Melt Flow Direction Research Articles

Thermal conductivity, electrical resistivity, mechanical, and rheological properties of thermoplastic composites filled with boron nitride and carbon fiber

AbstractHybrid composites consisting of boron nitride (BN) platelets and carbon fibers (CF) in a polybutylene terephthalate (PBT) matrix were melt‐compounded, and their thermal and electrical conductivity, tensile, and rheological properties were investigated. While it does not lead to an enhancement in thermal conductivity with respect to PBT/BN composites, the results indicate that a combination of BN and CF in PBT can significantly reduce electrical conductivity of the composites compared to that of PBT/CF composites. The relative low thermal conductivity of the hybrid composites is attributed to CF breakage that occurred during the extrusion and alignment of CF in melt flow direction, which is normal to the heat flow encountered during the thermal conductivity tests induced by injection molding. The hybrid composites were, however, found to have better tensile properties and processibility than PBT/BN composites at the same total filler content. POLYM. COMPOS., 26:66–73, 2005. © 2004 Society of Plastics Engineers

Relationship between shear stress and shear strain of polymer melt under vibrating force field

When a sinusoidal vibration was superimposed in parallel on the flow direction of a polymer melt being extruded through a capillary, the shear stress and shear rate of the polymer melt were analysed with a constant velocity type dynamic rheometer of capillary (CVDRC) devised by the authors. By measuring the instantaneous data of capillary entry pressure, capillary volume flux (or absolute velocity of piston rod) and their phase difference in a vibrating force field, it was found that the relationship between the pulsating amplitude value coefficient of entry pressure and that of volumetric flowrate was an approximate power series; the wall shear stress and wall shear rate of low density polyethylene (LDPE) melt extruded dynamically under various amplitudes and frequencies also exhibited a non-linear proportional relationship.

Morphology-tensile behavior relationship in injection molded poly(ethylene terephthalate)/polyethylene and polycarbonate/polyethylene blends (I) Part I Skin-core Structure

The skin-core structure of injection molded poly(ethylene terephthalate) (PET)/polyethylene (PE) and polycarbonate (PC)/PE blends was investigated. The results indicate that both shape and size of the PET and PC phases depended not only on the nature properties of PET/PE and PC/PE blends, but also on the injection molding parameters such as injection speed and the positions in the molded bars. The morphology in the section perpendicular to the melt flow direction included four layers, surface, sub-skin, intermediate layers as well as core zone. The surface layer was ignored in the present study. The sub-skin layer contained more or less fibrous structure and its thickness gradually decreased along the molded bar from the gate toward the non-gate end. At the same injection speed, the concentration of the injection-induced fibers in PC/PE blend was much higher than that in PET/PE blend. In the core region, the dispersed phase was mainly composed of spherical particles whose diameter increased along the melt flow pathway. Between these two layers, there was an intermediate layer where the dispersed particles mainly assumed the form of fibers, ellipsoids or spheres. Generally, no matter whether the dispersed particles were elongated or not during injection molding, the PET particles were larger than PC ones.

The manufacturing of thermoplastic composite parts by water-assisted injection-molding technology

This study investigated the manufacture of thermoplastic composite parts by a novel water-assisted injection-molding technology. A water injection unit, which included a water pump, water injection pin, control circuit, and water tank equipped with a temperature controller, was developed in our laboratory. Experiments were carried out on an 80-ton reciprocating injection-molding machine equipped with the lab-developed water injection unit. The materials used in the study were short glass-fiber filled polypropylene composites. To optimize the molding process, an experimental study, based on the Taguchi orthogonal array design, was conducted to characterize the effect of different processing parameters on the water-assisted injection-molding of thermoplastic composites, including melt temperature, mold temperature, short-shot size, water pressure, water temperature, and water injection delay time and hold time. After molding, the length of water penetration in the molded composites was determined. For the factors selected in the main experiments, short-shot size and water injection delay time were found to be the principal parameters affecting the manufacturing of water-assisted injection-molded composites. Internal rough surface was observed in molded parts. Composites molded by water-assisted injection-molding were found to have a shorter cycle time than those molded by gas-assisted injection-molding. In addition, the glass fibers in molded parts were found oriented mostly in the flow direction of the polymer melt. A moldability diagram was also developed based on the statistical results to give a first guideline for system optimization.

