连铸凝固过程热模拟实验方法及应用案例

凝固是制约冶金产品质量的重要环节,但因高温、不透明、大规模和连续化的生产特点,连铸生产条件下的凝固问题研究极为困难。目前的研究方法主要包括数值模拟、物理模拟和热模拟,其中热模拟方法因可以直接获取接近生产条件的实验数据而备受关注。本工作系统介绍了连铸凝固热模拟研究方法,简述了热模拟技术原理,并对结晶器热模拟方法及特征单元热模拟方法的原理和应用进行总结。其中,基于特征单元热相似性提出的连铸坯枝晶生长热模拟及凝固裂纹热模拟等方法成功地将十几吨铸坯的凝固过程“浓缩”到实验室用百克钢研究,不仅可以揭示钢液成分、浇注和冷却条件等因素对凝固过程、组织和元素分布的影响规律,而且还可以获得铸坯固液界面形貌、界面前沿溶质扩散和夹杂物演变、凝固裂纹形成的可能性及条件等其他手段无法得到而冶金界非常关注的问题。要点：(1)概述了连铸凝固过程研究方法。(2)重点介绍了连铸凝固过程热模拟方法的分类、原理和应用。(3)总结展望了连铸热模拟研究方法的研究方向。     

Simulation of non‐isothermal melt densification of polyethylene in rotational molding

Rotational molding involves powder mixing, heating and melting of powder particles to form a homogeneous polymer melt, as well as cooling and solidification. The densification of a loose powder compact into a homogeneous melt occurs over a wide range of conditions as the material passes from a solid state into a melt state. The numerical simulation of the non‐isothermal melt densification in the rotational molding process is presented in this work. The simulation combines heat transfer, polymer sintering and bubble dissolution models, and is based on an idealized packing arrangement of powder particles. The predictions are in general agreement with experimental observations presented in the literature for the rotational molding of polyethylenes. The simulation allows for systematic and quantitative studies on the effect of molding conditions and material properties on the molding cycle and molded part density. Results indicate that the densification process is primarily affected by the powder characteristics, which are accounted for in terms of the particle size and the particle packing arrangement. The material rheological properties become increasingly important as the powder characteristics lessen in quality. The simulation demonstrated that while certain combinations of processing conditions help reduce the molding cycle, they have a detrimental effect on the densification process.     

Numerical modeling of the cooling cycle and associated thermal stresses in a melt explosive charge

A comprehensive simulation tool is developed to describe and optimize the cooling cycle in the melt‐casting of Composition B. It comprises a multiphysics approach tackling heat, mass, and momentum transfers involved in the casting process. The highly nonlinear solidification step and development of thermal stresses are included. A V & V (Verification and Validation) approach was adopted whereby the model was verified against a benchmark problem and tested with a simple cylindrical geometry. Then, the approach was applied to a 105 mm caliber artillery shell and simulation results were in close agreement with experimental measurements. The model is equipped with a CZM function to account for adhesion between the solidified cast and the mold. During cooling, separation is possible and the size and location of gaps, depending on shrinkage and adhesion, are successfully emulated. The importance of controlled solidification is pointed out, especially regarding steep temperature gradients within the shell. © 2016 American Institute of Chemical Engineers AIChE J, 62: 3797–3811, 2016     

Orientation and crystallinity in film casting of polypropylene

In the cast film process a polymer melt is extruded through a slit die, stretched in air, and cooled on a chill roll. During the path in air the melt cools while being stretched. Film casting experiments were carried out with an isotactic polypropylene resin. The temperature and width distributions were measured along the draw direction. Further, the crystallinity and Hermans orientation factor were measured on the final film. The process was described by a simple thermomechanical model derived elsewhere. The evolution of the molecular orientation parameters was calculated on the basis of a dumbbell model coupled with velocity and temperature distributions provided by the thermomechanical model. The experimental crystalline orientations of the final films collapsed into a single step‐shaped curve (from low to high orientation) if plotted versus the stress calculated by the model at the frozen line. The experimental values of the crystallinity and Hermans orientation factors are discussed on the basis of predictions of the dumbbell model for melt orientation at the frozen line and the crystallinity data obtained in quiescent conditions under the same cooling rate. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 1981–1992, 2002; DOI 10.1002/app.10422     

