Crystallization behavior and molecular orientation of high density polyethylene parts prepared by gas‐assisted injection molding

The dependence of hierarchy in crystalline structures and molecular orientations of high density polyethylene parts with different molecular weights molded by gas‐assisted injection molding (GAIM) was intensively examined by scanning electron microscopy, two‐dimensional wide‐angle X‐ray scattering as well as dynamic rheological measurements. The non‐isothermal crystallization kinetics of the samples were also analyzed with a differential scanning calorimeter at various scanning rates. It was found that oriented lamellar structure, shish‐kebab and common spherulites were formed in different regions of the GAIM samples. The scanning electron microscope observations were consistent with the two‐dimensional wide‐angle X‐ray scattering results and showed that the molecular chains near the mold wall had strong orientation behavior, revealing the distribution of the shear rate of the GAIM process. The differences in crystal morphologies can be attributed to molecular weight differences as well as their responses to the external fields during the GAIM process. The formation mechanism of the shish‐kebab structure under the flow field of GAIM was also explored. Copyright © 2012 Society of Chemical Industry     

Self‐reinforcement of high‐density polyethylene/low‐density polyethylene prepared by oscillating packing injection molding under low pressure

This article reports the toughness improvement of high‐density polyethylene (HDPE) by low‐density polyethylene (LDPE) in oscillating packing injection molding, whereas tensile strength and modulus are greatly enhanced by oscillating packing at the same time. Compared with self‐reinforced pure HDPE, the tensile strength of HDPE/LDPE (80/20 wt %) keeps at the same level, and toughness increases. Multilayer structure on the fracture surface of self‐reinforced HDPE/LDPE specimens can be observed by scanning electron microscope. The central layer of the fracture surface breaks in a ductile manner, whereas the break of shear layer is somewhat brittle. The strength and modulus increase is due to the high orientation of macromolecules along the flow direction, refined crystallization, and shish‐kebab crystals. Differential scanning calorimetry and wide‐angle X‐ray diffraction find cocrystallization occurs between HDPE and LDPE. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 799–804, 1999     

A simple method for forecast of cooling time of high‐density polyethylene during gas‐assisted injection molding

Gas‐assisted injection molding (GAIM) is an innovative plastic processing technology, which was developed from the conventional injection molding, and has currently found wide industrial applications. About 70% of the whole gas‐assisted injection molding cycle is actually occupied by the cooling stage. The quality and production efficiency of molded parts are considerably affected by the cooling stage. Hence, it is necessary to study the solidification behaviors during the cooling stage. In this work, a simple experimental method was designed to simulate the solidification behaviors of high‐density polyethylene during cooling stage of GAIM. The enthalpy transformation approach, coupled with the control‐volume/finite difference techniques, was adopted to deal with the transient heat transfer problems with phase change effects. In situ measurements of the temperature decreases in the cavity were also carried out. Reasonable agreements between the experimental values and the simulated results such as cooling time, cooling rates, and temperature curves were obtained, which proved that this simple experimental method was effective. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Morphology of gas‐assisted and conventional injection molded polycarbonate/polyethylene blend

The skin‐core structure of the gas‐assisted and conventional injection molded polycarbonate (PC)/polyethylene (PE) blend was investigated. The results indicated that both the size and the shape of the dispersed PC phase depended not only on the nature of PC/PE blend and molding parameters, but also on its location in the parts. Although the gas‐assisted injection molding (GAIM) parts and conventional injection molding (CIM) part have the similar skin‐core structure, the morphology evolution of PC phase in the GAIM moldings and the CIM moldings showed completely different characteristics. In the section perpendicular to the melt flow direction, the morphology of the GAIM moldings included five layers, skin intermediate layer, subskin, core layer, core intermediate layer as well as gas channel intermediate layer, according to the degree of deformation. PC phase changed severely in the core layer of GAIM moldings, as well as in the subskin of CIM moldings. In GAIM parts, PC phase in the core layer of the nongate end changed far more intensely and aligned much orderly than that in the gate end. The morphology of PC phase in the GAIM part molded with higher gas pressure changed more severe than that in the GAIM part molded with lower gas pressure. In a word, PC phase showed more obvious fibrillation in the GAIM moldings than that in the CIM moldings. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3069–3077, 2006     

