The skin-core morphology and structure–property relationships in injection-molded polypropylene

Tensile bars of an isotactic propylene homopolymer and an ethylene–propylene copolymer were prepared by injection molding under a variety of melt temperatures and injection pressures. The effects of these processing variables on morphology and crystalline orientation were studied using optical microscopy and x-ray diffraction. Microscopy of microtomed thin sections of the tensile bars revealed the presence of three distinct crystalline zones, namely, a highly oriented nonspherulitic skin, a row or shear-nucleated spherulitic intermediate layer, and a typically spherulitic core. The thickness of the oriented skin layer is a function of the polymer melt temperature and varies inversely with temperature. The thickness of the intermediate layer varies with injection pressure, but in a complex manner. Preferred crystallite orientation in the skin and intermediate layers exerts profound effects on mechanical properties. Tensile yield strength, impact strength, and shrinkage increase with increasing combined thickness of the two oriented outer layers.     

Primary and secondary gas penetration during gas‐assisted injection molding. Part II: Simulation and experiment

Simulation and experimental studies have been carried out on the transient gas‐liquid interface development and gas penetration behavior during the cavity filling and gas packing stage in the gas‐assisted injection molding of a spiral tube cavity. The evolution of the gas/melt interface and the distribution of the residual wall thickness of skin melt along with the advancement of gas/melt front were investigated. Numerical simulations were implemented on a fixed mesh covering the entire cavity. The residual thickness of a polymer layer and the length of gas penetration in the moldings were calculated using both the simulation and model developed in Part I of this study and commercial software (C‐Mold). Extensive molding experiments were performed on polystyrene at different processing conditions. The obtained results on the gas bubble dynamics and penetration behaviors were compared with those predicted by the present simulation and C‐Mold, indicating the good predictive capability of the proposed model. Polym. Eng. Sci. 44:992–1002, 2004. © 2004 Society of Plastics Engineers.     

Fiber orientation and mechanical properties of short-fiber-reinforced injection-molded composites: Simulated and experimental results

A numerical simulation is presented that combines the flow simulation during injection molding with an efficient algorithm for predicting the orientation of short fibers in thin composite parts. Fiber-orientation state is represented in terms of a second-order orientation tensor. Fiber-fiber interactions are modeled by means of an isotropic rotary diffusion. The simulation predicts flow-aligned fiber orientation (shell region)near the surface with transversely aligned (core region) fibers in the vicinity of the mid-plane. The effects of part thickness and injection speed on fiber orientation are analyzed. Experimental measurements of fiber orientation in plaque-shaped parts for three different combinations of cavity thickness and injection speed are reported. It is found that gapwise-converging flow due to the growing layer of solidified polymer near the walls tends to flow-align the fibers near the entrance, whereas near the melt front, gapwise-diverging flow due to the diminishing solid layer tends to lign the fibers transverse to the flow. The effect of this gapwise-converging-diverging flow is found to be especially significant for thin parts molded at slower injection speeds, which have a proportionately thicker layer of solidified polymer during the filling process. If the fiber orientation is known, predictions of the anisotropic tensile moduli and thermal-expansion coefficients of the composite are obtained by using the equations for unidirectional composites and taking an orientation average. These predictions are found to agree reasonably well with corresponding experimental measurements.     

Blends containing liquid crystalline polymers: Preparation and properties of melt-drawn fibers,unidirectional prepregs,and composite laminates

This paper discusses the effect of melt drawing on the mechanical properties and morphology of liquid crystalline polymer (LCP) and thermoplastic polymer blends. Extruded fibers and films of LCP/polymer blends were melt drawn to develop uniaxial orientation of the dispersed LCP phase. The longitudinal modulus increased with increasing draw. The increase in modulus was due to higher aspect ratio of the LCP fibrils and improved molecular orientation of the LCP chains within the fibrils. Laminated composites were prepared using the extruded sheets as prepregs. The mechanical properties and the coefficient of thermal expansion (CTE) of the prepreg and the laminates agreed well with predictions from conventional composite lamination theories.     

