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The Warpage Simulation with In-Mold Constraint Effect in Injection Molding
This paper develops a new numerical approach to simulating the mold constraint phenomenon of warpage at the in-mold stage in injection-molded part of complex geometry and improves the accuracy of warpage analysis. Warpage in injection molding is the result of unequal volumetric shrinkage of material throughout the geometry of the piece as it cools from a melt to a solid state. Before the part is ejected, the deformation of warpage has been developed inside the mold. The influence of in-mold constraint for warpage is different according to the time of in-mold period. This in-mold constraint effect of warpage analysis technique is proved to improve the accuracy of warpage analysis for 3D/2.5D model. Several industrial cases with complex geometry are also studied to illustrate the capability of the proposed methodology.
Weld Lines Behaviour in Melt Blended and In-Situ Polymerised Nylon 6 Nanocomposites
Nylon 6 nanocomposites containing organically-modified montmorillonite made either by the melt blending or in-situ polymerised routes are investigated. Both nylon 6 nanocomposites are examined in X-ray diffraction and Transmission Electron Microscopy, to verify the level of intercalation / exfoliation of the organoclay layers in the nanocomposites. The materials are injected into tensile specimens either with single or double end-gated (with a weld line). Neat nylon 6 and single end-gated samples are used as a control, allowing comparison the performance of nylon 6 nanocomposites when weld line is present. The results show that the tensile strength and strain-to-failure of the neat nylon 6 exhibits little variation when comparing the single and double end-gated samples and thus no weld line behaviour is seen clearly. The melt blended nanocomposites show a greatly reduced in strain-to-failure for both single and double end-gated samples although the tensile modulus is increased significantly. In contrast, the in-situ polymerised nanocomposites show more ductile behaviour at single end-gated, compared to the double end-gated samples. Both nanocomposites show brittleness when the weld line is present. The fracture tensile surfaces are examined using Environmental Scanning Electron Microscopy and the results show that much larger micron-sized organoclay entities exist in the melt blended nanocomposites and that these, rather than nano-sized individual layers, cause the final behaviour. Whilst the dispersion is much finer for the in-situ polymerised than that of the melt blending nanocomposites.
Cell Development in Microcellular Injection Molded Polyamide-6 Nanocomposite and Neat Resin
The effects of nanoclay addition into polyamide-6 (PA-6) neat resin and processing parameters on cell density and size in microcellular injection molded components were investigated. The analyses were performed on the sprue section of standard ASTM D 638-02 tensile bars molded based on a fractional four factorial, three-level, L9 Taguchi design of experiments (DOE) with varying melt temperature, injection speed, super critical fluid (SCF) concentration, and shot size. It was found that the presence of nanoclay greatly reduced the cell size and increased the cell density when compared to neat resin processed under identical molding conditions. In addition, cell size distribution at the sprue center was, in general, the largest, gradually decreasing towards the skin for both neat resin and nanocomposite. Finally, in contrast to neat resin, in which shot size and injection speed were important to cell density and all molding parameters affected cell growth, the cell size and density for nanocomposite only depended strongly on shot size.
Core-Shell Rubber Modified Microcellular Polymide-6 Composite
This paper presents the effects of processing parameters and sub-micron core-shell rubber particle filler on the mechanical properties and cell morphology of microcellular injection molded polyamide-6 (PA-6) composites. Three types of materials were studied, namely, neat PA-6 resin, and 0.5% and 3.1% core-shell rubber polybutylacrylate-polymethylmethacrylate filled PA-6 composites. This study showed that the addition of a small amount (0.5%) of core-shell rubber particles improved the ductility and impact strength of microcellular injection molded PA-6 samples. In comparison to the microcellular injection molded PA-6 polymer-clay nanocomposite, the samples with a small amount (0.5%) of core-shell rubber had much higher impact strength and ductility. The small addition of core-shell rubber also reduced cell size and increased cell density of the microcellular injection molded PA-6 parts, in comparison to their neat resin counterparts. On the other hand, at higher core-shell rubber loading, the cell size and density were found to be similar to that of the neat resin.
