A coupled DEM/CFD analysis of the effect of air on powder flow during die filling

Die filling from a stationary shoe in a vacuum and in the presence of air was numerically analyzed using an Eulerian‐Lagrangian model, which employs a discrete element method (DEM) for the particles and computational fluid dynamics (CFD) for the air with a two‐way air‐particle interaction coupling term. Monodisperse and polydisperse powder systems have been simulated to explore the effect of the presence of air on the die filling process. For die filling with monodisperse powders, the influences of particle size and density on the flow behavior were explored. The numerical simulations revealed that the presence of air has a significant impact on the powder flow behavior, especially for systems with smaller and/or lighter particles. Flow has been characterized in terms of a dimensionless mass flow rate, and it has been shown that for die filling in a vacuum this is constant. The flow characteristics for die filling in air can be classified into two regimes. There is an air‐inert regime in which the particle size and density are sufficiently large that the effect of air flow becomes negligible, and the dimensionless mass flow rate is essentially identical to that obtained for die filling in a vacuum. There is also an air‐sensitive regime, for smaller particle sizes and lower particle densities, in which the dimensionless mass flow rate increases as the particle size and density increase. The effects of particle‐size distribution and adhesion on the flow behavior have also been investigated. It was found that, in a vacuum, the dimensionless mass flow rate for polydisperse systems is nearly identical to that for monodisperse systems. In the presence of air, a lower dimensionless mass flow rate is obtained for polydisperse systems compared to monodisperse systems, demonstrating that air effects become more significant. Furthermore, it has been shown that, as expected, the dimensionless mass flow rate decreases as the surface energy increases (i.e., for more cohesive powders). © 2008 American Institute of Chemical Engineers AIChE J, 2009     

Wood‐Plastic‐Composites: Rheologische Charakterisierung und Füllverhalten im Spritzgießprozess

Wood‐plastic composites (WPC) show a complex and completely different material and flow behavior in comparison to pure polymers. Especially the flow behavior is very important for the processing of these composites. In the following article results of rheological characterization and investigations of the filling behavior during the injection molding process are presented. Furthermore, appearing challenges and possible methods of resolution are discussed.     

Organische Synthese mit mikrostrukturierten Reaktoren

Organic Synthesis with Microstructured Reactors This article describes the chances microstructured reactors offer for chemical plant engineering. This suitability for chemical production is commonly regarded to be the key to the market penetration. Seen in the long term, there is potential that new plants can be equipped with microstructured reactors. Only economic balances, however, which draw up profitability, will open the door to the usage of chemical micro process engineering for plant construction. Main arguments for using microstructured reactors are thus enhanced conversion and selectivity, increased space‐time yields, waste reduction and more safety via small reactor volumes. Credit‐card sized reaction systems allow one to perform the screening of multi‐phase reactions. More prominent, similar screening is carried out for single‐step reactions. Moreover, safe processing with microstructured reactors in the explosive regime enlarges the traditional range of processing. The reaction guidance by microstructured reactors can further influence subsequent processing steps such as product purification and, in this way, can lower the energy costs of processes.     

Modeling and simulation of fiber‐reinforced polymer mold‐filling with phase change

A gas‐solid‐liquid three‐phase model for the simulation of fiber‐reinforced composites mold‐filling with phase change is established. The influence of fluid flow on the fibers is described by Newton's law of motion, and the influence of fibers on fluid flow is described by the momentum exchange source term in the model. A revised enthalpy method that can be used for both the melt and air in the mold cavity is proposed to describe the phase change during the mold‐filling. The finite‐volume method on a non‐staggered grid coupled with a level set method for viscoelastic‐Newtonian fluid flow is used to solve the model. The “frozen skin” layers are simulated successfully. Information regarding the fiber transformation and orientation is obtained in the mold‐filling process. The results show that fibers in the cavity are divided into five layers during the mold‐filling process, which is in accordance with experimental studies. Fibers have disturbance on these physical quantities, and the disturbance increases as the slenderness ratio increases. During mold‐filling process with two injection inlets, fiber orientation around the weld line area is in accordance with the experimental results. At the same time, single fiber's trajectory in the cavity, and physical quantities such as velocity, pressure, temperature, and stresses distributions in the cavity at end of mold‐filling process are also obtained. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42881.     

