Study on residual stresses of thin-walled injection molding

The residual stresses of the thin-walled injection molding are investigated in this study. It was realized that the behavior of residual stresses in injection molding parts was affected by different process conditions such as melt temperature, mold temperature, packing pressure and filling time. The layer removal method was used to measure the residual stresses at a thin-walled test sample by a milling machine. This simple method was demonstrated to be adequate for a thin-walled part. Moldings under different conditions were investigated to study the effects of the process conditions on the residual stresses of a thin-walled product using the elastic and viscoelastic models. The mold temperature was found to affect the size of the core region and residual stress on the surface layer of a thin-walled part in our studied range. The packing pressure was insensitive to the residual stresses in the studied high-pressure range. The residual stresses predicted by the viscoelastic model are about the same level and trend as compared to the experimental measurement.     

Molecular orientation induced by cooling stresses. Birefringence in polycarbonate: III. Constrained quench and injection molding

Residual stress and birefringence distributions are determined in polycarbonate samples obtained by quenching in a specially designed apparatus and by injection molding. The molecular orientation is distinguished from the thermally and pressure-induced residual stresses. The birefringence in the quenched samples is found to be positive and almost constant, independent of the quench temperature, but varying strongly with initial quench temperature between 150 and 180°C. The residual stress level, as determined by layer removal and sectioning, is very low. The birefringence distribution is mainly due to a tensile equibiaxial orientation induced by transient cooling stresses built up above T_g. The samples which are injection-molded with a high injection speed and without packing pressure display the same birefringence distribution as the quenched samples, apart from a local maximum beneath the surface due to the shear flow during filling. Apart from the flow during filling and packing, the frozen-in molecular orientation in injection-molded samples is also induced by transient thermal stresses present during vitrification. The birefringence from thermally induced orientation was found to be of comparable magnitude to that from flow-induced orientation. For a correct prediction of molecular orientation the thermal strains above T_g must therefore be included in simulation programs. Because of the low level of thermal stresses, the application of a packing pressure will lead to tensile stresses at the surface in general. © 1994 John Wiley & Sons, Inc.     

Influence of pineapple leaf fiber and it's surface treatment on molecular orientation in,and mechanical properties of,injection molded nylon composites

The objective of his work is to show that pineapple leaf fiber (PALF) can be used successfully to reinforce a high melting polymer such as nylon. One of the most important barriers to the utilization of lignocellulosic materials in polymer matrix composites is their limited temperature resistance. As a consequence, they are mostly used to reinforce low melting temperature polymers such as polyethylene and polypropylene as well as polystyrene. However, this work reveals that PALF can be used to reinforce nylon. This is because of its very low lignin content. Nylon 6/66 composites containing a fixed amount of 20 wt % PALF in the form of short and fine fibers were prepared with a laboratory twin screw extruder and then injection molded. The mechanical properties of three types of PALF, i.e. untreated, alkaline- and silane-treated, were studied. Significant improvements in modulus and heat distortion temperature were obtained. The crystalline structure and orientation in the injected composites were investigated with synchrotron wide angle x-ray scattering (WAXS). It was found that both PALF and nylon crystallites oriented well along the flow direction and this is the key factor for the improvements observed.     

CE chips fabricated by injection molding and polyethylene/thermoplastic elastomer film packaging methods

This study presents a new packaging method using a polyethylene/thermoplastic elastomer (PE/TPE) film to seal an injection-molded CE chip made of either poly(methyl methacrylate) (PMMA) or polycarbonate (PC) materials. The packaging is performed at atmospheric pressure and at room temperature, which is a fast, easy, and reliable bonding method to form a sealed CE chip for chemical analysis and biomedical applications. The fabrication of PMMA and PC microfluidic channels is accomplished by using an injection-molding process, which could be mass-produced for commercial applications. In addition to microfluidic CE channels, 3-D reservoirs for storing biosamples, and CE buffers are also formed during this injection-molding process. With this approach, a commercial CE chip can be of low cost and disposable. Finally, the functionality of the mass-produced CE chip is demonstrated through its successful separation of phiX174 DNA/HaeIII markers. Experimental data show that the S/N for the CE chips using the PE/TPE film has a value of 5.34, when utilizing DNA markers with a concentration of 2 ng/microL and a CE buffer of 2% hydroxypropyl-methylcellulose (HPMC) in Tris-borate-EDTA (TBE) with 1% YO-PRO-1 fluorescent dye. Thus, the detection limit of the developed chips is improved. Lastly, the developed CE chips are used for the separation and detection of PCR products. A mixture of an amplified antibiotic gene for Streptococcus pneumoniae and phiX174 DNA/HaeIII markers was successfully separated and detected by using the proposed CE chips. Experimental data show that these DNA samples were separated within 2 min. The study proposed a promising method for the development of mass-produced CE chips.     