Crystallization Behavior of Polyamide 6 Micocellular Nanocomposites
The crystallization behaviors of polyamide-6 and its nanocomposites undergoing the microcellular injection molding process were studied using Transmission Electron Microscopy (TEM), X-ray Diffractometer (XRD), Polarized Optical Microscopy (POM), and Differential Scanning Calorimetry (DSC). The relationships among the morphology, the mechanical property of the molded parts, and the crystallization behavior were investigated. With the addition of nanoclays in microcellular injection molded parts, the growth of ? form crystal is suppressed and the formation of ? form crystals is promoted. Both nanoclay and dissolved gas have big influence on PA-6 crystalline structures. The existence of nanoclay increases the crystallization rate. But with extra addition of nanoclays in the polymer matrix, the crystallization rate is reduced. Nanocomposites with proper amount of nanoclays posses the maximum crystallization activation energy and produce finer and denser microcell structure which leads to better mechanical properties.
The Investigation of Flow Behavior of Polymeric Melts in the Water Assisted Injection Molding
Water assisted injection molding (WAIM) is a pretty new way to fabricate hollow or more complicated parts. Basically, the process of WAIM is similar to gas assisted molding. If it is controlled correctly, WAIM can yield thinner, more uniform part walls. However, to obtain better products, a number of questions, regarding to the material properties and process conditions, need to be fully understood. In this study, the complex flow behavior of melts under the assistance of injected water is conducted by examining the coupling effects of the process conditions and the material properties numerically. Further, the verification is performed experimentally as well.
Fluid Assisted Polymer Processing in Extrusion and Injection Moulding: A Comparative Overview
A programme of research is described to compare gas, cryogenically cooled gas, water and super-critical carbon dioxide assisted polymer processing. Results indicate apparent reductions in melt viscosity are possible by adding super critical carbon dioxide. The issues associated with implementing water assisted injection moulding are highlighted. Reductions in residual wall thickness are possible in the gas assisted injection moulding process when cryogenically cooled gas is used over the conventional implementation.
A New Technique of Super Critical CO2-Asisted Surface Coating Injection Molding
A brand-new technique of injection molding using supercritical CO2 is proposed so as to modify the surface of the solidified plastic products. The principle of the CO2-asisted surface coating injection-molding techniques is described in this paper. The technique utilizes the plasticization effect of CO2 on polymer, solubility of low molecular substance in super critical CO2 and diffusion mechanism of CO2/low molecule mixture into the surface of injected polymer. To confirm the principles, experiments using dye pigment as a low molecular solute is conducted. The experimental results of CO2-asisted surface coating injection-molding techniques are given at the end.
Three-Dimensional Simulation of Transient Temperature Distribution for Lens Mold Embedded with Heaters
To avoid cost computation, the traditional simulation of mold temperature during the injection molding is usually conducted in a cycle-averaged approach plus one-dimensional transient variation. Boundary element method is frequent adopted as the major numerical scheme. In the present study, fully 3-D simulations of mold temperature variation based on both standard FEM in single CPU and VOF implemented in parallel computation scheme were executed for a lens mold embedded with heaters. Mold surface temperature distribution was measured by infrared thermal image system from which the temperature variations at interest locations can be obtained. Within steady state molding cycle, the predicted mold surface temperatures show a difference of 8° (cavity center), 6° (parting surface), and 18° (core cavity) after mold opening and part ejection. All simulations show quite precise predictions in accordance with experimental results.
A New Method for Simulation of Injection Molding
Simulation of injection molding originally used the so-called Hele-Shaw approximation in which the part is represented as a mesh of triangles, located at the midplane of the component, each of which has a defined thickness. The time required to create appropriate models for this type of analysis increased with the rapid acceptance of 3D solid geometry modeling. Several approaches to reduce the time to create a model have been introduced and include automatic midplane determination, dual domain finite element analysis (2.5D) and true 3D analysis. Each of these approaches ignores the fact that a given injection molded part often comprises a variety of regions some of which are better analyzed with 2.5D analysis while other regions require 3D analysis. In this paper we introduce a new type of simulation which automatically decomposes a 3D model into 2D and 3D regions. The appropriate analysis type is then used. The approach shows improved accuracy over 2.5D analysis yet is more efficient than full 3D analysis.