Influence of high molecular weight component on the hierarchical crystalline structures of injection‐molded bars of polyethylene

A small amount of high molecular weight molecules can have a dramatic influence on the flow‐induced crystallization kinetics and orientation of polymers. To elucidate the effects of the high molecular weight component under a real processing process, we prepared model blends in which high density polyethylene with a high molecular weight and wide molecular weight distribution was blended with a metallocene polyethylene with a low molecular weight and very narrow molecular weight distribution. To enhance the shear strength, gas‐assisted injection molding was utilized in producing the molded bars. The hierarchical structures and orientation behavior of the molded bars were intensively explored by using scanning electron microscopy and two‐dimensional wide‐angle X‐ray diffraction, focusing on effects of the high molecular weight component on the formation of the shish kebab structure. It was found that there exists a critical concentration of high molecular weight component for the formation of a shish kebab structure. The threshold was about 5.5–7.0 times larger than the chain overlap concentration, suggesting an important role of entanglements of the high molecular weight component. Moreover, the rheological properties of molten polyethylene melts were studied by dynamic rheological measurements and a critical characteristic relaxation time for shish kebab formation was obtained under the processing conditions adopted in this research. © 2013 Society of Chemical Industry     

Mold replication in injection molding of high density polyethylene

In order to deepen the mechanisms at the basis of mold surface replication onto the molded plastic surface, a novel experimental approach is proposed. Up to 20 different mold surface textures were made by machining with repetitive patterns of peaks and valleys. Mold replication tests were performed by over-molding of high density polyethylene (HDPE) on steel inserts. The surface morphology of inserts and injection molded parts was acquired by surface analyzer, and all the main roughness parameters were extracted and compared as well as the geometrical profiles. Surface morphology was also measured on molded samples after thermal relaxation at 100°C. As expected, a strong correlation was found between the roughness of mold insert and molded part over the full experimented range. Profiles on the molded surface have the same repetitive pattern of the corresponding insert surface but with lower peaks, higher valleys, and a horizontal shrinkage. Comparing molded HDPE surface profiles before and after thermal relaxation, it was observed a similar change to the one highlighted between mold insert and molded part. This occurrence suggests that the final surface appearance of the molded part is also a function of the relaxation mechanism during or immediately after injection molding.     

Structure development and interfacial interactions in high‐density polyethylene/hydroxyapatite (HDPE/HA) composites molded with preferred orientation

Composites of high‐density polyethylene (HDPE) filled with sintered and nonsintered hydroxyapatite (HA) powders, designated as HAs and HAns, respectively, were compounded by twin screw extrusion. Compounds with neoalkoxy titanate or zirconate coupling agents were also produced to improve interfacial interaction and filler dispersion in the composites. The composites were molded into tensile test bars using (i) conventional injection molding and (ii) shear‐controlled orientation in injection molding (SCORIM). This latter molding technique was used to deliberately induce a strong anisotropic character to the composites. The mechanical characterization included tensile testing and microhardness measurements. The morphology of the moldings was studied by both polarized light microscopy and scanning electron microscopy, and the structure developed was assessed by wide‐angle X‐ray diffraction. The reinforcing effect of HA particles was found to depend on the molding technique employed. The higher mechanical performance of SCORIM processed composites results from the much higher orientation of the matrix and, to a lesser extent, from the superior degree of filler dispersion compared with conventional moldings. The strong anisotropy of the SCORIM moldings is associated with a clear laminated morphology developed during shear application stage. The titanate and the zirconate coupling agents caused significant variations in the tensile test behavior, but their influence was strongly dependent on the molding technique employed. The application of shear associated with the use of coupling agents promotes the disruption of the HA agglomerates and improves mechanical performance. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2873–2886, 2002     

The relationship between mechanical properties and morphology of vibration‐injection‐molded polyethylene

This paper introduces a novel melt vibration‐injection molding. The effect of mid‐frequency melt vibration on mechanical properties was introduced, and SEM, WAXD and DSC investigations had been employed to provide evidence for explaining the relationship between mechanical properties and morphology of vibration‐injection‐molded specimens. The results show that the effect of vibration frequency is very different from that of vibration pressure amplitude. At a given vibration pressure amplitude, the increase of vibration frequency is beneficial for obtaining preferential orientation, more perfect lamellae and enhanced mechanical properties. For a given vibration frequency, increase of vibration pressure amplitude is a pre‐requisite for the achievement of a large‐scale lamella, more pronounced orientation, increase of cyrstallinity and high strength of high‐density polyethylene, but part of the toughness is lost. Copyright © 2004 Society of Chemical Industry     

