C形口模共挤出胀大的三维黏弹数值研究

运用有限元方法,采用PTT本构方程和Arrhenius黏度对温度依赖方程,对C形共挤口模中的聚丙烯(PP)和聚苯乙烯(PS)两熔体进行了三维非等温黏弹数值研究,主要研究了口模入口端熔体层间界面位置（r）对挤出胀大率和界面位置稳定性的影响,研究表明,随着r的增大,口模出口处熔体的二次流动程度减弱,挤出胀大率减小;在两熔体入口流率相等的情况下,取使得两熔体入口面面积近似相等的r值,能保证口模内及口模出口端熔体层间界面位置稳定性较好。     

Theoretical and experimental analysis of the die drool phenomenon for metallocene LLDPE

The die drool phenomenon, occurring as a result of the viscoelastic behavior of polymer melts during the flow, was experimentally investigated for metallocene‐based LLDPE on a specially designed annular extrusion die with the capability to control the stress state at the end of the die by the help of a specific type of cooling system. With the aim of understanding the die drool phenomenon in more detail, the flow at the end of the die was analyzed theoretically by the finite element method employing a modified White–Metzner model as the constitutive equation. It has been revealed that negative pressure occurs at the die lip area, which seems to be an important factor driving the onset of the die drool phenomenon. POLYM. ENG. SCI., 47:871–881, 2007. © 2007 Society of Plastics Engineers     

用双毛细管流变仪对 HDPE与LLDPE挤出压力振荡的研究

用RH2000恒速型双毛细管流变仪研究了3种HDPE和3种LLDPE的挤出压力振荡现象。通过压力振荡图、流动曲线和挤出物表观3方面,对两种类型聚乙烯压力振荡的差异进行了分析。发现HDPE的压力振荡很明显,振幅在2~3MPa,而LLDPE的振幅很小,甚至在压力-时间图上看不出来。6种聚合物的流动曲线都发生了断裂。在柱塞下降速度恒定下发生压力振荡时,流速并不恒定,粘界面条件下流速较小,滑界面条件下流速较大,并通过计算获得了3种HDPE发生压力振荡时对应于粘界面与滑界面的流速。HDPE和LLDPE在粘界面条件下挤出物都是鲨鱼皮,而滑界面条件下HDPE的挤出物类似于无规破裂,LLDPE挤出物表观较光滑,且没有鲨鱼皮。     

Conjugate transport in polymer melt flow through extrusion dies

A numerical study is carried out on the conjugate thermal transport in polymer and food melts flowing through extrusion dies. The simulation is performed to determine the influence of conduction through the die wall and of the thermal boundary conditions on the transport in the fluid and on the conditions at the outlet. An extrusion die with a uniform temperature or heat transfer coefficient specified at the outer surface is considered. It is found that, because of conduction in the solid wall, important physical variables such as centerline velocity, pressure drop, bulk temperature of the fluid and shear experienced by the fluid are strongly affected by the boundary conditions, as well as by the wall thermal conductivity and thickness. Channels of different geometries are used for the study. The flow in a circular straight tube with constant wall thickness is studied first. Flow and thermal transport in different, constricted, channels are studied next. Different wall materials are also considered. Comparisons with some experimental results are presented, indicating good agreement. The fluids considered in this study are highly viscous, polymer melts. Due to high viscous dissipation and temperature-dependent viscosity, the flow and heat transfer are coupled and the problem is quite complicated. The results show that, for some operating conditions, the bulk temperature can be high enough to cause significant heat transfer from the fluid to the wall. The downstream variation in the pressure and temperature are calculated. The thermal boundary conditions are found to have a strong influence on the temperature field and thus on the flow. The general dependence of pressure drop on temperature, flow rate, and geometry is investigated. Several other basic aspects of this problem are also discussed.     

Abnormal Rheological Behavior of Linear Low Density Polyethylene Melts at High Shear Rate

Abnormal rheological behavior is observed during capillary extrusion with a linear low density polyethylene (LLDPE). The nominal viscosity in the stick-slip flow regime and the gross melt fracture regime increase with a rise in temperature. This is attributed to slippage of the polymer melts. It is generally accepted that the occurrence of slippage can be detected by a Mooney analysis. However, it is found that the Mooney analysis is not applicable to detect the occurrence of slippage in the gross melt fracture regime. This is attributed to the main assumptions in the Mooney analysis being invalid in this regime, mainly due to the presence of a turbulent-like flow pattern. We suggest that failure of the time-temperature superposition principle can be used as an indication of slippage for polymer melts.     

