural stiffness of injection molded short fiber reinforced polycarbonate plates

The through-thickness fiber orientation distribution of injection molded polycarbonate plates was experimentally determined by light reflection microscopy and manual digitization of polished cross sections. Fiber length distribution was determined by pyrolysis tests followed by image analysis. A statistical analysis was done to determine the confidence limits of the fiber orientation results. The fiber orientation distribution was described by using second-order orientation tensors. The through-thickness stiffness variations were determined by the orientation averaging approach. This layer stiffness distribution was used to simulate the behavior of beams subjected to three point bending with a FEM Ansys model. The results were compared with experimentally determined flexural stiffness both in the flow direction and in the transverse flow direction. The effect of flow rate and melt-temperature on stiffness and fiber orientation is discussed.