Characterisation of injected EPBC plaques using the essential work of fracture (EWF) method

The influence of processing induced morphology, thickness and ethylene content (EC) of different ethylene–propylene block copolymers on fracture properties has been studied using the essential work of fracture (EWF) method. To analyse the influence of EC, four different materials were chosen with 0, 5.5, 7.8, and 8.4% in weight EC. Each material was injected in three different thicknesses (1, 2 and 3 mm). The resulting plaques were tested using the EWF method in both main orientations; the melt flow direction and transverse to melt flow direction. Different fracture behaviours have been observed, some of them preventing the applicability of the EWF method. Polarised light microscopy observations have revealed the existence of a skin/core structure, which is reduced with an increase in thickness and EC.

Mechanical properties, morphology, and crystallization behavior of new thermoplastic polyimide (N-TPI)/poly(ether sulfone) blends

Blends of wholly aromatic new thermoplastic polyimide (N-TPI) and poly(ether sulfone) (PES) were prepared by melt-mixing and subsequent injection molding. Their mechanical properties, morphology, and crystallization behavior were investigated. A synergistic effect on the flexural properties was observed for the N-TPI/PES blends over the whole compositions. Differential scanning calorimetric analysis confirmed that the blend is an immiscible system and that the crystallinity of the N-TPI component is very low (0.4–1.4%) irrespective of the composition. Scanning electron microscopic analysis suggested that the improvement in flexural properties is likely due to the reinforcement of the PES matrix by the fibrous N-TPI phase for the N-TPI/PES (40/60, 20/80) blends. For N-TPI/PES (80/20, 60/40), the orientation of the matrix N-TPI rather than the fibrillation of the PES phase along the melt-flow direction may contribute to the synergism observed. Also, study of the isothermal crystallization behavior of the blends revealed that the addition of 10 wt % PES accelerates the crystallization of N-TPI and further addition causes the retardation of it. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1366–1374, 2002

Thermal Expansion Anisotropy in Injection-Moulded Natural-Fibre Composites

Recently much effort has been devoted to the development of natural-fibre composites produced via injection moulding. Although several processing-cost benefits may be associated with the injection moulding process, the microstructures of injection-moulded composites can be quite complex and anisotropic. In this study, the thermal expansion anisotropy that may develop in injection moulded natural-fibre composites has been investigated. A thermal mechanical analyser (TMA) has been employed to determine the thermal expansion properties of jute and sisal-fibre reinforced polypropylene composites injection-moulded into the form of tensile bars and disks. Measurements indicated that thermal expansion coefficients of tensile bars in the melt-flow direction may be 5 to 7 times less than thermal expansion coefficients in the cross-flow direction.

Fatigue Fracture of Neat and Short Glass Fiber Reinforced Polypropylene: Effect of Frequency and Material Orientation

The influence of glass fibers content and testing conditions (frequency and mean load) on the fatigue crack propagation rate in injection molded neat and fiber reinforced polypropylene has been investigated. The resistance to fatigue crack propagation increases as the fiber volume fraction increases. The fatigue resistance of either neat and fiber reinforced polypropylene is tremendously dependent on the crack propagation direction, with higher values for crack propagating transversely to the melt flow direction in the molded plaques. A dramatic increase of the crack growth rate per cycle by decreasing frequency, at any given crack length was found for all the materials investigated. Analysis of data obtained at various frequencies and applied mean loads suggests that crack propagation is determined mostly by viscoelastic creep processes at the crack tip, the role of pure fatigue appearing quite secondary.

Manufacturing Process Optimization of Short Glass Fiber Reinforced Polycarbonate Composites in Injection Molding

This paper presents a fast and effective methodology for optimization injection molding process parameters of short glass fiber reinforced polycarbonate composites. Various injection molding parameters such as filling time, melt temperature, mold temperature and ram speed were considered. The methodology combines the use of Taguchi approach and a CAE flow simulation software which was used to simulate the injection molding process and to predict the fiber orientation. This method can replace the traditional "change-one-parameter-at-a-time" approach which is very inefficient, costly, time consuming and almost impracticable to yield an optimum solution. In the mean time, the microstructures were examined with a scanning electron microscopy to determine the fiber orientation and shear layer thickness to check the results of CAE simulation. The results indicated that three distinct layers (frozen layer, shear layer and core layer) are observed from surface to core at various injection molding conditions. The fiber orientation is perpendicular to the melt flow direction in frozen layer and core layer, but it has the opposite direction in shear layer. From the CAE analysis, we have gotten the optimum process parameters to obtain the thickest shear layer that was our target.

