Effect of low‐frequency melt vibration on HDPE morphology

BACKGROUND: A low‐frequency vibration‐assisted injection‐molding (VAIM) device was developed to explore the morphology of high‐density polyethylene (HDPE) injection moldings. Scanning electron microscopy, wide‐angle X‐ray diffraction and differential scanning calorimetry were used to characterize structure‐property relationships of final products prepared under different VAIM processing conditions (vibration frequency and vibration pressure amplitude) with conventional injection molding for comparison. RESULTS: It was found that increasing the vibration frequency at constant vibration pressure amplitude was beneficial for obtaining ‘shish‐kebab’ structures in the core region of VAIM specimens, and increasing the vibration pressure amplitude at constant vibration frequency was a prerequisite for achieving HDPE specimens with large‐scale lamellas, more pronounced orientation and high crystallinity. CONCLUSION: Both preferred orientation lamellas and increased crystallinity allow one to obtain strong injection moldings with the application of the melt vibration technique. Copyright © 2009 Society of Chemical Industry     

Improving the mechanical properties of polypropylene via melt vibration

The effect of melt vibration on the mechanical properties of polypropylene prepared by low-frequency vibration-assisted injection molding (VAIM) has been investigated. With the application of melt vibration technology, the mechanical properties of polypropylene are improved. The yield strength increases with the increment of the vibration frequency, and a peak stands at a special frequency for VAIM; the elongation at break decreases first and then increases with increasing vibration frequency, and a valley stands at a special frequency. The tensile properties increase sharply at an enlarged vibration pressure amplitude with sharply decreased elongation at break. The Young's modulus and impact strength also increase with the vibration frequency and pressure vibration amplitude. When it is prepared at 59.4 MPa and 0.7 Hz, the maximal yield strength is approximately 40 MPa versus 33.7 MPa for a conventional sample; an 18.7% increase in the tensile strength is produced. Self-reinforcing and self-toughening polypropylene molded parts have been found to be prepared at a high vibration frequency or at a large pressure vibration amplitude. Scanning electron micrographs have shown that, in the vibration field, the enhancement of the mechanical properties is attributable to more pronounced spherulite orientation and increased crystallinity in comparison with conventional injection moldings. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

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     

Improved mechanical properties and structure of PP/nano‐CaCO3 blends prepared by low‐frequency vibration injection molding

BACKGROUD: Melt vibration technology was used to prepare injection samples of polypropylene (PP)/nano‐CaCO₃ blends. It is well known that nano‐CaCO₃ particles are easy to agglomerate owing to their large surface energy. Improving the distribution of nano‐CaCO₃ particles in PP/nano‐CaCO₃ blends is very important for enhancing the mechanical properties. In this work, low‐frequency vibration was imposed on the process of injection molding of PP/nano‐CaCO₃ blends. The aim of importing a vibration field was to change the crystal structure of PP as we studied previously and improve the distribution of nano‐CaCO₃ particles. Furthermore, the mechanical properties were improved. RESULTS: Through melt vibration, the mechanical properties of PP/nano‐CaCO₃ samples were improved significantly. Compared with conventional injection molding, the enhancement of the tensile strength and impact strength of the samples molded by vibration injection molding was 17.68 and 175.96%, respectively. According to scanning electron microscopy, wide‐angle X‐ray diffraction and differential scanning calorimetry measurements, it was found that a much better dispersion of nano‐CaCO₃ in samples was achieved by vibration injection molding. Moreover, the crystal structure of PP in PP/CaCO₃ vibration samples changed. The γ crystal form was achieved at the shear layer of vibration samples. Moreover, the degree of crystallinity of PP in vibration samples increased 6% compared with conventional samples. CONCLUSION: Concerning the microstructure, melt vibration could effectively change the crystal structure and increase the degree of crystallinity of PP besides improving the distribution of nano‐CaCO₃ particles. Concerning the macrostructure, melt vibration could enhance the mechanical properties. The improvement of mechanical properties of PP/nano‐CaCO₃ blends prepared by low‐frequency vibration injection molding should be attributed to the even distribution of nano‐CaCO₃ particles and the formation of γ‐PP and the increase of the degree of cystallinity. Copyright © 2007 Society of Chemical Industry     

Control of polymer properties by melt vibration technology: A review

This paper reviews the technology of melt vibration (more specifically at low frequency) to reduce viscosity during processing of plastics and to enhance mechanical performance of the solidified parts. The effect of vibration frequency and amplitude on melt viscosity is explained in terms of shear-thinning criteria. The effect of pressure and temperature on shear thinning is also reviewed to predict how these variables interfere with melt vibration. Practical applications of the principles of melt vibration are provided in injection molding, extrusion and compression molding/thermoforming, from reduction of viscosity to lowering processing temperature and pressure to the elimination of melt defects and weld lines, to the enhancement of mechanical properties, stiffness and strength, by modification of the amorphous and semicrystalline texture and orientational state. Commercially available equipments are reviewed. Results showing the effect of melt vibration during processing for two classical polymers, polystyrene and polypropylene, are discussed. The paper concludes on the remaining challenges to bring the benefits of the new technology to full commercialization.     

