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1.
Microporous polysulfone hollow fibers were developed with the help of chemical blowing agents by means of extrusion. Two chemical blowing agents, azodicarbonamide and 5-phenyltetrazol, were selected, and the foam morphology dependent on the concentration of blowing agent was examined by scanning electron micrograph. By means of changing the processing parameters, e.g., temperature and screw speed, the structures of the foam, usable as membrane, can be controlled. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 69: 1753–1760, 1998 相似文献
2.
A new microcellular foam injection‐molding technology using non‐supercritical fluid physical blowing agents
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Atsushi Yusa Satoshi Yamamoto Hideto Goto Hiromasa Uezono Fumiya Asaoka Long Wang Megumi Ando Shota Ishihara Masahiro Ohshima 《Polymer Engineering and Science》2017,57(1):105-113
A new foam injection‐molding technology was developed to produce microcellular foams without using supercritical fluid (SCF) pump units. In this technology, physical blowing agents (PBA), such as nitrogen (N2) and carbon dioxide (CO2), do not need to be brought to their SCF state. PBAs are delivered directly from their gas cylinders into the molten polymer through an injector valve, which can be controlled by a specially designed screw configuration and operation sequence. The excess PBA is discharged from the molten polymer through a venting vessel. Alternatively, additional PBA is introduced through the venting vessel when the polymer is not saturated with PBA. The amount of gas delivered into the molten polymer is controlled by the gas dosing time of the injector valve, the secondary reducing pressure of the gas cylinder and the outlet (back) pressure of the venting vessel. Microcellular polypropylene foams were prepared using the developed foam injection‐molding technology with 2–6 MPa CO2 or 2–8 MPa N2. High expansion foams with an average cell size of less than 25 μm were prepared. The developed technology dispels arguments for the necessity to pressurize N2 or CO2 to the SCF to prepare microcellular foams. POLYM. ENG. SCI., 57:105–113, 2017. © 2016 Society of Plastics Engineers 相似文献
3.
This work explores the influence of a chemical blowing agent on different aspects of producing a short glass‐fiber‐reinforced polypropylene foam, examining the rheology of the system, the developed morphology of the part, and the resulting mechanical properties. Two different forms of an endothermic blowing agent, namely powder versus masterbatch, were compared to determine their effects on the process history and properties of an injection molded part. Samples were produced on an injection molding machine between 230 and 270°C using the low‐pressure foaming technique. Rheology of the resulting plasticized melt by the two different blowing agents was measured on an in‐line rheometer, showing a greater reduction in shear viscosity for the masterbatch additive, which correspondingly reduced the extent of fiber breakage observed. The final molded samples were analyzed for their foam structure (i.e., cell size, cell density, and skin thickness) as well as the properties of the glass fibers incorporated (namely, fiber length distribution). Tensile properties were found to diminish with increasing blowing agent content, though differences were observed based on the type of CBA used despite the similarities in foam structure produced. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4696–4706, 2006 相似文献
4.
The mechanical blending of polypropylene (PP) and low density polyethylene (LDPE) is an economical and simple method for producing new polymeric materials for specific applications. However, the reduction in strain‐at‐break of the blend is one of its main shortcomings. In this study, PP/LDPE foamed parts were fabricated by conventional injection molding (CIM) with azodicarbonamide as a chemical blowing agent (CBA) and tested for tensile properties at two test speeds. Also, the fracture surfaces of the parts were investigated by scanning electron microscopy (SEM). In addition, to investigate the underlying mechanism of the super‐ductility, the tested samples were carefully analyzed and compared, and further characterized by differential scanning calorimetry and SEM. The results suggest that fabricating PP/LDPE super‐ductile parts using CIM with a CBA is feasible. The results also indicate that there is a close relationship between the mechanical properties and morphological structures, which are deeply influenced by the dosage of CBA, the PP/LDPE ratio, and the packing parameters. Furthermore, compared to conventional injection molded solid parts, the ductility of the foamed parts can be dramatically improved by the formation of microfibrils in the PP phase, which come into being under certain processing conditions. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44101. 相似文献
5.