The effect of teflon™ coatings in polyethylene capillary extrusion

Two LLDPE resins were used in this work to determine the critical conditions for the occurrence of wall slip and melt fracture in capillary extrusion. It was found that the polymer-metal interface fails at a critical value of the wall shear stress of about 0.1 MPa and, as a result, slip occurs. At values of wall shear strees of about 0.18 MPa the extrudate surface appears to be matte, while small amplitude periodic distortions (sharkskin) appear on the surface of extrudates at wall shear stresses above 0.25 MPa. Using a special slit die, the polymer–wall interface was coated with Teflon? in order to examine the effect of this coating on the processability of polyethylenes. It was found that use of Teflon? promotes slip, thus reducing the power requirement in extrusion and, most importantly, eliminates sharkskin at high extrusion rates. © 1995 John Wiley & Sons, Inc.     

聚合物熔体在圆锥口型的挤出胀大方程

在聚合物熔体入口收敛流变中引入纯拉伸流变概念，应用张量理论进行了定量分析，导出了一个描述圆锥口型的挤出胀大方程。研究表明，入口收敛拉伸流变，是取合物熔体产生了强烈的弹性形变，导致了较大的挤出物胀大比。     

Oriented structure formation during polymer film extrusion

An experimental technique is described for producing fiberreinforced polymer films by inserting needle-like obstructions in a film-extrusion die. The, needles act as nucleation sites, generating a highly extensional local flow field, which causes sufficient orientation to induce the formation of oriented fibrillar crystallites embedded in a much less oriented matrix. To study the effectiveness of the above technique, linear low density polyethylene (LLDPE) and blends of linear low density with high density polyethylene (HDPE) were extruded through a film die with converging walls, with one or five needles inserted parallel to the extrusion direction, Microscopy observations, birefringence, and differential scanning calorimetry (DSC) measurements performed on the produced films showed in all cases that the presence of the needle induced the formation of a more oriented phase, which in most cases had a birefringence at least an order of magnitude higher than the film matrix. The best results were obtained in the case when blends of HDPE and LLDPE were extruded. The oriented structures obtained in this case consisted of HDPE and exhibited not only high birefringence but melting point elevation as well, indicating their fibrous nature.     

Use of a fluoroelastomer processing aid with non-olefin polymers

A die coated with fluoroelastomer (FE) has been used in the extrusion of thermoplastics other than conventional polyolefins, and including chlorinated polyethylene, polystyrene, polycarbonate, and an ionomer. Inverse gas chromatography was the source of acid/base interactions between FE and each of these matrix polymers. FE was found to be effective in reducing the apparent melt viscosity when the host polymer was acidic, or non-interactive. With strongly interactive, basic polymers the effectiveness of FE flow modifier was anulled. These polymers may be considered as effective purging agents for extrusion equipment containing FE residues.     

聚合物熔体在任意长径比圆锥口模的挤出胀大唯象研究

深入讨论了聚合物熔体在不同长径比、不同角度圆锥口模的挤出胀大现象及机理。对口模长径比较小的挤出胀大,由于熔体入口拉伸弹性变形来不及松弛,产生较大的挤出胀大;对长径比较大的口模,熔体在平直流道内停留时间较长,入口弹性形变逐渐松弛,这时主要是流动剪切应变引起的弹性变形,产生较弱的挤出胀大,比长径比小的挤出胀大来得小,并且聚合物熔体的挤出胀大随着长径比的增大而趋向一恒定值。结果还表明:聚合物熔体在圆锥口模的挤出胀大受到挤出口模入口角影响。当L/D较小时,挤出胀大与口模入口角有关;当L/D较大时,口模入口角对挤出胀大影响较小。     

Thermoplastic extrusion—the mechanism of the formation of extrudate structure and properties

Systems processed by thermoplastic extrusion can be regarded as heterophase polymer melts of incompatible water-plasticized biopolymers. In the process of thermoplastic extrusion, proteins and polysaccharides are melted at high pressure and temperature below the temperature region of their thermal decomposition. Dispersed particles of these systems can be deformed in flow. The mixed-melt anisotropic structure, formed in flow, is fixed by rapid conversion of the melt jet that lets the extruder die from a viscous state to a rubber-like state and then to a glassy state caused by cooling and drying. Incompatibility of proteins and polysaccharides in their water-plasticized melt mixtures impacts on structure formation and texturization during thermoplastic extrusion. Presented at the 20th ISF World Congress and 83rd AOCS Annual Meeting and Expo, May 10–14, 1992, Toronto, Ontario, Canada.     

