啮合同向双螺杆挤出机中组合螺杆性能的数值研究（Ⅰ）瞬态流场分析

采用聚合物流动分析软件POLYFLOW,数值模拟了聚合物熔体在组合式啮合同向双螺杆挤出机ZSK60的组合螺杆中的三维等温流动。在计算所得速度场和压力场的基础上,全面分析并讨论了由不同厚度和不同错列角的捏合块元件组成的组合螺杆的流场分布规律;研究了组合螺杆的输送性能和挤出稳定性;并分别采用平均剪切速率、平均特征剪切应力以及平均拉伸流动指数等瞬态混合指数表征了组合螺杆的瞬态混合特性。此外还考察了两种不同流变性质的聚合物熔体在组合螺杆中的瞬态流场分布规律。所得结论可为双螺杆挤出的数值模拟研究提供一定的方法指导,并为其工程实践提供一定的理论指导。     

新型偏心三螺杆挤出机流体混合特性分析

在传统的三螺杆挤出机的基础上,设计了一种新型偏心三螺杆挤出机。该三螺杆挤出机中具有螺杆几何结构偏心、螺槽构型呈梯度变化特殊以及较高的面积利用系数等特征。利用有限元法对聚丙烯（PP）熔体在新型偏心三螺杆挤出机中流动和混合规律进行三维数值模拟,给出偏心三螺杆挤出机中压力和速度分布规律,计算了3种偏心螺杆挤出流场的停留时间分布、分布指数、分离尺度、最大剪切应力等混合表征参数。结果表明,螺杆偏心距不仅决定了螺杆端面形状,也改变了螺槽梯度的变化程度。随着螺槽梯度的逐渐减小,挤出机内粒子团聚效应逐渐降低、物料剪切作用逐渐增强。在3种新型偏心三螺杆挤出机中,偏心距e=3 mm的新型偏心螺杆挤出机的混合性能相对较好。     

木塑复合材料单螺杆挤出机螺杆计量段的优化设计

从单螺杆挤出机的挤出过程及挤出理论出发,分析螺杆结构及其几何参数对螺杆塑化性能的影响,提出衡量螺杆塑化性能优劣的标准,并以此为优化目标,以竹粉/PP为例,对螺杆的相关几何参数进行优化。优化结果表明:木塑复合材料专用螺杆较普通塑料用螺杆更利于木塑复合材料熔体的输送,避免了因为木塑复合材料熔体黏度增加而出现的一系列问题。     

螺杆轴向移动对塑化流场均匀性影响的三维仿真研究

采用Polyflow软件对相同工艺条件下螺杆挤出机和往复式螺杆注塑机塑化时计量段流道内熔体进行了三维非等温流动模拟。比较两者中温度场、黏度场、速度场及压力场分布。结果表明,挤出螺杆塑化时的温度和黏度分布更加均匀;注射螺杆后退有利于提高熔体输送能力,但挤出螺杆塑化更稳定。     

螺杆式塑化3D打印装置的设计模拟及出丝性能研究

设计了螺杆、机筒、螺旋叶片等结构,建立了螺杆式塑化三维（3D）打印系统,利用Polyflow软件对压力场、速度场、剪切速率进行了数值模拟分析,并对粒状物料3D打印出丝性能进行了研究。结果表明,螺旋叶片结构通过施加轴向力实现了强制喂料;在整个螺杆挤出段上,熔融物料沿挤出方向随螺杆转速的提高压力逐步增加,梯形螺棱设计有利于熔体流动;材料熔融需要通过剪切作用与加热装置热量传递协同作用以降低熔体黏度;螺杆转速从0.1 r/s向1 r/s的变化过程中,丝材直径呈小幅度增加。     

新型场协同螺杆元件强化传质实验

介绍了一种新型场协同螺纹元件及可视化实验装置,在自行搭建的可视化实验平台上,开展了新型场协同螺杆元件强化传质的实验。对比观测了悬浮于高黏度硅油中示踪纤维在新型场协同螺杆元件和普通螺纹元件螺槽内的流动行为,通过对比分析新型场协同螺杆元件及普通螺纹元件内示踪纤维的取向变化,揭示了新型场协同螺杆元件在单螺杆挤出过程中熔体输送段的强化传质机理。实验结果表明,与普通螺纹元件相比,在场协同螺杆元件螺槽内形成了更为剧烈的涡旋流动,实现了螺槽顶部熔体与底部熔体的质量交换。而且,涡旋流动强化了螺槽内熔体的传质行为,特别是在螺槽深度方向的质量传递。     

