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Kinetic modeling is used to predict the rate of conversion of a partially cured thermosetting plastic subjected to catalytic conditions of temperature and pressure. Parameters for these models can be determined from experimental data obtained through differential scanning calorimetry (DSC). In the present study, kinetic parameters are determined from dynamic DSC scans of the Pro-Set 117LV/229 resin system. The primary contribution of this paper is an evaluation of the sensitivity of empirical kinetic parameters to the selection of reaction bounds. This study is performed using a finite difference approach to obtain parameter sensitivity. The results suggest that the kinetic parameters are insensitive to bound selection for this particular resin system. Additionally, the kinetic parameters obtained from dynamic scans are applied to isothermal DSC results, which indicates the importance of the accurate determination of kinetic parameters at low degrees of conversion and the evaluation of diffusion control. The effects of gelation and vitrification are also considered.
Sarah Stair, Russell Mailen, Theresa Vo, David Jack, May 2013
The use of carbon fiber reinforced composites is becoming increasingly implemented throughout the entire aerospace, automotive and many niche industries, in large part due to their high strength to weight ratios. Unfortunately, effective design of the fabrication process and the final part performance poses increased engineering difficulties over alternative manufacturing approaches. This work focuses on the residual strains that occur within the processed laminate as induced during manufacture. These residual strains are due to a combination of the curing kinetics of the thermoset and the induced thermal strains caused by a coefficient of thermal expansion mismatch for the resin and carbon fiber. In the present paper, we present results for a cross-ply (un-balanced) laminate. We use micromechanical theories to predict the stiffness and the coefficient of thermal expansion of an individual lamina from the constitutive properties for the fiber and the matrix, and couple the lamina results with a finite element structural and coupled thermal-structural analysis to predict the observed macroscopic deformation and curvature of an unloaded processed laminate. The finite element results are compared with the measured results and show exceptional qualitative agreement with experimental observations. Suggestions for improvements on the method are introduced along with a discussion of the need for the quantification of the relevant constitutive material properties for future modeling and experimental studies.
Yuyang Song, Umesh Gandhi, Hidetoshi Yomoda, May 2013
Application of chopped glass fiber reinforced polymer components from injection and compression molding process are rapidly increasing in automotive industry due to the light-weight and their mechanical properties. Material properties of such components strongly depend on fiber orientation. Therefore it is very important to estimate the fiber orientation distribution in such materials. In this paper we are presenting a practical method to estimate the fiber orientation from CT scan image. A 3D image of fibers is created and 2nd order orientation tensors are calculated using the coordinates of the end of fibers. The approach is demonstrated on a 3-D box shaped structure. The measured orientation tensors are compared with the predicted values from the mold-fill simulation. Key challenges in measuring the orientation tensor for the long fibers is also discussed.
Isik I. Nugay, Ido Offenbach, Mukerrem Cakmak, May 2013
An insert injection-compression molding process was used to encapsulate cholesteric liquid crystal displays with flexible and rigid lens. For this purpose, a complex hot runner mold was designed and constructed. Thermoplastic polyurethanes (TPU) were found to be the best candidates for this application after a vigorous search on desired physical properties for lenses including transparency, low melt viscosity and melting temperature and mechanical properties. Three different hardness grades of TPUs were selected to demonstrate the range of encapsulation from soft to hard. In this process, the encapsulation was accomplished in two steps: first, injecting the TPU on the front of the display from an edge pin gate and subsequently the back side of the display is encapsulated. A flow simulation is completed on Autodesk Moldflow Insight to help pre-visualize the changing process parameters effect on part quality and to compare with experimental results. The quality of encapsulation and shrinkage related problems and their elimination are discussed in this study.
Advances in modeling the mold filling process have led to better designs and manufacturing efficiencies of short?fiber fiber reinforced thermoset plastics. The flow characteristics during processing dictate the spatial varying fiber orientation, and thus the spatially varying mechanical properties. Knowledge of the fiber orientation allows for part optimization of both the mold design and the processing parameters. The classical fiber?fiber interaction model Folgar?Tucker (1984) for calculating the fiber orientation for fiber filled thermoplastics has seen extensive use within the industry for nearly 30 years. Recent modifications of the classical model include the reduced strain closure (Wang et al., 2008) and the anisotropic rotary diffusion model (Phelps and Tucker, 2009). Each of these models yield quite different characteristics in the resulting predicted fiber orientation. In this paper we suggest that the predicted flexural modulus from these different fiber interaction models may be quite different and may be an effective macroscopic structural test to identify acceptable diffusion models and possibly to quantify the appropriate empirical parameter ranges for acceptable interaction models. In this work, the velocity gradients are computed along streamlines of a complex domain of an injection?molded ASTM flex bar. The fiber orientation is then obtained along each streamline from the velocity gradients and then from an in?house code the local stiffness tensor is obtained as a function of position within the entire part. Lastly, the nodal stiffness values are imported back into the finite element software to perform structural bending tests as a function of length along the part. The results show a structural response that is significantly different between the different fiber interaction models, and thus lends credence to the possibility that flex tests may be a useful experimental technique to compare fiber interaction models.