Mechanical properties and morphological behavior of calcium carbonate‐filled polypropylene in dynamic injection molding

A custom‐made electromagnetic dynamic injection molding machine was adopted to study the mechanical properties and morphological behavior of calcium carbonate‐filled polypropylene (PP) in a dynamic injection molding process. The influence of vibration amplitude and frequency on the mechanical properties and morphological behavior of samples was investigated using tensile tests, notched Izod impact tests, differential scanning calorimetry, and scanning electronic microscopy. The tensile stress and the impact stress for all samples investigated were found to increase in a nonlinear manner with increasing vibration amplitude and frequency. The tensile stress reached a maximum value at about 8 Hz and 0.15 mm for neat PP and PP filled with 3, 20, and 30 wt% CaCO₃. For PP filled with 40 wt% CaCO₃, the tensile stress reached a maximum value at about 12 Hz and 0.2 mm. The impact stress reached a maximum value at about 12 Hz. From DSC experiments it was shown that the melting temperature slightly increased, but no new polymeric crystalline peak appeared under the vibration force field. The CaCO₃ particles were diffused easily and distributed evenly in the PP melt under the vibration force field, so it is very useful in improving the quality of injection products. Copyright © 2006 Society of Chemical Industry     

振动参数对HDPE注塑件力学性能的影响

采用自制的振动注射试验装置注射成型高密度聚乙烯(HDPE)制件,探讨振动频率、振动压力幅度对HDPE注塑件屈服强度、断裂伸长率、拉伸弹性模量和冲击强度的影响。结果表明,振动使得注塑件的综合力学性能得到改善;不同温度下振动频率或振动压力幅度对注塑件力学性能的影响程度不同,但其影响趋势是一致的。     

Effect of dynamic shear on the microcellular foaming of polypropylene/high‐density polyethylene blends

Dynamic shear in the axial direction of a rotor was vertically superposed on the melt flow direction, and its effects on the shear rate and melt strength were investigated theoretically. Polypropylene/high‐density polyethylene blends were microcellularly foamed with different vibration parameters. The experimental results were compared with those of a theoretical analysis, and the effects of dynamic shear on the foamability and ultimate cell structure were analyzed in detail. The theoretical results showed that the shear rate and melt strength increased with an increase in the vibration amplitude and frequency. The enhanced melt strength could effectively restrict cell growth, prevent cell rupture, and improve foamability. The experimental results showed that the cell orientation decreased and the cell structure was improved when axial dynamic shear induced by rotor vibrations was superposed on the melt flow direction. Furthermore, the cell diameter decreased and the cell density increased with increases in the vibration amplitude and frequency. The experimental results were very consistent with the theoretical analysis. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Extension of the orientation region of high density polyethylene molded by gas‐assisted injection molding: control of the thermal field

Wider zones with close‐knit orientation crystals in high density polyethylene (HDPE) parts prepared via the gas‐assisted injection molding (GAIM) process were obtained under high cooling gas pressure. In this study, compressed nitrogen, as a cooling medium, was introduced to retain a high cooling rate of the polymer melt. The high gas pressure leads to fast cooling of the polymer melt, which contributes to the stability of more oriented and stretched chains during the cooling stage. Then many more oriented structures are formed. SEM shows that many more oriented structures and interlocking shish‐kebab structures are achieved in parts under highest cooling gas pressure (P3). The P3 parts possess a higher degree of orientation than the corresponding regions of parts under lowest cooling gas pressure (P1). Moreover, tensile testing indicates that, compared with P1 parts, although P3 parts have lower crystallinity, the mechanical properties are improved because of the wider orientation zone and many more interlocking shish‐kebab structures. Combining the HDPE molecular parameters with the characteristics of the GAIM flow field and temperature field, the stability of oriented or stretched chains and the formation of orientation structures in various zones of the parts were analyzed. © 2014 Society of Chemical Industry     

Effect of packing pressure on fiber orientation in injection molding of fiber‐reinforced thermoplastics