Fluorine-containing arborescent polystyrene-graft-polyisoprene copolymers as polymer processing additives

Linear low density polyethylene (LLDPE) can suffer from melt extrusion defects including sharkskin, cyclic melt fracture, and gross melt fracture during processing. Arborescent polymers are dendritic macromolecules with characteristics, such as a compact structure and a rigid spherical topology, making them potentially useful as polymer processing additives (PPA) to alleviate melt extrusion defects. Arborescent polystyrene-graft-polyisoprene copolymer samples were synthesized from polystyrene substrates of linear and branched architectures functionalized with acetyl groups, and coupled with polyisoprene macroanions. A linear polyisoprene sample was also investigated for comparison. The polymers were hydrosilylated with (tridecafluoro-1,1,2,2-tetrahydrooctyl)dimethylsilane on 17-52% of the isoprene units and blended with LLDPE at 0.1 and 0.5% w/w to evaluate their performance as PPA by extrusion at different shear rates. All the samples led to some degree of improvement in the extrusion of LLDPE, albeit the performance of the branched additives was inferior to a commercial fluoroelastomer PPA. The lower molecular weight and more compact (G0 or comb-branched) PPA generally performed better than those with a high molecular weight. Several PPA samples induced the early onset of cyclic melt fracture but glossy, defect-free surfaces were obtained at higher shear rates. This suggests that a minimum shear rate is required for these additives to coat the extrusion die under the experimental conditions used.     

Sharkskin and oscillating melt fracture: Why in slit and capillary dies and not in annular dies?

The sharkskin and stick‐slip polymer extrusion instabilities are studied primarily as functions of the type of die geometry. Experimental observations concerning the flow curves, the critical wall shear stress for the onset of the instabilities, the pressure and flow rate oscillations, and the effects of geometry and operating conditions are presented for linear low‐density polyethylenes. It is found that sharkskin and stick‐slip instabilities are present in the capillary and slit extrusion. However, annular extrusion stick‐slip and sharkskin are absent at high ratios of the inside‐to‐outside diameter of the annular die. This observation also explains the absence of these phenomena in other polymer processing operations such as film blowing. These phenomena are explained in terms of the surface‐to‐volume ratio of the extrudates, that is, if this ratio is high, sharkskin and stick‐slip are absent. POLYM. ENG. SCI., 2008. © 2007 Society of Plastics Engineers     

Numerical study of nonisothermal polymer extrusion flow with a differential viscoelastic model

A numerical study of nonisothermal viscoelastic flow is conducted to investigate the complex flow characteristics of polymer melts in the extrusion process. A general thermodynamic model for the energy conversion related to viscoelastic fluid flow is introduced. The mathematical model for three‐dimensional nonisothermal viscoelastic flow of the polymer melts obeying a differential constitutive equation (Phan‐Thien and Tanner model) is established. A decoupled algorithm based on the penalty finite element method is performed on the calculation. The discrete elastic‐viscous split stress (DEVSS) algorithm, incorporating the streamline‐upwind Petrov‐Galerkin (SUPG) scheme is employed to improve the computation stability. Essential flow characteristics of polymer melts in the extrusion die for hollow square plastic profile is investigated based on the proposed numerical scheme with ignoring the outer thermal resource. The energy partitioning, which quantified the conversion of mechanical energy into thermal energy, is discussed. The effects of volume flow rate and die contraction angle upon the flow patterns are further investigated. POLYM. ENG. SCI., 2008. © 2007 Society of Plastics Engineers     

Flow of polymeric melts in short tubes

In an attempt to further understand the flow of polymeric melts through gates in injection molding, the present investigation deals with measurement of pressure drops during isothermal extrusion of fiber-filled and unfilled polystyrene, polypropylene, and polycarbonate melts in short tubes with sudden contraction at high shear rates typical of injection molding. Flow curves for these materials have been determined over a wide range of shear rates at various temperatures by using a capillary rheometer and extruder. Measurements indicate that rheological properties of fiber-filled melts after injection molding differ from those of fresh samples. Moreover, it has been found that decreasing the tube length increases the slope of the curve for pressure drop vs. Volumetric flow rate. Extra pressure losses due to end effects have been determined which show that at high shear rates these losses can reach levels as high as 100 bar, with the effect being higher for the fiber-filled melts. By using a viscoelastic consitutive equation, the extra pressure losses have been separated into entrance and exit losses. Model parameters required for this calculation have been determined from viscosity-shear rate curves for the melts. For various polymers, master curves useful for industrial applications have been constructed for the extra pressure losses.     