Assessing the effect of processing variables on the mechanical response of polysytrene molded using vibration‐assisted injection molding process

The present work is focused on the study of vibration‐assisted injection molding (VAIM) process, using polystyrene as a model polymeric system. This recently developed polymer processing operation is based on the concept of using motion of the injection screw to apply mechanical vibration to polymer melt during the injection and packing stages of injection molding process, to control the polymer behavior at a molecular level, which would result in improvements/alterations to the mechanical behavior of molded products. In this study, the afore‐mentioned concept was verified experimentally from monotonic tensile experiments and birefringence measurements of VAIM molded polystyrene in comparison with those of conventional injection molding process. The results of our study indicate that the actual degree of strength improvement depends on at least four parameters, namely, vibration frequency, vibration amplitude, vibration duration, and the delay time between the injection start and the vibration start. Furthermore, when these parameters were optimized, as much as a 28% strength improvement was observed, accompanied by an increase in toughness. Furthermore, birefringence measurements revealed that VAIM processing significantly altered the residual stress distribution throughout final products, but it did not, however, change the material density in the products. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

Antioxidant presence in thick‐walled high‐density polyethylene materials after rotational molding

High‐density polyethylene (HDPE) sample tanks manufactured by using a rotational molding process were used in a study to determine the presence and characteristics of antioxidants at the inside and outside surfaces of the tanks. The sample tanks were manufactured by using three different processing times to create undercooked, ideal, and overcooked tanks. Differential scanning calorimetry (DSC) was used to perform oxidation induction time (OIT) studies by using specimens cut from the surfaces of the tank. The OIT portion of the analysis exposes the melted HDPE specimens to an oxygen environment at 200°C for over 2 h. The DSC monitors change in the energy transfer rate to or from the specimen because of chemical reactions. A numerical integration process was used to analyze the DSC‐OIT data and to obtain additional information about the energy levels measured during the analysis. Fourier transform infrared (FTIR) studies were also performed to determine the chemical characteristics of specimens cut from the processed tanks. Results showed increased degradation at the inside surfaces of the overcooked tanks because of a lack of antioxidants. The results also showed that metal ions from the mold wall could react with the outside surfaces of the tanks and influence the level of antioxidants at that surface. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3052–3066, 2004     

Computer simulation of the filling process in gas‐assisted injection molding based on gas‐penetration modeling

Gas‐assisted injection molding can effectively produce parts free of sink marks in thick sections and free of warpage in long plates. This article concerns the numerical simulation of melt flow and gas penetration during the filling stage in gas‐assisted injection molding. By taking the influence of gas penetration on the melt flow as boundary conditions of the melt‐filling region, a hybrid finite‐element/finite‐difference method similar to conventional‐injection molding simulation was used in the gas‐assisted injection molding‐filling simulation. For gas penetration within the gas channel, an analytical formulation of the gas‐penetration thickness ratio was deduced based on the matching asymptotic expansion method. Finally, an experiment was employed to verify this proposed simulation scheme and gas‐penetration model, by comparing the results of the experiment with the simulation. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2377–2384, 2003     

Bi‐axial self‐reinforcement of high‐density polyethylene induced by high‐molecular weight polyethylene through dynamic packing injection molding

Previously, bi‐axial self‐reinforcement of high‐density polyethylene (HDPE) was achieved through a uni‐axial shear stress field introduced by dynamic packing injection molding technology. Here, further improvement of tensile strength along the flow direction (MD) was achieved by blending a small amount of high‐molecular‐weight polyethylene (HMWPE) with HDPE, while the tensile strength along the transverse direction (TD) still substantially exceeded that of conventional moldings. Tensile strengths in both flow and transverse directions were considerably enhanced, with improvements from 23 MPa to 76 MPa in MD and from 23 MPa to 31 MPa in TD. The effect of HMWPE content and molding parameters on tensile properties was also investigated. The tensile strength along MD was highly dependent on HMWPE content, oscillating cycle, mold temperature, melt temperature and packing pressure, while that along TD was insensitive to composition and processing parameters within the selected design space. According to the stress–strain curves, samples with HMWPE produced by dynamic packing injection molding had a special tensile failure mode in MD, different from both typical plastic and brittle failure modes. There were no yielding and necking phenomena, which are characteristic during tensile testing of plastic materials, but there was still a considerably higher elongation compared to those of brittle materials. However, in TD, all dynamic injection molding samples exhibited plastic failure as did typical conventional injection molding samples. Copyright © 2006 Society of Chemical Industry     

