Flow balancing in die design of wood flour/HDPE composite extrusion profiles with consideration of rheological effect

This article presents an experimental study on the flow balance of an extrusion die for various wood flour (WF)/high‐density polyethylene (HDPE) compositions. Flow balancing, in the design of a thermoplastic extrusion die, is dependent on the material rheological properties so that a change in the material, in some cases, may result in a total redesign of the die. To investigate the importance of this particular effect, the flow balance of an extrusion die, with a U‐shaped profile having uneven wall thicknesses, was undertaken. The main feature of the die was an adjustable restrictor implemented for fine balancing similar to that employed in the slit dies. The rheological influence of wood plastic composites (WPCs) on the flow balance of the die via loading various WF contents, 40, 60, and 70% by weight, was experimentally investigated; flow balancing was performed via varying the height of the restrictor bar. Interestingly, the results showed that, for a high WF content (above 60%), the issue of flow balancing for an uneven wall thickness profile is much less complicated because of the plug flow behavior of the composite. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

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.     

基于流动平衡的塑料挤出模优化设计

在构建流道三维参数化模型的基础上,将有限元数值分析和优化技术相结合,以口模出口处型材截面上各子区域平均流速相等为优化目标,以压缩段入口壁厚和平直段长度为设计变量,对挤出模优化设计进行了研究。并以实例说明了优化设计的具体过程,验证了所用方法的正确性和有效性。     

In‐line versus off‐line rheological measurements on S‐PVC formulations releant for flow in extrusion dies

The supermolecular structures present in S‐PVC during processing are dependent on the thermomechanical history and affect the rheological behavior. Motivated by the need to have reliable rheological data relevant for numerical modeling of 3D flows in extrusion dies, we compared standard off‐line capillary measurements with‐inline measurements obtained from an instrumented die mounted on an extruder. Different PVC formulations showed different slip behavior in‐line, and also differences in comparison with the off‐line results. Cogswell analysis on entry pressure drops was used in order to obtain elongational information. For one formulation, with plug flow both in off‐line and in‐line measurements, an analysis based on the resistance to flow in contracting zones was the only way to get flow data that could be used in 3D profile simulations. The temperature sensitivity of measurements was used in order to discuss our results in terms of structural changes within the melt due to differences in thermomechanical history and formulation. Finally, an example of a 3D simulation of the flow of this formulation in an industrial extrusion die is given and compared with observations.     

Flow behavior of polypropylenes with different molecular structures in a coat‐hanger die

An experimental investigation of the flow behavior of three polypropylene melts with different molecular structures during extrusion through a coat‐hanger die is presented. Two linear and one long‐chain branched material, rheologically characterized in shear and elongation, were investigated. Using laser–Doppler velocimeter measurements of the velocity profiles across the gap height were performed at five various locations along the die. The uniformity of the velocity distribution along the die has been assessed using the maximum velocities v₀ of the corresponding velocity profiles across the gap. The velocity distribution along the die changes with throughput and temperature. Regarding the rheological properties, it was found that the power‐law index of the viscosity as a function of shear rate has a decisive influence on the uniformity of flow but that the pronounced strain hardening in elongation typical of the long‐chain branched polypropylene is not reflected by the velocity distribution along the die. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

Rheological properties,flow visualization and extrudate swell of NR compound by rotating‐die rheometer

An experimental apparatus coupled with a rotating die system was especially designed and manufactured to study the rheological properties, flow patterns and swelling behavior of natural rubber (NR) compound for different shear rates and die rotating speeds at a test temperature of 110°C, the results being compared with those by the static capillary die. It was found that NR compound used exhibited psuedoplastic non‐Newtonian behavior. The rotation of the capillary die could reduce the extrusion load. The wall shear stress for any given shear rates increased with increasing die rotating speed. The fluctuation of the entrance pressure drop increased with increasing die rotating speed. The flow pattern development in the rotating‐die rheometer was different from that observed in the static die. The flow patterns in the rotating die were clearly unstable and contained two flow components which included axial flow along the barrel and circumferential flow at the die entrance. The size and shape of the axial and circumferential flows were more dependent on the piston displacement. It was found that the swelling ratio of the NR compound decreased with increasing die rotating speed. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers.     