Resin transfer molding process optimization using numerical simulation and design of experiments approach

The art of resin transfer molding (RTM) process optimization requires a clear understanding of how the process performance is affected by variations in some important process parameters. In this paper, maximum pressure and mold filling time of the RTM process are considered as characteristics of the process performance to evaluate the process design. The five process parameters taken into consideration are flow rate, fiber volume fraction, number of gates, gate location, and number of vents. An integrated methodology was proposed to investigate the effects of process prameters on maximum pressure and mold filling time and to find the optimum processing conditions. The method combines numerical simulation and design of experiments (DOE) approach and is applied to process design for a cylindrical composite part. Using RTM simulation, a series of numerical experiments were conducted to predict maximum pressure and mold filling time of the RTM process. A half‐fractional factorial design was conducted to identify the significant factors in the RTM process. Furthermore, the empirical models and sensitivity coefficients for maximum pressure and mold filling time were developed. Comparatively close agreements were found among the empirical approximations, numerical simulations, and actual experiments. These results were further utilized to find the optimal processing conditions for the example part.     

Verfahrenstechnische Fortschritte für die Herstellung neuer Materialien – Foliengießen aus Nanopartikeln

It is shown by the example of a ceramic tape casting process, how advances in the field of process engineering lead to improved material properties. One great advance is the possibility of producing nanoparticles in stirred media mills. The nano‐sized particle suspensions can be directly processed to ceramic tapes. The processing of nanoparticles requires specifically adjusted process conditions, but leads to a drastic improvement of the final product properties. Hence, dense and crack‐free ceramic tapes with a higher mechanical strength, a lower surface roughness and a translucent character compared to tapes from micro‐sized powders can be obtained.     

Enhanced polymer filling and uniform shrinkage of polymer and mold in a hot embossing process

Low filling efficiency and large thermal stress are two important problems that limit the wide use of hot embossing especially in fabricating high aspect ratio patterns. Two types of flow barriers, the first being an accessorial slot on the mold (SFB), the other was a block on the hot embossing machine (BFB), were designed to enhance polymer filling and their performances were simulated with the finite element method. The numerical simulation results show that two kinds of flow barriers can also accelerate the polymer filling speed and improve filling efficiency. The BFB has a better promoting effect and can be easily used as a quasi close‐die embossing process. The shrinkage of the polymer and mold is made uniform with a designed polymer grip holder to minimize the thermal stress. The polymer was clipped at a temperature in a cooling step and its deformation was fixed; thus, the shrinkage of the polymer can be equal to the mold at a special temperature. An improved hot embossing machine was designed and the hot embossing process was modified to satisfy these requirements. At last successful fabrication of the light guide plate verified the improvements. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Nonisothermal transient filling simulation of fiber suspended viscoelastic liquid in a center‐gated disk

The present study numerically investigates a fiber orientation in injection‐molded short fiber reinforced thermoplastic composite by using a rheological model, which includes the nonlinear viscoelasticity of polymer and the anisotropic effect of fiber in the total stress. A nonisothermal transient‐filling process for a center‐gated disk geometry is analyzed by a finite element method using a discrete‐elastic‐viscous split stress formulation with a matrix logarithm for the viscoelastic fluid flow and a streamline upwind Petrov–Galerkin method for convection‐dominated problems. The numerical analysis result is compared to the experimental data available in the literature in terms of the fiber orientation in center‐gated disk. The effects of the fiber coupling and the slow‐orientation kinetics of the fiber are discussed. Also, the effect of the injection‐molding processing condition is discussed by varying the filling time and the mold temperature. POLYM. COMPOS., 2011. © 2010 Society of Plastics Engineers     