Comparative assessment of slow strain rate, 4-pt bend and constant load test methods for measuring environment stress cracking of polymers

The environment stress cracking (ESC) susceptibility of polycarbonate and acrylonitrilebutadiene-styrene (ABS) in a number of environments has been evaluated using slow strain rate, four-point bend and constant load tests. Testing was conducted for the materials in the as-processed and annealed state. For the as-processed materials, the longitudinal direction of the specimen was generally perpendicular to the melt flow direction. Nevertheless, a few tests were carried out with the specimen orientation parallel to the flow direction. The ESC techniques gave consistent ranking of aggressivity of environments although the extent of relaxation of the ABS when exposed to particular environments limited the significance of results from the bend tests. Otherwise, the results validate further the use of the slow strain rate technique as an accelerated test method for assessing the ESC of plastics, albeit primarily as a ranking technique. The threshold stress for crazing was highest for the slow strain rate method which can be attributed partly to the particular strain rate adopted. There was no effect of molecular orientation on ESC for the limited range of test conditions evaluated. The difference in threshold stress measured for the annealed and as-processed specimens indicated the existence of a net tensile residual stress in the very near-surface region, not detected by the layer removal method of residual stress measurement.

Optimization of the injection molding process of short glass fiber reinforced polycarbonate composites using grey relational analysis

This paper presents a fast and effective methodology for the optimization of the injection molding process parameters of short glass fiber reinforced polycarbonate composites. Various injection molding parameters, such as filling time, melt temperature, mold temperature and ram speed were considered. The methodology combines the use of the GRA (grey relational analysis) method and a CAE flow simulation software, to simulate the injection molding process and to predict the fiber orientation. This method can replace the traditional “change-one-parameter-at-a-time” approach, which is very inefficient, costly, time consuming and almost impracticable to yield an optimum solution. At the same time, the fiber orientation was examined by CAE simulation to forecast the shear layer thickness, and simultaneously to check the accuracy of the GRA. The results indicated that three distinct layers (a frozen layer, a shear layer and a core layer) are observed from the surface to the core for various injection molding conditions. The fiber orientation is perpendicular to the melt flow direction in the frozen layer and the core layer, but it has the opposite direction in the shear layer. From the CAE analysis, the optimum process parameters to obtain the thickest shear layer are found, which is the target of the present research.

Thermophysical characterization of the deformation behavior of polyethylene/kaolin composites

The deformation behavior at room temperature of injection-molded and blow-molded samples of high-density polyethylene (HDPE) differing in orientation (with respect to the melt flow direction) and in filler content (untreated and surface-treated kaolin, respectively) were characterized by the stretching calorimetry technique. Samples with longitudinal and transversal orientations were examined. Strain-softening and strain-hardening phenomena were observed, respectively, in the range of inelastic strains above the yield point. The depression of yield stresses for the filled composites compared to those for the corresponding neat polymers is associated with the onset of debonding of the matrix polymer from the filler surface. The analysis of the energy balance of the debonding process suggests that the fraction of a debonded interface is smaller in samples containing a coupling agent and larger filler particles.

Composition-dependent properties of polyethylene/Kaolin composites. III. Thermoelastic behavior of injection-molded samples

Injection-molded samples of high-density polyethylene (HDPE) differing in the orientation pattern (with respect to the melt flow direction) and in filler content (untreated and surface-treated kaolin, respectively) were characterized by wide-angle X-ray scattering, microhardness, and stretching calorimetry techniques. The crystallinity of the polymer matrix in filled samples shows the same value as that found for the neat polymer regardless, filler content, and/or filler surface treatment. The thermoelastic behavior of all samples in the strain interval below the apparent yield point e * is quantitatively discussed in terms of classical equations for elastic solids. Analysis of thermoelastic parameters of the boundary interphase (BI) reveal an unusually stiff, highly oriented structure of the matrix polymer within BI. Discrepancies of experimental values for the internal energy increment in the inelastic strain interval above e * between unfilled and filled samples is explained in terms of the filler debonding process. The latter process is discussed in light of the formation of a polymer-free filler surface and of the concomitant inelastic deformation (plastic flow) of a polymer matrix in the interstitial space between filler particles.