Undercooled Melt Formation and Shaping of Alumina–Yttrium Aluminum Garnet Eutectic Ceramics

Undercooled melt was produced in the alumina-rich portion of the Al₂O₃–Y₂O₃ system, heating the metastable eutectic structure to above the metastable eutectic temperature but below the equilibrium eutectic temperature, followed immediately by solidification along the equilibrium path in the undercooled melt. Coupling the melting and solidification enabled a nearly adiabatic transformation from the metastable eutectic structure to the equilibrium eutectic structure. The eutectic structure produced through the solidification was uniform and fine throughout the castings. A fin-type casting with a thickness of 300 μm was successfully produced. This paper proposes a novel casting process using the undercooled melt formation.     

Modeling and simulation of thermally induced stress and warpage in injection molded thermoplastics

Thermally induced stress and the relevant warpage caused by inappropriate mold design and processing conditions are problems that confound the overall success of injection molding. A visco-elastic phase transformation model, using a standard linear solid for the solidified polymer and a viscous fluid model for the polymer melt, of 2-D finite element scheme with 8 noded overlay isoparametric elements was used to simulate and predict the residual stress and warpage within injection molded articles as induced during the cooling stage of the injection molding cycle. Computed results are in good agreement with published experimental data. The approach proposed here is to examine and simulate the injection molding solidification process with the intent of understanding and resolving more inclusive and realistic problems.     

Melt solidification in polymer processing

This paper treats two cases of polymer melt solidification in rectangular geometry. The cases treated are the one of static solidification and that of solidification during flow in a narrow gap channel. Both cases are solved using the method of Dussinberre, which reduces the two-phase moving boundary case to a single phase problem, simplifying the mathematics considerably. The numerical solutions are based on a combination of the concept of flow analysis network (FAN), a finite element method developed for solving polymer flow problems, with a Crank-Nicolson implicit finite difference scheme. The methods may be used in computing the cooling down period and preventing “short” conditions in injection molding dies. Examples of solidification of high density polyethylene illustrate the applicability of the method.     

Three‐dimensional simulation of primary and secondary penetration in a clip‐shaped square tube during a gas‐assisted injection molding process

This article proposes a generalized Newtonian model to predict the three‐dimensional gas penetration phenomenon in the GAIM process, where the gas and melt compressibility are both taken into account and hence the primary and secondary penetrations in GAIM processes are able to be quantitatively predicted. Additionally, an incompressible model requiring no outflow boundary is also presented to emphasis the influence of gas compressibility on the primary penetration. Based on a finite volume discretization, the proposed numerical model solves the complete momentum equation with two front transport equations, which are employed to track the gas/melt and air/melt interfaces. The modified Cross‐WLF model is adopted to describe the melt rheological behavior. The two‐domain modified Tait equation is exploited to represent the melt compressibility, while a polytropic model is employed to express the gas compressibility. The proposed schemes are quantitatively validated by the gas penetration characteristics in a clip‐shaped square tube, where good prediction accuracy is obtained. The influences of five major molding parameters, such as the injection pressure, mold temperature, melt temperature, delay time, and melt material on the gas penetration characteristics in the same clip‐shaped square tube via the proposed numerical approach are extensively presented and discussed. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

板坯连铸凝固过程动态耦合数值模拟

基于实际生产测得的数据,采用连续动态三维耦合模型对板坯连铸凝固过程的流场、温度场和凝固进行模拟. 结果表明,由浸入式水口进入的钢液在结晶器内冲击形成上下两股回流,凝固促进了流动速度的衰减,提高拉速扩大了回流区域;结晶器内铸坯宽面偏角部100~150 mm处存在局部过热,在结晶器出口,拉速由0.02 m/s增加到0.025 m/s,坯壳厚度减小约3 mm.     