Profile Extrusion: Experimental Assessment of a Numerical Code to Model the Temperature Evolution in the Cooling/Calibration Stage

Designing an industrial calibration system that ensures fast and uniform cooling of a complex extruded profile is always a difficult task. Numerical tools are a valuable aid for the design of these systems in order to achieve fast, low‐cost, and effective solutions. This work comprises the experimental validation of a numerical code, previously developed to support the design of profile extrusion calibration systems, using data collected from an industrial extrusion line. The referred numerical code was developed in the open‐source computational library, OpenFOAM, being able to predict the heat exchanges between the polymer and the calibrator during the profile extrusion cooling stage. The good agreement obtained between the numerical code predictions and the experimental measurements allowed us to confirm the code accuracy. Subsequently, and in order to demonstrate the usefulness of the numerical code, a numerical study was carried out based on an optimization algorithm aimed at improving the performance of the studied calibration system. As a result, the study led to the proposal of an alternative setup, which presents a simpler constructive solution and a better performance than the existing one. POLYM. ENG. SCI., 59:2367–2376, 2019. © 2019 Society of Plastics Engineers     

Die design for rigid PVC—the effect of die land length on extrudate swell

PVC profile extrusion compounds have a unique morphology. While other polymers gradually decrease in extrusion die swell with increasing length/thickness (L/D) ratio, PVC profile extrusion compounds have a low die swell, quite independent of the die's L/D ratio in the range of 5 to 20. The fact that the die land length can be changed without changing the extrudate swell is an important consideration, which makes die design and balancing dies simpler and easier for PVC profile extrusion compounds. While other polymers substantially increase extrudate swell with increased shear rate, the swell of the PVC profile compounds is not much affected by shear or extrusion rate. This unique behavior allows wider processing latitude in profile extrusion and faster extrusion rates than with other polymers. Another unique factor in the rheology of PVC profile extrusion compounds is that extrusion die swell increases with increasing melt temperature, while other polymers have decreasing die swell with increasing melt temperature. The unusual rheology of PVC profile extrusion compounds is attributed to its unique melt morphology, where the melt flow units are 1 um bundles and molecules that have low surface to surface interaction and entanglement at low processing temperatures but increased melting and increased entanglement at higher processing temperatures. Other polymers, unlike PVC, have melt flow at the molecular level.     

发射药药料离模膨胀和流动均匀性模拟及在模具设计中的应用

为了分析挤出成型过程中模具结构参数对七孔硝基胍发射药离模膨胀率及流动均匀性的影响规律,采用计算流体力学方法,对挤出成型过程进行模拟计算,讨论了模具各结构参数重要性的主次关系;对七孔发射药制备模具进行了结构优化,并进行了实验验证。结果表明,模具收缩角对膨胀率和药料出口速度均匀性的影响最大,压缩段高度次之,成型段长度的影响最小。模具优化后流道出口端速度分布均匀性提高36.53%,表明该模拟计算的可靠性与实用性。     

Effects of Extrusion Rate,Temperature, and Die Diameter on Melt Flow Properties During Capillary Flow of Low-Density-Polyethylene

The melt flow properties of a low-density polyethylene were measured at test temperatures varying from 140 to 170°C and in a wide range of extrusion rates by means of a capillary rheometer, to identify the influence of extrusion conditions (such as temperature, shear rate, and die diameter) on the melt flow behavior in the present paper. The results showed that the entry pressure drop increased nonlinearly with an increase of the piston speeds, and it decreased with an addition of the die diameter. The melt shear flow obeyed roughly the power law and the melt shear viscosity decreased approximately linearly with an increase of the true shear rates in a bi-logarithmic coordinate system. The dependence of the melt shear viscosity on temperature accorded approximately the Arrhenius expression. Under these experimental conditions, the entrance pressure drop increases as an exponential function with an addition of the channel contraction ratio.     