Mechanical and thermal properties of poly(acrylonitrile-butadiene-styrene) copolymer reinforced with potassium titanate whiskers

Potassium titanate (K2Ti6O13) whisker treated with tetrabutyl orthotitanate was used to improve the mechanical and thermal properties of the poly(acrylonitrile–butadiene–styrene) (ABS) copolymer. The composites were prepared in a twin-screw extruder followed by injection molding. Static tensile measurements showed that both the modulus and breaking stress of ABS/K2Ti6O13 composites increase considerably with increasing whisker content; the strain at break of ABS was almost unaffected by the incorporation of a whisker content up to 15 wt %. Izod impact tests indicated that the composites showed a decrease in the impact strength with increasing whiskers content. Thermogravimetric analysis showed that the K2Ti6O13 whisker additions have little effect on the thermooxidative stability of ABS. Scanning electron microscopic observations revealed that the whiskers were aligned along the melt-flow direction in the thin surface layer, whereas the whiskers were oriented randomly as well as perpendicular to the injection direction in the thick core region of the composites. The Tsai–Halpin equation was used to evaluate the moduli of the ABS/K2Ti6O13 composites. The theoretical calculations generally correlated well with the experiment data by assuming K2Ti6O13 whiskers to have an aspect ratio of 12. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2985–2991, 1999

Performance of potassium titanate whisker reinforced polyamide-6 composites

Composites of polyamide-6 reinforced with potassium titanate whiskers were prepared in a twin-screw extruder, followed by injection moulding. The whiskers were surface treated with tetrabutyl orthotitanate prior to blending. The tensile, impact, morphology and thermal properties of the injection moulded composites were investigated. Static tensile measurements showed that the tensile strength and modulus of the composites increased with increasing whiskers content. However, Izod impact tests indicated that the composites showed a decrease in the impact strength initially with whiskers content up to 25 wt%; thereafter they showed a slight increase when the whiskers content reached 35 wt%. Scanning electron microscopic observations revealed that the whiskers within the composites were oriented uniformly along the melt flow direction. Finally, thermogravimetric analysis showed that the heat resistance of the composites tended to increase with increasing whiskers content. The results were analysed and discussed in terms of established models of the behaviour of short-fibre reinforced composites.

The influence of magnesium hydroxide morphology on the crystallinity and properties of filled polypropylene

Two magnesium hydroxide fillers of different morphology, but almost identical surface area, were incorporated into polypropylene to assess their nucleation properties. The platelet filler was found to reside parallel to the flow direction, whereas the pseudospherical filler resided isotropically within the matrix. The plate plane (001) of magnesium hydroxide was found to be responsible for causing nucleation, the polypropylene b axis growing parallel to the melt flow direction. Vinyl silane modification reduced this effect. Filler loading level was found to be influential in governing crystallization initiation, growth, and final crystallinity levels. Low filler loadings produced fast crystallization rates but low crystallinity contents, whereas high filler contents produced the opposite effects. Mechanical properties were found to be influenced by matrix morphology; however, filler content became the major factor at loading levels of 40% and above. © 1998 John Wiley & Sons, Inc. Adv in Polymer Techn 17: 53–62, 1998

