A theoretical analysis of non-isothermal flow in wire-coating co-extrusion dies

A theoretical study of non-isothermal superimposed flow of two polymer melts in wire coating co-extrusion dies has been carried out. Numerical methods have been employed to solve the coupled momentum- and energy-balance equations. Various combinations of three polymers—namely, high density polyethylene (HDPE), polystyrene (PS) and low density polyethylene (LDPE) have been studied and least squares curve fitted quadratic polynomials have been used for constitutive equations for all three polymers in non-Newtonian high shear rate regions. A multitude of thermal and mechanical boundary conditions can be treated by this algorithm. It was found that temperature and velocity profiles in the die depend significantly on the arrangement of the polymers. Maximum temperature rise has been noted to increase sharply with wire velocity but it can be reduced by increasing the die radius. When the thickness of the outer layer is increased from zero, the shear stress at the wall undergoes a dramatic change (if the viscosities of the polymers are different) at small values of the flow rate ratio and it reaches an asymptotic value at large values of flow rate ratio. It was also found that viscosity ratio at the interface can be reduced by changing the initial temperatures of the liquids. It was observed in some cases that large errors in the calculation of rheological and thermal variables for this problem can be made if temperature rise due to viscous dissipation is not considered.     

The flow distribution of molten polymers in slit dies and coathanger dies through three-dimensional finite element analysis

The isothermal flow of power-law fluids in slit dies and coathanger dies is studied. A general three-dimensional finite element code is developed for the purpose of flow analysis. The pressure distribution, the velocity distribution, and the transverse flow rate distribution are obtained. The effect of the die geometry on the flow distribution is critically discussed. It is found that a die channel with cross section of dog bone profile produces a flatter transverse flow rate distribution.     

Rheological analysis of oligomeric perfluoropolyethers

The novelty lies on the molecular level investigation of the end group functionality on rheological properties of fractionated, monodisperse oligomeric perfluoropolyethers (PFPEs) with various molecular weights and chain-end functionalities in this study. A sharp transition observed in the slope of the shear viscosity at a critical molecular weight for PFPEs was interpreted based on temporal tube mechanism caused by end group agglomeration of strong functional polar end groups. The temperature dependence of the shear viscosity was found to yield an Arrhenius form, determining the flow activation energy and hydrodynamic volume. The flow activation energy was also compared to the activation energy for surface diffusion to examine the role of end group-solid surface interaction. Modified Cole-Cole plots for storage and loss moduli along with polymer relaxation show the microstructural changes due to the interaction of PFPE end-group, which alters effective molecular weights.     

Rheological analysis of ceramic pastes

This paper is concerned with a number of means of characterising the rheological properties of a ceramic paste. The intrinsic flow behaviour of the paste, during upsetting, is studied experimentally by using multi coloured paste samples, as well as by a finite element numerical computation. The flow behaviour of the paste is approximated by an elasto-viscoplastic material constitutive model and implemented by using an established finite element code. The material flow properties, which are necessary for the implementation of the numerical model, were obtained using the squeeze film and hardness indentation test configurations. The flow fields generated by the simulation are shown to be a good accord with the experimental observations. The experimental procedure for selecting the material parameters which are necessary for the implementation of the numerical model is described. The accuracy of the numerical method described is also evaluated by comparing the simulation results with experimental data obtained from the net upsetting force against the imposed relative displacement behaviour and the flow visualisation of deformed coloured layers. In these respects, a comparison of the finite element model predictions and the experimental results demonstrates a good mutual agreement.     

Numerical simulation of wire-coating low-density polyethylene: Theory and experiments

The wire-coating process was analyzed numerically making use of a particular die design employed in highspeed industrial operations. Both the lubrication approximation theory and a fully two-dimensional finite element analysis were applied under isothermal and nonisothermal conditions, respectively. Particular emphasis has been given to the heat transfer effects between the melt arid the solid and free boundaries. A variety of thermal boundary conditions was studied, ranging from adiabatic to constant temperature walls. The influence of dimension less groups such as Peclet, Nahme, and Biot numbers is examined. Oscillation-free solutions are obtained for the temperature field by using a standard finite element Streamline-Upwind/Petrov-Galerkin technique. Rheological data for a wire-coating low-density polyethylene (LDPE) resin (Alathon-3535) were used in the analysis. The predictions include pressure and temperature distributions, shear stresses and shear rates both at the die wall and the wire, and wire tension for different wire speeds. The numerical results are compared with a set of experimental data for LDPE in a typical die used by Du Pont Co. It is found that the isothermal lubrication approximation for power-law fluids overestimates pressure distributions when applied at die operating temperature. The nonisothermal finite element analysis gives better predictions, especially when realistic thermal boundary conditions are imposed, with the experimental results lying between those found from simulations assuming isothermal walls (upper limit) and adiabatic walls (lower limit).     

自流耐火烧注料流变学分析

利用宾汉姆体模型对耐火浇注料的流变参数进行了分析。认为研制自流耐火浇注料应兼顾以下四个方面：（１）颗粒级配；（２）超细粉的品种及加入量；（３）减水剂的品种及加入量；（４）水泥的矿物组成、细度和相应的添加剂     

The effect of melt compressibility on a high-speed wire-coating process

The effects of melt compressibility on a wire-coating process have been investigated, assuming that the compressible behavior of a polymeric melt obeys the Spencer-Gilmore equation of state. The compressible model is distinctly different from the incompressible model in two ways: (1) it has substantially lower pressure build-up within the die, and (2) the location of the maximum velocity is closer to the traveling wire position. As a result, the velocity profile within the die may change from a parabolic shape to a shape somewhat similar to that observed in a drag-flow case; and the shear stress generated by the fluid on the moving wire is no longer constant. Calculations indicate that the effect of melt compressibility during wire coming may not be neglected if the wire speed Ls greater than 50 cm/s (100 ft/min). In addition, the relationships between processing parameters and product coating thickness for both compressible and incompressible fluids are quite different.     

