粘弹性流体在挤出口模中入口区长度的确定

讨论了粘弹性流体在圆孔口模流动中入口区的流动状态。分析结果表明,当流体进入口模后的停留时间为其弛豫时间的７倍时,流体将达到充分发展流动状态。在此基础上,导出一估算挤出口模入口区长度的表达式。考察了两种混炼胶经毛细管挤出时的入口区长度,发现入口区的长径比约为１￣２。     

Effects of wall slip on the rheological measurement and extrusion die design of a filled rubber compound

We investigate the rheological properties of a filler-reinforced rubber compound using a rotorless shear rheometer specially designed for rubber and a rotational rheometer with parallel-plate geometry. Our aim is to evaluate the effects of wall slip on both the oscillatory and steady shear modes of the rotational rheometer. The rheological measurements show that the slip generally does not affect the oscillatory shear but does exist in the steady shear flow and tends to make the measured shear viscosity lower than the true value. Besides, we extend the investigation into engineering applications. The extrusion die for a given extrudate profile is obtained using a finite-element model in which the wall slip is considered as a boundary condition. To validate the die design, an extrusion experiment is carried out and the results confirm that, for filled elastomers, it is necessary to consider the effect of wall slip in the extrusion die design.     

The effect of wall slip on the performance of fiat extrusion dies

Flat extrusion dies are commonly used in a wide variety of film. Sheet and coating applications. Although flal dies can be designed to produce an exit flow distribution that is very uniform across most of the width, there will usually be a region along each side where it drops gradually to zero. This often requires trimming the edges of the film or sheet downstream in order to meet product specifications. It is commonly believed that treating the land area of the die with coatings that promote a small amount of wall slip will reduce the size of this edge effect and therefore improve die performance. This analysis shows that slip over the entire land region of the die will adversely affect die performance. Better performance is possible but only if the sides of the land are treated.     

挤出机头内流场对玻纤取向和分布的影响

本文分析了平直和发散两种挤出机头内的流场,推得发散流道内熔体周向拉伸应变速率的表达式;研究了两种挤出流率下由这两种机头挤出的制品壁内玻纤的取向和分布,并通过流道内的剪切和周向拉伸应变速率,对玻纤取向和分布的形成机理进行解释。结果表明：经平直机头挤出的制品内,玻纤在剪切作用下基本沿流动方向排列。发散机头内熔体受剪切和周向拉伸的共同作用,使制品壁厚方向形成了“表层-次表层-芯层-次表层-表层”的五层结构,并首次发现芯层呈“W”形排列。玻纤的排列不仅受流动过程中的应变影响,更取决于应变速率的大小。     

挤出机机头中压力降的研究

将挤出机机头流道中各个平行于横截面的几何形状，转化成理想状态的窄缝或圆形，通过分析计算得出一个纠正函数。由此应用简单的分析方程计算挤出机机头中实际的压力降。     

三元乙丙橡胶在简单口型中的正向挤出分析及逆向口型设计

采用Polyflow软件、Bird-Carreau纯黏性模型对三元乙丙橡胶(EPDM)在简单口型(正三角形、矩形)及其流道中进行了正向挤出过程模拟和逆向口型设计分析,并加工了相应的口模,通过挤出机进行了挤出试验,试验数据与模拟预测结果比较接近,逆向计算所得到的口型比较合理。结果表明,可以采用Polyflow软件、Bird-Carreau纯黏性模型近似地模拟计算EPDM的正向挤出过程和逆向口型设计。     

压力驱动微流道内流动电势及壁面滑移效应

This study investigated the streaming potential and wall slip effects on pressure-driven liquid flow in hydrophobic microchannels.The Poisson-Boltzmann equation for the electrical double layer(EDL)and Navier-Stokes equation for incompressible viscous fluid were established.For those microchannels with high wall zeta potential, the traditional Debye-H點kel linear approximation for solving the potential distributions of EDL would produce big error, therefore, analytical expression for potential distributions and Navier slip boundary condition were introduced to solve the N-S equation analytically, then analytical solution of streaming potential could be obtained by using the electrical current balancing condition.The influences of electrokinetic parameter(K), wall zeta potential and slip coefficient on streaming potential and velocity distributions were discussed in detail.The results showed that streaming potential decreased with increasing electrokinetic parameter, while increased significantly with increasing slip coefficient.It also tended to reach a maximum value at a certain zeta potential and then decreased rapidly with increasing zeta potential.Streaming potential and wall slip both affected fluid flow in microchannels, the former retained the development of liquid flow, but the latter accelerated flow velocity.Wall slip effect played a major role at lower zeta potentials, that is, flow velocity increased at lower zeta potentials when the combined effects of streaming potential and hydrodynamic slippage appeared in microchannels.Wall slip velocity gradually reduced to zero at higher zeta potential, then wall slip effect on pressure-driven flow in microchannels could be ignored.     