振动力场作用下三螺杆挤出机计量段中聚合物挤出的模拟

建立了同向旋转三螺杆挤出机计量段实际流道真实尺寸的三维非牛顿模型，利用ANSYS软件中的FLOTRAN CFD分析功能对该流场进行数值模拟，分析了振动力场作用下挤出机计量段内的黏度分布、压力分布、速度分布以及振幅对流量的影响，同时与无振动状态的挤出特性进行了比较。研究结果表明：振动力场的引入使熔体黏度和流量呈周期性变化，熔体黏度降低，螺棱两侧压差随振幅增大而增大，剪切作用增强，分散性混合能力和分布性混合能力提高。     

振动场作用下双波螺杆混炼性能数值模拟

借助于三维数值模拟思想,对双波螺杆混炼段构建物理简化模型,并应用聚合物流动分析软件POLYFLOW及粒子示踪法对比研究该混炼段在稳态与动态条件下的混炼性能。结果表明,双波螺杆螺槽的特殊结构导致熔体粒子各流场参数波动周期性变化;引入振动场可使熔体粒子受到的压力和作用力降低,并可使流场获得稳定的黏度;存在最佳振动强度,使得波状螺槽内熔体粒子受到剪切作用达到最大值及粒子受力实现稳定降低。     

单螺杆挤出机熔融输送段冲蚀磨损特性研究

采用离散相位法(DPM),建立单螺杆挤出机熔融输送段熔体输送模型,分析流场特性并预测螺杆易发生冲蚀的区域,研究螺杆转速、填料粒径和填料质量分数对螺杆冲蚀的影响。结果表明,流场压力从入口向出口逐渐升高,且螺棱推力面处的压力大于螺棱背面的压力;流场速度分布沿着径向从机筒向螺杆表面逐渐增加;螺棱推力面顶部、螺棱背面顶部和螺棱背面根部为螺杆易发生冲蚀的位置;螺杆的最大冲蚀率随着螺杆转速、填料粒径和填料质量分数的增加而增大。     

螺杆构型对UPVC注塑混合效果影响的研究

运用基于有限元的CFD分析软件POLYFLOW,并根据注塑机的实际工作状态对适合UPVC注塑的3种螺杆构型计量段流道中的熔体进行三维瞬态模拟,其中包括普通螺杆、销钉混炼头螺杆和波状螺杆;对模拟结果进行后处理得到熔体的压力场、速度场、剪切速率场、剪切应力场和黏度场,这些场量是反映熔体分散混合和非分散混合性能的重要指标,通过分析这些场量来比较3种不同螺杆构型对UPVC注塑混合效果的影响;混合效果是影响注塑制品质量的关键因素,从而优选出最适合UPVC注塑的螺杆构型。     

锥形双螺杆挤出过程菠萝型混合元件三维流场分析（Ⅱ）

利用ANSYS有限元软件对啮合异向锥形双螺杆挤出机中的新型螺杆元件——菠萝型混合元件进行了三维流场的模拟计算，对其速度场、压力场、粘度场以及剪切速率、剪切应力进行了分析，绘制出更为直观的展开图。同时，将其和常规螺纹元件进行了比较。     

Effect of process variables on melt velocity profiles in extrusion process using single screw plastics extruder

Abstract

Single screw extruders are used to generate a continuous flow of molten polymer in many industrial polymer processes. The melt velocity profile as extruded is important in determining the properties of the final product and influences process related phenomena such as die swell and the onset of sharkskin. The factors that influence the velocity profile would be expected to be the melt temperature (this affecting the viscosity of the melt), the screw and die geometry, and the output rate from the extruder. In the present work a thermocouple mesh sensor coupled with a cooled stainless tube has been used to determine velocity profiles in melts exiting from the screw of a single screw extruder. The results show that the technique can be used successfully to determine velocity profiles in the extrusion process.

It was found that the main influence on the magnitude of the melt velocity was the extruder screw speed. Melt temperature, and hence melt viscosity, were found to have little effect on the velocity profiles measured. The flow in the centre of the duct was retarded slightly owing to the flow across the screw tip and no rotational component of flow was observed. The velocity profiles measured seemed to be reasonably stable, only small changes being observed in the velocity profiles as the melt flowed along a duct of uniform cross-section, although these changes were limited in nature. Die diameter and length had a limited effect on the velocity profiles generated, although the die entry angle did have a significant effect on the shape of the velocity profile at higher screw speeds.     