Liquid crystalline polymer laminates

On the basis of a poly(ethylene terephthalate) copolyester containing 60 mol % p-hydroxybenzoic acid, thin liquid crystalline films (160 μm thick) are obtained by melting the polymer at 300°C and chilling at 0°C. The undrawn films obtained have a high degree of orientation as evidenced by X-ray measurements. Due to molecular orientation, these films are characterized by their excellent mechanical properties. In order to avoid losses in the mechanical strength due to increase in their thickness, laminates are prepared using thin liquid crystalline films. Lamination is carried out by annealing under pressure at 170°C for 6 h, resulting in samples with excellent mechanical properties regardless of their thickness. A method is proposed that makes it possible to combine the unique mechanical properties of thin films of liquid crystalline polymers with a lamination process in order to obtain thick and very strong materials.     

A gradient structure formed in injection‐molded polycarbonate in situ hybrid composites and its corresponding performances

Three polycarbonate (PC) composites that were reinforced, respectively, with liquid crystalline polymer (LCP), glass fibers, and both of them were prepared by a single injection‐molding process. The role of LCP in improving the processibility of the composites was characterized by torque measurement test. The transitions of LCP morphology in two‐ and three‐component composites were investigated by using polarizing optical microscopy and scanning electron microscopy. The micrographs showed a skin–core gradient structure in all three systems investigated, and the addition of glass fiber to the PC/LCP blend affected the morphological transition and content distribution of dispersed LCP phase through the thickness of the injection‐molded samples. These results were correlated well with the measurements of tensile mechanical properties and dynamic mechanical analysis. How to fully use the dispersed LCP phase in PC in situ hybrid composites was discussed for the thickness change of core layer and the heterogeneous distribution of more LCP in the core. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 625–634, 2004     

The effect of copolymer composition on the spatial structural hierarchy developed in injection molded bacterial poly(3-hydroxybutyrate-co-3-hydroxyvalerate) parts

The effects of copolymer composition on structure development in injection molded bacterial poly(3-hydroxybutyrate-co-3-hydroxyvalerate)(PHBHV) parts were investigated. The increase of hydroxyvalerate co-monomer content lowers the melting temperature as it disrupts the crystalline order as a result, depth variation of melting behavior in the injection molded samples were found to depend primarily on the co monomer composition. At lower HV concentrations, the skin regions were found to exhibit a single melting peak that is also higher than those in the interior of the parts where generally bimodal melting behavior is observed. At higher HV content bimodal melting prevails throughout the injection molded parts including the skin and shear regions. This unusual behavior was attributed to the flow induced crystallization in extended chain formation at the skin and ease of inclusion of HV defects in the HB crystals that formed at slower cooling conditions in the interior creating thinner HV rich crystals with lower melting and thicker HV poor crystals with higher melting peaks.Depth profiling micro beam wide angle X-ray diffraction studies revealed that these polymers exhibit two distinct orientation behaviors depending on the distance from the surface. At the skin, invariably the chain axes are oriented along the flow direction. Beyond the transition layer located between the shear layer and core, the orientation does not disappear as expected from fast crystallizing polymers but rather preferential orientation of crystals with their a-axes along the flow direction was observed. At low HV content, the materials exhibit unusually high preferred orientation behavior throughout the thickness even for thick moldings, resembling the orientation behavior of polymers with low orientation relaxation behavior such as thermotropic liquid crystalline polymers. This is partly attributed to the unusually low injection melt temperature employed in these materials to avoid thermal degradation. The increase of HV content in the copolymers was found to change this c+a type orientation gradient across the thickness to gradual decrease of c-axis oriented crystals. This change was attributed to the decrease of crystallizability with the addition of HV and increasing melt fraction in the melt stream as the overall melting temperature decreases with the increase of HV content.     