Thermoplastic foaming within a mold cavity was visualized as it was conducted in an 85‐ton core‐back injection‐molding machine. The core‐back molding process moved a section of the mold just after injecting a molten polymer into the cavity, quickly reducing the pressure to enhance the bubble nucleation. The foaming behavior during core‐back was observed directly through the glass windows of the mold. In the experiments, impact copolymer polypropylene was foamed with carbon dioxide. The effects of the gas concentration and the core‐back rate on bubble nucleation and growth were investigated. It was experimentally confirmed that the bubbles disappeared when the cavity was fully packed and that bubble nucleation occurred when the mold plate was moved and the cavity pressure dropped. Faster core‐back rates and higher gas concentrations increased the number of bubbles while decreasing their size. To analyze the experimental results, a bubble nucleation and growth model was employed that was based on batch foaming. The numerical results were a reasonable representation of the experiments, and this study demonstrated the applicability of the conventional free foaming model to the industrial core‐back molding process. Many aspects of the foaming in the core‐back molding aresimilar to the behaviors observed by batch foaming. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers 相似文献
6.
This study presents the development of dynamic models for gas injection pressure that may be implemented in the design of control systems for gas‐injection units. A nonlinear dynamic model was first derived and then verified by experimental measurements. This was done by using a laboratory‐built, gas‐assisted injection unit. The agreement between the prediction and measurement indicates that the present nonlinear dynamic model adequately predicts the dynamic behavior of gas injection pressure during the process. Although the resulting model is useful for understanding the behavior of the process and the effects of different process variables, its complexity may cause difficulties in a real control application. Therefore, a second‐order model based on the basic characteristics of the nonlinear model was proposed to approximate the gas injection pressure. In order to determine the model parameters, the algorithm of recursive least‐square system identification was employed. A comparison of simulated results of an identified model with experimental data showed that the model accurately predicted the transient behavior of gas injection pressure. Consequently, this low‐order model can be easily implemented into the control system design of a gas‐injection unit. 相似文献
7.
This study was aimed at understanding how the process conditions affect the weld‐line strength and microstructure of injection molded microcellular parts. A design of experiments (DOE) was performed and polycarbonate tensile test specimens were produced for tensile tests and microscopic analysis. Injection molding trials were performed by systematically adjusting four process parameters (i.e., melt temperature, shot size, supercritical fluid (SCF) level, and injection speed). For comparison, conventional solid specimens were also produced. The tensile strength was measured at the weld line and away from the weld line. The weld‐line strength of injecton molded microcellular parts was lower than that of its solid counterparts. It increased with increasing shot size, melt temperature, and injection speed, and was weakly dependent on the supercritical fluid level. The microstructure of the molded specimens at various cross sections were examined using scanning electron microscope (SEM) and a light microscope to study the variation of cell size and density with different process conditions. 相似文献
8.
Wood‐fiber‐reinforced plastic profiles are growing rapidly in nonstructural wood‐replacement applications. Most manufacturers are evaluating new alternative foamed composites, which are lighter and more like wood. Foamed wood composites accept screws and nails better than their nonfoamed counterparts, and they have other advantages as well. For example, internal pressures created by foaming give better surface definition and sharper contours and corners than nonfoamed profiles have. In this study, the microfoaming of polypropylene (PP) containing hardwood fiber was performed with an injection‐molding process. The effects of different chemical foaming agents (endothermic, exothermic, and endothermic/exothermic), injection parameters (the mold temperature, front flow speed, and filling quantity), and different types of PP (different melt‐flow indices) on the density, microvoid content, physicomechanical properties, surface roughness, and microcell classification of microfoamed PP/wood‐fiber composites were studied. A maleic anhydride/polypropylene copolymer (MAH‐PP) compatibilizer was used with the intention of improving the mechanical properties of microfoamed composites. The microcell classification (from light microscopy) and scanning electron micrographs showed that an exothermic chemical foaming agent produced the best performance with respect to the cell size, diameter, and distance. The polymer melt‐flow index and the variation of the injection parameters affected the properties and microstructure of the microfoamed composites. The density of the microfoamed hardwood‐fiber/PP (with a high melt‐flow index) composites was reduced by approximately 30% and decreased to 0.718 g/cm3 with an exothermic chemical foaming agent. Tensile and flexural tests were performed on the foamed composites to determine the dependence of the mechanical properties on the density and microvoid content of the foamed specimens, and these properties were compared with those of nonfoamed composites. MAH‐PP improved the physicomechanical properties up to 80%. With an increase in the mold temperature (80–110°C), the surface roughness was reduced by nearly 70% for the foamed composites. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1090–1096, 2005 相似文献
9.
Identification of cell‐nucleation mechanism in foam injection molding with gas‐counter pressure via mold visualization
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Vahid Shaayegan Lun Howe Mark Chul B. Park Guilong Wang 《American Institute of Chemical Engineers》2016,62(11):4035-4046
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 相似文献
10.
Fabrication of highly expanded thermoplastic polyurethane foams using microcellular injection molding and gas‐laden pellets
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Xiaofei Sun Hrishikesh Kharbas Lih‐Sheng Turng 《Polymer Engineering and Science》2015,55(11):2643-2652
Thermoplastic polyurethane (TPU) is one of the most widely used and versatile thermoplastic materials. TPU foams have been extensively applied in various industries including the furniture, automotive, sportswear, and packaging industries. In this study, two methods of producing highly expanded TPU injection molded foam were investigated: (1) microcellular injection molding (MIM) with N2 as a blowing agent, and (2) a novel gas‐laden pellet/MIM combined process with N2 and CO2 as co‐blowing agents. Two designs of experiments (DOEs) were performed to learn the influences of key processing parameters and to optimize foam quality. By using N2 and CO2 as co‐blowing agents, a bulk density as low as 0.20 g/cm3 was successfully achieved with a hysteresis compression loss of 24.4%. POLYM. ENG. SCI., 55:2643–2652, 2015. © 2015 Society of Plastics Engineers 相似文献
11.
In this study, we developed a foaming control system using the Gas Counter Pressure (GCP) combined with mold temperature control during the microcellular injection molding (MuCell) process and investigated its influence on the parts' surface quality and foams structures. The results revealed that under GCP control alone when GCP is greater than 10 MPa, part surface roughness for transparent polystyrene (PS) improved by 90%. When GCP increased, the skin thickness also increased, the weight reduction decreased and the average cell size reduced to about 30 μm. For black PS parts, when GCP is greater than 10 MPa, the part gloss reaches the same value as that molded by conventional injection molding. By increasing gas holding time, the cell density decreased and the cell size distribution became more uniform. The increase in amount of supercritical fluid foaming agent also increased the cell density. Applying mold temperature control alone with temperature in the range of 90–120°C (near Tg), the surface roughness improved by 65%. Increasing mold temperature decreased the skin thickness; however, the cell size distribution became significantly nonuniform. It was found that thin skin, small and uniform cell size as well as good surface quality can be achieved efficiently by simultaneous combining of GCP and mold temperature control. The proposed innovative approach may lead to a significant improvement and a more broad application for MuCell process. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013 相似文献
12.
Numerical simulations have been undertaken for the film‐blowing process of viscoelastic fluids under different operating conditions. Viscoelasticity is described by an integral constitutive equation of the K‐BKZ type with a spectrum of relaxation times, which can fit the experimental data well for the shear and extensional viscosities and the normal stresses measured in shear flow. Nonisothermal conditions are considered by applying the Morland–Lee hypothesis, which incorporates the appropriate shift factor and pseudotime into the constitutive equation. The momentum and energy equations are expressed in the machine direction only by using a quasi‐one‐dimensional approach introduced earlier by Pearson and Petrie. The resulting system of differential equations is solved using the finite element method and the Newton‐Raphson iterative scheme. The method of solution was first checked against the Newtonian and Maxwell results for various film characteristics given earlier. The simulations are compared with available experimental data and previous simulations in terms of film shape, velocity, stresses, and temperature. The present results show that the existing modeling of force balances is inadequate for quantitative agreement with the experimental studies. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007 相似文献
13.
We investigated , by visual observation and numerical calculations , the foaming behavior of polypropylene within a foam injection mold cavity with the environmentally benign physical blowing agents nitrogen (N2) and carbon dioxide (CO2) . An 85‐ton core‐back injection‐molding machine with temperature and pressure monitoring systems as well as a high‐pressure view cell was used for the investigation . The experiments showed a prominent difference in bubble nucleation and growth between N2 and CO2 injection foaming . Even when the weight concentration of N2 dissolved in polymer was one‐third that of CO2 , N2 injection foaming provided a bubble number density that was 30 times larger and a bubble size that was one‐third smaller compared to CO2 injection foaming . Classical bubble nucleation and growth models developed for batch foaming were employed to analyze these experimental results . The models reasonably explained the differences in injection foaming behavior between N2 and CO2 . It was clearly demonstrated by both experiments and numerical calculations that N2 provides a higher number of bubbles with a smaller bubble size in foam injection molding compared to CO2 as a result of the lower solubility of N2 in the polymer and the larger degree of super‐saturation . POLYM. ENG. SCI., 2011. ©2011 Society of Plastics Engineers 相似文献
14.
15.
Hrishikesh Kharbas Paul Nelson Mingjun Yuan Shaoqin Gong Lih‐Sheng Turng Rick Spindler 《Polymer Composites》2003,24(6):655-671
This paper presents the effects of process conditions and nano‐clay fillers on the microstructure (namely, size, density, and distribution of microcells within samples) and the resulting mechanical properties of microcellular injection molded polyamide‐6 (PA‐6) nanocomposite and its neat‐resin counterpart. Based on the design of experiments (DOE) matrices, samples were molded at various supercritical fluid (SCF) levels, melt temperatures, shot sizes, melt plastication pressures (MPP), and injection speeds. These samples were then subjected to scanning electron microscope (SEM) analysis, tensile testing, and impact testing. For both materials, the microstructure and the mechanical properties of the molded samples were found to be dependent on the process conditions and presence of nano‐clay, which could serve as microcell nucleating agent. At higher weight reductions, the nanocomposite samples exhibit much smaller microcells and higher cell densities than those obtained in the neat‐resin samples. The SEM micrographs reveal noticeable differences in microcell surface roughness between the nanocomposite and the neat resin. A statistical design analysis was used to identify the optimal process conditions that would result in desirable cell size and density and, thus, better mechanical properties. For example, the highest tensile strengths have been observed at the highest levels of shot size, MPP, injection speed, and SCF level, and at the lowest level of melt temperature. 相似文献
16.
Three different types of characteristic structures‐microcells, nanoclay, and crystallite lamella‐exist in injection molded polyamide‐6 microcellular nanocomposites. These structures are in completely different scales. The spatial orientation of these microscale structures crucially determines the material's bulk properties. Based on scanning electron microscopy, transmission electron microscopy, and two‐dimensional X‐ray diffractometry measurements, it was found that the nanoclay and the crystallite formed special geometric structures around the microcells and near the part skins. The nanoclay platelets lay almost parallel to the surfaces of the molded parts. Preferred orientation of the crystallites was induced by the presence of the nanoclay. A molecular‐based model is proposed to describe the structural hierarchy and correlations among the microcells, nanoclay, and crystallite lamella. From the small‐angle X‐ray scattering experiments, it was found that microcellular injection molding produces relatively smaller crystallite lamella than that of conventional injection molding, and that for both solid and microcellular neat resin parts the crystallite lamella thickness at the part skin is smaller than that at the core. Polarized optical microscopy results also indicated that the size of crystallites in the microcellular neat resin and nanocomposite parts is smaller than that in the corresponding solid parts. POLYM. ENG. SCI., 47:765–779, 2007. © 2007 Society of Plastics Engineers 相似文献
17.
Christian Gertz 《European Journal of Lipid Science and Technology》2004,106(11):736-745
Despite slimness mania and acrylamide scare, the market of fried products is still growing. Frying is an extremely effective way to cook food. A fried product tastes good, has a good flavour and is prepared within a few minutes. Every effort has been made to optimise the frying process. With regard to the quality of the fried food, the quality of the frying oil is very important. In the past, important characteristics of industrial frying oils were oxidative stability, high smoke point and low foaming. Nowadays, new frying fats with various additives, with a healthier fatty acid profile and higher heat stability are emerging. Emulsifiers, anti‐polymerising agents, and natural and synthetic antioxidants improve the performance during frying. Sesamol, rosemary and other natural extracts display strong stabilising effects during the frying operations. Filtration and the use of heat‐stabilising additives help to retard fat degradation and give the producer a larger time‐window for optimum frying. The effectiveness of the treatment with filter aids or mineral adsorbents and the stabilising effects of synthetic and natural agents were compared by using the Rancimat test for testing oxidative stability and the OSET (oxidative stability at elevated temperature) test to determine the stability at the frying temperature. 相似文献
18.
In this work, nonisothermal film blowing process analysis for non‐Newtonian polymer melts has been performed theoretically by using minimum energy approach and the obtained predictions were compared with both, theoretical and experimental data (internal bubble pressure, take‐up force, bubble shape, velocity and temperature profiles) taken from the open literature. For this purpose, recently proposed generalized Newtonian model depending on three principal invariants of the deformation rate tensor, D, and its absolute defined as $\sqrt {D \cdot D}$ has been used. It has been found that film blowing model predictions are in very good agreement with the corresponding experimental data. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011 相似文献
19.
In this paper the stagnation phenomenon occurred in the coat‐hanger die is investigated using a three‐dimensional finite element method to simulate the polymer fluid flow in the die. The stagnation zone is defined to evaluate the degree of the stagnation. The effects of the inlet flow rate, the slot gap, the manifold angle, and the power‐law index on the stagnation are then analyzed numerically. It is found that the manifold angle and the geometric abrupt change between the manifold and the slot have significant influence on the stagnation, and a coat‐hanger die with tear‐dropped manifolds to be capable of diminishing the stagnation. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 相似文献