Dynamic Packing Injection Molding Research Articles

Largely Improved Mechanical Properties of a Poly(styrene-b-isoprene-b-styrene) Thermoplastic Elastomer Prepared under Dynamic-Packing Injection Molding

In this work, a poly(styrene-b-isoprene-b-styrene) thermoplastic elastomer was processed via a novel processing technique, dynamic-packing injection molding (DPIM). The shear cessation time is variable during DPIM, and a conventional injection-molded sample (C-sample) was prepared for comparison. For the dynamic-packing injection-molded sample (D-sample) with a short cessation time, a high Young’s modulus was obtained but with no significant enhancement of the break strength relative to the C-sample. However, a significant improvement of the break strength was achieved for the D-sample with longer cessation time. The mechanical performance of both the C-sample and D-samples was interpreted from the contribution of polyisoprene (PI, as the matrix) and polystyrene (PS, as a dispersed phase) segments. It is proposed that a short cessation time (strong shear) can induce the formation of a stretched PI network, while prolonged cessation time (more relaxation) could result in the segregation of PS and PI microdomains and the formation of a perfect parallel orientation of the PS cylinder in a hexagonal lattice.

Preparation of polypropylene/graphite nanocomposite with the aids of rotating solid‐state mixing and dynamic packing injection molding

In this work, to achieve good dispersion state of graphite in the nonpolar intractable polymer, polypropylene (PP), two specific compounding/molding techniques, rotating solid‐state mixing (RSSM), and dynamic packing injection molding (DPIM), were used during the preparation of PP/graphite nanocomposite. The enhanced dispersion/exfoliation of graphite substance in PP matrix induced by RSSM and/or DPIM treatment was well identified by a combination of polarized optical microscopy, wide‐angle X‐ray diffraction, and scanning electronic microscopy. A comparative analysis indicated that RSSM mainly pulverizes graphite pristine‐particles into tactoids with significantly decreased diameter‐size, while DPIM offers strong melt shear force for exfoliating graphite tactoids into nanosheets whose thicknesses are about tens nanometers and length of 1 micron. Uniform dispersion of graphite nanosheets leaded to substantial increase in heterogeneous crystallization rate and mechanical properties. Our present study proposes a facile effective approach for large‐scale preparation of polymer/graphite nanocomposite with high‐performances. POLYM. COMPOS., 35:1943–1951, 2014. © 2014 Society of Plastics Engineers

Mechanical Properties and Morphology of Polypropylene/Nano-Montmorillonite Composites Prepared by Dynamic Packing Injection Molding

In this article, dynamic packing injection molding (DPIM) technology was used to prepare injection samples of pure polypropylene and its composites (PP/OMMT). The DPIM technology led to remarkable mechanical enhancement from 43.23 MPa and 26.16 J/g of conventional injection-molded PP(CIM-PP) to 60.46 MPa and 29.03 J/g of DPIM-PP for tensile strength and impact strength, respectively. More importantly, the samples containing OMMT (termed PP/OMMT) exhibited excellent dispersion under the same flow condition. This special structure gave rise to further reinforcement from 40.91 MPa and 39.23 J/g of CIM-PP/OMMT to 54.87 MPa and 168.19 J/g of DPIM-PP/OMMT for tensile strength and impact strength, respectively. The morphology of DPIM samples was characterized through differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and scanning electron microscopy (SEM). It was found that a much better dispersion of nano-OMMT was achieved by DPIM. Moreover, γ-crystal was generated in shear layer of DPIM samples. The crystallinity of PP matrix in DPIM sample increases by 7% compared to CIM sample. The excellent mechanical properties of DPIM PP/OMMT nanocomposites sample could attribute to the homogeneous distribution of OMMT particles, the intercalation or exfoliation of montmorillonite lamellas as well as the morphology change of PP matrix under the influence of dynamic shear stress.

Oscillatory shear-accelerated exfoliation of graphite in polypropylene melt during injection molding

In this study, good dispersion status of graphite in a nonpolar, intractable polymer, i.e. polypropylene (PP), was realized in melt processing by using a specific dynamic packing injection molding (DPIM) technique. The exfoliation extent of graphite increased remarkably from the skin zone to the core zone of the molded part, as confirmed by combination of WAXD, SEM and TEM analyses, indicating an accelerated exfoliation occurred during the DPIM processing. This phenomenon is due to decreased melt flow channel and increased melt viscosity as the solidification takes place from the wall into the center, which leads to greatly increased shear force. The good dispersion of graphite results in obvious reinforcements of both tensile strength and impact strength by adding moderate amount of graphite. The present study proposes a promising route for realizing the large-scale fabrication of structural parts of polymer/exfoliated-graphite nanocomposites with excellent mechanical properties.