Injection molding of fiber‐reinforced polymeric composites is increasing with demands of geometrically complex products possessing superior mechanical properties of high specific strength, high specific stiffness, and high impact resistance. Complex state of fiber orientation exists in injection molding of short fiber reinforced polymers. The orientation of fibers vary significantly across the thickness of injection‐molded part and can become a key feature of the finished product. Improving the mechanical properties of molded parts by managing the orientation of fibers during the process of injection molding is the basic motivation of this study. As a first step in this direction, the present results reveal the importance of packing pressure in orienting the fibers. In this study, the effects of pressure distribution and viscosity of a compressible polymeric composite melt on the state of fiber orientation after complete filling of a cavity is considered experimentally and compared with the simulation results of Moldflow analysis. POLYM. COMPOS. 28:214–223, 2007. © 2007 Society of Plastics Engineers     

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     

Vibration-Induced Self-Reinforcement of Injection Molding Samples of High-Density Polyethylene

A pressure vibration injection machine with vibration frequency of 0–1.5 Hz and vibration pressure of 0–75 MPa was developed to investigate self-reinforcement of high-density polyethylene (HDPE). The effect of vibration frequency and vibration pressure on tensile strength and elongation of HDPE DGDA6098 vibration molding samples, which were obtained at different melt temperatures, was studied, and SEM and WAXD measurements were conducted. Experimental results showed that vibration changed the crystal structure of vibration samples and enhanced their orientation. Instead of spherulites of static samples, crystal structure of vibration samples was lamellae that was orientated along melt flow direction. When vibration frequency was high, lamella size was small and orientation degree was low. When vibration pressure was high, lamella size was large and orientation degree was high. Therefore, vibration samples were self-reinforced with increasing vibration frequency and vibration pressure, where the maximum increment of tensile strength was 41.0 %.     

Structure and physical property relationships in processed polybutene‐1

The effect of SCORIM was investigated on three grades of polybutene‐1 and one grade of ethylene–butene‐1 copolymer. The methods and processing conditions used for injection molding and the properties of the moldings are reported. Phase transformations and their relationship with mechanical properties are discussed in detail. Both, conventional and shear‐controlled orientation injection molding (SCORIM) were employed to produce moldings. SCORIM is based on the application of specific macroscopic shears to a solidifying melt. The multiple shear action enhances molecular alignment. The moldings were investigated by performing mechanical tests, fractographic analysis, differential scanning calorimetry studies, wide‐angle X‐ray diffraction, polarized light microscopy, and atomic force microscopy. The application of SCORIM improves the mechanical performance. Molecular orientation results in the formation of shish‐kebab morphology. One grade of polybutene‐1 exhibited a greater than fivefold increase in Young's modulus. The application of high cavity pressures favored the formation of the stable Form I' in polybutene‐1. The formation of Form I' led to a decrease in crystallinity and mechanical properties. However, this loss was by far smaller than the gain obtained via the formation of shish‐kebab morphology. The relationship between mechanical properties and micromorphologies of the investigated materials is explained. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 814–824, 2003     

Fatigue performance of injection‐molded short e‐glass fiber reinforced polyamide‐6,6. II. Effects of melt temperature and hold pressure

Tensile and fatigue properties of an injection molded short E‐glass fiber reinforced polyamide‐6,6 have been studied as a function of two key injection molding parameters, namely melt temperature and hold pressure. It was observed that tensile and fatigue strengths of specimens normal to the flow direction were lower than that in the flow direction, indicating inherent anisotropy caused by injection molding. Tensile and fatigue strengths of specimens with weld line were significantly lower than that without weld lines. For specimens in the flow direction, normal to the flow direction and with weld line, tensile strength and fatigue strength increased with increasing melt temperature as well as increasing hold pressure. The effect of specimen orientation on the tensile and fatigue strengths is explained in terms of the difference in fiber orientation and skin‐core morphology of the specimens. POLYM. COMPOS., 2011. © 2010 Society of Plastics Engineers.     

Self‐interference flow of an isotactic polypropylene melt in a cavity during injection molding. I. Effect of the self‐interference flow on the properties

The self‐interference flow (SIF) of a melt in a cavity during injection molding is introduced. It comes from two streams of the melt being split by a patented mold gate called a twin gate. The effects of this flow on the static and dynamic mechanical properties, thickness distribution, and shrinkage in the transverse direction (TD) of injection‐molded isotactic polypropylene parts are discussed. SIF has an influence on the static mechanical properties, especially the impact strength. There are slight increases in the tensile strength and Young's modulus and an increase of approximately 70–90% in the impact strength in comparison with the properties of samples obtained by a conventional flow process with a common pin gate. Dynamic mechanical thermal analysis studies show an increase in the storage modulus for SIF samples. Results obtained from research into the effect of the mold temperature and injection pressure on the impact strength show that the impact strength of SIF specimens has a weaker dependence on the mold temperature and injection pressure. In addition, the flow brings a more uniform thickness distribution and a smaller shrinkage in the TD to SIF samples. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2784–2790, 2003     

