Microstructure and mechanical behavior of reinforced reaction injection molded (RRIM) polyurethane

A study was carried out to characterize the microstructure and distribution of some mechanical properties in reinforced reaction injection molding panels (RRIM). The panels were prepared under a variety of processing conditions. Scanning electron microscopy, differential scanning calorimetry, and Fourier transform infrared analysis were employed for microstructure characterization. The following mechanical tests were carried out: dynamic mechanical, tensile, and impact. The results indicate significant relationships between processing conditions, microstructure, and mechanical properties. In particular, the skin/core structure of the panels and the size distribution of bubbles in the matrix have an important effect on the impact properties. Furthermore, the balance between the distributions of cure and crystallinity, which is difficult to define clearly, plays an important role in determining panel behavior.     

Properties of mat reinforced reaction injection molded materials

Reinforced reaction injection molding, RRIM, using preplaced glass-fiber mats is an attractive method for production of composite materials. The effect of the glass-fiber mat on the properties of a styrene-dimethacrylate copolymer (SDM) is analyzed. Thermal conductivity as a function of porosity was measured. A simple method to obtain the thermal coefficient of linear expansion using the Rheometrics System Four is presented. Dynamic mechanical properties were measured using torsion and three-point bending of rectangular bars. Adding the glass-fiber mat increased the storage modulus (G′) by a factor of 5 below T_g and 100 above it. Empirical correlations for the composite modulus as a function of glass-fiber content below and above T_g are presented. The increase in glass-transition temperature with increasing glass-fiber content is discussed.     

Shrinkage anisotropy in fiber reinforced injection molded products

This paper includes a systematic study of the effect of fiber concentration and molding conditions on fiber orientation and shrinkage in injection molded composites. Closed-form expressions were derived to relate shrinkage and internal stresses to the molding pressure and fiber orientation. The shrinkage predictions were seen to agree well with experimentally measured shrinkages.     

Fracture toughness of injection molded glass fiber reinforced polypropylene

The 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 K_c, 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.     

Strength of short-fiber reinforced composites

The structural utility of short, glass fiber-reinforced epoxy composities is experimentally investigated for fiber volume fractions from 0.15 to 0.5. The strength and stiffness of systems with randomly oriented fibers are compared with those of similar composites with aligned fibers. The ultimate strength of both types of material increses in a reasonably linear fashion with volume fraction up to 0.5. For all volume fractions in this range, strength of the random composites is slightly higher than the longitudinal and much higher than the transverse strength of equivalent compsites with aligned fibers. The modulus of the random system is approximately two-thirds the longitudinal and twice the transverse modulus of the unidirectional material. The structural utility of the flow molded material is greatest in uniaxial, stiffness critical situations. The greater strength and planar isotropy of the random composites make them preferable in all strength limited or multiaxial applications.     

Creep and physical aging of injection molded,fiber reinforced polypropylene

Tensile creep data have been obtained at 23°C as a function of physical age from test pieces of glass fiber reinforced polypropylene cut from injection molded square plaques in directions parallel and transverse to the flow direction. Smaller compliances were measured for loading parallel to the flow than for loading in the perpendicular direction, reflecting the preferential alignment of the fibers with the flow. Two creep functions have been used to model the creep behavior over both a limited timescale where the age of the test piece remains effectively constant and a wider timescale where significant further aging of the material occurs. The results of these analyses reveal significant differences between modeling injection molded polypropylene and compression molded material.     

Microstructure and mechanical properties of microcellular injection molded polyamide-6 nanocomposites

The microstructure and mechanical properties of microcellular injection molded polyamide-6 (PA6) nanocomposites were studied. Cell wall structure and smoothness were determined by the size of the crystalline structure, which, in turn, were based on the material system and molding conditions. The correlation between cell density and cell size of the materials studied followed an exponential relationship. Supercritical fluid (SCF) facilitated the intercalation and exfoliation of nanoclays in the microcellular injection molding process. The orientation of nanoclays near the surface of microcells and between microcells was examined and a preferential orientation around the microcells was observed. Nanoclays in the microcellular injection molding process promoted the γ-form and suppressed the α-form crystalline structure of PA6. Both nanoclays and SCF lowered the crystallinity of the parts. Microcells improved the normalized toughness of the nanocomposites. Both microcells and nanoclay had a significant influence on the mechanical properties of parts depending on the molding conditions.     