The Morphology and Tensile Strength of High Density Polyethylene/Nano-Calcium Carbonate Composites Prepared by Dynamic Packing Injection Molding

In this article, dynamic packing injection molding (DPIM) was used to prepare pure high density polyethylene (HDPE) and its composite with nano-CaCO3 samples, whose mechanical properties were improved significantly. Compared with conventional injection molding (CIM), the enhancement of tensile strength of dynamic HDPE/ nano-CaCO3 samples are 122%. According to the SEM, WAXD, and DSC measurement, it was found that a much better dispersion of nano-CaCO3 was achieved by the technology of DPIM. More importantly, DPIM also caused an obvious increase in orientation of HDPE matrix. The crystallinity degree of HDPE in dynamic sample increased by 5.52% compared with a conventional one. The improvement in mechanical properties for dynamic HDPE/ nano-CaCO3 is attributed to the even distribution of nano-CaCO3 particles, the orientation of HDPE crystals, as well as increase of crystallinity degree under the influence of DPIM.

The effect of multiple variables on tensile property of injection‐molded polypropylene through the combination of orthogonal design and variance analysis

AbstractIn this article, a combination of orthogonal design and variance analysis was used to study the systematical effect of vibration pressure, melt temperature, packing cycle, and mold temperature on the tensile property of polypropylene in dynamic packing injection molding (DPIM). The tensile measurement results show that all variables had significant influence on the final tensile property under the present molding condition. Meanwhile, the optimal molding condition to achieve high tensile strength was also obtained in the investigated ranges. Further quantitative statistical analysis revealed the degree of influence of four variables on the tensile property of polypropylene in DPIM. The sequence of the degree of influence from maximum to minimum is as follow: vibration pressure, melt temperature, packing cycle, and mold temperature. These findings provide the experimental and statistical evidences to illustrate that the application of orthogonal design and variance analysis might be an effective approach to systematically study the effect of multiple variables on the final mechanical property. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Influence of Carbon Nanotubes on the Crystallization and Directional Tensile Behavior of High-Density Polyethylene Prepared by Dynamic-Packing Injection Molding

The influence of multi-walled carbon nanotubes (MWCNTs) on the crystallization and directional tensile properties of high-density polyethylene (HDPE) was studied for samples prepared by dynamic-packing injection molding (DPIM). Oscillatory shear was imposed on the gradually cooled melt during the packing solidification stage of DPIM. For the oriented composites containing 1.8 wt% MWCNTs, the tensile fracture behavior showed typical brittle features along the flow direction (FD) and perpendicular direction (PD), which were almost the same as those that occurred in oriented pure HDPE. The elongation at break along both directions decreased due to the incorporation of MWNCTs in the oriented composites compared with the oriented pure HDPE. However, the tensile strength of the oriented HDPE/MWCNT composites was greatly improved along the FD due to the presence of carbon nanotubes; meanwhile, it was not weakened along the PD. In scanning electron microscopy observations, it was found that there were some oriented hybrid shish-kebab structures in a nanometre scale in the oriented HDPE/MWCNT composites, but not in its isotropic composites. This suggests that MWCNTs were involved in the shear-induced crystallization of HDPE. Differential scanning calorimetry measurements confirmed that the crystallinity of oriented HDPE composites with 1.8 wt% MWCNTs was higher than those of isotropic HDPE and isotropic composites, but was not obviously higher than that of oriented pure HDPE. These findings demonstrate that MWCNTs indeed affected the formation of crystalline structures, but did not greatly influence the crystallinity of HDPE under shear flow. The transition of crystalline morphology might be the reason for change in tensile behavior for the oriented HDPE/MWCNT composites compared with the oriented pure HDPE.