On the difference in material structure and fatigue properties of nylon specimens produced by injection molding and selective laser sintering

This paper describes the influence of dynamic tension/compression loading on notched and unnotched nylon specimens fabricated by Injection Molding (IM) and Selective Laser Sintering (SLS). The main objective of this work is to analyze and describe the differences in material structure and fatigue properties of as-built nylon parts produced by IM and SLM from the same polyamide 12 powder. The differences in dimensional quality, density, surface roughness, crystal structure and crystallinity are systematically measured and linked to the mechanical fatigue properties. The fatigue properties of the unnotched SLS specimens are found to be equal to those of the unnotched IM specimens. The presence of pores in the sintered samples does not lead to rapid failure, and the microvoid coalescence failure mechanism is delayed. The notched specimens show more brittle failure and increased fatigue resistance which is caused by local notch-strengthening. The results enable improved understanding of the difference in material structure and fatigue behavior of selective laser sintered and injection molded polyamide.     

Morphological comparison of isotactic polypropylene parts prepared by micro‐injection molding and conventional injection molding

The morphological feature of microparts evolved during micro‐injection molding may differ from that of the macroparts prepared by conventional injection molding, resulting in specific physical properties. In this study, isotactic polypropylene (iPP) microparts with 200 µm thickness and macroparts with 2000 µm thickness were prepared, and their morphological comparison was investigated by means of polarized light microscopy (PLM), scanning electron microscopy (SEM), differential scanning calorimeter (DSC), and wide‐angle X‐ray diffraction (WAXD). The results presented some similarities and differences. PLM observations showed that the through‐the thickness‐morphology of micropart exhibited a similar “skin–core” structure as macropart, but presented a large fraction of shear layer in comparison to the macropart which presented a large fraction of core layer. The SEM observation of shear layer of micropart featured highly oriented shish‐kebab structure. The micropart had a more homogeneous distribution of lamellae thickness. The degree of crystallinity of the micropart was found to be higher than that of the macropart. High content of β‐crystal was found in micropart. The 2D WAXD pattern of the core layer of macropart showed full Debye rings indicating a random orientation, while the arcing of the shear layer indicates a pronounced orientation. The most pronounced arcing of the micropart indicates the most pronounced orientation of iPP chains within lamellae. Copyright © 2011 John Wiley & Sons, Ltd.     

Non-destructive measurement of cavity pressure during injection molding process based on ultrasonic technology and Gaussian process

Non-destructive measurement of the cavity pressure is of great importance for monitoring, optimizing and controlling the injection molding process. However, to date, almost all researches have relied on embedded pressure probes, and holes have to be drilled in the molds. In this paper, a non-destructive cavity pressure measurement method is proposed based on ultrasonic technology and a Gaussian process. According to the pressure-volume-temperature profile, the cavity pressure of a given polymer can be treated as a function of the density and the temperature. Moreover, the cavity pressure is significantly affected by injection hydro-cylinder pressure. Ultrasonic technology is employed to detect the variation of polymer density during injection molding. The Gaussian process is adopted to model the functional relationships between the cavity pressure, the ultrasonic signal, the mold temperature and the injection hydro-cylinder pressure. Experimental results show that the proposed Gaussian process regression model has a better modeling performance than that of the neural network regression model, and the proposed measurement method is capable of measuring the cavity pressure at different processing conditions and measurement locations during injection molding. In general, the proposed method offers several advantages: (1) non-destructive, (2) flexible, (3) no wires, (4) low-cost, and (5) health and safety, so it has great application prospects in injection molding.     

Morphology and mechanical behavior of isotactic polypropylene with different stereo‐defect distribution in injection molding