Computer simulation of stress‐induced crystallization in injection molded thermoplastics

Injection molding of semicrystalline plastics was simulated with the proposed stress‐induced crystallization model. A pseudo‐concentration method was used to track the melt front advancement. Stress relaxation was considered using the WFL model. Simulations were carried out under different processing conditions to investigate the effect of processing parameters on the crystallinity of the final part. The simulation results reproduced most of the experimental results in the literature. Comparison is made between the slow‐crystallizing polymer (PET) and fast‐crystallizing polymer (PP) to demonstrate the effect of stress on the crystallization kinetics during the injection molding process for materials with different crystallization properties. The results show that for fast‐crystallizing plastics, stress has little effect on the final crystallinity in the injection molded parts.     

Pressure solidification of amorphous thermoplastics

It is assumed that solidification of a polymer melt is possible by applying pressure. Therefore, the influence of pressure on various properties of amorphous polymers was studied as reported in this article. Pressure‐volume‐temperature (pvT)‐measurements were made on liquid and glassy polycarbonate as well as polymethylmethacrylate and cycloolefin‐copolymer to calculate the thermal expansion coefficient and the shift of glass transition temperature with pressure. The existing pvT‐apparatus was modified to measure the compression heating and the temperature of the polymer during the isothermal solidification process. All results are in good agreement with literature and show the feasibility of the process approach of pressure solidification for amorphous thermoplastics. In addition, the flow temperature of the materials was measured in a plate‐plate‐rheometer to identify the lowest process temperature possible. In addition, the results of the isothermal vitrification are discussed. It is shown that solidification occurs due to compression and that it can be proven by a change in compressibility and the volume course. POLYM. ENG. SCI., 2009. © 2008 Society of Plastics Engineers     

Numerical Simulation for Effects of Injection Mold Cooling on Warpage and Residual Stresses

Abstract

The dimensions quality of the injection‐molded parts is the result of a complex combination of material, part, and mold designs and process conditions. In this article, warpage prediction relies on the calculation of residual stresses developed during the molding process. The solidification of a molten thermoplastic between cooled parallel plates is used to model the mechanics of part warp in the injection‐molding process. Flow effects are neglected, and a thermorheologically simple thermoviscoelastic material model is assumed. The warp and residual stresses numerical simulation with finite element method (FEM) is time dependent. At each time step, the material properties can be temperature and pressure dependent. Mold temperature or mold‐cooling rate effects on part warp have been numerically predicted and compared with experimental results. By showing the mold‐cooling effects, it was concluded that mold cooling has a significant effect on part warpage, and mold‐cooling parameters, such as mold temperature, resin temperature, cooling channels, etc., should be set carefully.     

Melt‐blowing thermoplastic polyurethane and polyether‐block‐amide elastomers: Effect of processing conditions and crystallization on web properties

Melt‐blown webs from ester and ether thermoplastic polyurethanes and polyether‐block‐amide (PEBA) elastomers were produced at different die‐to‐collector distances (DCD) to study the correlation between the polymer type and hardness, melt‐blowing process conditions, and web properties. An experimental set up was built to measure the air temperature and velocity profiles below and across the melt‐blowing die to correlate the fiber formation process and polymer crystallization behavior to process conditions and web properties. It was shown that air temperature and velocity profiles follow similar trends with increasing distance below the melt‐blowing die: both drop rapidly until reaching a plateau region approximately 5–6 cm below the die. Thereafter, they remain relatively constant with further increasing distance. It was found that crystallization onset and peak temperatures of all block copolymers in this study fall within this region of rapid velocity and temperature drop. This suggests that the polymers have already started to crystallize and solidify before reaching the collector, the extent of which depends on the crystallization kinetics of the polymer. The strong influence of the crystallization kinetics on web strength was clearly demonstrated in the PEBA series. In particular, the hardest grade produced the lowest web strength mainly because of its high crystallization rate and crystallization onset temperature. It is concluded that the melt‐blown web strength is strongly dependent on the degree of fiber‐to‐fiber adhesion within the web, which is determined by the amount of fiber solidification that occurs prior to the collector. The crystallization kinetics of the polymer and the distances traveled between the die and collector or the exposure time of the polymer melt to process and ambient air were shown to be critical in the amount of fiber solidification attained. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Thermal Stress Analysis of Fused-Cast AZS Refractories during Production: Part I, Industrial Study