Structure evolution and orientation mechanism of isotactic polypropylene during the two‐stage solid die drawing process

High oriented isotactic polypropylene prepared by self‐designed two‐stage die drawing apparatus was explored through kinds of methods, including differential scanning calorimetry, two‐dimensional wide‐angle X‐ray diffraction, polarized light microscope, and tensile test. The results showed that there was a great difference between the orientation mechanism and structure evolution of two‐stage solid die drawing process and single‐stage solid die drawing process. All samples would undergo free drawing process after die drawing process. Die drawing and free drawing process were of equal importance to single‐stage die drawing process while die drawing process showed an prominent contribution to the two‐stage die drawing process. Drawing speed showed beneficial influence on die drawing process for both single‐ and two‐stage die drawing process routes. Under the same processing condition, tensile strength and modulus of samples prepared by two‐stage die drawing process were higher than those prepared by single‐stage die drawing process and the maximum value could reach 241.93 MPa and 3.57 GPa, respectively. Moreover, two‐stage die drawing samples showed better dimensional stability than single‐stage die drawing samples especially under low draw rate. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46581.     

Modeling and simulation approaches in the resin transfer molding process: A review

A review of current approaches in modeling and simulation of the resin transfer molding (RTM) process is presented. The processing technology of RTM is discussed and some available experimental techniques to monitor the process cycle are presented. A master model is proposed for the entire process cycle consisting of mold filling and curing stages. This master model contains the fundamental and constitutive sub‐models for both stages. The key elements of the master model discussed in this study are: flow, heat and mass balance equations for fundamental sub‐models, permeability, cure kinetics, resin viscosity and void formation for constitutive sub‐models. At the end, numerical methods widely used to simulate the filling process are presented and published simulation results of mold filling and process cycle are reviewed.     

基于田口试验法的熔融沉积成型PLA制品工艺优化

为了提高熔融沉积成型(FDM)制品的质量,采用田口试验法研究填充密度、沉积方向和填充图案三个因素对聚乳酸(PLA)制品拉伸性能的影响。结果表明,加工工艺参数对拉伸弹性模量和拉伸强度的影响程度均为沉积方向>填充密度>填充图案,其中拉伸弹性模量最优化的参数组合为填充密度100%、沉积方向45°、填充图案网格形状,拉伸强度最优化的参数组合为填充密度70%、沉积方向45°、填充图案网格形状。通过扫描电子显微镜对拉伸试验失效断面进行观察分析,探究了FDM打印工艺参数之沉积方向、填充密度对PLA试样拉伸失效的影响及其失效机理,其表征分析与试验结果高度吻合,表明该研究结果具有较好的借鉴意义。     

Analysis of pellet shaping kinetics at the die opening in underwater pelletizing processes

Underwater pelletizing of thermoplastic materials has gained high importance within the chemical and the compounding industry. It is known for producing comparatively spherical pellets with good bulk characteristics. Pellet shapes are determined by the viscous and elastic properties of the melt as well as various process conditions. Therefore, deformation of the melt strands because of the water flow at the die‐hole opening may result in undesired shapes. This is particularly the case when processing polymers of low viscosity. This paper defines an analytical framework describing the pellet formation at the die‐hole opening. Furthermore, a nondimensional DS number is developed as a measure for the deformation sensitivity of different polymer materials. In order to validate this approach, a new camera system and video recording method has been developed, which allows for a visual analysis of the cutting process in underwater pelletizing systems. Examining different materials at various process parameters, this paper identifies the main parameters responsible for pellet deformation and demonstrates how the DS number can be used to anticipate these effects. POLYM. ENG. SCI., 55:1170–1176, 2015. © 2014 Society of Plastics Engineers     

The Effect of Processing Parameters on the Crystalline,Orientation, and Mechanical Properties of Two‐Stage Die‐Drawn Polypropylene