Computational Prediction of PVC Degradation during Injection Molding Radial Flow
Polyvinyl chloride (PVC) materials have been widely used because of their excellent weather ability, chemical resistance, and flame retardant properties. However, polymer degradation may occur, especially in high shear injection molding processes. A computational model has been developed to calculate the degradation of PVC during injection molding of discs. The effects of the injection speed, melt temperature, shot sizes, and material properties were examined. It was found that the injection speed is the most important factor influencing PVC degradation. Furthermore, the computational and experimental results were compared. The model could be used to help design the process to avoid degradation.
Numerical Simulation for Modeling Non-Isothermal Cell Growth in Microcellular Injection Molding
This paper presents the mathematical modeling and numerical simulation of non-isothermal cell growth during the post-filling stage of microcellular injection molding. The model combines two numerical techniques, namely, finite-volume method to solve the transient heat transfer based on the equation of energy and finite-difference method to solve the continuity and momentum equations for the pressure field and the diffusion equation for the concentration of dissolved gas within the polymer-gas solution. The unit-cell" model employed in this study takes into account the effect of injection and packing pressures melt and mold temperatures and variable material properties for the polymer-gas solution. The numerical results in terms of cell size across the sprue diameter agree fairly well with the experimental data for microcellular injection molded polyamide 6 samples."
Prediction of Cooling Time in Injection Molding by a Simplified Semi-Analytical Equation
A semi-analytical equation, used successfully in food freezing/chilling time prediction, is proposed as a potential simple alternative for cooling time prediction in injection molding of polymer parts, amorphous or semi-crystalline. This equation is based on a convective boundary condition for the mold-part interface and requires information on the thermal contact resistance (TCR) at this interface. By incorporating a literature based heat balance method in the proposed equation, it is possible to use it as a standalone predictor of polymer cooling time. Its performance was tested against data generated with C-MOLD™ for four polymers, Polystyrene (PS), Polycarbonate (PC), Polypropylene (PP), and Polyethylene (PE). The % mean error and its SD calculated this way were respectively – 9.44 and 0.97 for PS, -9.44 and 0.83 for PC, -14.22 and 5 for PP and –20.12 and 1.38 for PE. The proposed equation can be used successfully to predict the cooling time of the selected semi-crystalline or amorphous polymers with the accuracy being higher for amorphous polymers and practically independent of the precise knowledge of the TCR, provided the latter is smaller than 0.001 m2K/W.
Shear Effects and Dimensional Stability in Injection Molding
A special mould (RCEM) was used to impose a controlled shear action on polypropylene molded discs. This was obtained by superimposing an external rotation to the pressure driven advancing flow front, during the molding filling stage. The resulting shear rate and stress were computed using a non-isothermal model that combines the simple and rotation shear components.The results evidence the importance of the magnitude and of the homogeneity of the imposed shear field on the material structure development and on the dimensional stability of the obtained moldings. This was evaluated quantitatively by the total shear stress level along the flow length and by the distortion angle for different rotating speeds.
Fabrication and Mechanical Properties of Textile Insert Injection Moldings
In this study, in order to extend one-unity composites to injection molding field, textile insert injection molding was developed. Knitted fabric was chosen as reinforcement configuration, which is one of the textile configurations. PE/PE one-unity composites were prepared inserting knitted polyethylene (PE) fiber as reinforcement and PE was injected as matrix by using injection and injection-compression molding process. Tensile properties of PE/PE one-unity knitted composites were investigated. From these results, it was found that the tensile properties were dependent on resin impregnation state into fiber bundles. Tensile properties of injection-compression specimens were higher than that of injection specimens.