The effct of asymmetries of die exit geometry on extrudate swell and melt fracture

Experimental investigations were performed to see how the die exit geometry and the extrusion velocity influence on extrudate swell and melt fracture for several polymer melts [low-density polyethylene, styrene-butadiene rubber (SBR) and SBR/HAF (carbon black) compound]. Four different types of die exit geometry were considered; 0° (symmetric. usual capillary die), and 30°, 45° and 60° (asymmetric dies) were chosen for the die exit angle. Extrudate diameters were measured without draw-down under isothermal condition. Polymer melts were extruded into an oil that has the same density and temperature as those of the extrudate. Extrudate swells from dies with different diameters were correlated with volumetric flow rates. It was observed that the extrudate swell increases with increasing volumetric flow rate and exhibits through a minimum value at about 45° die exit angle. As to the fracture phenomena, it was observed that the critical shear for the onset of melt fracture increases with the increasing die exit angle up to 45°. However, for 60° die exit angle, the onset of melt fracture is again similar to that of 0° exit angle.     

Effect of molecular structure on extrudate swell behavior for different thermoplastic melts in an electro‐magnetized die

The extrudate swell ratio of five different thermoplastic melts flowing in a constant shear rate rheometer having a capillary die with and without application of magnetic field was studied. The effects of the magnetic flux direction and density, die temperature, and wall shear rate on the extrudate swell and flow properties were investigated. The experimental results suggested that an increasing wall shear rate increased the swelling ratio for the polystyrene (PS), LLDPE, and PVC melts, but the opposite effect was observed for the ABS and PC melts. The extrudate swell ratio for the PS, ABS, PC, and LLDPE melts decreased with increasing die temperature, the effect being reversed for the PVC melt. Thermoplastic melts having high benzene content in the side‐chain and exhibiting anisotropic character were apparently affected by the magnetic field, the extrudate swell ratio increasing with magnetic flux density. The effect of the magnetic field on the extrudate swell ratio decreased in the order of PS → ABS → PC. The extrudate swell ratio for the co‐parallel magnetic field system was slightly higher than that for the counter‐parallel magnetic field system at a high magnetic flux density. POLYM. ENG. SCI., 47:270–280, 2007. © 2007 Society of Plastics Engineers.     

Extrusion characteristics of thermoplastic melts containing fluoropolymers

This paper investigates the influence of fluoropolymers on the extrusion characteristics of three different thermoplastic melts. The presence of a small amount of fluoropolymer reduced the pressure drops, extrudate swell, and retarded the onset of surface defects in linear low density polyethylene (LLDPE), while little or no effect was found in polystyrene (PS) and polyamide 12 (PA 12). Among the fluoropolymers investigated in this study, vinylidene fluoride-hexafluoropropylene (VDF-HFP) copolymer was the most effective additive in reducing the pressure drop and extrudate distortion of LLDPE, polyvinylidene fluoride (PVDF) intermediate, and tetrafluoroethylene-hexafluoropropylene (TFE-HFP) copolymer exhibited the least effectiveness. Contact angle measurements on the die surface indicated that all fluoropolymers preferentially deposit on the die surfaces during the extrusion of LLDPE and PS blends with small amounts of fluoropolymers.     

Computer-aided experiment of using real-time small angle light scattering image processing technique for visual characterization flow field of polymer melts

The phenomenon of small angle light scattering (SALS) has been applied to the actual extrusion molding process. The current study utilizes a self-designed mold with built-in windows for observation of polymer melts within a slit die. A high-speed charge-coupled-device (CCD) camera is used to record the SALS images in real-time with different process conditions for subsequent analysis. Modification algorithm has been proposed to eliminate the effect of multiple scattering. Flow behavior of polymer melts is simulated and analyzed by real-time SALS image processing technique. Visualization is performed via a high-performance computer-aided analysis software which allows on-line data acquisition and characterization the flow field of polymer melts.     

Thermomechanical analysis and modeling of the extrusion coating process

During the extrusion coating process, a polymer film is extruded through a flat die, stretched in air, and then coated on a substrate (steel sheet in our case) in a laminator consisting of a chill roll and a flexible pressure roll. The nip, i.e. the area formed by the contact between the pressure and the chill rolls, constitutes the heart of the extrusion coating process. Indeed, in this region, some of the most critical properties, such as adhesion, barrier properties, optical properties, are achieved or lost. In this article, we first present an experimental investigation of the coating step, which enables to characterize the leading thermomechanical phenomena. It is shown that there is no polymer macroscopic flow in the nip, but a local flow within the asperities of the steel substrate surface. This microscopic flow, at the interface between the film and the substrate, is slowed by strong cooling conditions in the nip. Several models are then proposed, giving access to the temperature profile through polymer thickness and substrate, the pressure distribution in the nip as well as the behavior of the polymer melt in the nip at the interface with the substrate. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers