Die geometrical effect on annular die swelling

Die geometrical effect on Newtonian annular jet swelling was studied using a finite element method. Numerical result shows what we expected in the limiting cases of die swelling with annular die geometries, i.e., variation of the die gap for straight dies follows the limiting behavior of capillary and planar die swellings. Also the effect of upstream die geometry was investigated. The result shows that final extrudate dimensions are generally influenced by its previous history in the die, i.e., not only by shearing action but also by elongational effect caused by narrowing die gap.     

Experimental observation on entrance flow inside extrusion dies

A flow visualization technique was employed to observe the flow fields at the entrance region inside extrusion dies. Four types of solutions, i.e., Newtonian, shear-thinning, Boger, and viscoelastic fluids, were used as test fluids. Entrance flows inside center-fed and end-fed dies of different configurations were examined. It was found that, even though the swirling motions appear in the entrance region of center-fed dies, no vortex develops. Shear-thinning behavior tends to enhance swirling, but fluid elasticity suppresses the swirling. On the other hand, vortices exist at certain end-fed die geometries, and vortices will grow if the fluid exhibits shear-thinning, yet fluid elasticity reduces the size of vortices. Several inlet designs effective for eliminating vortices were studied.     

如何解决挤出模头渗料难题

模头咬模现象也称为口模泄料、模头渗料、积垢,是挤出加工过程中一种常见的故障问题。这种故障会引起多种产品缺陷,如可见挤出产品表面缺料、加工流程中断和产品完全开     

Some theoretical results of swelling in fiber or particle filled polymers

The problem of inhomogeneous swelling of polymers around particles or fibers is treated analytically within the context of Gaussian statistics for polymer chains and the Flory-Huggins approximation for the interaction between polymer and solvent. The resulting differential equation for the displacement field around the inclusions is quite simple; it contains a homogeneous part and a nonlinear part. The nonlinear part of the differential equation is preceded by a parameter n(1 ? 2_χ), where n is the molecular weight between crosslinks and χ is the Flory-Huggins interaction parameter. Accordingly, the displacement field for the case of theta solvent, i.e. χ equals 0.5, can be solved exactly. In good solvents, the problem can be treated as a perturbation of the theta solvent condition. The amount of deviation from the theta solvent solution is obtained through a numerical technique and the results are tabulated as a function of n(1 ? 2_χ).     

三元乙丙橡胶挤出压力振荡现象的影响因素

采用恒速型双筒毛细管流变仪研究了剪切速率、口模温度和含氟弹性体偏氟乙烯-六氟丙烯-四氟乙烯共聚物(PPA)对三元乙丙橡胶(EPDM)挤出压力振荡现象的影响.结果表明,EPDM高速挤出时,升高温度可以推迟挤出压力振荡现象的发生;振荡周期和振幅均随剪切速率的增加而减小;加入PPA可以改善EPDM挤出物的外观,且挤出压力低于纯EPDM,PPA质量分数越大,其降低幅度越大.     

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。     

Effects of die temperature on extrudate swell in screw extrusion

Extrusion of a hot polymer melt through a cooler die zone substantially increases the extrudate swell of some thermoplastics. This effect was examined for commercial samples of low-density polyethylene, polypropylene, and polystyrene. Two conflicting effects come into play during extrusion of a thermoplastic. Colder melt temperatures promote increased extrudate swell, but the same conditions also facilitate molecular disentanglement and reduced melt elasticity and die swell. Since the extrusion process itself may affect the relation between die swell and melt temperature, laboratory-scale measurements for the design of processes like blow molding are better carried out with small-scale screw extruders than with capillary rheometers. For some applications it may be advantageous to use a polymer whose die swell is particularly responsive or unresponsive to die temperature variations. The procedure described in this article can be used effectively to monitor this characteristic.     

Some studies on the swelling behavior of polyethylene

When polymers are extruded from a capillary, the amount of extrudate swelling is generally found to be a function of shear rate and capillary dimensions. Experiments with polyethylene now show that when shear rate is replaced by an average transit time in the capillary, postextrusion swell becomes a function of transit time only. Plots of swell versus transit time are independent of the applied stress and reduce to a single line for capillaries of different dimensions. The swell was found to be dependent upon the average molecular weight and the molecular weight distribution. Little correlation, however, was found with the amount of high molecular weight fractions.     

Die swell from capillary die and slit die: A theoretical study

Assuming polymer fluids obey the CEF equation, equations concerning die swell from capillary die and slit die were derived. The die swell of polymer increased with increasing shear rate and recoverable shear. Our theoretical predictions conformed well with the experimental data of die swell for PP and PS.     

Experimental study on extrudate swell and die geometry of profile extrusion

The objective of this study was to determine the die swell behavior of a polymer melt and to design a die for forming a polymeric extrudate with a desired shape using profile extrusion. Polystyrene pellets were chosen to perform the profile extrusion experiments. First, the polystyrene pellets were melted and pushed through a quarter ring profile. The profile of the swelled extrudate agreed with the numerical predictions. A modified die was designed to produce a quarter ring profile extrudate based on the direct extrusion problem (DEP) prediction. Polystyrene pellets were also melted and pushed through the modified die. The experimental results were close to the computational results. The melting temperature, die length, and melting residence time affect die swell behavior. The die swell ratio becomes smaller as the melting temperature and melting residence time are increased. As the die length is increased, the die swell ratio is lowered. According to the die geometry predictions, an extrudate with the desired profile can be made precisely.     

Some comments on chain dimensions in the semi-crystallized state

In view of recent experimental and theoretical work on the conformation of semicrystalline polymers analytical calculations of the mean chain dimensions are performed for models containing short crystalline sequences and alternating amorphous sequences. The results are analysed and illustrated by means of the parameters for polyethylene and isotactic polystyrene. The invariance of the radius of gyration with crystallinity can only be explained by introducing chain folding in the crystalline sequence. The number of folds required is discussed considering the data obtained both in the Guinier and intermediate ranges of scattering vectors.     

Some comments on the microstructure of hardened cement pastes

The effect of water/cement ratio of hardened cement pastes in the range 0.23 to 0.71 on the micropore structure as revealed by mercury intrusion and nitrogen adsorption porosimetry is discussed. The relationship between these micropores and the solid minerals as revealed by electron microscopy is also discussed.     

Some comments on the hydrodynamics of thin liquid films

This note discusses the effect of gas phase shear stress on the hydrodynamics development of thin liquid films, and presents some experimental data for film thickness in the entrance region.     

Die lines in plastics extrusion: Film blowing experiments and numerical simulation

Die lines are annoying surface striations that can destroy extrudate aesthetics and compromise product properties. The available literature on die lines has been previously reviewed to explore how die lines arise and how to minimize them. Many factors have been found to influence die line formation. Some techniques are effective in suppressing die lines, mainly discovered through manufacturing experience. Here, we first briefly introduce how to measure and characterize die lines (die line metrology). Film blowing experiments are then conducted to study how various die defects, material properties and operating conditions affect die lines. The empirical relations between die lines and die defects, material properties and operating conditions are explored using dimensionless groups. We also investigate the fluid mechanics in die line formation. The free surface flow of polymer melt leaving the die lips is simulated. The qualitative relations between die defects and die lines are explored. The results are compared with film blowing experiments. Finally, new ways to suppress die lines and prospects for future studies are proposed. Polym. Eng. Sci. 44:1811–1827, 2004. © 2004 Society of Plastics Engineers.     

Effect of boron nitride on melt fracture phenomena and processability of lldpe during extrusion

Melt fractures related to processing instabilities limit processing rates in many commercially important polymer processing operations, such as fiber spinning, film blowing, extrusion, and various coating flows. Therefore, melt fracture is responsible for deteriorating the quality and the mechanical properties of final products for rates greater than a critical processing one at which melt fracture occurs. In this study, a commercial linear low-density polyethylene (LLDPE) was modified by adding a small amount of boron nitride (BN) during extrusion in order to improve processability. Capillary rheometry was used to assess processability at various temperatures, levels of applied shear rate, and the length-to-diameter (L/D) ratio for both the pure resin and resins containing boron nitride. Also, parallel-plate rheometry was used to evaluate the dynamic rheological properties of these resins. The relationship between the characteristic relaxation time and the critical shear rate for the onset of melt fracture and slip is discussed.     

影响挤出膨胀因数和收缩率的工艺及设备因素

阐明影响挤出胎面膨胀和收缩的主要工艺及设备因素是口型板的结构和尺寸,挤出条件以及设备。口型的冠部截面大,口型圆角Ｒ小,流胶口大,排胶温度高,挤出速度慢,则膨胀因数小,反之,膨胀因数大。冷却效果好,胎面稳定性高,收缩率小。认为：要减小胎面变形,口型总宽度应大于半成品总宽度。差值为３０￣５０ｍｍ,冠宽略小于半成品冠宽,差值为３￣１０ｍｍ;口型圆角Ｒ取值以２￣３ｍｍ为宜。