Fatigue propagation behaviour of polystyrene/polyethylene blends

Fatigue crack propagation (FCP) of injection-moulded polystyrene (PS) and 95/5, 85/15 and 70/30 PS/high-density polyethylene (HDPE) blends at loading frequencies of 2 and 20 Hz was studied. The FCP results showed that increasing the HDPE content caused a progressive reduction of the fatigue crack growth rates, especially when a styrene/ethylene– butylene/styrene (SEBS) terpolymer was added as a compatibilizer. Increasing the loading frequency also led to a fatigue crack growth rate reduction. Moreover, the fatigue crack growth rates were lower at a given cyclic stress intensity factor range, ΔK, when the crack propagated normal, instead of parallel, to the melt-flow direction during injection moulding. Fractographic observations indicated that discontinuous growth bands (DGBs), associated with the fracture of crazes in the plastic zone, were present through most or all of the fracture surfaces of the PS/HDPE specimens. In the presence of sufficient HDPE, these DGBs were formed by the initiation, growth and coalescence of large dimples initiated at HDPE particles ahead of the microscopic crack front, similar to a multiple crazing effect. The loading frequency effect on the FCP behaviour of these blends is attributed to a time-dependent deformation process. It is concluded that the FCP behaviour of these blends is strongly affected by the loading direction with respect to the matrix and minor phase orientation, by the presence of a compatibilizer, by the composition of the blend and by the testing conditions. © Chapman & Hall.

Tribological characteristics of short glass fibre reinforced polycarbonate composites

Polymeric composite material products are manufactured by injection moulding and are valued for their good mechanical and tribological properties. Various filler reinforcements are frequently added to the base materials to improve the wear and impact properties. The present study analyses the wear behaviour of polycarbonate reinforced with 20 wt.% short glass fibres. The specimens were prepared under different moulding conditions, with varied filling time, melt temperature, and mould temperature. Friction and wear tests were performed with a Schwingum Reibung Verschleiss (SRV oscillation friction wear) ball-on-plane tester. The worn surface was examined with scanning electron microscopy. The results indicate that the wear volume loss in the sliding direction perpendicular to the melt flow direction is usually lower than that in the sliding direction parallel with the melt flow direction. The optimum injection moulding conditions for lowest wear volume loss are a filling time of 5 s, melt temperature of 290°C, and mould temperature of 80°C, when the sliding direction is parallel with the melt flow direction. The effects of wear volume loss are influenced significantly by the thickness of the frozen layer and the fibre orientation with respect to the sliding direction. The higher the wear volume loss, the higher is the friction coefficient. The wear volume loss in the sliding direction parallel with the melt flow direction is in good agreement with the distribution of the frozen layer thickness. Grooves, cracks, cutting of fibres and debonding of fibres are the major wear mechanisms found in wear tests of short glass fibre reinforced polycarbonate.

Fracture toughness of injection molded glass fiber reinforced polypropylene

AbstractThe fracture behavior of polypropylene reinforced with 30% by weight of short glass fibers was studied using single and double feed plaque moldings. Plaques were injection molded using several gate types and gate positions. Fracture toughness Kc, was calculated at different positions in the plaque moldings using single edge notched tension specimens. Fracture toughness was assessed in the directions parallel and perpendicular to the mold fill direction through measurements of the load to produce complete fracture. Results indicated that the value of fracture toughness is affected by the type of gate as well by size of gate. Position of the specimen also affected fracture toughness. Generally, specimens taken from positions near cavity walls gave higher toughness values than those taken from the center of the moldings. Furthermore, fracture toughness in the transverse direction was consistently higher than in the melt flow direction. Finally, in the case o double feed moldings, a much higher fracture toughness was obtained when the initial crack was perpendicular to the weld line than when it was placed inside the weld line.

Thermal Diffusivity of Injection-molded Polpropylene Plate Filled with Glass Fiber

Anisotropy in composition structure of injection-molded polypropylene plate filled with glass fiber was studied by means of measuring that thermal diffusivity. Two types of test films, whose thickness direction was vertical to melt flow direction (MD) or transverse direction to the melt flow (TD), were cut from the injection-molded plates, with a diamond cutter and a microtome. Glass fiber contents in the test films were 0, 5, 10 and 20 weight percent. Those thermal diffsivity was measured by a simple non-steady state method, using AC Joule heating, which had been developed in our laboratory. Then, the thermal diffsivity of injection-molded plate was evaluated as anisotropic in the place near the gate. Further, it was clarified by the obervation of dispersion and orientation states of glass fiber with polarizing light microscopy, that the orientation of glass fiber caused the anisotropy in the thermal diffusivity, rather than weight percent of the glass fiber.