A study has been conducted to understand and prevent the formation of cracks in alumina-zirconia-silica (AZS) refractory blocks during solidification processing. A fundamental approach has been taken, centered on the development of a three-dimensional mathematical model to predict heat flow and stress generation in fused-cast AZS refractory blocks. In the first part of a two-part study, the "voidless" casting process has been carefully examined in an industrial setting. From a survey of the distribution, frequency of occurrence, and fracture surface morphology of cracks, an attempt was made to link the crack types found in the study to process variables. In-mold temperature data collected for a single casting throughout the normal cooling period have been used to validate the heat-flow model which is described in Part 11. The stress analysis, cause of the different cracks, and remedial action are also presented in Part 11.     

Structure and thermal behavior of poly(L‐lactic acid) clay nanocomposites: Effect of preparation method as a function of the nanofiller modification level

The structural and thermal characteristics of poly(L ‐lactic acid)/layered‐silicate hybrid materials that were produced via two different routes, namely by solvent casting and by melt mixing, were compared in association with the degree of clay modification. Investigation of the produced materials' structure revealed that, at low modification levels, melt blending is necessary in dispersing the amine‐treated clay into the polymer matrix. At intermediate degrees of modification, both techniques are capable of swelling the silicate clay with the solution casting to be a more effective method. Thermal measurements showed that the clay modification level influences significantly the thermal stability of both solution and melt processed hybrids. Moreover, the material derived from melt mixing displayed a higher onset decomposition temperature. The glass transition temperature of the polymer was not significantly affected by the preparation method followed. However, the crystallization process was found to be strongly dependent on both the preparation method and the degree of clay modification. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Pressure drop estimation for polyamide 6 flow through spinnerets and filters

The pressure drop resulting from polyamide 6 flow through industrial spinnerets and wire‐mesh filters was examined as a possible parameter for improving spinning process constancy with experimental techniques and a numerical approach. The rheological characterization of the polymer melt was performed with a capillary rheometer and a controlled‐stress rotational rheometer equipped with a high‐temperature oven cell. Measurements in a nitrogen atmosphere were carried out at different temperatures and at various moisture contents to determine the effect of the postcondensation process on the rheological properties of the polymer melt. These experiments were used to collect all basic material information necessary to fit the data with the purely viscous Cross model and the viscoelastic Kaye, Bernstein, Kearsley, Zappas (K‐BKZ) model. A spinning pilot plant (consisting of an extruder, a gear pump, a pressure sensor, and a spin beam with several spin packs installed) was used to measure pressure drop values through industrial spinnerets and through two types of filters: (1) Dutch twilled weave filters and (2) sintered filters. Pilot plant tests on filters showed that in the examined range of melt throughputs, the pressure drop increased linearly with an increase in the melt flow rate for all the filters considered. The results with respect to the spinneret geometry led to the conclusion that the numerical simulations gave satisfactory predictions even for experimental data coming from complex systems such as spinning plants, as long as extensional properties were accounted for by the model. On the contrary, pressure drop predictions obtained from the Cross model underestimated the pilot plant values by approximately 20% because of the inability of the model to consider the extensional component of the flow. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1577–1587, 2006     

Predictive Modeling of Non-Isothermal Crystallization in the Solidification of Polyoxymethylene Melt