The effect of screw geometry on melt temperature profile in single screw extrusion

Experimental observations of melt temperature profiles and melting performance of extruder screws are reported. A novel temperature sensor consisting of a grid of thermocouple junctions was used to take multiple temperature readings in real time across melt flow in a single screw extruder. Melt pressure in the die and power consumption were also monitored. Three extruder screws at a range of screw speeds were examined for a commercial grade of low density polyethylene. Results showed melt temperature fields at low throughputs to be relatively independent of screw geometry with a flat‐shaped temperature profile dominated by conduction. At high throughputs, melting performance and measured temperature fields were highly dependent upon screw geometry. A barrier‐flighted screw with Maddock mixer achieved significantly better melting than single flighted screws. Low temperature “shoulder” regions were observed in the temperature profiles of single‐flighted screws at high throughput, due to late melting of the solid bed. Stability of the melt flow was also dependent upon screw geometry and the barrier‐flighted screw achieving flow with lower variation in melt pressure and temperature. Dimensionless numbers were used to analyze the relative importance of conduction, convection, and viscous shear to the state of the melt at a range of extrusion conditions. Polym. Eng. Sci. 46:1706–1714, 2006. © 2006 Society of Plastics Engineers     

The effect of melt viscosity on thermal efficiency for single screw extrusion of HDPE

In this work, a highly instrumented single screw extruder has been used to study the effect of polymer rheology on the thermal efficiency of the extrusion process. Three different molecular weight grades of high density polyethylene (HDPE) were extruded at a range of conditions. Three geometries of extruder screws were used at several set temperatures and screw rotation speeds. The extruder was equipped with real-time quantification of energy consumption; thermal dynamics of the process were examined using thermocouple grid sensors at the entrance to the die. Results showed that polymer rheology had a significant effect on process energy consumption and thermal homogeneity of the melt. Highest specific energy consumption and poorest homogeneity was observed for the highest viscosity grade of HDPE. Extruder screw geometry, set extrusion temperature and screw rotation speed were also found to have a direct effect on energy consumption and melt consistency. In particular, specific energy consumption was lower using a barrier flighted screw compared to single flighted screws at the same set conditions. These results highlight the complex nature of extrusion thermal dynamics and provide evidence that rheological properties of the polymer can significantly influence the thermal efficiency of the process.     

Development of an in-line viscometer in an extrusion molding process

In this work, an in-line viscometer to measure the viscosity of polymer melts under extrusion molding processes was developed. The in-line viscometer contains a stress sensor and a shear rate sensor which were installed between the screw and the die of an extruder. In this way, the flow line after the screw cannot be changed, unlike the present in-line capillary rheometer which can change the diameter of the pipe of the flow line and hence influence the throughput. All data acquisition is done by a computer such that the melt viscosity can be calculated automatically. The shear-thinning behavior of a low-density polyethylene (LDPE) under three different temperatures is presented in all experiments. It is concluded that the melt viscosity can be effectively monitored. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 919–924, 1997     

管壁厚度对微挤出成型的影响分析

基于Bird-Carreau黏度模型,运用有限元方法对三维等温微管挤出成型流动模型进行了数值分析,主要研究了管壁厚度对微管挤出成型过程中挤出胀大、速度分布、剪切速率和口模压降等重要指标的影响。结果表明,当熔体入口体积流率相等时,随着管壁厚度的增大,挤出物挤出胀大率和横截面尺寸变化量增大;口模出口端面上熔体的二次流动增强,但挤出速度和剪切速率减小;熔体在口模内的压力降明显下降;适当增加管壁厚度,有利于提高微管挤出质量。     

Developing laminar morphology in higher‐viscosity‐ratio polyblends by controlling flow fields in a single‐screw extruder

The morphology of high‐density polyethylene (HDPE)/polyamide‐6 (PA‐6) blends with different melt‐shear‐viscosity ratios (VRs), prepared by combining a single‐screw extruder with a convergent die was studied. Two different screw geometries, metering and mixing screws, and three screw speeds, 20, 40 and 60 rpm, were evaluated to investigate their effects on the morphology of extruded ribbons. Two mixing screws with low and high shear intensity, respectively, were used. Both the geometry and speed of screw were found to have an important role in the morphological changes of the blends. In contrast to previous studies, the results shown in this work reveal that it is possible to develop a laminar structure of PA‐6 in an HDPE matrix with a VR larger than one by controlling the flow fields, through appropriately combining the type and shear intensity of the screw with its speed. A well‐developed laminar PA‐6 phase with an aspect ratio of about 100 was obtained under the optimum combination. Copyright © 2004 Society of Chemical Industry