Analysis of asymmetric morphology evolutions in iPP molded samples induced by uneven temperature field

Mold surface temperature has a strong effect on the amount of molecular orientation and morphology developed in a non‐isothermal flowing polymer melt. In this work, a well‐characterized isotactic polypropylene was injected in a rectangular mold cavity asymmetrically conditioned by a thin electric heater specifically designed. The cavity surface was heated at temperatures ranging from 80 to 160°C for different times (0.5, 8, and 18 s) after the first contact with the polymer. Asymmetrical thermal conditions have a strong influence on the melt flow, by changing its distribution along the cavity thickness, and final part deformation. The morphology distribution of the molded samples was found strongly asymmetric with complex and peculiar features. Optical and Electron microscopy confirmed the complete reorganization of the crystalline structures along the sample thickness. X‐rays analysis reveals that molecular orientation of the sample surface decreases with the mold temperature and the heating time. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2699–2712, 2016     

Effects of molding conditions on the electromagnetic interference performance of conductive ABS parts

Polymers filled with conducting fibers to prevent electromagnetic interference (EMI) performance have recently received great attention due to the requirements of 3C (computer, communication, and consumer electronics) products. In the present article, the effect of fiber content and processing parameters, including melt temperature, mold temperature, and injection velocity, on the electromagnetic interference shielding effectiveness (SE) in injection molded ABS polymer composites filled with conductive stainless steel fiber (SSF) was investigated. The influence of fiber orientation and distribution resulting from fiber content and molding conditions on EMI performance was also examined. It was found from measured results that fiber content plays a significant role in influencing part EMI SE performance. SE value can reach the highest values of approximately 40 dB and 60 dB at 1000 MHz frequency for fiber content 7 wt % and 14 wt %, respectively, under the best choice of molding conditions. Higher melt and mold temperature would increase shielding effectiveness due to a more uniform and random fiber orientation. However, higher injection velocity leading to highly‐orientated and less uniform distribution of fiber reduces shielding effectiveness. Among all molding parameters, melt temperature affects SE performance most significantly. Its influence slightly decreases as fiber content increases. Injection speed plays a secondary importance in affecting SE values, and its influence increases as fiber content increases. Upon examination of fiber distribution via optical microscope and subsequent image analysis, it was found that the fiber becomes more densely and random distributed toward the last melt‐filled region, whereas fiber exhibits less concentration around the middle way of the flow path. This can be attributed to the combined effects of fountain flow, frozen layer thickness, and gapwise melt front velocity. The results indicate that molding conditions, instead of fiber content alone, are very important on the SE performance for injection molded SSF filled ABS composites. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1072–1080, 2005     

Molecular orientation distributions during injection molding of liquid crystalline polymers: Ex situ investigation of partially filled moldings

The development of molecular orientation in thermotropic liquid crystalline polymers (TLCPs) during injection molding has been investigated using two‐dimensional wide‐angle X‐ray scattering coordinated with numerical computations employing the Larson–Doi polydomain model. Orientation distributions were measured in “short shot” moldings to characterize structural evolution prior to completion of mold filling, in both thin and thick rectangular plaques. Distinct orientation patterns are observed near the filling front. In particular, strong extension at the melt front results in nearly transverse molecular alignment. Far away from the flow front shear competes with extension to produce complex spatial distributions of orientation. The relative influence of shear is stronger in the thin plaque, producing orientation along the filling direction. Exploiting an analogy between the Larson–Doi model and a fiber orientation model, we test the ability of process simulation tools to predict TLCP orientation distributions during molding. Substantial discrepancies between model predictions and experimental measurements are found near the flow front in partially filled short shots, attributed to the limits of the Hele–Shaw approximation used in the computations. Much of the flow front effect is however “washed out” by subsequent shear flow as mold filling progresses, leading to improved agreement between experiment and corresponding numerical predictions. POLYM. ENG. SCI.,, 2011. © 2011 Society of Plastics Engineers     

Processability and mechanical performance of hybrid composites based on poly(ether sulfone) modified with a glass fiber–reinforced liquid crystalline polymer

Hybrid composites, based on poly(ether sulfone) (PES) and glass fiber–reinforced copolyester liquid crystalline polymer (gLCP) up to 40% gLCP, were obtained by injection molding: these polymers were immiscible. Despite its higher viscosity, the gLCP acted as a processing aid for PES. The Young's modulus of the composites increased linearly with gLCP content, attributed to the opposing effects of increasing skin thickness and decreasing orientation of the fibrillated LCP in the skin. The break properties decreased with increasing gLCP content, mainly because of the lack of adhesion between the phases. The notched impact strength increased substantially on addition of 10% gLCP, suggesting that the dispersed rigid particles changed the fracture behavior of PES. The composite with 10% gLCP appeared to be the most attractive because it showed an increase in stiffness of 18%, 6.5‐fold impact strength, and a tensile strength similar to that of PES. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 854–860, 2004     