The predictive modeling of non-isothermal crystallization of polyoxymethylene (POM) melt was proposed based on an isothermal crystallization experiment. The development of crystallinity and crystallization morphological evolution was simulated by considering the process of crystallization includes the three steps: nucleus, growth and ripening and the pre-ripened degree was considered explicit simultaneously. Double-scale computational modeling of the solidification phase transition of the semi-crystalline polymer POM melt was then numerical implemented. The heat flow equation coupling the evolution of crystallinity was computed by the finite different numerical method. The result of the cooling stage of POM melt shows the method can produce more visual information about the processing of crystallize and a relative reasonable detail about the temperature field.     

Numerical and experimental studies for gas assisted extrusion forming of molten polypropylene

In this study, the experiments of gas‐assisted extrusion (GAE) for molten polypropylene were carried out under different gas pressures, the different extrudate deformations and sharkskin defects of melt were observed. To ascertain the effects of gas on melt extrusion, non‐isothermal numerical simulation of GAE based on gas/melt two‐phase fluid model was proposed and studied. In the simulations, the melt extruded profile, physical field distributions (velocities, pressure drop, and first normal stress difference) were obtained. Numerical results showed that the deformation degree of melt increased with increasing gas pressure, which was in good agreement with experimental results. It was demonstrated that the influence of gas pressure on the melt extrusion could be well reflected by GAE simulation based on gas/melt two‐phase fluid model rather than simplified‐GAE (SGAE) based on full‐slip wall boundary condition used in the past time. Experimental and numerical results demonstrate that the gas pressure induced first normal stress difference is the main reason of triggering flow behavior changes, extrudate deformations, and sharkskin defects of melt. Therefore, the reasonable controlling of gas pressure is a key in practice of GAE, and the gas layer and its influence should be considered in GAE numerical simulation. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42682.     

Electric‐field induced filler association dynamics and resulting improvements in the electrical conductivity of polyester/multiwall carbon nanotube composites

The effects of electric fields on the filler response dynamics and electrical percolation of poly(ethylene succinate)/multiwall carbon nanotube (MWCNT) composites are studied. When subjected to AC electric fields in their melt state, PESu/MWCNT composites exhibit dramatic improvements in their transverse electrical conductivity. More importantly, the elevated conductivity values are preserved after matrix solidification. Overall, the experimental results show that the electrified composites exhibit the same electrical conductivity levels as their non‐electrified counterparts at approximately threefold less filler content. The dynamics of the insulator‐to‐conductor transition under an electric field also are studied for these composites and correlate reasonably well with operating parameters, such as electric field intensity, matrix viscosity, and filler content through a relatively simple model. Such a model can serve as an enabling tool in the determination of process conditions for the manufacturing of electrically conducting MWCNT/polymer composites. POLYM. COMPOS., 38:1571–1578, 2017. © 2015 Society of Plastics Engineers     

Finite element analysis for wavelike flow marks in injection molding

Wavelike flow marks are a kind of surface defect that can arise during the filling stage of the injection molding process. In this study, we performed a numerical analysis using a finite element method to predict the conditions under which flow marks are generated. To simplify the analysis, a two dimensional flow through a channel between two parallel plates was considered. The viscosity of the polymer melt was modeled by the Cross‐WLF equation. For the finite element analysis, a velocity–pressure formulation was used to simultaneously solve the continuity and momentum equations. The calculation domain for the numerical analysis keeps changing with time due to the advancing melt front. To handle the free surface more accurately, a moving grid method was employed. A numerical mesh was generated at each time step using an automatic mesh generation scheme. An analytical model was developed to correlate the effects of process variables to the flow mark geometry. Results of the numerical analysis were compared with the available experimental data. The estimated geometry of the flow marks were in good qualitative agreement with experimental observations. Parametric studies have been performed to examine the effects of various processing conditions and the material properties on flow mark size. POLYM. ENG. SCI., 47:922–933, 2007. © 2007 Society of Plastics Engineers