Effect of vibration extrusion on the structure and properties of high‐density polyethylene pipes

BACKGROUND: The axial strength of a plastic pipe is much higher than its circumferential strength due to the macromolecular orientation during extrusion. In this work, a custom‐made electromagnetic dynamic plasticating extruder was adopted to extrude high‐density polyethylene (HDPE) pipes. A vibration force field was introduced into the whole plasticating and extrusion process by axial vibration of the screw. The aim of superimposing a vibration force field was to change the crystalline structure of HDPE and improve the molecular orientation in the circumferential direction to obtain high‐circumferential‐strength pipes. RESULTS: Through vibration extrusion, the circumferential strength of HDPE pipes increased significantly, and biaxial self‐reinforcement pipes could be obtained. The maximum increase of bursting pressure and tensile yield strength was 34.2 and 5.3%, respectively. According to differential scanning calorimetry and wide‐angle X‐ray diffraction measurements, the HDPE pipes prepared by vibration extrusion had higher crystallinity, higher melting temperature, larger crystal sizes and more perfect crystals. CONCLUSION: Vibration extrusion can effectively enhance the mechanical properties of HDPE pipes, especially the circumferential strength. The improvement of mechanical properties of HDPE pipes obtained by vibration extrusion can be attributed to the higher degree of crystallinity and the improvement of the molecular orientation and of the crystalline morphology. Copyright © 2008 Society of Chemical Industry     

Effect of melt vibration on structure and mechanical properties of HDPE/nano-CaCO3 blend

Melt vibration technology was used to prepare injection sample of HDPE/nano-CaCO₃ blend, whose mechanical properties were improved significantly. Compared with conventional injection molding, the enhancements of the tensile strength and impact strength of the sample molded by vibration injection molding were 41.2 and 43.2%, respectively. According to the SEM, WAXD, and DSC measurement, it was found that a much better dispersion of nano-CaCO₃ in sample was achieved by vibration injection molding. Moreover, crystal orientation degree of matrix HDPE increased under the effect of melt vibration. The crystallinity degree of HDPE in vibration sample increased by 5.5% compared with conventional one. The improvement of mechanical properties of HDPE/nano-CaCO₃ blend prepared by low-frequency vibration injection molding attributes to the even distribution of nano-CaCO₃ particles and the orientation of HDPE crystals and increase of crystallinity degree under the influence of melt vibration.     

An electromagnetic dynamic film blowing technology for mLLDPE

Metallocene linear low‐density polyethylene (mLLDPE) has superior physical and mechanical properties. However, mLLDPE has very poor film blowing processibility. To overcome this shortcoming, an electromagnetic dynamic extruding film blowing system for mLLDPE was developed. A vibration force field was superposed on the entire extruding process through the screw. The die pressure, the screw load, and the power consumption decreased and the melt strength increased as the vibration frequency and amplitude increased, implying that the bubble stability enhanced, which resulted in the improvement of the processibility. In addition, experimental data show that the film strength in the transverse direction greatly increased and the film mechanical properties in machine and transverse directions became more uniform, so the film quality was improved finally. This rule was confirmed by using two additional materials, high‐density polyethylene and low‐density polyethylene. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 83–89, 2006     

Self-Reinforced Isotactic Polypropylene Prepared by Melt Vibration Injection Molding

Polypropylene (PP) has a good combination of properties, but at low temperatures it is friable and its impact ductility is very low. To improve impact strength, a vibration injection molding (VIM) technology was used to investigate the mechanical properties of polypropylene. Yield strength is upgraded with an increment in vibration frequency and a peak stands at a special frequency for VIM; the elongation at break and impact strength are also enhanced by increased vibration frequency. The wide-angle x-ray diffraction (WAXD) curves and the scanning electronic microscopy (SEM) micrographs have shown that, in the vibration field, the enhancement of mechanical properties is attributed to the occurrence of γ-phase crystals and more pronounced spherulite deformation than those seen in conventional injection moldings (CIM), and the smaller spherulites with the existence of β-phase crystals are favored for improving toughness. With the application of vibration injection molding, the mechanical properties of isotactic PP are improved. To prepare self-reinforcing and self-toughening polypropylene molded parts it has been concluded that high vibration frequency is required. Increasing vibration pressure amplitude obviously significantly improves the yield strength and impact strength.