An investigation specifically focusing on the relationship among processing parameters, structure, and mechanical properties of solid two‐stage die‐drawn polypropylene (PP) has been done in this study, which aimed at providing a reliable reference in the choose of appropriate conditions that can be able to meet a specific processing requirement. PP sheets were drawn through a self‐design solid two‐stage die‐drawing apparatus to various drawing ratios (DRs) at different speeds and with different die‐exit thicknesses. The results showed that tensile strength, modulus, and the degree of orientation significantly increased with the nominal DR and could be further improved by increasing the drawing speed. While the crystallinity increased prior and decreased to near a constant value when the drawing speed was over 150 mm/min in the researched scope. Larger the die‐exit thickness was, lower the orientation degree was but higher the crystallinity could be obtained. Compared with crystallinity, orientation degree plays a leading role in the final product properties during a two‐stage solid die‐drawing process. POLYM. ENG. SCI., 59:2347–2355, 2019. © 2019 Society of Plastics Engineers     

Die filling optimization using three-dimensional discrete element modeling

Inhomogeneous density distributions after die filling are a ubiquitous problem in powder technological part production. In this paper, numerical simulations of the die filling process in realistic, three-dimensional (3D) cavities are presented using the discrete element method with both multi-sphere and single sphere grain models. Good agreement was found between calculated and experimentally measured density distributions. The formation of an inhomogeneous distribution is discussed by a time-resolved analysis of the filling process. Grain rearrangement and densification during subsequent feeding shoe passages are characterized. The shoe velocity was tested for its influence on the density homogeneity. Suggestions for density homogenization with the application of cavity oscillations or volumetric filling are given. A density homogeneity index is introduced. The application of a coarse graining scheme circumvents the intrinsic difficulty of non-manageable grain numbers in 3D filling simulations. The validity and limitations of this scheme are discussed.     

Influence of the melt viscosity and operating conditions on the degree of filling,pressure, temperature,and residence time in a co‐kneader

The effect of the melt viscosity and operating conditions on processing parameters in a co‐kneader with a discharge die was experimentally investigated. Filling ratio, pressure, temperature, and residence time distribution were measured. Experiments were performed with polypropylene resins. The viscosity of the melt was varied either by changing the regulation temperature of the kneader or the molecular weight of the polymer. The filling pattern in the co‐kneader shows the conveying capability of the various elements without any effect of the melt viscosity. Experimental residence time distributions remain the same at a given feed rate and screw speed, regardless of the viscosity of the material. The global degree of filling in a zone combining conveying elements, kneading elements, and restriction ring was found to be nearly constant in the conditions of this study and therefore a simple relation exists between the mean residence time and the feed rate. Beside expected variations of the die pressure and melt temperature when the viscosity, screw speed or feed rates change, a model based on heat equation and experimental data demonstrate the high capability of the co‐kneader for heat exchange and for the control of self‐heating during mixing. POLYM. ENG. SCI., 58:133–141, 2018. © 2017 Society of Plastics Engineers     

Optimization of filling process in RTM using a genetic algorithm and experimental design method

In the resin transfer molding (RTM) process, preplaced fiber mat is set up in a mold and thermoset resin is injected into the mold. An important issue in RTM processing is minimizing the cycle time without sacrificing part quality or increasing the cost. In this study, a numerical simulation and optimization process for the filling stage was conducted in order to determine the optimum gate locations. The control volume finite element method (CVFEM), modeled as a 2‐dimensional flow, was used in this numerical analysis along with the coordinate transformation method to analyze a complex 3‐dimensional structure. Experiments were performed to monitor the flow front to validate the simulation results. The results of the numerical simulation corresponded with that of the experimental quite well for every single, simultaneous, and sequential injection procedure. The optimization analysis of the sequential injection procedure was performed to minimize fill time. The complex geometry of an automobile bumper core was chosen. A genetic algorithm was used to determine the optimum gate locations in the 3‐step sequential injection case. Taguchi's experimental design method was also used for determining the pressure contribution of each gate. These results could provide the information on the optimum gate locations and injection pressure in each injection step and predict the filling time and flow front.     