Morphologies and Mechanical Properties of High-Density Polyethylene Induced by the Addition of Small Amounts of Both Low- and High-Molecular-Weight Polyolefin under Shear Stress Applied by Dynamic Packing Injection Molding

This paper focuses on the mechanical properties and crystal morphology of a self-reinforced high-density polyethylene 5000S (HDPE 5000S) by simultaneously blending with 9 wt% high-molecular-weight polyethylene (HMWPE) and 9 wt% low-molecular-weight polyethylene (LMWPE) (A9) under the shear stress field which was engendered by a self-made dynamic packing injection molding (DPIM) machine. The results of mechanical properties, differential scanning calorimetry, and scanning electron microscopy characterization were as follows: (1) The tensile strength of the dynamic samples increased to 112.1 MPa, 4.85 times as much as that of static packing injection molding (SPIM) samples (23.1 MPa), as a result of realizing polyethylene's self-enhancement; (2) Shish-kebab structure was found in the dynamic samples; (3) The crystallinity of the DPIM A9 sample reached 68.6%, on increase by 18.7% compared with that of the SPIM sample. The formation of the shish-kebab structure and enhancement of mechanical properties are explained.

Effect of shearing on the orientation, crystallization and mechanical properties of HDPE/attapulgite nanocomposites

High density polyethylene (HDPE)/attapulgite (AT) nanocomposites, prepared by conventional injection molding (CIM) and dynamic packing injection molding (DPIM), were investigated with focus on AT-induced crystallization and orientation under shear. Infrared spectroscopy (FTIR) analysis showed there is no special chemical interaction between HDPE and AT, but shear induced significant changes on the material structure and properties. Differential scanning calorimetry (DSC) analysis showed strong nucleation effect by AT especially under shear. And more, shear will induce much better dispersion of AT in the DPIM sample vs. CIM. AT nanorods and lamellae of HDPE are more organized in the DPIM sample while there is only random distribution in the CIM sample. Most AT nanorods embed in the HDPE lamellae and form a brush-like hybrid structure due to shear. The shear-induced orientation will be enhanced with higher AT loading. The mechanical performance of the composites was significantly improved via DPIM.

Mechanical properties and morphology of polypropylene‐calcium carbonate nanocomposites prepared by dynamic packing injection molding

AbstractIn this article, dynamic packing injection molding (DPIM) technology was used to prepare injection samples of Polypropylene‐Calcium Carbonate (PP/CaCO3) nanocomposites. Through DPIM, the mechanical properties of PP/nano‐CaCO3 samples were improved significantly. Compared with conventional injection molding (CIM), the enhancement of the tensile strength and impact strength of the samples molded by DPIM was 39 and 144%, respectively. In addition, the tensile strength and impact strength of the PP/nano‐CaCO3 composites molded by DPIM increase by 21 and 514%, respectively compared with those of pure PP through CIM. According to the SEM, WAXD, DSC measurement, it could be found that a much better dispersion of nano‐CaCO3 in samples was achieved by DPIM. Moreover, γcrystal is found in the shear layer of the DPIM samples. The crystallinity of PP matrix in DPIM sample increases by 22.76% compared with that of conventional sample. The improvement of mechanical properties of PP/nano‐CaCO3 composites prepared by DPIM attributes to the even distribution of nano‐CaCO3 particles and the morphology change of PP matrix under the influence of dynamic shear stress. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Shear induced fiber orientation, fiber breakage and matrix molecular orientation in long glass fiber reinforced polypropylene composites

Long fiber reinforcement is well known to offer thermoplastic materials with high performances. But little work has been done to systematically investigate the effect of shear on the structures and properties of long glass fiber reinforced thermoplastics. The purpose of this work is to investigate the effect of shear on fiber orientation, fiber breakage and matrix molecular orientation in long glass fiber reinforced polypropylene composites, and to construct the structure–property relations. A so-called dynamic packing injection molding (DPIM) technique, which imposed oscillatory shear (10 s −1) on the gradually cooled melt during the packing solidification stage, was used to prepare the dynamic samples, and optical microscope (OM), scanning electron microscope (SEM), micro-Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) were used to characterize the samples. The results show that despite the well-published enhanced fiber and matrix orientation, shear will cause a remarkable fiber breakage. Combined effect of fiber breakage and orientation on the mechanical properties of the composites was qualitatively investigated and tensile properties of the dynamic samples, the conventional injection molded samples and the short glass fiber reinforced samples were compared. It is demonstrated that shear will induce more severe fiber breakage in long glass fiber reinforced polypropylene than in short glass fiber reinforced polypropylene, and that compromising the decreased strength caused by the severe fiber breakage is very difficult.