Xun Chen, JeongIn Gug, Margaret SobkowiczKline, May 2013
Plastics Engineering Department, University of Massachusetts Lowell Nanocomposites based on biodegradable poly(butylene succinate) (PBS) and silica fillers were prepared by a melt-blending process. Two types of unmodified fumed silica and octadecyltrichlorosilane (OTS) functionalized silica were used as fillers. Rheology was used to study relaxation dynamics and viscoelastic properties of these nanocomposites in the melt state. The effects of polymer-particle and particle-particle interactions on viscoelastic properties of nanocomposite materials were investigated. Linear viscoelastic data indicate a transition to a solid-like response at low oscillation frequencies for particle weight fractions as low as 5%. The long-time response upon a step shear strain demonstrates that liquid-like behavior persists in the nanocomposites below 5 wt% loading, which is related to the relaxation of the temporal polymer-particle network. Dynamic viscoelastic and dynamic mechanical thermal analysis (DMTA) measurements of the PBS/silica nanocomposite reveal that fumed silica with the smallest primary particle size has the largest dynamic moduli over the testing temperature range. The hydrophobic functionalization of silica filler does not appreciably change the thermal transition temperatures in the nanocomposites.
Powders for additive manufacturing processes such as Selective Laser Sintering (SLS) are currently produced by costly cryogenic milling or precipitation processes for a limited selection of resins. A nowel technique allows production of pellets in the micrometer-scale by extruding a polymer melt through a capillary and perturbing it with a hot air stream which can be used for the SLS processes. Due to the fact that this micropelletization process is used to produce powder particles from different materials, the developed software is very helpful to predict the flowability of these different particle shapes and different materials and its usage for the SLS process. This software will determine the flowability characteristics of different grain shapes and will be evaluated by comparison of results provided by both, software and experimentation. In a second step, different micropelletized materials surface smoothnesses will be compared and evaluated towards their competitiveness. This validation will show a comparison of quality and performance between the different powder production processes. The goal of this work is, to produce more cost effective but competitive powders out of different micropelletized materials.
Ahmad Zohre vand, Abdellah Ajji, Frej Mighri, May 2013
Nanocomposites porous film based on polypropylene (PP) and titanium dioxide (TiO2) nanoparticles were prepared by the uniaxial stretching method. Effects of drawing temperature, extension rate, stretching ratio and composition of the base films on final properties and microstructure of the stretched films were studied. Water vapor permeability results showed significant decrease in permeability of the films stretched at temperatures higher than 60°C. Water vapor permeation in the porous nanocomposite films had a direct relation with nanoparticle content, extension rate and stretching ratio. Study on morphology of stretched films, using SEM, revealed that the pores form due to PP/TiO2 interfacial debonding at lower stretching ratio while higher stretching ratios cause an enlargement of the pores and formation of PP fibril structure parallel to the stretching axis. Apparent porosity values, BET surface area measurement and quantification of dye absorption on pores showed smaller dependency on process parameters.
A rapid thermocycling blow mold with electric heating was used to produce automobile spoiler in the present work. In order to achieve high heating efficiency as well as uniform temperature distribution on mold cavity surface, a multi-objective optimization model was proposed and a hybrid method consisting of design of experiment (DOE), artificial neural network (ANN) and multi-objective genetic algorithm (GA) was developed to optimize the heating system of the mold. The results showed that the method can effectively give the optimal values of design variables. Further, the temperature distribution uniformity on the mold cavity surface was largely improved and the heating efficiency was also guaranteed by using optimal design results.
The use of in-mold pressure and surface temperature sensors was investigated to determine whether they reduced variation in part weight when variation in material viscosity and check ring leakage were introduced to the process. Velocity to pressure transfer when the part was not quite full (2-stage, pack with second stage), after the part was packed with a fast velocity (2-stage, pack with first stage), and after the part was packed with a slow velocity (3-stage) were the injection strategies evaluated. It was found that surface temperature sensors toward the end of fill were the most beneficial in all cases studied.
Crosslinked plastic parts demonstrate prominent performance advantages over the non-crosslinked articles for a wide array of applications. New crosslinkable polyethylene compositions were recently developed which showed outstanding processibility and excellent thermal and mechanical properties, such as increased impact strength, high modulus, and enhanced environmental stress cracking resistance (ESCR). The advancement in balance of melt processibility and solid properties is ideally suited for the rotational molding process, opening opportunities to produce high-performance end-use products, including sporting boat, large agricultural and chemical containers, all-plastic cars, and other outdoor products.