Morphologies and mechanical properties of HDPE induced by small amount of high‐molecular‐weight polyolefin and shear stress produced by dynamic packing injection molding

To better understand the effect of a small amount of high‐molecular‐weight polyethylene (HMWPE) on the mechanical properties and crystal morphology under the shear stress field, the dynamic packing injection molding (DPIM) was used to prepare the oriented pure polyethylene and its blends with 4% HMWPE. The experiment substantiated that the further improvement of tensile strength along the flow direction (MD) of high‐density polyethylene (HDPE)/HMWPE samples was achieved, whereas the tensile strength along the transverse direction (TD) still substantially exceeded that of conventional molding. Tensile strength in both flow and TDs were highly enhanced, with improvements from 23 to 76 MPa in MD and from 23 to 31 MPa in TD, besides the toughness was highly improved. So, the samples of HDPE/HMWPE transformed from high strength and brittleness to high strength and toughness. The obtained samples were characterized via SEM and TEM. For HDPE/HMWPE, the lamellae of the one shish‐kebab in the oriented region may be stretched into other shish‐kebab structures, and one lamella enjoys two shish or even more. This unique crystal morphology could lead to no yielding and necking phenomena in the stress–strain curves of HDPE/HMWPE samples by DPIM. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Integrated simulations of structural performance,molding process,and warpage for gas‐assisted injection‐molded parts. III. Simulation of cyclic,transient variations in mold wall temperatures

Whether it is feasible to perform an integrated simulation for structural analysis, process simulation, as well as warpage calculation based on a unified CAE model for gas‐assisted injection molding (GAIM) is a great concern. In the present study, numerical algorithms based on the same CAE model used for process simulation regarding filling and packing stages were developed to simulate the cooling phase of GAIM considering the influence of the cooling system. The cycle‐averaged mold cavity surface temperature distribution within a steady cycle is first calculated based on a steady‐state approach to count for overall heat balance using three‐dimensional modified boundary element technique. The part temperature distribution and profiles, as well as the associated transient heat flux on plastic–mold interface, are then computed by a finite difference method in a decoupled manner. Finally, the difference between cycle‐averaged heat flux and transient heat flux is analyzed to obtain the cyclic, transient mold cavity surface temperatures. The analysis results for GAIM plates with semicircular gas channel design are illustrated and discussed. It was found that the difference in cycle‐averaged mold wall temperatures may be as high as 10°C and within a steady cycle, part temperatures may also vary ∼ 15°C. The conversion of gas channel into equivalent circular pipe and further simplified to two‐node elements using a line source approach not only affects the mold wall temperature calculation very slightly, but also reduces the computer time by 95%. This investigation indicates that it is feasible to achieve an integrated process simulation for GAIM under one CAE model, resulting in great computational efficiency for industrial application. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 339–351, 1999     

The hierarchical structure of water-assisted injection molded high density polyethylene: Small angle X-ray scattering study

High density polyethylene (HDPE) was molded by a new polymer processing method, that is, water-assisted injection molding (WAIM), and its hierarchical structure was studied by two-dimensional small angle X-ray scattering (SAXS). For comparison, the hierarchical structure of HDPE molded by conventional injection molding (CIM) was also characterized. The result shows that the WAIM part exhibits a distinct skin-core-water channel structure which is different from the skin-core structure for the CIM part. In the skin layer of both WAIM and CIM parts, the shish-kebab structure was formed due to the shear stress brought by melt filling, but the lamellar orientation parameter of CIM part is smaller than that of WAIM part. The spherulites with random lamellar orientation are dominant at the core of both parts owing to the low cooling rate and feeble shear stress therein. Interestingly, the shish structure and the lamellae with low level of orientation can be found at the water channel layer of WAIM part. They are attributed to the shear stress brought by water penetration. Moreover, the lamellar orientation parameter in water channel layer is smaller than that of skin layer. In addition, the long period of WAIM part first increases and then decreases with the elevating distance from the skin surface, while that of CIM part tends to increase monotonously. In a word, one can conclude that the rapid cooling rate and shear brought by the injected water have significant influence on the structural evolution for the WAIM part. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Biaxially self‐reinforced high‐density polyethylene prepared by dynamic packing injection molding. II. Microstructure investigation