Impact fracture behavior of injection molded long glass fiber reinforced polypropylene

In this work the variation in Izod impact strength with spatial location was examined for injection molded long glass fiber polypropylene composite plaques. These plaques were fabricated at different sets of processing conditions, with injection speed and melt temperature being varied. By carefully machining test specimens, fifteen different plaque locations both in the in-flow and cross-flow directions were tested. The part morphology was described with the use of characteristic layer thickness ratios, i.e., the shell and the core to part thickness ratios, which were measured experimentally. It was shown that the variation in impact strength with sample location strongly correlates to shell to part thickness ratio. In addition, it was observed that different failure mechanisms exist for different fiber orientations, i.e., for fibers oriented transversely to the crack plane or on the crack plane itself. Scanning electron microscopy (SEM) of the fracture surface was conducted and the results supported our findings on the microstructural level.     

Fracture toughness of a short-fiber reinforced thermoplastic

Thin plates of carbon short-fiber reinforcement polycarbonate were injection molded. The mold was designed to produce a uniform melt flow across the cavity and an extended knock-out pin was incorporated to form a circular hole at the center of the molded plate. The elastic constants of the plaques were determined using sections cut from the plate at different angles to the direction of flow. Analysis of the data showed that the plates could be treated macroscopically as being orthotropic. Microscopic observations revealed that the fiber orientation was primarily in the flow direction and was tangential in the vicinity surrounding the hole. The fracture toughness, as measured by the stress intensity (K), was determined using the compliance method. Experimental calibration curves were constructed at 0° and 90° to the axis of flow by loading specimens containing saw cuts of varying length. The resultant curves were non-dimensionalized by incorporation of the elastic moduli, thickness, and width. The fracture toughness values were determined using a razor notch as a starter crack. The crack growth during testing was found to be stable, which could allow several determinations to be made on each plate. The effects of crack length, flow in the cavity, and fiber orientation around the hole were investigated. The fracture toughness was found to decrease with increasing crack length, but was not found to reach a limiting value within the practical range of testing. The effect of flow was also found to be significant. Specimens oriented 90° to the axis of flow showed higher toughness values. This was attributed to the fibers being oriented perpendicular to the axis of the crack. The samples tested with razor notches cut at the edge of the molded holes had still higher apparent toughness values. Similarly, this effect was explained by the higher fiber orientation shown with photomicrographs of specimens cut near the edge of the hole.     

Processing and properties of injection molded thermoplastic composites reinforced with melt processable glasses

This work was concerned with evaluating the properties of injection molded composites comprising polyetherimide (PEI) and polyetheretherketone (PEEK) reinforced with various lower T_g melt processable phosphate glasses. Composites were produced utilizing a variety of glass and resin combinations in order to ascertain the effects of factors such as glass concentration and viscosity of the components on the mechanical properties of the composite blends. Changes in the rheological and interfacial properties of the blends obtained by varying the resins and phosphate glasses used during processing resulted in a variety of reinforcing morphologies consisting of glass beads, ribbons, and an interpenetrating network structure. The large variations in the glass phase morphologies obtained during injection molding led to composites that displayed a wide range of properties. Generally, it was found that the use of resin/glass combinations that minimized the viscosity difference between the components resulted in composites displaying the best overall mechanical properties. The stiffness of the composites was found to increase with glass concentration with loadings up to 45 vol% glass, leading to moduli 3‐4 times greater than those of the neat resins. While the addition of the phosphate glasses produced significant enhancements in the stiffness of the composite blends, the strength often fell to values 2‐3 times lower than those of the neat resins.     