Shawn Shi, Ramani Narayan, Shilpa Manjure, May 2013
Polylactic Acid (PLA) is the most widely available, renewable and compostable polymer with several unique features. However, PLA is poor in its ability to withstand elevated use temperatures above 55 °C. As such it is common practice to either compound PLA with additives that improve its heat deflection temperature or increase its crystallinity in mold or in an extra annealing step for use in injection molded applications. The objective of this research was to study the crystallization of three PLA grades and its effect on thermal properties including compostability. Crystallization was studied using DSC and Talc was used as a nucleating agent. Crystallinity was found to vary from 25% to 60% for the various grades. The PLA was converted into test bars and cutlery and its heat distortion temperature was tested before and after annealing. Additionally, the crystallized cutlery was sent to a local composting facility and was found to disintegrate within 4 weeks, which is much sooner than the requirements of the ASTM D6400 standard of 12 weeks.
Friction behavior of polyethylene terephthalate (PET) in either its amorphous or semi-crystalline form can depend on several factors. Surface microstructure and the resulting forces (Van der Waals), residual stress, the molecular weight distribution (MWD), and contaminants can influence polymer friction. A semi-crystalline polymer, PET exhibits an interesting microstructure sensitive to processing. PET is used for a range of applications with different forms. In some cases it is essential to determine the friction coefficients on molded parts. The current work presents a unique fixture built for measuring the friction of amorphous PET in the form of bottle preforms and stretched PET in the form of empty bottles, collected from high speed injection stretch blow molding (ISBM) equipment. The test frame was developed to accommodate samples with a range of contact areas, contact force, and time. The friction behavior of polymers is known to be dependent on the time and speed of the test. In the current work, dynamic friction between the molded parts were measured at a fixed rotation speed (13 RPM) and averaged over multiple samples. Data has been collected across several variables including change in chemistry, MWD, and processing changes related to cooling time. Friction on amorphous parts was higher than semi-crystalline ones. Time dependent friction behavior was observed irrespective of the microstructure. This suggests the presence of transitory micro surface features, at least on pristine surfaces.
A novel combination approach to producing quality foamed injection molded parts has been investigated. By combining extruded, gas-laden pellets with microcellular injection molding, the processing benefits and material characteristics of using both nitrogen (N2) and carbon dioxide (CO2) as co-blowing agents can be realized, thus yielding features superior to that of using either N2 or CO2 alone. Using an optimal content ratio for the blowing agents, as well as the proper sequence of introducing the gases, foamed parts with a much better morphology can be produced. In particular, extruding N2 gas-laden pellets, followed by microcellular injection molding with higher amounts of CO2, produces a cellular structure that is very fine and dense. In this paper, the theoretical background is discussed and experimental results show that this combined approach leads to significant improvements in foam cell morphology for low density polyethylene (LDPE), polypropylene (PP), and high impact polystyrene (HIPS) using two different mold geometries.
During the demolding process, high ejection forces can cause a local damage of the plastic part. The plastic part design as well as the design of the demolding system can be verified according to occurring stresses in the part by applying an integrative simulation already in early stage of the product development. This approach consists of a coupling of injection molding simulation and structural simulation. That way it is possible to take account of the process influence on the residual stresses in the plastic part, which may increase the contact pressure between part and mold core. For high ejection speed, it is possible to model a strain rate dependent mechanical material model to get a higher accuracy of the stress results. The validation of the simulation results in real injection molding trials show a good accordance concerning the measured and simulated demolding force.
The numerical simulation of fiber orientation during compression molding (CM) is a crucial and valuable tool for predicting the mechanical reliability of the final part. Considering LFT, advanced models are necessary to obtain accurate results for fiber orientation. The objective of our research is to verify new models which consider reduced strain closure (RSC) and anisotropic rotary diffusion (ARD). Since the initial fiber orientation in the charge will be considered, the first step of this project is to measure it. Using micro computerized tomography (?CT) and Volume Graphics’ (VG) VGStudio MAX image processing software for CT data analysis, the initial fiber orientation of the charge will be obtained. With the help of interpolation functions such as radial basis functions (RBF) the initial fiber orientation can be evaluated at each node of the charge mesh in the CM simulation. Model parameters are fitted by matching the experimental fiber orientation results.