Study of the effect of organic clay on the shear‐induced crystallization of high‐density polyethylene through dynamic‐packing injection molding

AbstractOrganic nanoparticles as heterogeneous nucleators have a great effect on the crystallization of polymer matrices in nanocomposite systems, and the effect will be enhanced under shear flow. A home‐made dynamic‐packing injection molding (DPIM) device was developed to explore the effect of organic clay on the shear‐induced crystallization of high‐density polyethylene (HDPE). Differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD) and scanning electron microscopy (SEM) were used to characterize the flow‐induced crystalline structure of HDPE/clay nanocomposite injection moldings. It was found that higher crystallinity and thicker crystal planes which contribute to the improvement of mechanical properties were achieved in HDPE/clay nanocomposite samples prepared by DPIM. DSC results clearly showed that an increase of about 16% in crystallinity was achieved in dynamic HDPE/clay nanocomposite samples compared with traditional unfilled HDPE samples. WAXD confirmed that dynamic HDPE/clay nanocomposite samples had maximum crystal sizes at the (110) and (200) planes of 335 and 305 Å, respectively. SEM images indicated that the arrangement of crystalline structures in dynamic HDPE/clay samples was altered slightly compared with unfilled HDPE samples prepared using the same processing parameters. The results showed that organic clay was beneficial for increasing crystallinity and crystal size in the HDPE/clay nanocomposite system under shear flow. Meanwhile the arrangement of crystalline structures was insignificantly affected by the organic clay, and the preferred regular arrangement of lamellae could still be formed in the dynamic HDPE/clay nanocomposite system. Copyright © 2010 Society of Chemical Industry

MORPHOLOGY AND MECHANICAL PROPERTIES OF PA1010/PP BLENDS OBTAINED VIA DYNAMIC PACKING INJECTION MOLDING

The effect of phase morphology on mechanical properties of the immiscible blends of PA1010 /PP have been studied via dynamic packing injection molding ( DPIM).The tensile strength,tensile modulus and notched impact strength were found largely increased by means of shear.When the content of PP was 20% ,the izod impact strength of dynamic samples was 21.3 kJ /m2,which was more than 3 times that for the static samples.The tensile strength of dynamic samples was 50.9 MPa when the content of PP was 20% ,which was 1.5 times that for the static samples.SEM results showed that the shear-induced morphology with core in the center and shearing layer surrounding the core was observed in the cross-section areas of the specimens.SEM results also indicated a remakably decrease of domain size and more uniform dispersion of the dynamic molded specimens,especially when the content of PP was 20% ,the domain size decreased from about 3.9 μm to about 1.4 μm.The observed higher mechanical properties in dynamic samples could be due to the effect of phase morphology under shear stress.The tensile strength of dynamic samples is higher than that of static samples in the whole composition region due to the molecular orientation induced by shear stress in DPIM,and the toughing mechanism lies in shearing-induced smaller domain size and the molecular orientation induced by shear stress in DPIM.

STUDIES ON THE CRYSTAL STRUCTURE OF POLYPROPYLENE OBTAINED BY COOLING MELT NEAR MELTING POINT UNDER SHEAR FLOW

In order to study the crystal structure of polypropylene obtained by cooling melt near melting point under shear flow,samples were prepared by using a dynamic packing injection molding (DPIM) device which was designed by ourselves. With the aid of scanning electron microscopy ( SEM ),differential scanning calorimetry (DSC) and wide-angle x-ray diffraction (WAXD),crystal stuctures and morphologies were studied from skin to shear regions in these samples obtained by cooling melt near melting point under shear flow (DL). Compared with the samples molded with traditional injection molding (SL)and the samples molded with high melt temperature( DH ),SEM results indicated that crystal structures and morphologies in DL had been greatly affected. Oriented crystal stuctures were formed from skin to shear regions in DL,and no shish-kebabs could be found in these samples. DSC results showed that the dimension of lamellae in DL is bigger than that in SL and DH. WAXD test exhibited,compared with SL and DH,the dimension of crystal plane (110,040 and 130) is not greatly changed in DL.