Microstructure and tensile properties of injection molded thermoplastic polyurethane with different melt temperatures

The use of injection molding technology to prepare heterogeneous interlayer film of laminated glass holds strong applicable potential. This article aims to investigate the effects of melt temperature and melt flow on the microstructure evolution and tensile properties of thermoplastic polyurethane (TPU) specimens during the injection molding process. The tensile properties of the TPU specimens show dependency on the melt temperature and melt flow direction. The results of birefringence indicate that melt flow and lower melt temperature induce higher stretching deformation of the molecular chain network. Small-angle X-ray scattering analysis approves that besides the melt temperature and flow direction, the testing position on the cross section of the specimen has great influence on the microstructure of the TPU sheet. Further analysis and conclusions can be made using wide-angle X-ray scattering method. The above results demonstrate that both the tensile properties and microstructure of the injection molded TPU specimens tend to be isotropic with the increase of melt temperature. © 2020 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48891.     

Morphology and mechanical investigation of microcellular injection molded carbon fiber reinforced polypropylene composite foams

Employing microcellular injection molding technology, carbon fiber (CF)/polypropylene (PP) composite foams have been prepared. The influences of injection molding conditions and CF amounts relating to the flexural and impact performances have also been studied. X-ray computed tomography scanning has been used for morphological observation. For the flexural specimens, although the solid skin and foamed core layers can be confirmed significantly, the intermediate layer is indistinct. Moreover, the stretched cells can be confirmed dramatically for the Charpy impact specimens. The cell density increases to 12.0 × 10³ cell/cm² when the nitrogen content is 1%. By contrast, the cell densities decrease with the injection speed and CF content increasing accordingly. Further, the maximum specific flexural modulus and Charpy impact strength of the foams can achieve 14 GPa/(g/cm³) and 6.2 kJ/m², respectively, at the CF content of 30 wt%. Finally, the microcellular structure with the highest cell density can be confirmed with the nitrogen content of 1 wt%, the injection speed of 50 mm/s and the CF content of 10 wt%. Obviously, the CF contents have shown strong influences on the mechanical behaviors of the CF/PP composite foams compared with nitrogen contents or injection speeds.     

Mechanical properties of glass fiber reinforced polypropylene injection molded with a rotation mold

A special mold (Rotation, Compression, and Expansion Mold) was used to impose a controlled shear action during injection molding of short glass fiber reinforced polypropylene discs. This was achieved by superimposing an external rotation to the pressure‐driven advancing flow front during the mold filling stage. Central gated discs were molded with different cavity rotation velocities, inducing distinct levels of fiber orientation through the thickness. The mechanical behavior of the moldings was assessed, in tensile and flexural modes on specimens cut at different locations along the flow path. Complete discs were also tested in four‐point flexural and in impact tests. The respective results are analyzed and discussed in terms of relationships between the developed fiber orientation level and the mechanical properties. The experimental results confirm that mechanical properties of the moldings depend strongly on fiber orientation and can thus be tailored by the imposed rotation during molding. POLYM. ENG. SCI. 46:1598–1607, 2006. © 2006 Society of Plastics Engineers.     

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

A 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.     

Stress relaxation properties of some reaction injection molded (RIM) and reinforced RIM systems

This study examined the stress relaxation properties of six urethane reaction injection molded (RIM) systems, both reinforced and non-reinforced RIM systems from room temperature to 100°C. Data is presented which shows the dynamic mechanical (DMS), differential scanning calorimetry (DSC), physical properties, and stress relaxation data. The major conclusion from this work is that the state of hard segment (amorphous or crystalline) makes a vast difference in the polymer's reaction to stress.     

Two-dimensional mapping of tensile strength for injection molded composites of glass fiber reinforced polypropylene

An array of polypropylene composites were injection molded into rectangular plaques having a single side gate. As a consequence of non-symmetrical flow patterns, giving complex distribution of fiber orientations, measured tensile strength of test specimens cut along X-Y directions produced nonlinear and anisotropic trendlines. Chemical coupling, glass fiber content, and the choice of short or long glass fiber reinforcement are shown to be material variables influencing the magnitude of the tensile strength at any given localized position. These data verify the notion that tensile strength evaluation along just the flow direction is insufficient for anticipating end use performance.     