Large amount of work has been published on the isotacticity–properties relationship of isotactic polypropylene (iPP). However, the stereo‐defect distribution dependence of morphology and mechanical properties of iPP injection molding samples is still not clear. In this study, two different isotactic polypropylene (iPP) resins (PP‐A and PP‐B) with similar average isotacticity but different stereo‐defect distribution were selected to investigate the morphology evolution and mechanical properties (tensile and notching) of their injection molding samples using differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), 2D wide angle X‐ray diffraction (2D‐WAXD), and scanning electron microscope (SEM). The results of DMA showed that the molecular movement ability of PP‐A (with less uniform distribution of stereo‐defect) was stronger than that of PP‐B, meanwhile the analysis of DSC and SEM suggested that after injection molding, smaller spherullites, and crystals with higher perfection had formed in the specimens of PP‐A. The resulting of tensile properties of PP‐A were found to be better than that of PP‐B. The results of morphology evolution by SEM observation and 2D‐WAXD showed that PP‐A is more likely to occur interspherulite deformation and can disperse the tensile stress more efficiently, and therefore, its crystal structure can withstand a greater force when tensile stress is applied. On the other hand, PP‐B has larger spherulites and boundaries, and low perfection of lamellaes, and the intraspherulte deformation tend to take place. It is easier for the crystal of PP‐B to be broken up and reoriented along the tensile direction. Copyright © 2014 John Wiley & Sons, Ltd.     

The banded spherulites of iPP induced by pressure vibration injection molding

Isotactic polypropylene(i PP) samples obtained by pressure vibration injection molding(PVIM) and conventional injection molding(CIM) were studied by polarized-light microscopy(PLM), respectively. It was found that the alternating bright and dark banded spherulites were generated in the transitional region of PVIM parts. It is the first time that the banded spherulites of isotactic polypropylene were observed in polymer processing. What's more, the banded spherulites were proved to be constituted of ?-form crystal by hot stage polarized-light microscopy(HT-PLM) and wide angle X-ray diffraction(WAXD). Morphology of the banded spherulites was also studied by scaning electronical microscopy(SEM).     

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

The 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.     

Luminance and brightness field distribution of light guiding plate for backlight panel (BLP) by micro molding

This research investigates the luminance and the brightness field distribution of the microstructure of a light guiding plate (LGP) by micro injection molding (MIM) and micro injection‐compression molding (MICM). The process of manufacturing a LGP includes photo‐etching, MIM, MICM, and optical field measurement. The results show that the luminance of microstructure of LGPs produced using MICM is better than those made using MIM. The results also indicate that the most important processing parameter is the mold temperature for the luminance distribution of the LGP whether made by MIM or MICM. The maximum luminance of the LGP is 80 Nit (cd/m²) on micro molding. The brightness field distribution of the LGP made using MICM is more uniform than those made using MIM for the same processing parameters. MICM is a more suitable process than MIM for the fabrication of a LGP on a backlight panel (BLP). Copyright © 2008 John Wiley & Sons, Ltd.     

Residual stresses and birefringence in injection molding of amorphous polymers: Simulation and comparison with experiment

A physical modeling and a two‐dimensional numerical simulation of the injection‐molding of a disk cavity by using a hybrid finite element method (FEM) and finite difference method (FDM) are presented. Three stages of the injection‐molding cycle––filling, packing, and cooling––are included. The total residual stresses are taken to be a sum of the flow stresses calculated using a compressible nonlinear viscoelastic constitutive equation and the thermal stresses calculated using a linear viscoelastic constitutive equation. The total residual birefringence is taken to be the sum of the flow birefringence related to the flow stresses through the stress–optical rule, and the thermal birefringence related to the thermal stresses through the photoviscoelastic constitutive equation. The Tait equation is used to describe the P‐V‐T relationship. The simulation shows that without packing the birefringence in the surface layer of moldings, with its maximum near the surface, is caused by the frozen‐in flow birefringence (flow stresses) and in the core region by the frozen‐in thermal birefringence (thermal stresses). With packing, a second birefringence maximum appears between the center and the position of the first maximum due to flow in the packing stage. The predicted birefringence profiles and extinction angle profiles are found to be in fair agreement with corresponding measurements in literature for disk moldings. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 622–639, 2006     

Compression testing of continuous fiber reinforced polymer composites with out-of-plane fiber waviness and circular notches

We investigated the face-stabilized Open-Hole Compression (OHC) test method for evaluating the effects of fiber waviness on the compression strength of continuous carbon fiber reinforced polymer composites. Temporal evaluations of the load-deformation response, acoustic emissions and optical microscopy are used to understand the failure modes and damage progression in the OHC specimen. The failure modes observed are structurally correlated to matrix failure and kink zone formation leading to fiber fracture. The results show how the resin pocket plays a more critical role than the layup in influencing the initiation of damage in the composite specimens.     

Analysis of orientation fluctuation in fluids by small angle X-ray scattering

The small angle X-ray scattering of molecular fluids contains information on particular aspects of their orientational order. Examples are given for the case of the isotropic, nematic and cholesteric phases of mesogenic molecules. It is shown that the distribution of the molecular centers relative to the direction defined by the molecular long axes can be analysed by means of small angle X-ray scattering. An approximate expression for the circulation correlation function is given.     