Hierarchical structure of injection-molded bars of HDPE/MWCNTs composites with novel nanohybrid shish–kebab

Injection-molded products usually show hierarchical structure from skin to core due to the existence of shear gradient and temperature gradient. Investigating the hierarchical structure is helpful to better understand the structure-property relationship of injection-molded sample, which is important for design and preparation of polymer products with high performance. In this work, the hierarchical structures of injection-molded bars of high-density polyethylene (HDPE)/multi-walled carbon nanotubes (MWCNTs) composite were explored by examining the microstructure and crystal morphology, layer by layer, along the sample thickness, using SEM, DSC and 2D-WAXS. To enhance the shear effect, a so-called dynamic packing injection molding (DPIM) technique was used to prepare the molded bar with high orientation level. Interestingly, SEM revealed that in the skin and core zones, the lamellae of PE anchored randomly on the surface of MWCNTs, while well-defined nanohybrid shish–kebab (NHSK) entities, in which fibrillous carbon nanotubes (CNTs) act as shish while HDPE lamellae act as kebab, exist in the oriented zone. The changed NHSK crystal structure along the thickness direction of molded bar is considered as due to the shear gradient and thermal gradient in injection molding. And the underlying origin of in situ formation of NHSK under shear effects is discussed based on experimental observations.

The effect of shear on mechanical properties and orientation of HDPE/mica composites obtained via dynamic packing injection molding (DPIM)

AbstractThe interfacial interaction and orientation of filler play important roles in the enhancement of mechanical performances for polymer/inorganic filler composites. Shear has been found to be a very effective way for the enhancement of interfacial interaction and orientation. In this work, we will report our recent efforts on exploring the development of microstructure of high density polyethylene (HDPE)/mica composites in the injection‐molded bars obtained by so‐called dynamic packing injection molding (DPIM), which imposed oscillatory shear on the melt during the solidification stage. The mechanical properties were evaluated by tensile testing and dynamic mechanical analysis (DMA), and the crystal morphology, orientation, and the dispersion of mica were characterized by scanning electron microscopy and two‐dimensional wide‐angle X‐ray scattering. Compared with conventional injection molding, DPIM caused an obvious increase in orientation for both HDPE and mica. More importantly, better dispersion and epitaxial crystallization of HDPE was observed on the edge of the mica in the injection‐molded bar. As a result, increased tensile strength and modulus were obtained, accompanied with a decrease of elongation at break. The obtained data were treated by Halpin–Tsai model, and it turned out that this model could be also used to predict the stiffness of oriented polymer/filler composites. Copyright © 2009 John Wiley & Sons, Ltd.

Correlation of Oscillation Cycles and Crystallization in HDPE Blends with Small Amounts of HMWPE Prepared by Dynamic-Packing Injection Molding

The effect of oscillation cycles on crystal morphology was investigated for high-density polyethylene (HDPE) in blends with 4 wt% high molecular weight polyethylene (HMWPE) (labeled B4) in samples prepared through dynamic-packing injection molding (DPIM). With the aid of differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), a weblike shish-kebab morphology that markedly increased stiffness and toughness was found at a specific oscillation cycle. The DSC and SEM results showed that crystal morphology was altered with changes in the oscillation cycle. The SEM and TEM results showed that a much better weblike shish-kebab structure, in which most of the lamellae connect different columns compared with conventional shish-kebabs, was formed in the B4 samples when the oscillation cycle was 10s. These results show that a proper oscillation cycle favors the improvement of crystal structures in HDPE blends induced by a small amount of HMWPE.

Mechanical Properties and Crystal Structure of HDPE Induced by Small Amount of High-molecular-weight and Low-molecular-weight Polyethylene under Shear Stress Produced by Dynamic Packing Injection Molding