Modeling material property heterogeneity in fiber‐reinforced injection molded plastic parts

Fiber reinforced plastic parts manufactured by injection molding have heterogeneous stiffness and strength behavior due to the molding process influence on the fiber orientations. This paper presents a methodology for determining the process‐dependent anisotropic and inhomogeneous mechanical properties of injection‐molded parts using a thickness‐wise layered homogenization technique. This technique produces an equivalent laminated meso‐scale representation at any location in the part and enables point‐wise application of the existing laminated plate and shell theories. The methodology is demonstrated by illustrating property variations in edge‐gated and center‐gated plaques. Spatial variations of elastic moduli, shear modulus, and Poisson's ratio are modeled. The model can be conveniently embedded within finite element structural analyses accounting for the process‐dependent material heterogeneities within the structure. POLYM. COMPOS., 26:98–113, 2005. © 2004 Society of Plastics Engineers     

Fiber orientation structures and mechanical properties of injection molded short glass fiber reinforced ribbed plates

In this paper we describe a study of the fiber orientation structures present within a model ribbed injection molded plate. The details of the fiber orientation at each chosen location on the injection molded parts were measured using an in-house developed image analysis system, which enabled large areas to be scanned (up to 200 mm²) up to a limit of 1 million fiber images. Two materials were used for these experiments, short glass fiber filled PBT and short glass fiber filled nylon 66. First, a comparison was made between the fiber orientation at an identical position, 28 mm from the injection gate on a transverse rib, on two plates made from glass fiber filled PBT. It was found that the fiber orientation in these two separately manufactured components was virtually identical when comparing the whole scanned area, but the differences became more significant when comparing areas on the length scale of an individual fiber (∼ 200 μm). Second, the fiber orientation at the same position was compared for two plates made using the glass/PBT and glass/nylon 66 materials. The differences for the complete scanned areas were small, confirming that mold geometry plays a crucial role in determining fiber orientation structures, and that matrix properties are secondary. Third, the fiber orientation structures at various positions across one of the glass/PBT plates were examined in greater detail, in particular across a number of the transverse ribs: the chosen ribs were of various widths and heights. Differences in structure were found depending on the local rib geometry. Finally, the effect of the measured fiber orientation structures in determining the mechanical properties of the ribbed plate was investigated using simple modeling schemes. While the stiffness of the rib/web assembly was found to depend on the average fiber orientation of the two parts, the different thermal expansions of the web and the rib, caused by the different fiber orientation in the two regions, led to significant warpage of the rib/web assembly. Polym. Compos. 25:237–254, 2004. © 2004 Society of Plastics Engineers.     

Microstructure in injection molded samples of liquid crystalline poly(P-hydroxy-benzoic acid-Co-ethylene terephthalate)

The microstructure of injection molded bars (2.9 and 5.8 mm thick) of thermotropic liquid crystalline poly(p-hydroxy-benzoic acid-co-ethylene terephthalate) has been studied by SEM on samples etched with n-propylamine, SEM fractography, DSC, IR, ESCA, WAXS and polarized microscopy. The 2.9 mm bar consists of three different layers: a highly oriented surface skin, an oriented intermediate layer and a non-oriented core. The 5.8 mm bar has a more complex microstructure and is composed of five different layers: a highly oriented surface skin, an oriented layer just beneath, a non-oriented layer, another oriented layer and a non-oriented core. The thicknesses of the different layers vary, significantly, with distance from the mold gate. The thickness of the core increases, significantly, with increasing distance from the mold gate at the expense of the oriented layers. The structure within the different morphological layers is not perfectly uniform. Tensile testing demonstrated the mechanical anisotropy of the surface material (a ratio of almost 20 between the longitudinal and transverse moduli) and the isotropy of the central core material.     

The effect of processing variables on microstructure of injection molded short fiber reinforced polypropylene composites

Injection molded composites of polypropylene reinforced with short glass fibers were obtained under a variety of injection molding conditions. The microstructure of the moldings was determined using a variety of experimental techniques, including optical and scanning electron microscopy, differential scanning calorimetry, Fourier transform infra-red spectroscopy, and thermogravimetry. Thus, it was possible to obtain a detailed characterization of the crystallinity, morphology, and orientation distribution in the matrix, in addition to the distribution of fibers and their orientation of the fibers in the composite. The influence of molding conditions on the above microstructural characteristics is summarized in an effort to explain the experimental observations.