基于三维有限元模拟的聚合物复合型材共挤出过程分析

建立了两种聚合物熔体在圆柱形流道中共挤出流动的三维数值计算模型,采用有限元方法数值模拟了复合棒材(LDPE/PS)的共挤出成形过程,并对共挤出界面的形状、入口与出口的速度、流动过程的压力降以及入口流量对界面的影响等进行了分析。结果表明,在对接棒材的共挤出过程中,当入口流量相同时,共挤出界面偏向低黏度熔体一侧;入口流量较小的变化即引起共挤出界面的形状与位置较大的变动,在对接棒材的成形过程中为获得稳定的流动,须严格控制入口流量的变化。最后将数值计算得到的界面与实验结果进行了对比,表明数值计算的结果与实验结果是一致的。     

气辅共挤出流道中聚合物流动过程的数值分析

建立了气辅条件下两种聚合物在矩形流道中共挤出流动的三维非等温数值分析模型。用粘弹性流体模型(PTT模型)描述熔融聚合物的特性,Arrhenius方程表示流动对温度的依赖性,并且考虑聚合物相对于流道壁面的滑移以及不相容聚合物熔体间滑移的边界条件。用有限元方法数值模拟了聚合物成型过程,将计算结果与普通共挤出成型流动进行了对比分析。结果表明,气垫层的加入,将使聚合物熔体的压力降减低20%～40%;使流道出口处的速度场分布均匀,速度场的最大值下降约50%;气垫区聚合物的自由流动还将对共挤出界面的形状和位置有一定的影响。     

双层聚合物共挤出过程的数值模拟

以HDPE/LDPE共挤出型材为例,用有限元方法对非牛顿流体在共挤出流道内的等温流动进行了二维数值模拟。分析了模具流道收敛角、入口体积流量以及模具结构形状对流道内压力场、速度场以及界面偏移的影响。结果表明:当流道收敛角在0°<α<90°区间变化时,角度的大小对熔体界面影响不大;但当流道收敛角为90°时,共挤出区开始段出现流动死角。流道入口平流区可以使流道内的压力场均匀,有降低流道压力降的作用。HDPE/LDPE两种聚合物熔体共挤出时,其入口流率比对界面位置的影响很大。     

聚合物气辅共挤成型中挤出胀大的数值模拟

以一矩形截面共挤型材为例,采用Giesekus本构方程和Navier滑移模型建立数值模型,使用EVSS、SU等有限元方法对气辅共挤和传统共挤时两种聚合物熔体在口模内外的等温粘弹流动做了三维数值模拟,得到了气辅共挤和传统共挤时的挤出胀大率、速度场、应力场及剪切速率分布。对模拟结果进行了分析和对比,结果表明,气辅共挤能消除挤出胀大和模外熔体偏转流动现象;气辅共挤时两相熔体的速度场均匀一致,熔体流动稳定,呈柱塞状挤出;熔体表面的切向和法向应力为零,因而可有效提高挤出速率,并防止制品表面"鲨鱼皮"现象的出现。     

复合共挤成型中挤出胀大的三维粘弹数值模拟

采用Phan-Thien and Tanner(PTT)本构方程,建立了矩形截面共挤口模内外两种聚合物熔体流动的三维粘弹数值模型,有限元模拟了聚丙烯/聚苯乙烯(PP/PS)共挤过程中的挤出胀大现象,并用实验验证了模拟结果。研究表明:当入口体积流量相同时,两熔体挤出口模后会朝向黏度较高的PS熔体一侧偏转,型材截面呈非对称畸变。两熔体在垂直挤出方向上的速度分布导致了挤出胀大过程中熔体的偏转流动,而口模出口处的剪切速率分布基本决定了共挤型材截面的形状。实验结果与模拟结果基本相符,模拟所得挤出胀大率比实际值大8.6%。等温假设是影响共挤出胀大数值模拟准确度的主要因素。     