In order to better understand the effect of small amount of both high-molecular-weight polyethylene (HMWPE) and low-molecular-weight polyethylene (LMWPE) 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 samples and its blends ones with different contents of HMWPE and LMWPE. The experiment substantiated that the further improvement of tensile strength and impact stength along the flow direction (MD) of HDPE/HMWPE/LMWPE samples was achieved, while the tensile strength along the transverse direction (TD) still substantially exceeded that of conventional molding. When the contents of HMWPE and LMWPE were respectively 8% in blends, the tensile strength in both flow and transverse directions of the samples were highly enhanced, with improvements from 27.75 MPa to 115.43 MPa (about 316%), in MD and from 23MPa to 32.74 MPa (about 42.34%), in TD; besides the impact strength was improved from 21.55 KJ/m2 to 72.6 KJ/m2 (about 236.89%), in MD but decreased from 17 KJ/m2 to 6.92 KJ/m2 in TD. The obtained samples were characterized via DSC, WAXD and SEM. For HDPE/HMWPE/LMWPE, the shish-kebab structure which is composed of stretched chains (shish) and lamellae (kebab) was seen in the oriented region of DPIM samples and the spherulites existed in the oriented region of SPIM samples. Furthermore, the appropriate amount HMWPE and LMWPE (about 8%, respectively) blended into mixture can improve the thickness and the length of lamellae, and the degree of crystallinity in shear region by DPIM which were approved by DSC and SEM, at the same time, it can also enhance the intensity of orientation of lamellae in shear region confirmed by SEM and WAXD. The reason of improvement of mechanical properties is the existence of these thicker, longer and more orientated lamellae in shear region.

Combined effect of shear and nucleating agent on the multilayered structure of injection‐molded bar of isotactic polypropylene

AbstractMolten polymers are usually exposed to varying levels of shear flow and temperature gradient in most processing operations. Many studies have revealed that the crystallization and morphology are significantly affected under shear. A so‐called “skin‐core” structure is usually formed in injection‐molded semicrystalline polymers such as isotactic polypropylene (iPP) or polyethylene (PE). In addition, the presence of nucleating agent has great effect on the multilayered structure formed during injection molding. To further understand the morphological development in injection‐molded products with nucleating agent, iPP with and without dibenzylidene sorbitol (DBS) were molded via both dynamic packing injection molding (DPIM) and conventional injection molding. The structure of these injection‐molded bars was investigated layer by layer via SEM, DSC, and 2 days‐WAXD. The results indicated that the addition of DBS had similar effect on the crystal size and its distribution as shear, although the later decreased the crystal size more obviously. The combination of shear and DBS lead to the formation of smaller spherulites with more uniform size distribution in the injection‐molded bars of iPP. A high value of c‐axis orientation degree in the whole range from the skin to the area near the core center was obtained in the samples molded via DPIM with or without DBS, while in samples obtained via conventional injection molding, the orientation degree decreased gradually from the skin to the core and the decreasing trend became more obvious as the concentration of DBS increased. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Molecular Weight Dependence of Hybrid Shish Kebab Structure in Injection Molded Bar of Polyethylene/Inorganic Whisker Composites

In our previous work, a hybrid shish kebab structure, with polyethylene (PE) crystal lamellae periodically decorated on the surface of an inorganic whisker (SMCW) and aligned approximately perpendicular to the long axis of the whisker, has been observed in the injection molded bar of PE/SMCW composites. To investigate the effect of the molecular weight of the PE matrix on the formation of the hybrid shish kebab structure and the corresponding physical properties of HDPE/SMCW composites, in this work, three types of PE with different molecular weights were used to prepare the composites. They were first melt blended and then subjected to dynamic packing injection molding (DPIM), in which the prolonged shear was exerted on the melt during the solidification stage. An obvious hybrid shish kebab (HSK) structure, with PE crystal lamellae closely packed on the surface of the SMCW, was found in the samples with a low molecular weight PE (LMW-PE) matrix and a medium molecular weight PE (MMW-PE) matrix. However, in samples with a high molecular weight PE (HMW-PE) matrix, an incomplete HSK structure with PE crystal lamellae loosely decorated on the surface of the SMCW was observed. Furthermore, DSC results indicated that SMCW served as a good nucleating agent only for the composite with a LMW-PE matrix and the nucleation efficiency decreased with increasing PE molecular weight. Correspondingly, the tensile strength of the PE/SMCW composites was significantly improved by adding SMCW for the samples with a LMW-PE or MMW-PE matrix. Especially for samples with a LMW-PE matrix, the tensile strength was remarkably enhanced by the presence of only 1 wt % SMCW. For the composites with a HMW-PE matrix, the addition of SMCW had almost no reinforcing effect on the composites. The molecular weight dependence of the formation of HSK and property enhancement was discussed on the basis of the chain mobility and crystallization capability of the PE matrix.