Effects of stereo-defect distribution on the crystalline morphology and tensile behavior of isotactic polypropylene prepared by compression molding process

In this study, the aggregation morphology, tensile behavior, and morphology evolution during the tensile test of two isotactic polypropylene (iPP) samples with similar molecular weight and average isotacticity but different uniformities of stereo-defect distribution are investigated by differential scanning calorimetry (DSC), two-dimensional wide angle X-ray diffraction (2D-WAXD), and scanning electronic microscopy (SEM). The results revealed that the uniformity of stereo-defect distribution of iPP determines the crystalline structure and aggregation morphology, and further influences the tensile behavior and morphology evolution during the tensile test. For PP-A with less uniform stereo-defect distribution, its ability of crystallization is stronger compared with PP-B, resulting in smaller spherulite sizes, higher melting point and degree of crystallinity, and narrower distribution of lamellar thickness of the compression molding specimens. During the tensile test, mainly the inter-spherulite deformation takes place at the early stage for deformation, which further results in drastic deformation of lamellar and high degree of reorientation at the strain increases, exhibiting higher yield strength and elastic modulus, and lower elongation at break compared with PP-B; for PP-B with more uniform stereo-defect distribution, larger spherulite sizes, lower melting point and degree of crystallinity in its compression molding sample are observed. During the tensile test, intra-spherulite deformation mainly takes place, which can disperse the tensile stress more uniformly. As the strain increases, lower degree of crystalline destruction and reorientation of the crystallites take place. The yield strength and elastic modulus of PP-B is lower than PP-A, and its elongation at break is higher.     

Methodologies for the characterisation of glass fibre orientation and distribution in large components moulded from sheet molding compounds (SMC)

Sheet Moulding Compounds (SMC) made of unsaturated polyester resin and other additives, reinforced with glass fibres, have emerged as a substitute for steel automotive outer panels. For a better understanding of the compression moulding of SMC, it is necessary to characterise the flow of material and particularly of the glass fibres. The aim of this work is to develop methodologies for characterising local mass fraction and orientation of short glass fibres in compression moulded composite components such as SMC. Pyrolysis, photographs by transmission of visible light and X-ray photographs analysed with homemade software for characterising the orientation give valuable and interesting information concerning the flow of fibres.     

Characterization and corrosion behavior of injection molded 17-4 PH steel electrochemically coated with poly[trans-dichloro(4-vinylpyridine)ruthenium]

The present study describes preliminary results on the corrosion resistance of injection molded 17-4 PH stainless steel potentiostatically coated with poly {trans-[RuCl₂(vpy)₄]}, where vpy (4-vinylpyridine) acts as a ligand. The coated electrodes were characterized by scanning electron microscopy and energy dispersive spectroscopy, as well as by electrochemical techniques. The microstructural analysis indicated that the films reached up to 100 μm thickness. Cyclic voltammetry was carried out in 0.1 M tetrabutylammonium hexafluorophosphate/methyl isobutyl ketone. The results revealed a scan rate dependent wave corresponding to a Ru²⁺/Ru³⁺ redox reaction, thus confirming the presence of ruthenium in the films. Anodic polarization tests were performed in 3% NaCl in order to estimate the anodic dissolution current density and the corrosion rate of the material. The results indicated that coating the substrate improved the corrosion resistance of the material. Received: 13 May 1999 / Accepted: 14 October 1999     

Morphology evolution and crystalline structure of controlled‐rheology polypropylene in micro‐injection molding

In this paper, the microstructural evolution of controlled‐rheology polypropylene (CRPP) with different melt viscoelasticities was investigated by polarized optical microscopy, scanning electronic microscopy, differential scanning calorimeter, and wide‐angle X‐ray diffraction. It is found that a typical “skin‐core” structure formed in CRPP microparts and the thickness of oriented layer of CRPP microparts decreases notably with the addition of peroxide. The thickness of oriented layer and the distribution of different layers strongly depend on the melt flow properties and the corresponding relaxation time (λ). Furthermore, the mechanisms of the suppressed formation of oriented layers during the micro‐injection molding process are discussed mainly from the viewpoint of rheology and thermodynamics. It is revealed that the shear‐induced orientation is one of the key factors for the formation of oriented molecular structure (row nuclei). The final thickness of the oriented layer is the result of the competition between the orientation behavior and the disorientation behavior. Copyright © 2015 John Wiley & Sons, Ltd.