LDPE熔体在挤出口模流动时入口区流型的观测

应用毛细管流变仪测量了在接近实际挤出工艺条件下低密度聚乙烯熔体的流变性质，应用流动可视化技术，观测了试样熔体流经突然收缩的轴对称口模时的入口收敛流型，发现，在口模入口前区的两侧存在明显的环流区，环流区长度随着挤出速率和流道收缩比的增加而增大，而随着温度的升高而减小，采用环流区长度的公式估算了实验条件下试样的环流区长度值，结果表明，预测值与实测值之间有较好的一致性。     

Pressure drop through tapered dies

A simple method is presented for calculating the pressure drop for the flow of power law liquids in dies with a wide slit profile and with vertical and/or lateral tapers, as well as in dies with the shape of a circular truncated right cone. Tapered dies are known to give improved extrudate quality and/or higher output rates without encountering extrudate defects which occur in dies with parallel channels at similar extrusion pressures. A possible ultimate optimization of the extrusion process—as far as die design is concerned—is discussed. It is suggested that this be based upon an extension of the method from dies with a rectilinear convergent taper to dies with a curvilinearly converging channel aspect the generation of which latter is indicated.     

Effect of dynamic cross‐linking on melt rheological properties of polypropylene/ethylene‐propylene‐diene rubber blends

Study of melts rheological properties of unvulcanized and dynamically vulcanized polypropylene (PP)/ethylene‐propylene‐diene rubber (EPDM) blends, at blending ratios 10–40 wt %, EPDM, are reported. Blends were prepared by melt mixing in an internal mixer at 190°C and rheological parameters have been evaluated at 220°C by single screw capillary rheometer. Vulcanization was performed with dimethylol phenolic resin. The effects of (i) blend composition; (ii) shear rate or shear stress on melt viscosity; (iii) shear sensitivity and flow characteristics at processing shear; (iv) melt elasticity of the extrudate; and (v) dynamic cross‐linking effect on the processing characteristics of the blends were studied. The melt viscosity increases with increasing EPDM concentration and decreased with increasing intensity of the shear mixing for all compositions. In comparison to the unvulcanized blends, dynamically vulcanized blends display highly pseudoplastic behavior provides unique processing characteristics that enable to perform well in both injection molding and extusion. The high viscosity at low shear rate provides the integrity of the extrudate during extrusion, and the low viscosity at high shear rate enables low injection pressure and less injection time. The low die‐swell characteristics of vulcanizate blends also give high precision for dimensional control during extrusion. The property differences for vulcanizate blends have also been explained in the light of differences in the morphology developed. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1488–1505, 2000     

聚合物双腔微管直角挤出模具非对称流动平衡设计方法

针对聚合物双腔微管直角挤出模具非对称流道结构,基于聚合物流动平衡原理,建立了直角挤出模具非对称流道结构参数关系。在直角挤出模具的挤出成型方向上,采用反向流道的方法计算流道长度,并对反向流道的终点进行拟合,基于拟合偏差构建了目标函数,对模具非对称流道结构进行了优化设计。根据优化结果设计制造了双腔微管直角挤出模具,并且进行了挤出成型实验。实验结果表明挤出模具非对称流道设计是合理的,证明了聚合物双腔微管直角挤出模具非对称流动平衡设计方法的正确性和有效性。     

Investigation of die drool phenomenon for HDPE polymer melt

In this work, the die drool phenomenon analysis has been performed for HDPE polymer melt by using a specific type of experimental set-up, digital image analysis technique and rheological tools. It has been revealed that the thermally induced degradation occurring inside the processing equipment may lead to HDPE melt elasticity enhancement, which promotes unwanted material accumulation at the end of the extrusion die.     

马鞍型异型材挤出成型过程三维等温粘弹性的数值模拟

基于PTT粘弹性本构模型,通过马鞍型异型材挤出成型过程的全三维稳态等温有限元数值模拟,系统研究了聚合物粘弹性流变性能参数和成型工艺参数对异型材口模挤出成型过程的影响规律,并揭示了其影响机理.研究结果表明,聚合物异型材口模挤出离模膨胀是由口模出口处的二次流动引起,离模膨胀比随着口模出口处的二次流动强度增加而增大.聚合物异型材口模挤出离模膨胀随着进口流量和聚合物熔体松弛时间的增加而增加,而随着聚合物熔体材料常数和粘度比的增大而减小.     