Effects of wall slip on ZrO2 rheological behavior in micro powder injection molding

The effects of wall slip on the rheological behavior of ZrO₂ feedstock flowing through various channels were studied. As compared to conventional no-slip condition, a power-law wall slip model was established to simulate the filling behavior of ZrO₂ feedstock flowing through the mold. The effects of wall slip on pressure distribution of the micro spool mandrils during filling was compared with the injection molded micro components. Experimental results verified that the simulation including wall slip yielded better prediction for the pressure gradient as well as crack propagation for the micro components. Increasing mold temperature not only enhanced the feedstock temperature flowing through the micro channels, but also reduced the temperature difference of the micro components and likely the ensuing thermal stress as well as cracks. Powder-binder separation is more sensitive to the mold temperature variation than melt temperature variation. Defect-free micro spool mandrils were injection molded using the optimized parameters.     

影响天线罩性能的主要工艺因素

本文主要从工艺方面出发，对地面雷达天线罩在生产过程中所需要控制的指标进行了分析，指出罩壁厚度和均匀性是主要的工艺指标，并分析各项指标是如何对雷达天线罩性能产生影响。     

Influence of slip on the flow of a yield stress fluid around a flat plate

This experimental study deals with the influence of slip on the non‐inertial flow of a viscoplastic fluid around a flat plate moving at a constant velocity. The bulk and interfacial properties of the fluid have been finely characterized. The drag force has been analyzed with regards to the flow velocity and for two tribological conditions: adherence and slip. This force decreases with the velocity and is reduced in the presence of slip. Kinematic fields have also been measured by Particle Image Velocimetry (PIV), to determine the influence of both the velocity and the tribological conditions on the liquid and solid regions of the flow. The results highlight no significant influence of the flow velocity on the thickness of the boundary layer and rigid zones. The wall shear stresses along the plate obtained from force measurements and slip velocities are then compared to rheometrical measurements. © 2015 American Institute of Chemical Engineers AIChE J, 62: 1356–1363, 2016     

Analysis of the fluid–structure interaction in the optimization‐based design of polymer sheeting dies

A polymer‐sheeting‐die‐design methodology is presented that integrates a simulation of the polymer melt flow and die‐cavity deformation with numerical optimization to compute a die‐cavity geometry capable of giving a nearly uniform exit flow rate. Both the polymer melt flow and sheeting‐die deformation are analyzed with a general‐purpose finite‐element program. The approach includes a user‐defined element that is used to evaluate the purely viscous non‐Newtonian flow in a flat die. The flow analysis, which is simplified with the Hele–Shaw approximation, is coupled to a three‐dimensional finite‐element simulation for die deformation. In addition, shape optimization of a polymer sheeting die is performed by the incorporation of the coupled analyses in our constrained optimization algorithm. A sample problem is discussed to illustrate the die‐design methodology. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3994–4004, 2007     

聚丙烯腈-二甲基亚砜溶液挤出胀大的研究

使用毛细管流变仪对聚丙烯腈(PAN)的二甲基亚砜(DMSO)溶液的挤出胀大进行了研究。研究发现,孔口胀大比(B)随表观剪切速率(γ)、溶液浓度的增大而增大;随毛细管长径比(L/D)增大、温度(T)升高、滤布层数(N)增多而减小;L/D增大或γ增大时,孔口胀大比活化能(EB)下降,C％增大,EB也增加。     

实现稳定挤出成型的流变分析

采用流变学方法分析挤出成型过程，讨论实现稳定挤出的一些工艺措施。指出保证供料速度均匀，适当加长螺杆、降低螺槽深度、减少机筒与螺杆突棱的间隙，采用小口径机头和适当降低挤出温度，易实现稳定挤出。     

