Effects of processing conditions on birefringence development in injection molded parts. II. Experimental measurement

The birefringence of injection molded parts was measured using a digital photoelasticity system, which combines a digital image analysis technique and the half-fringe photoelasticity (HFP) method The effects of processing conditions, including melt temperature, mold temperature, filling time and packing pressure, on the birefringence development in the molded parts were investigated. It was found that temperature and pressure are the two dominant factors that determine the birefringence development in the parts during the molding process. Frozen-in birefringence of the molded parts decreases with increasing melt temperature, mold temperature and injection speed. Birefringence of the parts also increases with increased packing pressure, especially around the gate area. Numerical simulations using the Leonov viscoelastic fluid model predict similar dependence of birefringence of parts on processing conditions. Simulated results are also consistent with measured values.     

Effect of polymer properties on the structure of injection-molded parts

In this study, the distributions of both molecular orientation and crystallinity along the flow direction as well as across the thickness direction of injection-molded specimens of poly(ethylene terephthalate) (PET) molded at different mold temperatures were investigated. The degree of molecular orientation at the surface of the specimens was compared with that of other injected materials (polystyrene, high density polyethylene, liquid crystal polymer) showing different thermal, rheological, and crystallization characteristics. It was found that the molecular orientation at the skin layer of the molding increases with the polymer relaxation time, the rigidity of the polymer molecules, and the crystallization rate of the polymer. Moreover, in the case of PET, it was found that the crystallinity at the skin layer and in the core of the molding depends on the mold temperature. For low mold temperatures, near the gate, the maximum of crystallinity was observed at the subskin layer because of the “shear-induced crystallization” generated during the filling stage. On increasing the mold temperature, the maximum of crystallinity was found to shift to the skin layer as a result of the decrease of the thickness of this layer. For low mold temperatures, the variation of the molecular orientation in the thickness direction was found to be much the same as for the crystallinity of the polymer. This result indicates that the shear-induced crystallization process improves the degree of molecular orientation in the flow direction since it inhibits the relaxation process of the polymer molecules.     

Effect of processing conditions on the structural gradients developed in injection-molded poly(aryl ether ketone) (PAEK) parts. I. Characterization by microbeam X-ray diffraction technique

Depending on the processing conditions, poly(arylene ether ketone) exhibits unique structural gradients as a result of its thermomechanical history when it is injection-molded. Gapwise structure gradients change from a fully amorphous to multilayer amorphous–semicrystalline–amorphous and, finally, to a uniformly semicrystalline one when mold temperature is increased. When injection speed is decreased, the crystallized layers become thicker, and at very slow injection speeds, the crystalline layers developed near the two surfaces of the parts approach each other at the core. These structure variations were characterized by differential scanning calorimetry, optical microscopy, and a matrixing microbeam X-ray diffraction (MMBX) technique developed in our laboratories. The relationship between the structure gradients developed and the processing variables and the cavity geometry are discussed. © 1993 John Wiley & Sons, Inc.     

注塑制品质量的影响因素概述

注塑成型工艺过程是一个复杂大系统,影响成型过程和制品质量的因素多而复杂。本文概括了影响成型过程和制品质量的因素,并系统地从材料、产品、模具和工艺四个方面进行了分析。     

Integrated simulations of structural performance,molding process,and warpage for gas-assisted injection-molded parts. I. Analysis of part structural performance

Whether it is feasible to perform an integrated simulation for structural analysis, process simulation, as well as warpage calculation based on a unified CAE model for gas-assisted injection molding (GAIM) is a great concern. In the present study, numerical algorithms based on the same finite element mesh used for process simulation were developed to simulate the bending performance of gas-assisted injection-molded parts. Polystyrene and nylon plates designed with five different channel geometries were gas-assisted injection-molded. Part flexible strength was measured via bending tests. It was found that part stiffness basically increases linearly with the inertia moment of the plate. Gas channel design results in part structural reinforcement by introducing an additional moment of inertia determined by the shape and the dimension of the channel section as well as the hollowed-core geometry. An analysis algorithm based on VRT/DKT elements superimposed over beam elements representing gas channels of various section geometries was developed to evaluate part bending behavior. An equivalent diameter was assigned to the beam element so that both the original gas channel and the circular beam have the same moment of inertia. The simulated results were also verified with ANSYS 3-D and 2 ½-D analysis. The simulations show reasonable accuracy as compared with measured results and predictions from ANSYS. This investigation indicates that it may be feasible to achieve an integrated simulation for GAIM under one CAE model, resulting in great computational efficiency for industrial application. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 417–428, 1998     

Anisotropy in injection-molded ethylene-propylene-diene rubbers. Part I

Anisotropy and molecular orientation are well-known phenomena in the field of thermoplastics. In the case of rubber materials only a few studies have described anisotropy. Injection molding has been shown to give rise to higher anisotropy than compression molding. The anisotropy in the rubber material is assumed to be due to molecular orientation and is strengthened by carbon black. In order to understand the mechanism of anisotropy in rubber materials, an extensive study has been performed. In this paper, results from two injection-molded ethylene-propylene-diene (EPDM) rubbers, compounded both with and without carbon black, are presented. The polymers had different molecular weight distributions and the compounds were injected into center-gated 1- and 4-mm-thick disks. The properties measured in different directions were mechanical, dynamic mechanical, and swelling. These measurements show that anisotropy can be a very important factor to take into account. The origin of anisotropy is presumably the molecular orientation which arises during the filling of the mold with the rubber melt.     

Distribution of some physical properties in injection-molded thermoplastic parts

During the injection molding process, moldings undergo shear and elongational stresses. They are also exposed to thermal gradients of varying intensity. Invariably, molded parts, or at least some sections thereof, freeze before the polymer chains can relax to a random, non-oriented configuration. As a result, injection molded parts contain frozen strains and exhibit anisotropic physical properties. In the present work, a variety of experimental techniques have been employed to determine the three-dimensional variation of the following properties of injection-molded, thermoplastic, rectangular parts: density, heat shrinkage, birefringence, and tensile strength.     

Effect of processing conditions on the structure development in the welding region of injection-molded poly(arylene ether ketone). III

Effect of injection-molding process conditions including mold temperature and injection speed on the structure developed at the weld region of poly(arylene ether ketone) was investigated using MMBX (matrixing micro beam X-ray diffraction) and image analysis techniques. The polymer exhibited multilayer structural formations at and near the weld region, and these were found to be alternating highly crystalline and low crystalline regions. With the exception of the converging section of the cavity, no significant orientations were observed through the X-ray diffraction method. However, polarized light microscopy studies revealed that the bands of higher crystallinity possess higher birefringence as compared to the intervening regions. This suggested that these bands are formed under an orientational flow field. The geometrical pattern of these multilayer formations changes depending on the injection-molding condition particularly with injection speed and mold temperature. Tensile test results indicate that the samples with no weld regions generally possess higher tensile strength. This difference between the tensile properties of the samples possessing weld and no weld decreases at high injection speeds. Izod impact strength exhibited an unusual behavior: The welded parts actually possess higher impact strength. This was attributed to the formation of multilayer structures at the weld zone. © 1995 John Wiley & Sons, Inc.     

Effects of molding conditions on properties of injection-molded polycarbonates

Statistically designed experiments were carried out to study the effects of molding conditions on the properties of two types of polycarbonate, which were synthesized by the solvent process and the melt process, respectively. The properties tested in this study were classified into two groups with respect to the effect of molding conditions. One, which included birefringence, heat shrinkage at 180°C, and surface resistance to Taber abrasion, was mainly affected by stock temperature and was slightly affected by holding pressure. The other, which included resistance to solvent crack, Rockwell hardness, density, and heat shrinkage at 120°C, was affected by mold temperature and holding pressure. Mechanically isotropic moldings with a low degree of frozen orientation could be molded at a high stock temperature and at a low holding pressure, where stock temperature was more effective than holding pressure. Moldings with low residual stresses could be molded at a high mold temperature and at a low holding pressure. Essentially there was no difference in the molding conditions and properties by the method of synthesis. However, under the same molding conditions polycarbonate synthesized by the melt process gave a higher degree of frozen orientation and somewhat more rigid moldings.     

注塑件熔接痕位置预测技术的研究进展

介绍了注塑件充填过程熔接痕形成的数学模型,综述了数值模拟技术、数值模拟技术结合数学规划理论、阀式浇注塑大型注塑成型制品多浇口进料顺序控制技术等3种注塑件熔接痕位置预测、优化和控制方法,介绍了该技术的分析流程、预测方法及国内外的研究进展和应用实例,熔接痕控制技术,尤其是熔接痕消除技术的完善是今后熔接痕研究的一个重要方向。     

Effect of processing conditions on electromagnetic shielding and electrical resistivity of injection-molded polybutylene terephthalate compounds

This research introduces an analysis of the anisotropic electrical resistivity (ER) and its relation to the electromagnetic shielding effectiveness (EMSE) for two injection-molded carbon-fiber-reinforced polybutylene terephthalates (PBTs). The properties were measured for 2-mm thick injection moldings considering the effect of melt temperature, injection velocity, and flow distance. The results for one compound showed an EMSE in the range of 30–40 dB, while EMSE for a compound with lower filler content is in the range of 45–75 dB. A combination of higher temperature and higher velocity leads to an increase of EMSE for both compounds in the range of 3%–8.5%. However, the increase in flow path reduced the EMSE for both compounds up to 10%. A novel experimental apparatus was used to measure the anisotropic ER in the three directions, that is, parallel, perpendicular, and transversal to flow. It is evident that injection molding induced high anisotropy for both compound specimens, and, depending on the processing conditions, produced similar longitudinal resistivity (0.2–4 Ω.cm) but higher transversal resistivity (8–22 Ω.cm). ER properties were compared with EMSE, evidencing an inverse relation as expected. Furthermore, it was found that the longitudinal resistivity is the main contributor to the specimens shielding.     

Effect of process conditions on the weld‐line strength and microstructure of microcellular injection molded parts

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.     

Applications of n.m.r. In process development of acrylates. I. Quantitative analysis in the process for methyl acrylate

An n.m.r. method is developed for the quantitative analysis of the components in the compositions obtained in a process for the production of methyl acrylate. The average error of the method is about ± 2.0%.     

Effect of processing conditions on morphology and mechanical properties of injection-molded poly(l-lactic acid)

This work investigates the relationship among the processing, morphology, and the mechanical properties of injection-molded poly(L -lactic acid) (PLLA). Melt processing temperature, mold temperature, injection flow rate, and holding pressure were systematically changed following a design of experiments array. The thermomechanical environment imposed during processing was estimated by computer simulations for the mold-filling phase, which allows the calculation of shear stress, shear rate, and the thickness of frozen skin layer. The morphology was characterized by differential scanning calorimetry and hot recoverable strain measurements. The analysis of variance results of influence of processing factors on the morphology are in good agreement with the analysis of thermomechanical parameters on the morphology. The primary factor for inducing the crystallinity in PLLA product was the stress-induced crystallization, whereas the thermal induced crystallization had a little effect. The morphology–mechanical property relationships were established. The crystallinity developed during processing has little effect on elastic modulus, increases the yield strength, and severely decreases the elongation at break. The level of molecular orientation developed during processing has little effect on elastic modulus, but increases both the yield strength and the elongation at break. POLYM. ENG. SCI., 47:1141–1147, 2007. © 2007 Society of Plastics Engineers     

Hierarchical structure gradients developed in injection-molded PVDF and PVDF–PMMA blends. I. Optical and thermal analysis

The structural gradients developed along and across the flow direction of injection-molded PVDF and PVDF/PMMA parts were investigated by optical microscopy and thermal analysis techniques. The spatial variation of crystallinity across the thickness direction was found to be insensitive to the process variables: injection speed and mold temperature. This relatively flat crystallinity profile across the thickness of the parts was found to decrease with the increase of PMMA concentration. The blends become noncrystallizable beyond about 40–45% PMMA concentration. The influence of flow history on the structural evolution across the thickness was observed in the peak position of the cold crystallization region. This peak temperature showed a minimum at depths where shear effects are at their maximum. This was attributed to the increased levels of chain orientation frozen in the amorphous portions of these regions which crystallize at lower temperatures upon heating. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 909–926, 1998     

Numerical analysis of a reactive extrusion process. Part I: Kinetics study on grafting of vinylsilane to polyethylene

For controlling a reactive extrusion process in the subsequent study. the model equations of reaction kinetics and shear viscosity were studied. We focused on a free radical reaction between the molten polyethylene and vinylsilane. The kinetics model was expressed as a reaction rate equation with an apparent rate constant. The shear dependent of reacted polyethylene was formulated by employing the modified Cross model proposed in our early study. In addition, the average molecular weight was considered to correlate the shear viscosity with the reaction kinetics, leading to a series of rheo‐kinetics formulas. The experiments were carried out in a specific batch mixer suitably designed for sampling in arbitrary periods. Their reaction conversions, molecular weight distributions, and shear viscosity were measured, respectively, with an induced coupled plasma (ICP) emission spectrochemical analyzer, a high temperature gel permeation chromatography (GPC), and a capillary rheometer. Determining the parameters in each model, a simulator is set to investigate an engineering extrusion process.     

The influence of surface heating on the birefringence distribution in injection molded parts

In order to reduce the frozen-in orientation, which is usually present in injection molded products, the effect of high-performance mold surface heaters was investigated. The heaters are capable of changing their surface temperature by 70°C within a few tenths of a second typically, which minimizes their effect on the cooling time. The effects of the heating time, the instant of heating, and the heating power on the birefringence distribution of a polystyrene resin were studied. Reductions of the birefringence peak by a factor 4 to 7 were observed. The birefringence is removed most effectively by heating briefly before and during the injection stage. A heating pulse of about one second and with a power density of 20 W/cm² then seems to be sufficient. A minimum power density of 10 W/cm² is needed for the relaxation to occur in this specific system.     

Temperature-dependent influence of molecular orientation and internal stresses on the deformation of injection-molded polypropylene parts

Local orientation measures by infrared dichroism and thermal stress measures by dilatometric analysis of image (DAI) have made it possible to quantify the evolution of the structure versus the thickness of the injected plastic parts. The stiffness local properties on the one hand, and thermal expansion on the other hand, have then been established. Deformation measures made in temperature have pointed out its deformation induced by thermal stresses and that attributed to the gradient of molecular orientation and cristallinity of the polymer. A calculation of plastic part deformation constituted of elementary layers whose Young's modulus and coefficients of thermal expansion values have been previously evaluated, has enabled us to corroborate experimental results. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1661–1669, 1998