Effect of shear history on flow instability at capillary extrusion for long‐chain branched polyethylene

Effect of applied processing history on flow instability at capillary extrusion is studied using a commercially available low‐density polyethylene (LDPE) having long‐chain branches. It is found that processing history in an internal mixer in a molten state depresses long‐time relaxation mechanism associated with long‐chain branches, which is known as “shear modification.” Consequently, the onset of output rate for melt fracture increases greatly. Furthermore, it should be noted that the sample having intense shear history shows shark‐skin failure without volumetric distortion, although it has been believed that LDPE exhibits gross melt fracture at capillary extrusion. The reduction of elongational viscosity by the alignment of long‐chain branches along to the main chain is responsible for the anomalous rheological response. As a result, the sample shows shark‐skin failure like a linear polyethylene at a lower output rate than the critical one for gross melt fracture. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Studies on wire coating extrusion. I. The rheology of wire coating extrusion

Wire coating extrusion was studied, both experimentally and theoretically, using a pressure-type die. For the experimental study, a wire coating apparatus of laboratory scale was constructed, consisting of a pay-off device, extruder, cross-head and pressure-type die, cooling trough, and take-up device. The materials used were low- and high-density polyethylenes and thermoplastic rubber. The following measurements were taken during the experiments: (1) the axial pressure profiles in the die, (2) melt flow rate, and (3) take-up speed. The measurements were then used to determine the effect of the rheological properties of the polymers on the performance of the wire coating operation. It was found that a reduction in axial pressure gradient and a reduction in the recoverable elastic strain of a molten polymer at the die exit can be realized as the speed of the wire is increased. For the theoretical study, using a power-law model, the equations of motion were solved numerically to predict the volumetric flow rate as functions of the pressure gradient in the die and the rheological properties of the polymer being extruded. Solution of the system equations permitted us to predict the velocity profile and shear stress distributions of a molten polymer inside a pressure-type wire coating die.     

Rheological properties of metallocene isotactic polypropylenes

Rheological properties of metallocene‐catalyzed isotactic polypropylenes (MET‐PP) were evaluated in comparison with those of Ziegler–Natta‐catalyzed isotactic polypropylenes (ZN‐PP) and MET‐PP was generally characterized in a rheological aspect. Based on the characterization, various flow processibilities and their effect on the higher order structure and product properties of the processed article were estimated. The capillary flow properties at various temperatures, elongational flow properties, and dynamic viscoelasticities of MET‐PPs and ZN‐PPs with various melt flow indexes (MFIs) were measured. Furthermore, as an example of application of rheological analysis, the selection of proper raw resin and processing conditions in the sheet‐extrusion of MET‐PP was studied. MET‐PP shows the following rheological features due mainly to the narrow molecular weight distribution in comparison with ZN‐PP with equivalent MFI to that of MET‐PP: while the viscosities at low shear rates are lower, those at high shear rates are higher. Although there is little difference in the loss modulus G″ (viscosity), the storage modulus G′ (elasticity) is very (about one decade) lower. The die swell is much smaller. The entrance pressure loss and end correction coefficient are lower. The critical shear rate at which a melt fracture begins to occur is lower. The melt tension, elongational viscosity, and melt flow index ratio are lower. The flow activation energy is slightly lower. The zero‐shear viscosity obeys the 3.4th‐power law independent of catalyst. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2157–2170, 2002     

Rheological effects of the incorporation of chlorinated polyethylene compatibilizers in a HDPE/PVC blend

Small‐amplitude oscillatory measurements, creep and recoil experiments, capillary extrusion flow and shrinkage measurements have been performed to elucidate the effect of block and random chlorinated polyethylene (CPE) on the rheological properties of a ternary high density polyethylene (HDPE)/ poly(vinyl chloride) (PVC)/CPE system. It is observed that the storage modulus, the complex viscosity and the steady stale viscosity at low shear rates decrease when a small amount of CPE is incorporated to 50/50 (wt.) HDPE/PVC binary blend. However, at high shear rates, in experiments performed in extrusion flow, the trend is reversed, and the incorporation of CPE to the binary blend increases viscosity. The high melt elasticity of HOPE is severely reduced when this polymer is mixed with PVC, but when CPE is included as a third component, elastic recovery is considerably increased. All these rheological results, which are independent of type (block or random) of CPE used, are explained considering the morphological changes produced by CPE and during extrusion flow.