Foamed rubber profiles have found widespread industrial applications. Their cellular structure offers savings potential with respect to component weight, material consumption and costs. The foam structure is generated using chemical blowing agents, which are added to the rubber mixture before processing. Foaming and vulcanisation occur simultaneously in the curing unit. Both processes are adjusted by modification of the rubber mixture, which is currently state of the art for the calibration of the foam quality. There are little systematic approaches to an optimisation of the cellular structure during processing. In a current research project, correlations between processing parameters, polymer structure and foam structure of chemically foamed rubber profiles are systematically analysed and evaluated. A lab scale pin barrel extruder is used for production of chemically foamed rubber profiles with an EPDM-based rubber compound and Azodicarbonamide (ADC) - based blowing agents. In first experiments, the thermal stress of the rubber compound during processing and the extrusion speed have been identified as influencing factors on the cellular structure. A high thermal stress caused by increased processing temperatures leads to a faster scorch of the compound and thus to finer cellular structures with smaller average cell sizes and higher cell densities. With increasing extrusion speed, a finer cellular structure is formed.
A methodical analysis of influencing factors on the cellular structure allows an optimisation of the foamed profiles in terms of their cellular structure and will lead to a guideline for processing. Primary objective of further investigations is the analysis of influencing factors of the curing equipment used on the profile quality.
Silicone rubber profiles are used in an increasing amount of applications, for example in sealing or damping technologies. A cellular structure of the silicone rubber profile enhances the damping properties and leads to a reduction of component weight, material consumption and cost at the same time. For production of cellular rubber profiles, the use of chemical blowing agents is state of the art. However, physical blowing agents such as nitrogen offer ecological, economical and procedural advantages. So far, an application of a physical foaming process on silicone rubber has been difficult due to the sorption properties of the material.
In a current research project, a continuous foaming process for silicone rubber is developed using a modified lab scale extrusion plant. A constant foaming process was realised in first experiments. The silicone rubber profiles are analysed in terms of cellular structure, surface quality and density. Experimental investigations show that the amount of blowing agent injected has a significant influence on density, foam structure and surface roughness. The use of a nucleating agent improves the cellular structures in terms of a smaller average cell size and increased homogeneity. Besides, the foam structure can be influenced by the pressure in the extrusion die. Primary objective of further investigations is the analysis of the influence of silicone rubber material properties on the profile quality.
Christian Hopmann, Clemens Behmenburg, Ulf Recht, May 2013
Injection molding is the most important process to manufacture complex polymer parts. Today, however the injection molding of elastomers almost entirely produces solid workpieces. Functionalized complex hollow components, e. g. for the conduction of medias, are often manufactured in costly multistep processes. The projectile injection technique offers a new approach to easily produce elastomers in a one-step process into complex hollow parts.
In addition to the well-known process variations of the gas (GIT) and water injection technology (WIT), the projectile injection technology (PIT) provides special advantages for the production of elastomeric continuous fiber reinforced hollow bodies, especially considering the residual wall thickness. When using the PIT, a projectile is initially placed on an injector. The melt is injected into the mold cavity afterwards. Subsequently a fluid is then injected into the component interior. The PIT uses the projectile for the actual melt displacement. It is driven by means of the fluid through the device and thereby forms the interior cavity.
Because of the fundamentally different mechanism of melt displacement and cavity formation when using the PIT in comparison to GIT or WIT, in thermoplastic processing standard materials which do not need to be specially modified for FIT can be used. So far, the PIT has not been used for the manufacturing of elastomeric hollow bodies. The target of ongoing investigations is to clarify whether or not the PIT is suitable for elastomeric processing.
Avraam I. Isayev, Tian Liang, Todd M. Lewis, May 2013
The present study is devoted to ultrasonic devulcanization of tire rubber particles of 10 and 30 meshes by means of a new ultrasonic twin-screw extruder. Ultrasonic amplitude and devulcanization temperature were varied at a fixed frequency of 40 kHz. The die pressure and ultrasonic power consumption during devulcanization were recorded. Degree of devulcanization was investigated by measuring the crosslink density, gel fraction and revulcanization behavior. Rubber of 30 mesh exhibited a lower die pressure and higher degree of devulcanization than those of rubber of 10 mesh. Due to the higher level of devulcanization and lower viscosity of devulcanized rubbers at higher amplitudes, the temperature of devulcanized rubbers at the die was reduced with an increase of the ultrasonic amplitude. Cole-Cole plots, crosslink density and gel fraction of devulcanized and revulcanized rubbers, revulcanization behavior, and modulus of revulcanizates separated in two distinct groups based on the level of devulcanization and effect on molecular structure of devulcanized rubber. Revulcanizates with a greater degree of devulcanization exhibited higher elongation at break, while those with a lower degree of devulcanization exhibited higher strength and modulus. Revulcanizates of rubber of 30 mesh exhibited a consistently higher elongation at break. The normalized gel fraction versus normalized crosslink density was described by a unique function independent of the processing conditions and rubber particle size.
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