聚合物共挤出过程的挤出胀大有限元分析

采用有限元方法模拟了非牛顿流体在共挤出过程中的挤出胀大现象,分析了模具流道收敛角、模具内壁的表面质量以及平直段长度对共挤出聚合物挤出胀大的影响。结果表明:当流道收敛角α在0°~30°区间变化时,角度的大小对两种熔体的挤出胀大率均有一定的影响;当30°≤α≤90°时,角度的变化对熔体挤出胀大率几乎没有影响。模具内壁的表面质量对挤出胀大率影响较大,改善模具壁面质量可以有效地减小挤出胀大率。适当增加流道出口平直段长度可以显著减小挤出胀大率。     

非等温气辅共挤出胀大的三维粘弹数值模拟

采用PTT本构方程和Arrhenius黏度对温度依赖方程,运用有限元方法,对低密聚乙烯(LDPE)/高密聚乙烯(HDPE)熔体的共挤过程进行了三维非等温粘弹数值模拟,对比分析了两熔体在传统和气辅共挤过程中的速度场、剪切速率分布和层间界面形貌。研究表明,气辅共挤成型在口模出口处不存在二次流动,且在挤出方向流速均匀,剪切速率分布均匀且数值比传统共挤小得多,说明气辅共挤能有效消除传统共挤过程中的挤出胀大和界面偏移现象。     

气辅挤出胀大比与滑移段长度的关系

采用参数渐变法和Thompson变换,对粘弹性高分子熔体在不同气体辅助挤出口模内的流动进行了数值模拟研究。考察了体积流量、松弛时间和滑移段长度对挤出物挤出胀大比的影响。研究表明,熔体在滑移段的停留时间与材料松弛时间之比与挤出胀大比之间存在指数衰减关系,其实质是熔体在滑移段处于形变衰减过程。理论分析与数值模拟具有高度的一致性,表明该方程可用于指导气辅口模设计。     

橡胶轮胎部件挤出过程数值模拟分析

以某轮胎部件胶料的挤出过程为研究对象,采用Phan-Thien-Tanner(PTT)本构方程对挤出过程进行三维等温数值模拟。分析了胶料在挤出机内的流动情况,并考察分析壁面滑移系数、流量和牵引速率对出口速率分布和挤出胀大的影响。结果表明,在自由挤出的条件下,壁面滑移程度对胶料离开口模时的速率分布影响较大,主要表现为对挤出胀大的影响,而流量的变化对挤出胀大的影响较小;当施加牵引时,随着牵引速率的增加,挤出胀大比减小,当牵引速率超过一定范围后会导致挤出物的形状发生畸变。     

轮胎胎面胶料共挤出成型的有限元仿真研究

以胎面胶料复合挤出过程为研究对象,采用Phan-Thien and Tanner(PTT)本构模型和Navier滑移模型对胎面胶/下胎面胶(TWS/FB)的共挤出过程做了三维数值模拟,分析了流道长度对胶料共挤出质量的影响。结果表明:当两熔体的入口体积流率一致时,黏度差异是导致两种熔体的熔体界面向高黏度熔体偏转的主要原因之一。适当增加流道的长度,可以减小胶料出口处的速度场、剪切速率场的集中程度并使之趋于均匀,能够有效降低挤出胀大和模外熔体偏转流动现象。     

Experimental and finite element simulation methods for rate-dependent metal forming processes

An experimental procedure and a finite element simulation method for rate-dependent metal forming processes are developed. The development includes the formulation of a tangential stiffness matrix for an axisymmetric solid finite element with four node, eight degree of freedom, quadrilateral cross-section. The formulation includes the effects of elasticity, viscoplasticity, temperature, strain rate and large strains. The solution procedure is based on a Newton-Raphson incremental-iterative method which solves the non-linear equilibrium equations and gives temperatures and incremental stresses and strains. Three examples are studied. In example 1, finite element simulation for the upsetting of a cylindrical workpiece between two perfectly rough dies is performed and the results are compared with alternative finite element solutions. In examples 2 and 3, both experimental and finite element studies are performed for the upsetting of a cylindrical billet and the forging of a ball, respectively. Annealed aluminium 1100 workpieces are used in both examples. For the finite element analysis, uniaxial compression tests are first performed to provide the material properties. The tests generate elastic moduli and two sets of stress-strain curves (quasi-static and constant strain rate), which are used to establish a rate-dependent material model for input. For both examples 2 and 3, comparisons between the experimental and finite element simulation results for the forming force vs. die displacement relations and also for the deformed configurations show good agreement. The versatility of finite element methods allows for displaying detailed knowledge of the metal forming process, such as the distributions of temperature rise, yield stress, effective stress, plastic strain, plastic strain rate, forming forces and deformed configurations, etc. at any instance during the forming process.     