Differential scanning calorimetry, wide‐angle X‐ray diffraction, small‐angle X‐ray scattering, and transmission electron microscope are employed to study the microstructure of biaxially self‐reinforced high‐density polyethylene (HDPE) prepared in uniaxial oscillating stress field by dynamic packing injection molding. The results indicate that the biaxial self‐reinforcement of HDPE is mainly due to the existence of interlocking shish‐kebab morphology (i.e., zip fastener structure), along with the orientation of lamellae and molecular chains and the enhanced crystallinity. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1591–1596, 2004     

The hierarchy structure and orientation of high density polyethylene obtained via dynamic packing injection molding

The evaluation of microstructure and crystal morphology in injected-molded bar becomes much complicated because of the existence of a shear gradient and a temperature gradient from the skin to the core of the samples. To understand the relationship between shear rate-molecular weight and oriented structure of injection molded bar, in this work, the hierarchy structure and the effect of molecular weight on the formation of shish-kebab structure were investigated by examining the lamellar structure of injection molded samples of high density polyethylene (HDPE) with different melt flow index (MFI), layer by layer, along the sample thickness. To enhance the shear effect, so-called dynamic packing injection molding (DPIM), in which the melt is firstly injected into the mold and then forced to move repeatedly in a chamber by two pistons that move reversibly with the same frequency as the solidification progressively occurs from the mold wall to the molding core part, was used to obtain the molded bar. Furthermore, a small amount of ultra-high molecular weight polyethylene (UHMWPE) was added into HDPE to explore the effect of UHMWPE on the crystal morphology and orientation. Our results indicated (1) that the overall orientation in the molded bar increased with decreased MFI, and a small amount of UHMWPE could enhance substantially HDPE orientation; (2) at the skin, there existed intertwined lamellae constituting an interlocked lamellar assembly, a typical shish-kebab structure gradually developed from the subskin-layer to the core, with increased shish content toward the center, but in the core was a spherulite-like superstructure with randomly distributed lamellae; (3) UHMWPE played an important role not only in the formation of shish, but also in the transformation from spherulite to shish-kebab oriented structure for HDPE with a low molecular weight (high MFI).     

Identification of cell‐nucleation mechanism in foam injection molding with gas‐counter pressure via mold visualization

The mechanisms of cell nucleation and growth are investigated in foam injection molding (FIM) using gas‐counter pressure (GCP). An in‐situ mold visualization technique is employed. The application of GCP suppresses cell nucleation, and prevents the blowing agent from escaping during mold‐filling. The inherent structural heterogeneity in the regular FIM can be improved because of the uniform cavity pressure when employing GCP. The cavity pressure profiles show much faster pressure‐drop rates using GCP, because the single‐phase polymer/gas mixture has a lower compressibility than the two‐phase polymer/bubble mixture. Therefore, both the cell nucleation and growth rates are significantly increased through a higher pressure‐drop rate on the removal of the GCP. The effect of GCP magnitude on the cell morphology is explored. When the GCP is lower than the solubility pressure, bimodal foaming occurs. As the GCP increases above the solubility pressure, the cell density increases because of the higher pressure‐drop rate. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4035–4046, 2016     

Mechanical properties and structure of high‐density polyethylene samples prepared by injection molding with low‐frequency vibration

To better understand the formation of different crystal structures and improve the mechanical properties of high‐density polyethylene samples, melt vibration technology, which generally includes shear vibration and hydrostatic pressure vibration, was used to prepare injection samples. Through melt vibration, the crystal structure changed from typical spherulites of the traditional injection sample to obviously orientated lamellae of vibration samples. Sizes and orientation degrees of lamellae were different according to different vibration conditions. Crystallinity degrees of vibration samples increased notably. Therefore, the tensile strength of vibration samples increased with increasing vibration frequency and vibration pressure, whereas elongation of vibration samples decreased during the first stage and then continued to increase as the vibration frequency increased. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 818–823, 2005