Orientation parameters for the specification of effective properties of heterogeneous materials

A general formulation is presented for the orientation averaging of the properties of representative volume elements of heterogeneous materials. Approximations are introduced that result in the identification of four independent orientation parameters that are required to specify a general state of three-dimensional orientation. The general relationships are reduced further for the special case of “planar” and “axial” distribution and applied to an aggregate averaging scheme for short-fiber composites.     

The thermomechanical environment and the microstructure of an injection moulded polypropylene copolymer

The microstructure of an injection moulding propylene copolymer is varied through systematic changes on the processing conditions (melt and mould temperatures and injection flow rate). The skin-core structure was characterised by several experimental techniques. The skin ratio was assessed by polarised light microscopy. The morphological features of the skin layer (level of crystalline phase orientation, degree of crystallinity, β-phase content and double texture) were evaluated by wide-angle X-ray diffraction. The core features (degree of crystallinity and lamella thickness) were assessed by differential scanning calorimetry. The thermomechanical environment imposed during processing was characterised by mould filling simulations. The thermal and shear stress levels were evaluated by a cooling index and the wall shear stress. The results show the relationship between these and the microstructural features. The microstructure development is then interpreted considering the constrictions imposed during processing, being assessed by thermomechanical indices. Furthermore, the direct connections between these indices and the degree of crystallinity of the core and the level of orientation of the skin are verified.     

Primary and secondary gas penetration during gas‐assisted injection molding. Part I: Formulation and modeling

A theoretical study has been carried out on the transient gas‐liquid interface development and gas penetration behavior during the cavity filling and gas packing stage in the gas‐assisted injection molding (GAIM) of a tube cavity. A mathematical formulation describing the evolution of the gas/melt interface and the distribution of the residual wall thickness of skin melt along with the advancement of gas/melt front is presented. The physical model is put forward on the basis of Hele‐Shaw approximation and interface kinematics and dynamics. Numerical simulation is implemented on a fixed mesh covering the entire cavity. The model and simulation can deal with both primary and secondary gas penetrations. The predicted and measuredresults are compared in Part II of this study to validate the theoretical model. Polym. Eng. Sci. 44:983–991, 2004. © 2004 Society of Plastics Engineers.     

Optical properties of high‐density polyethylene in injection press molding for IR system lenses

An experimental investigation of injection press molding (IPM) was conducted to assess high infrared radiation (IR) transmittance with an opaque state (low‐visibility ray (VR) transmittance) necessary for IR system lenses as a target high‐density polyethylene (HDPE) IR transmission material. The changed conditions were the cavity open distance and delay time considering the polymer melt flowability. Other molding conditions were held constant. Mold surface roughnesses of two kinds were used. Data for IR and VR transmittance were evaluated using measurements or observation results obtained for surface roughness, thickness, differential scanning calorimetry (DSC), crystallinity, and the internal structure. Results show that the surface roughness and thickness of molded parts did not influence IR or VR transmittance. For thin skin layers with low orientation of molecular chains, the IR transmittance was higher for longer delay times. For low molecular chain orientation in the shear–core layer, the VR transmittance was also low. The molecular chain orientation differed depending on IPM conditions. Setting a longer delay time produced a uniform distribution of the molded part thickness. Furthermore, thickness became a constant value when a mold with high surface roughness was used. POLYM. ENG. SCI., 2017. © 2017 Society of Plastics Engineers     

Numerical simulation and experimental investigation of injection mold filling with melt solidification

A two-phase model is presented for simulating the injection mold filling process including the effect of transient melt solidification, i.e., the phase change effect. The liquid region is governed by Hele-Shaw flow for a non-Newtonian fluid using a modified Cross model to describe viscosity under non-isothermal conditions. Further, the energy equation of the solid phase is dominated by a transient condition. The interfacial energy balance equation is also proposed to predict the solidified layer thickness and temperature profile. Two well-characterized semicrystalline materials, polypropylene and polyethylene, were used in the present work. Good agreement is obtained between the predicted results and experimental observations from this study and the previous literature concerning the thickness of solid layer, the shape of, advancing melt front, and the pressure traces. In particular, the predicted pressure based upon the two-phase model is higher than that in terms of the single-phase model by about 13 percent. Finally, the semicrystalline structure of the frozen skin layer and the central core were investigated with a scanning electron microscope to verify the two-phase model.     