流体密度对纳通道内流动滑移的影响

基于纳通道内的二维Couette流,采用分子动力学方法研究了流体密度对纳通道内流体流动滑移的影响,分析讨论了流体密度变化对纳通道内流体的微观结构、速度分布以及滑移长度的影响。研究结果表明,由于受到固体表面势作用,流体密度在表面附近呈“振荡”分布,随着流固密度比的减少,密度分布的振荡幅度和分层数也相应减少,各向同质区域范围随之扩大。并且,流固密度比的减小使得固体表面的运动状态难于传递给流体,能量传递穿过流固界面时的损耗增加,导致边界速度滑移程度增加。与之相对应,流体流动的滑移长度将随着流固密度比的减小而增加。     

Effect of vibration extrusion on the structure and properties of high‐density polyethylene pipes

BACKGROUND: The axial strength of a plastic pipe is much higher than its circumferential strength due to the macromolecular orientation during extrusion. In this work, a custom‐made electromagnetic dynamic plasticating extruder was adopted to extrude high‐density polyethylene (HDPE) pipes. A vibration force field was introduced into the whole plasticating and extrusion process by axial vibration of the screw. The aim of superimposing a vibration force field was to change the crystalline structure of HDPE and improve the molecular orientation in the circumferential direction to obtain high‐circumferential‐strength pipes. RESULTS: Through vibration extrusion, the circumferential strength of HDPE pipes increased significantly, and biaxial self‐reinforcement pipes could be obtained. The maximum increase of bursting pressure and tensile yield strength was 34.2 and 5.3%, respectively. According to differential scanning calorimetry and wide‐angle X‐ray diffraction measurements, the HDPE pipes prepared by vibration extrusion had higher crystallinity, higher melting temperature, larger crystal sizes and more perfect crystals. CONCLUSION: Vibration extrusion can effectively enhance the mechanical properties of HDPE pipes, especially the circumferential strength. The improvement of mechanical properties of HDPE pipes obtained by vibration extrusion can be attributed to the higher degree of crystallinity and the improvement of the molecular orientation and of the crystalline morphology. Copyright © 2008 Society of Chemical Industry     

Investigation of combination of finite element formulation and element type on the accuracy of 3D modeling of polymeric fluid flow in an extrusion die

The aim of this work is to investigate the effect of finite element formulation and element type on the accuracy of 3D modeling of generalized Newtonian fluid flow in complex domains. Computer models based on three finite element solution schemes (mixed, continuous, and discrete penalty), and two element types (hexahedral and tetrahedral) in a 3D framework were developed. The well‐known Carreau model was used to reflect the rheological behavior of the fluid. To determine the validity of the developed computer simulations, the flow of two high‐density polyethylene (HDPE) melts with different viscosities through an extrusion die was simulated and compared with experimentally measured data. Comparison showed that the three methods produced nearly the same results with the hexahedral elements. However, continuous penalty method using tetrahedral elements demonstrated an extreme discrepancy from the experimental data. Discrete penalty method was unable to predict secondary variable (pressure) accurately using tetrahedral elements. The best results were obtained by the use of mixed method in conjunction with tetrahedral elements. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Effect of electrode geometry on the uniformity of plasma-polymerized methyl methacrylate coatings

Plasma polymerization is widely used to deposit functionalized coatings which properties can be tuned by different operational parameters. Although plasma-deposited coatings have many interesting properties, the influence of these parameters on the polymerization process is not yet well understood. In this study, plasma-polymerized methyl methacrylate (MMA) coatings are produced and carefully planned experiments are conducted to evaluate the influence of discharge power and electrode geometry on the chemical composition and thickness of the deposited films. For this purpose, polymerization processes are carried out in two plasma reactors with different electrode geometries (parallel plate versus mesh-to-plate) and the influence of discharge power is examined for both reactor types. This study reveals that the chemical composition of the deposited films is strongly influenced by the power of the discharge: at high powers, the coatings contain less ester functional groups, which is most likely due to monomer fragmentation at these elevated powers. It is also shown in this work that the reactor set-up strongly determines the uniformity of the deposited films: in the parallel plate reactor, film thickness, surface morphology and chemical composition depend on the location of the substrate in the discharge region, while in the mesh-to-plate reactor the deposited films are physically and chemically uniform. Taking into account the latter statement, the mesh-to-plate reactor might have important industrial applications.     

高适应性挤出模具机头结构设计

探讨了挤出模具机头结构对模具适应性的影响,提出了对配方、工艺条件有高适应性模具结构的设计方法。