Numerical simulation of wire-coating: The influence of temperature boundary conditions

A finite element program has been used to analyze the wire-coating process of an MDPE melt. The melt is modeled by a nonisothermal Carreau model. The emphasis is on predicting an accurate temperature field. Therefore, it is necessary to include the heat conduction in the metal parts. A comparison is made with the results of a simulation that models the heat conduction in the metal head by means of a Biot boundary condition. The influence of the wire velocity, inlet temperature and power-law index will be examined.     

A network flow analysis of extrusion dies and other flow systems

A method is presented to calculate the slow viscous flow distribution in systems of passages, for which the major velocity components are substantially parallel to the axes of those passages. That condition is generally satisfied in flat extrusion dies, disc filters, in-line filters, and other now devices. A finite difference matrix method is initially used to determine the flow distribution for an assumed viscosity distribution. That flow distribution is next used to determine a new distribution of resistances, now based on a specified rheological equation. This process is iterated until there is no significant change in the flow distribution. The passages are subdivided in this method and replaced by a network of resistances. A few unknowns are introduced at one end of the network, which are solved at the other end, using a matrix marching routine. The method is described for newtonian flow through a flat die with equalizing channel, for which the analytic solution is known. Results are shown for that die for flow of power law liquids.     

结构施工时变内力分析

将使用阶段的结构作为受力分析对象是目前结构设计的常规做法,其结果是结构的最终内力、变位等与施工过程无关,这不符合工程实际情况,而现行的结构工程施工内力有限元法并没有提供相应的公式能解决这个问题。利用施工过程时变有限元分析方法对结构施工内力进行几何非线性分析,使计算结果更符合结构施工的实际情况,全过程跟踪分析结构在施工各阶段以及施工完毕时的变量,指出结构工程设计考虑施工过程时变效应影响的重要性。     

橡胶止水带受力状态下的有限元分析

应用大型有限元分析软件ANSYS,建立了橡胶止水带于混凝土中在不同受力形式下的有限元模型,分析了在各种不同工况下止水带变形及应力的分布,描述了其可能出现裂纹的位置。通过分析计算得出,当止水带在混凝土中受力而产生应力时,对止水带伸缩性起主要作用的是中心孔的横梁结构,此部位承受了较大的应力。与此同时,计算得出在各种工况下止水带所受最大应力以及止水带中心孔横梁结构中心节点的最大反力。分析了各种工况下最大应力与位移的关系,以及止水带中心孔横梁结构中心节点反力与位移的关系。为止水带的设计、生产、安装及模型优化提供了依据。     

Flow distribution in spiral mandrel dies

An analysis of flow of a power law fluid in a spiral mandrel die is presented. The analysis is applied to study the effect of various die design parameters on the flow distribution at the end of the spiral mandrel section. Three variables that have a very strong effect on the flow distribution are the number of grooves, the initial clearance, and the groove helix angle. The distribution is improved by increasing the number of grooves, by using a non-zero initial clearance, and a relatively small helix angle. Two more variables that have a significant effect on the flow distribution are the taper angle and the initial groove depth. The optimum taper angle was found to be between 1 and 3 degrees. The distribution uniformity improves with the initial groove depth, while the pressure drop reduces at the same time.     

A three-dimensional analysis of the effect of die body deflection in the design of extrusion dies

Flat extrusion dies operating under high pressure often experience a significant amount of distortion. Satisfactory designs of such dies require an accurate prediction of the deflection of the die body as well as the flow in the internal channels of the die. Here a three-dimensional deflection analysis is coupled to a three-dimensional flow analysis. Such analyses are now being used by design engineers on personal computers for routine design of deflecting dies.     

Rheological analysis of polyvinyl butyral near the glass transition temperature

In many polymer manufacturing operations, the material is processed near the glass transition temperature (T_g). Examples are thermoforming, blow molding, film blowing, hot embossing, forging, plastic welding, and de‐airing in safety glass lamination. In these processes, solid‐like behaviors such as strain hardening and yielding play important roles. These material properties cause the material to flow (or deform) in a way that substantially differs from a polymer melt. In order to understand the flow behavior near the T_g, polyvinyl butyral (PVB), a rubbery polymer used in safety glass lamination, was studied in this work. The flow properties of the polymer above the T_g were characterized by using both shear and elongational rheometers, and a tensile tester. The measured flow properties were described by a viscoelastic constitutive model.     

Rheological analysis of stabilized cerium-gadolinium oxide (CGO) dispersions

The objective of the present work is to generate general rheological criteria to investigate high solid loading dispersions suitable for the shaping of homogeneous ceramic bodies. Systematic analysis of the rheological properties of moderately low specific surface area (SSA) Ce_0.9Gd_0.1O_3-δ (CGO10) dispersions was performed in rotational and oscillatory modes. The dispersant content was optimized to attain fully stabilized dispersions. A critical upper limit for the ceramic content was introduced and denoted ϕ_h. It defines the limit to non-Newtonian flow and corresponds to the highest feasible volume fraction to which reproducible dispersions are achieved. The method proposed for its determination is based on the analysis of the flow index as function of the ceramic volume fraction. For the CGO dispersions formulated in this work, ϕ_h was found to be around (0.34 ± 0.04). The maximum volume fraction (ϕ_m) was also estimated and found to be (0.55 ± 0.01).     

Plastic compaction of porcelain products in porous dies

Translated from Steklo i Keramika, No. 8, pp. 18–19, August, 1990.