Finite element analysis of polymer melt flow in cable-covering crossheads

A finite element method is presented for the analysis of isothermal non-Newtonian polymer melt flow in narrow channels of complex shape. The particular application considered is flow in cable-covering crossheads. The geometric flexibility of the finite element method allows a mesh of triangular elements to be constructed to suit the shape of the flow channel. Computed results obtainable from the analysis include the distribution of polymer layer thickness on the finished cable, together with the extrusion pressure required to maintain a given flow rate of melt. Some typical thickness distribution results are presented as an introduction to experimental verification of the method and its application to crosshead design.     

Distribution of higher-order structures in injection moldings of particulate-filled polypropylenes

Flexural test specimens were injection-molded from polypropylenes filled with flaky talc or spherical calcium carbonate at levels of 0-40 wt % under cylinder temperatures of 200-320°C. Distributions in the flow direction of higher-order structures such as thickness of skin layer, a*-axis-oriented component fraction [A*], and crystalline orientation functions and distributions in the thickness direction of higher-order structures such as crystallinity, β-crystal content, [A*], and crystalline orientation functions were studied. These higher-order structures are inhomogeneous in the flow and thickness directions also for injection moldings of particulate-filled polypropylenes, which strongly influences the product properties such as mechanical and thermal properties. Molecular orientation process in injection molding was theoretically analyzed from a viewpoint of growth of recoverable shear strain at the gate and its relaxation in the cavity, which could considerably well explain the influences of particulate filling and cylinder temperature on not only the mean values but also the changes in the flow and thickness directions, of the quantities such as thickness of the skin layer and crystalline orientation functions, which express the degree of molecular orientation.     

The potential mechanical response of macromolecular systems—A composite analogy

A variety of macromolecular systems including crystalline and oriented thermoplastics, block copolymers, and flexibilized thermosetting resins can realistically be viewed as composite systems. This paper examines the utility of using predictive methods developed for two-component engineering composites to predict the mechanical properties of macromolecular systems. The concepts presently available for the prediction of stiffness and expansion coefficients of short-fiber rein-forced plastics are reviewed with respect to their engineering accuracy in structural systems design. These techniques are then applied to predict the stiffness of a hybrid polymer system lying midway between an engineering composite and a crystalline polymer. The hybrid consists of a polymer matrix (butadiene-acrylonitrile copolymer) reinforced with in-situ crystallized, low-molecular-weight filler (acetanilide). Finally, the composite approach is applied to the prediction of stiffnesses and expansion coefficients of crystalline polyethylene as a function of volume fraction crystallinity and crystallite morphology.     

Hybrid particulate and fibrous injection molded composites: Carbon black/carbon fiber/polypropylene systems

The electrical properties of injection molded composite systems based on a polypropylene matrix and two types of carbonaceous fillers—carbon black (CB) and carbon fibers (CF)—were investigated. In addition to conductivity as a function of system compositions, conductivity profiles were studied. Inhomogeneous spatial distribution of CB particles in moldings containing either CB as a single filler or in combination with CF was found. Furthermore, unexpected fiber orientation transverse to the melt flow direction and disappearance of skin‐core orientation pattern, typical for injection molded fiber filled composites, were observed in the two filler samples. An amplification of the shear‐thinning behavior, characteristic for the polypropylene (PP) matrix, imposed by the inhomogeneity of the CB distribution resulting in flattening of the advancing melt front and velocity profile is suggested as underlying the observed phenomena. POLYM. COMPOS., 26:454–464, 2005. © 2005 Society of Plastics Engineers