Adaptive 2D finite element simulation of metal forming processes

The development and integration of available current methods and the development of new methods for an adaptive finite element analysis of metal forming processes are presented. The analysis includes large-strain, elastic–plastic, and thermal effects. Many numerical methods such as mesh generation, simulation of the contact between the workpiece and tool and die, and optimization of the finite element mesh are integrated and incorporated. In addition, an algorithm is developed which can detect certain severely distorted elements where the area of integration is approaching zero. The advantage of correcting these regions of locally distorted elements is demonstrated. These numerical methods are implemented in a finite element program developed for simulating metal forming processes, with the emphasis on automating the analysis. Examples include an axisymmetric stress simulation of a coldheading process, a plane strain simulation of an extrusion process and a plane strain simulation of orthogonal metal cutting, all with noticeable thermal effects. The orthogonal cutting forces and feed forces calculated are compared with two sets of experimental data, with good agreement.     

线型聚乙烯熔体的剪切弹性行为表征

利用稳态剪切柔量，可回复剪切应变以及熔体挤出胀大比等参数，定量计算和定性表征了４种线型聚乙烯熔体的剪切弹性行为，研究了各表征参数之间及其与分子量，分子量分布等分子参数的关系，讨论了剪切弹性行为对加工性能的影响。     

Numerical Analysis on Rotary Forging Mechanism of a Flange

A numerical simulation on the rotary forging process of a flange is conducted by three-dimensional rigid-plastic finite element method. The states of stress and strain rate in the workpiece are analyzed and the forging mechanism of the flange is reveal6d. Moreover, the influence of the die configuration on the material flow is also analyzed.     

Modeling fatigue crack and spalling for rolling die under hot milling

In hot milling process, rolling die is subjected to nonsteady conditions which can rise the combinations of fatigue and spalling damage mechanism. An understanding about the failure mechanism of the rolling die is essential under hot rolling process. Fatigue crack growth and spalling process are governed by highly concentrated strain and stress in the crack tip region. Based on the theory of elastic‐plastic fracture mechanics, an analytical model are presented in this paper to determine the elliptical crack growth rate and spalling damage mechanism. The model includes new proposed constitutive equations for fatigue and spalling crack growth. To verify the models, finite element simulation and experimental data are considered. The results show good agreement with finite element simulation and experimental data.     

Dynamics of Polymeric Fluids 1. The Molecular Theory of Die Swell： A Set of Equation on Swelling Ratio in Extrudates

The structural model of the multiple-transient networks and the mechanism of the multiple-reptation entangled chains due to the dynamic reorganization in the entangled sites were extensively applied on the die swell of polymeric liquids in the steady simple shear flow. The total (recoverable and unrecoverable) viscoelastic free energy of deformation and flow, the constitutive equation and the expression of the simple shear (tanψ)were deduced from the conformational probability distribution function of the entangled polymer chains. It found that: (1) the magnitudes of simple shear (tanψ) depended not only on the free recoil (or recoverable strain) but also on the viscous heating (or unrecoverable strain); (2) the total recoil may be resolved into the instantaneous and delayed recoil. Based on these facts, the functions of the partition and two experiential fractions of the recoverable (1(-W)γ) and the unrecoverable ((-W)γ) conformations for the recoil and viscous heating of polymeric liquids were defined correspondingly. Then the correlation of the instantaneously and ultimately (or total) recoverable strains to the (N1/τ12)w and the fraction of trans-form conformation was obtained. After introducing the condition of uniform two-dimensional extension (αx=αy=α, αz=α-2) and the swell ratio (B=α), two sets of equations on the instantaneous and ultimate swelling ratios (BE, BEVT) were obtained, and a method to determine the fraction of the recoverable transform conformation were proposed.The equations of BE and BE5 were verified by the experimental data of HDPE (high denisity polyethylene)at two different high temperatures. It shows that the molecular theory of die swell can be used to predict the correlation of the swelling to the (N1/τ12)w and the fraction of trans-form conformation.