Improvement of processing parameters by using processing aids during the linear low‐density polyethylene film blowing process

Polymer processing aids are used to improve processing properties in the polyethylene industry. These materials improve not only the physical and mechanical properties of the final products but also their processing properties. This paper studies some of the processing variables such as die pressure, melt temperature, masterbatch activity, and die gap by examining the functions of polymer processing aids and, last but not least, the effects on the film blowing process of two‐component processing aids containing a perfluorinated additive and polyoxyethylene. J. VINYL ADDIT. TECHNOL., 2010. © 2009 Society of Plastics Engineers     

Ultrasonic oscillations effect on rheological and processing properties of metallocene‐catalyzed linear low density polyethylene

The effects of ultrasonic oscillations and die materials on die pressure, productivity of extrusion, melt viscosity of metallocene‐catalyzed linear low density polyethylene (mLLDPE), as well as their mechanism were studied in a special ultrasonic oscillations extrusion system developed in our lab. Die materials used in our experiment included steel, brass, and polytetrafluoroethylene (PTFE)_. The experimental results showed that ultrasonic oscillations as well as die materials have great influence on the rheological and processing behavior of mLLDPE. Ultrasonic oscillations can greatly increase the productivity of mLLDPE melt extruded through different dies, and can decrease the die pressure and the melt viscosity of mLLDPE. Compared with steel or brass die, mLLDPE melt extruded through PTFE die is more sensitive to ultrasonic oscillations. A possible mechanism for the improved processability of mLLDPE is proposed in this article. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1873–1878, 2003     

Modeling the chemorheological behavior of epoxy/liquid aromatic diamine for resin transfer molding applications

The analysis of the chemorheological behavior of an epoxy prepolymer based on a diglycidylether of bisphenol‐A (DGEBA) with a liquid aromatic diamine (DETDA 80) as a hardener was performed by combining the data obtained from Differential Scanning Calorimetry (DSC) with rheological measurements. The kinetics of the crosslinking reaction was analyzed at conventional injection temperatures varying from 100 to 150°C as experienced during a Resin Transfer Molding (RTM) process. A phenomenological kinetic model able to describe the cure behavior of the DGEBA/DETDA 80 system during processing is proposed. Rheological properties of this low reactive epoxy system were also measured to follow the cure evolution at the same temperatures as the mold‐filling process. An empirical model correlating the resin viscosity with temperature and the extent of reaction was obtained to carry out later a simulation of the RTM process and to prepare advanced composites. Predictions of the viscosity changes were found to be in good agreement with the experimental data at low extents of cure, i.e., in the period of time required for the mold‐filling stage in RTM process. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4228–4237, 2006     

Viscosity analysis of polypropylene‐kaolin composites measured using single‐screw extruder

Deviations between data for the apparent and true viscosity measurement are commonly observed in the rheological studies of thermoplastics. To validate the significance on the usage of these data on the processing side, filling analysis for the apparent viscosity and true viscosity of polypropylene‐kaolin (PP‐kaolin) composites was conducted by using CadMold® software on the dumbbell Computer Aided Design model for comparative purposes. The raw apparent viscosity data were generated from a single‐screw extruder by using different sets of die geometries. Bagley and Rabinowitsch corrections were implemented to get a corrected set of true viscosity measurements. It was found that the true viscosity curves are lower compared with the apparent viscosity curves of PP‐kaolin. By constructing the corresponding data from PP‐kaolin composite rheological results, it was discovered that the high viscosity of the apparent viscosity melt data has shown a large variation in the mold distributions filling temperature, reduces the total pressure loss and melt shear stress, and increases the melt velocity during mold filling. Although the corrections were made on the calculated results, however, the significance is relatively small and it does not prove the changes in the overall physical property of the composite. Therefore, correcting the data by using the Bagley and Rabinowitsch corrections is not necessary in mold‐filling analysis. J. VINYL ADDIT. TECHNOL., 20:275–283, 2014. © 2014 Society of Plastics Engineers