The effect of orientation on the mechanical properties of injection molded polystyrene

The deformation and fracture behavior of injection molded plaques have been determined, and the results interpreted in terms of the effect of molecular orientation on the crazing and shear yielding behavior. The molecular orientation was characterized by optical birefringence. A range of injection molding conditions and two mold thicknesses were Used and this resulted in a large variation in the molecular orientation, particularly through the sheet thickness. Tensile tests were made on samples cut at different angles to the injection molding direction. The moldings are considered to consist of a composite of layers of material with different orientation, and the properties of the samples cut from the molding are analyzed in terms of the properties of each layer. Results from material oriented unidirectionally by hot drawing have been used to predict the composite properties, and good agreement has been obtained.     

Study on mechanical properties and material distribution of sandwich plaques molded by co‐injection

In the sandwich injection molding process (co‐injection), two different polymer melts are sequentially injected into a mold to form a part with a skin/core structure. Sandwich molding can be used for recycling, improving barrier and electrical properties, or producing parts with tailored mechanical properties. In this study the evaluation of flexural modulus and impact strength of co‐injected plaques have been investigated. Virgin and short glass fiber reinforced (10 and 40%) polypropylene were used in six different combinations of sandwiched layers. The skin and core thicknesses were measured by optical microscopy and used to calculate the theoretical flexural modulus, which was compared to the experimentally measured modulus. Fiber orientation states were also observed by scanning electronic microscopy (SEM) at some specific locations and their effect on mechanical properties discussed. The experimental results indicate that an important improvement in transverse modulus, near the gate, is obtained when the virgin polypropylene (PP) is used as a skin and 40% short glass fiber polypropylene (PP40) as core. When both skin and core are made of PP40, the flexural moduli are slightly higher than conventionally injected PP40. POLYM. COMPOS. 26:265–275, 2005. © 2005 Society of Plastics Engineers.     

The effect of flake aspect ratio on the flexural properties of mica reinforced plastics

Two naturally occurring micas, phlogopite and muscovite, were ground and classified according to their average aspect ratios (flake equivalent diameter to thickness ratio). These flakes were then used to reinforce a polystyrene copolymer and a polyester resin. The compression molded test pieces were tested in flexure and the flexural strengths and flexural moduli determined for each aspect ratio. The experimental results indicate a strong dependence of strength and modulus on the flake aspect ratio up to a value of 100–200 for these systems. At high volume fractions, 0.6 to 0.7, high aspect ratio mica composites yielded flexural strengths of 35–45,000 psi with flexural moduli of 10–14 million psi. Notched Izod values were in the range of 0.5–1.4 ft lb per inch of notch. These results were compared with the theoretical treatments of Padawer and Beecher, and Riley.     

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     

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.     

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     

Fiber orientation in injection molding with rotating flow

The 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     

Frozen-in birefringence and anisotropic shrinkage in optical moldings: II. Comparison of simulations with experiments on light-guide plates

The thermally-, flow-induced and total birefringence components and anisotropic shrinkages in LGP moldings were simulated by using a combination of a CV/FEM/FDM technique nonlinear viscoelastic and photoviscoelastic constitutive equations and orientation functions, as described in Part I of this study. The simulated results were compared with measurements on LGP moldings of a polystyrene (PS) and two optical grade polycarbonates (PCs) OQ1030 and OQ3820 having low and high molecular weights. The thermally-induced birefringence was simulated by a combination of constrained and free cooling during molding. In LGP moldings of PS, the simulated thermally-induced birefringence indicated a minor variation with location in the mold plane, a parabolic shape in the core region and an increase towards the wall. Compared to the flow-induced birefringence, the thermal birefringence provided a minor contribution to the total transverse birefringence Δn₁₂. In LGP moldings of PCs, the simulated thermally-induced birefringence showed a significant variation with location in the mold plane, nearly constant value in the core region and high value in the wall region. In LGP moldings of both PCs, the contributions of the thermally- and flow-induced birefringence to the total transverse birefringence Δn₁₂ were significant. The effect of processing conditions on the development of the normal birefringence in LGP moldings of PCs was ranked from most to least: the packing pressure, mold temperature, melt temperature, injection speed and packing time. However, in LGP moldings of PS the packing time effect was significant due to a longer gate freezing time. Simulated and measured normal birefringence along the flow direction was in fair agreement, but simulations were unable to describe the observed birefringence maximum arising near the gate. The averaged luminance of LGP moldings exhibited some correlation with the averaged normal birefringence. LGP moldings of PC OQ1030 indicated a pronounced maximum in the simulated transverse flow birefringence in the core but a low value near the wall. In contrast, the LGP molding of PC OQ3820 showed a high simulated birefringence near the wall and a low value of maximum in the core. The simulated and measured total transverse birefringence in LGP moldings was in fair agreement. LGP molding of both PCs showed similar tendency in shrinkage variation with processing conditions. However, the thickness shrinkage was higher in LGP moldings of PC OQ3820. The effect of processing conditions on the development of shrinkage in LGP moldings of both PCs was ranked from most to least: the packing pressure, melt temperature, mold temperature, injection speed and packing time. In LGP moldings of PS, the thickness shrinkage slightly increased with increasing melt temperature and significantly increased with reducing packing time. A good agreement between the simulated and measured anisotropic shrinkages in LGP moldings at various processing conditions was observed.     

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.     

Ejection force in tubular injection moldings. Part I: Effect of processing conditions

The development and manufacture of injection molds for high quality technical parts are complex tasks involving the knowledge of the injection molding process and the material changes induced by processing. In the case of some specific shapes (boxes, tubular fittings), the shrinkage is partially restricted by the mold. The molding shrinks against the core, inserts or pins. Thus, upon ejection, it will be necessary to overcome the frictional forces resulting from the shrinkage. The knowledge of the ejection force is a useful contribution to optimizing the design of molds with these features, and to guaranteeing the structural integrity of the moldings. A study on the effect of conditions on the ejection force required for deep tubular moldings is described for the cases of three common thermoplastic polymers. The studies were based on tubular moldings (60 mm diameter, 146 mm length, and 2 mm thickness). The injection unit cell consisted of a 1 MN clamp force injection molding machine, thermal regulator, and material dryer. During processing, pressure, temperature and ejection force evolutions were recorded. The results show that the processing conditions noticeably influence the ejection force. Polym. Eng. Sci. 44:891–897, 2004. © 2004 Society of Plastics Engineers.     

Processing–mechanical property relationships in injection moldings of polybutylene

Two unfilled nonpigmented extrusion grades of polybutylene have been injection-molded into a tensile bar mold under a wide range of barrel and mold temperatures. The overall structure of the moldings has been determined and correlated with processing conditions. The short term tensile mechanical properties of the moldings have been ascertained and correlated with molding structure. For low mold temperatures, the Young's modulus and tensile strength of injection moldings of polybutylene are controlled by the extent of and structure within the highly oriented skin. Low barrel temperatures can give rise to highly crystalline thick skins that treble the Young's modulus and fracture stress, when compared to high barrel temperature moldings. Increasing the mold temperature introduces a brittle response in polybutylene injection moldings. Modulus is controlled, at the high mold temperatures, by the skin thickness and by the crystallinity of the material comprising the core of the molding.     

Effect of complex flow kinematics on the molecular orientation distribution in injection molding of liquid crystalline copolyesters

Wide-angle X-ray scattering (WAXS) is used to probe the molecular orientation in steady isothermal complex channel flows (in situ) and in injection molded plaques (ex situ) of a new, low-cost aromatic copolyester based on the mesogen 4,4′-dihydroxy-α-methylstilbene (DHαMS). Complex orientation states arise from the competition of inhomogeneous mixed shear and extension in isothermal flows. Slit-contraction flows lead to a significant but temporary increase in the average degree of molecular orientation, suggesting that this polymer is of the ‘shear-tumbling’ type. Conversely, bimodal orientation states are observed in slit-expansion flows, where transverse extension leads to a strong reduction in the average degree of molecular orientation along the flow direction. Similar bimodal orientation states are observed in injection molded plaques, suggesting that these kinematic concepts translate rather directly to the more complex transient non-isothermal case of injection molding. Variations in orientation state induced by changes in plaque thickness may be rationalized by systematic changes in the relative importance of shear and extension. These results suggest a complementary perspective on ‘skin-core’ morphologies in liquid crystalline polymer moldings, and provide a clear conceptual link between more fundamental studies in isothermal flows and structure development during processing.     

Formation mechanism of high-pressure water penetration induced fiber orientation in overflow water-assisted injection molded short glass fiber-reinforced polypropylene

Recently, there has been growing interest in water-assisted injection molding (WAIM) not only for its advantages over gas-assisted molding (GAIM) and conventional injection molding (CIM), but also for its great potential advantages in industrial applications. To understand the formation mechanism of water penetration induced fiber orientation in overflow water-assisted injection molding (OWAIM) parts of short glass fiber-reinforced polypropylene (SGF/PP), in this work, the external fields and water penetration process within the mold cavity were investigated by experiments and numerical simulations. The results showed that the difference of fiber orientation distribution in thickness direction between WAIM moldings and CIM moldings was mainly ascribed to the great external fields generated by water penetration. Besides, fiber orientation depended on the position both across the part thickness and along the flow direction. Especially in the radial direction, fiber orientation varied considerably. The results also showed that the melt temperature is the principal parameter affecting the fiber orientation along the flow direction, and a higher melt temperature significantly facilitated more fibers to be oriented along the flow direction, which is quite different from the results as previously reported in short-shot water-assisted injection molding (SSWAIM). A higher water pressure, shorter water injection delay time, and higher melt temperature significantly induced more fibers to be orderly oriented in OWAIM moldings, which may improve their mechanical performances and broaden their application scope.     

The properties of mica-filled polypropylenes

Water-ground Phlogopite micas were classified into narrow particle-size distributions containing flakes with well-defined diameters and thicknesses in order to evaluate the influence of particle size and flake aspect ratio on the mechanical properties of mica-filled polypropylenes, For the purposes of comparison, most of the injection-molded specimens contained 40 percent (by weight) mica. As expected, the flexural and tensile modulus values increased in proportion to the aspect ratio over the range from 30 to 60 to a maximum of 8 GPa. The measured tensile strengths of the mica-filled polypropylenes increased substantially as the flake diameter became smaller, but did not correlate with the flake aspect ratio. The attainable properties were frequently dependent upon the method of mixing, and considerable care was necessary to ensure proper dispersion and adequate coupling. Intensive mixing, as in a Gelimat Mixer, may cause in situ delamination and particle-size reduction of the mica filler particles, leading to a marked increase in tensile strength of the resulting composite. The mica-filled compounds could be reprocessed many times without significant loss of properties, particularly compounds having mica particles less than 40 μm in diameter. The fracture energies (notched Izod) and the heat-distortion temperatures were not appreciably influenced by the size or aspect ratios of the mica within this range. Increased fracture toughness could be achieved by reducing the mica concentration or employing a polypropylene copolymer. Guidelines are presented to indicate the preferred characteristics of mica fillers and the influence of mixing conditions on performance.     

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.     

Physical properties of moldings from liquefied wood resins

The liquefied wood resins obtained by liquefying wood in the presence of phenol using phosphoric acid as a catalyst were applied to prepare the moldings by using hexamine as a hardener. The effects of the molding conditions and the moldings' compositions on flexural properties and water-sorption kinetics of the moldings were investigated. It was found that the liquefied wood resins had satisfactory and almost uniform curing reactivity, although they were composed of different kinds of wood components. The flexural properties of the liquefied wood moldings were enhanced with an increase in the amount of combined phenol within the liquefied wood and became comparable to those of the commercial novolak when the amounts of combined phenol were larger than 75%. Furthermore, it was also found that with an increase in the content of wood fillers the flexural properties of the liquefied wood moldings were enhanced more effectively than were the cases of the commercial novolak moldings, exhibiting that the liquefied wood resins could gain a greater reinforced effect from compounding with the wood fillers than did the commercial novolak resins, and the greater the amount of combined phenol, the higher the reinforcing performance of wood fillers. In addition, water-sorption measurements and the SEM observations of the moldings indicated that the liquefied wood resins had much greater hydrophilicity than that of the novolak and revealed a greater compatibility with wood fillers. © 1995 John Wiley & Sons, Inc.     

Mesostructure development during molding of sheet molding compounds

Experiments have been carried out with sheet molding compounds in which the amount of shear developed during molding was varied by changing the charge size. The effects of pressure and temperature were also investigated. The moldings were characterized by their mesostructures and the influence of the mesostructures on the Izod toughness and flexural strength was examined. It was found that high degrees of shear resulted in fiber orientation and the spreading of the fiber bundles. However, this did not improve properties, and it was concluded that for optimum properties, small amounts of shear were desirable.     

Structure and properties of injection moldings of polypropylene/polystyrene blends

A homoisotactic polypropylene (PP) was melt blended with 0–30 wt % of three kinds of polystyrene (PS) with melt flow indexes lower than, similar to, and higher than that of PP. The blends were injection molded at cylinder temperatures of 200–280°C, and the structure and properties of the injection moldings were studied. With PS blending, although the PP molding whitened, no surface defect such as layer peeling and pearl-like appearance occurred. The rigidity and dimensional accuracy of the molding improved without much deterioration in impact strength and heat resistance. At the same time the fluidity also improved. The injection moldings of PP/PS blends did not show clear skin/core structure under a polarizing microscope. The degrees of crystallinity and crystalline c-axis orientation decreased with PS blending. PS particles were the smallest when the ratio of the viscosity of the PS to that of PP at molding shear rate was slightly lower than unity. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 1015–1027, 1997