The SPE Library contains thousands of papers, presentations, journal briefs and recorded webinars from the best minds in the Plastics Industry. Spanning almost two decades, this collection of published research and development work in polymer science and plastics technology is a wealth of knowledge and information for anyone involved in plastics.
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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.
Mechanical Properties of Weldline in Injection Molded PC/ABS
Relationship between morphology and mechanical properties of weldline in injection-molded PC/ABS blend was investigated by testing specimens sliced along the flow direction. The diameter of ABS particles at the weldline decreased along the direction from the center to the surface of the specimen. The most outside slice showed the lowest strength among all the slices due to stress concentration effect by surface v-notch. In contrast, the adjacent one exhibited the highest strength among all, then the strength decreased gradually toward the center. Thus, the decrease of the diameter of ABS particles coincided with the increase of the strength of the weldline.
The Effect of Processing Factors on Weld Lines in Injection Moulded Nylon 6 Nanocomposites
The effect of process factors on weld lines of nylon 6 nanocomposites was investigated. Eight process factors were evaluated: injection pressure, holding pressure, holding time, back pressure, screw rotational speed, cooling time, melt and mould temperatures. A modified L16 orthogonal array of the Taguchi method with three levels was designed to run injection moulding experiments using a smaller number of samples to determine the most influential parameters. The experimental results were analysed by sensitivity test signal-to-noise (S/N) ratio and analysis of variance (ANOVA). The results show that the principal process factors for single end-gated tensile samples are different to those of double end-gated samples when the weld line occurs. The important process factors of the single end-gated samples are injection pressure and mould temperature, whilst for the double end-gated samples are mould temperature and melt temperature. The temperature factor plays an important role in processing to heal the weld strength by promoting the ability of polymer molecules to diffuse across the weld line boundary, enhancing interfacial strength at the weld region.
The Effects of Tooling Surfaces on Injection Molded Nanofeatures
The composition and surface properties of tooling materials become more critical as the size of the molded features decreases. This work investigates the effect of tooling surfaces with micro and nanoscale features. These tooling surfaces were employed as inserts for micro injection molding. Insert materials included etched and coated silicon wafers with pattern depths of 600 nm and minimum features of 200 nm. Electroformed nickel-based digital versatile disk (DVD) masters were employed as a control because this tooling currently can reproduce features that are 140 nm in depth. The micro and nano-featured parts were molded with high flow polycarbonate over a range of processing conditions. Atomic force microscopy (AFM) was used to characterize the surface topography of molded samples. The goal of this study was to explore the effect of different tooling materials on molded plastic parts with nanoscale features in terms of replication quality and durability of mold surface.
Effects of Materials when Injection Molding Nano-Scale Features
Although micro parts and features are routinely molded, the performance of polymer melts is not well understood when the part wall thickness is less than 1 mm. In this study, the effects of molding conditions and material properties were determined for the replication of nanoscale features via injection molding. The nanoscale features were part of a thin insert incorporated into a micromold. The performance of high-flow grades of polypropylene, polystyrene, polycarbonate, and polymethylmethacrylate were examined using a design of experiments designed to investigate the effects of melt temperature, mold temperature, injection velocity, and packing pressure on depth ratio and surface quality. Atomic force microscopy (AFM) was employed to measure the molded parts. As expected, higher melt and mold temperatures provided better feature replication. Replication was also material dependent with polypropylene providing the best feature replication.
Injection Molding of Micro-Structured Surfaces
Communication and information technology are branches of industry with a high potential for growth and innovation. Micro-structured light guiding elements made from plastics can e.g. help improving display technology referring to illumination. On the one hand the investigations considered different polymers (PMMA, PC, POM, COC) and on the other hand several test structures. The processing parameters were varied systematically. Especially a high mold surface temperature is a precondition for the accurate reproduction of microstructures, but leads to increased cycle times. Therefore, within the investigations the use of a dynamic heating system by induction was analyzed to heat the cavity surface efficiently. The aim is to improve the molding accuracy and to reduce the formation of orientations in the molded part. Furthermore, new demolding technologies are analyzed using different demolding principles.
Water Assisted Injection Molding for Thermoplastic Materials
This study investigated the water-assisted injection molding of thermoplastic materials. The first part of this report was to develop a water assisted injection-molding system, which included a water pump, a water injection pin, a water tank equipped with a temperature regulator, and a control circuit. Two types of water injection pins were designed and made to mold the parts. The second part of this report is to test the moldability of the developed system on various thermoplastic materials, including polystyrene, polyethylene, polypropylene, and acrylonitrile-butadiene-styrene. A comparison has been made between the parts molded by water assisted injection molding and gas assisted injection molding. The final goal of this research is to gain better understanding of the moldability of water assisted injection-molded parts, so that steps can be taken to optimize the process. This would provide significant advantages in improving parts quality.
In-Situ Observation of Birefringence during Vibration-Assisted Injection Molding
The phenomenon of birefringence has been widely used in the study of steady state and transient polymer flows as well as for stress analysis but has seldom been applied to the actual injection molding process. The current study utilizes a custom designed mold with built-in windows for observation of the polymer melt within the cavity. A polystyrene melt is viewed through crossed polarizers to reveal the birefringence pattern in the melt during the molding cycle. A high-speed CCD camera is used to record the birefringence patterns in real time throughout the cycle for subsequent analysis. The use of birefringence yields information regarding the molecular orientation of the polymer that can be compared under different processing conditions.
In-Mold Coating of Thermoplastic Substrates: Further Developments
In-mold coating (IMC) is carried out by injecting a liquid low viscosity thermoset material onto the surface of the thermoplastic substrate while it is still in the mold. A computer code based on the Control Volume based Finite Element Method (CV/FEM) has been developed to predict the fill pattern and pressure distribution during the coating flow assuming the coating to be a power law fluid. A packing module is being added to further improve the pressure prediction and achieve desired coating thickness, the preliminary results are presented. Regression based statistical analysis is used to demonstrate the significance of various control variables used in a 1-D IMC flow condition. Data envelopment analysis (DEA) is used to find the optimal compromises between multiple performance measures (PMs) to prescribe the settings of IMC process variables in the 2-D IMC flow case and the location of the injection point on a real part. Case studies are presented for this purpose.
Multi-Component Laminate Moulding (MLM): Process Evaluation
Multi-component Laminate Moulding (MLM) is a novel injection moulding process that can produce multi-layered injection mouldings from at least two thermoplastic compounds. The principle of the process and initial results were presented at the Antec Meeting in 2002. This paper will describe recent developments in the process and the results from moulding trials carried out on combinations of thermoplastics including several engineering thermoplastic compounds. The discussion will highlight the potential benefits of the technology. Reference will be made to the relationship between the mechanical properties and the level of compatibility between the separate components. An assessment of the potential cost saving from the use of the MLM process with engineering resins will also be presented.
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