Effect of Material Velocities and Temperatures on the Morphology and Flexural Properties of Sandwich Parts: Statistical Study
Co-injection molding (or sandwich molding) is a process in which two or more dissimilar polymers are injected sequentially or simultaneously in a mold. Using a virgin polymer with a good surface aspect for the skin and fiber reinforced polymer for the core, one can obtain interesting final properties.In this study a statistical analysis of the relationship between processing parameters and final properties of co-injected molded plaques has been performed applying experimental design concepts. The plaques were co-injected with a virgin polypropylene (PP) as skin and a 40 % short glass fiber reinforced polypropylene (PP40) as core. Four co-injection molding parameters (independent variables), skin and core injection velocity and skin and core temperature were varied in two levels. Flexural tests and skin/core ratio measurements (dependent variables) were carried out on samples to investigate the effect of processing parameters. It has been found that the skin injection velocity has the most significant effect on the skin thickness distribution and on the mechanical properties studied.
Materials Distribution and Mechanical Properties of Incompatible Bi-Material Monosandwich Injection Moldings
Rectangular plates were injection molded in two incompatible materials (polypropylene, PP, as the outer material and high density polyethylene, HDPE, in the core) by the monosandwich technique. The molding program included variations of the injection flow rate, the two materials ratio, and both melt temperatures, accordingly to a L8 Taguchi orthogonal array. The effect of processing on the materials distribution was analyzed, namely for common defects, such as incomplete molding, finger-like and break-through anomalies. The microstructure of the moldings was observed by polarized light microscopy, and the material ratio assessed. The mechanical behavior of the specimens was assessed at 50 mm/min and 23 °C. The experimental results were analyzed by ANOVA statistical tool, being identified the main processing parameters, their percentage of contribution and their effects on the materials distribution and molding mechanical properties.
Processing Characteristics of PSU/PC Blend Associated with Degradation in Injection Molding
This study investigates the effect of degradation of the PSU/PC blend (SMA-8™) on quality characteristics of the molded parts. Experiments were conducted at two sets of melt temperatures - ‘normal’ temperature, where no severe degradation is expected (T-zone 1); and ‘elevated’ temperature, where significant degradation should occur (T-zone 2). ASTM D638 tensile bars were used for molding and measured for quality parameters of part weight, warpage, and tensile strength. Small fluctuations in the process variables were purposely introduced for Tzone 1 and T-zone 2. As the results, T-zone 1 provided more consistent shot-to-shot part properties and less process disturbances for all the parameters than T-zone 2. The subsequent baking of the molded parts rendered most of the parts excessively warped for T-zone 2. Back pressure was an important variable regarding shot-to-shot variations in all the parameters.
Advances in Injection Molding Process / Quality Control
Injection molding process/quality control has been an active research area for many years, especially when part quality requirement is becoming more stringent due to increasing applications of plastics. This paper reviews the state-of-the-art research and developments in injection molding control. It has been found that all the prior studies can be organized into a multiple-level structure system, which consists of one feedforward loop (process setup) and three feedback loops (machine control, process control, and quality control). The three feedback loops are cascaded so that the output from the previous controller becomes the command to the next controller. Numerous variables, models, and control methods have been proposed and employed for different levels of control. However, the real on-line quality control without human's intervention has not been realized primarily due to lack of thorough understanding of the relationship among machine, process, and quality variables and lack of transducers for on-line quality measurement. Based on the research progress to date, it has been concluded that process/quality model and quality sensors are the two most important areas for further advancement in injection molding control.
Evaluation of Weldline Strength of Injection Molded Polystyrene by Surface Milling
Weldline strength of injection-molded polystyrene was evaluated using surface-milled specimens. The surface including a V-notch on an adjacent flow weldline occurring behind an obstructive pin was mechanically milled off and its tensile strength was measured. The strengths of specimens milled 1/5 of the thickness off from each surface were almost the same independent of the distance from the pin. On the other hand, the strengths of specimens without milling decreased once and then increased along the flow direction. It demonstrated that the strength of weldline was dependent on the properties of the surface layer of weldline influenced by stress concentration effect of V-notch and effect of flow stagnation at the area near the pin. The size of the surface layer could be evaluated by the variation of milling depth. The tensile strength increased with an increase of milling depth up to an intrinsic value and hence increased little despite the specimen was milled off deeper. It demonstrated that there was an area in which the bonding of the resin was insufficient at the weldline interface below the V-notch. The depth of the area was ten times larger than the V-notch depth at the area near the pin and decreased along the flow direction.
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