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.     

锚式搅拌槽中高粘弹性流体的流速分布及其功率消耗

用照相法测定了锚式搅拌槽中高粘弹性流体的流型和流速分布,另测定了搅拌功率消耗,结果发现:1.与牛顿流体相比,在低Re数下,粘弹性流体的切向速度较大,而径向速度则较小.2.转速相同时,在高剪切率区域,粘弹性流体的剪切率大于牛顿流体.由CEF方程导出功率计算式N_pRe_af_s~(1-n)=k_pf_vf_s~2[1+F_1avf_s~(m-n-3)Wi/K_s~2]用实验数据确定f_(?)和F_(1av),得到可适用于牛顿流体、假塑性流体和粘弹性流体的普适功率计算式,计算结果与实验值比较接近.     

Flow patterns in conical and cylindrical hydrocyclones

Using a two beam, 300 mW laser Doppler velocimeter, the tangential and axial velocity fields were determined for the water flow in a 102-mm modular hydrocyclone. The body of the equipment could be changed to transform it into a conical or a flat bottom hydrocyclone. During the tests the pressure drop and the diameter of apex and vortex were varied and the axial and the tangential velocities and their turbulence intensity were measured. The results shows that the inlet pressure affects only the magnitude of the velocities, but does not change the flow pattern. The tangential velocity is similar in both types of hydrocylones while the axial velocity is different. In both hydrocyclones the axial velocity is a function of the radial position but, while it is a linear function of the vertical coordinate in the cylindrical hydrocyclone, this is not the case for the conical vessel.     

铝粉高填充材料混合流场数值模拟及实验研究

应用Polyflow软件模拟计算了4种总长度为440 mm的螺杆熔体混合段内铝粉高填充复合材料的三维混合流场。通过对各个螺杆组合流道的剪切速率、剪切应力、轴向速度分布等指标进行分析,研究了这4种螺杆组合的流场分布规律和混合性能;分别对每个螺杆组合上的不同元件的混合特性进行直观的对比。数值模拟与实验分析结果表明,带有反向螺纹元件和捏合块的螺杆对填充材料中的超细铝粉具有更优的综合混合能力。     

Flow characteristics of hyperboloid stirrers

The flows in a fully-baffled vessel with a diameter T = 144 mm driven by hyperboloid stirrers of diameters D = 773 and 27/3 have been visualised and characterised by local measurements of velocity and turbulence and by power number. The results were obtained for a range of rotational speeds from 6 to 40 rev/s. The visualisation showed that the larger stirrer gave rise to a radial jet and that the smaller stirrer formed a jet inclined towards the base of the vessel so that there was a tendency for the system of two vortices, one above and one below the jet, to give way to a single vortex as the clearance between the stirrer and the base of the vessel was reduced. The velocity measurements revealed bulk-flow values an order of magnitude less than that of the maximum radial velocity in the jet, that the maximum radial velocity was 24% of the circumferential velocity of the tip of the stirrer, and that the radial velocities were proportional to the rotational speed. The flows generated by the hyperboloid stirrer were less vigorous than those of Rushton impellers of similar radius and were associated with power numbers 28 times less. The power number did not vary with rotational speed or with clearance within the measured range. The contrast with propeller and disc stirrers is less pronounced, but the hyperbolic profile is likely to find application and the present results provide a basis for choice.     

浸没式撞击流反应器流场涡特性的数值研究

利用大涡模拟（LES）方法研究了撞击流反应器内流场涡特性,分析撞击区域流体流动特征。改变进口速度、喷嘴间距,讨论流场速度、涡量和平面涡能量分布规律,并分析了流场流型、涡演化过程和涡核形式。在反应器内靠近撞击驻点的涡尺寸小、脉动性高,随着撞击距离的增加,流体速度逐渐减小,涡影响范围变大。平均涡量和平均涡能量随进口速度的增加,先增加后减小。结合Q判据分析了反应器内涡的演化过程和流体流型。根据径向射流涡的演变过程,得到径向射流两侧涡演化的周期,在0.15~0.20 s之间。撞击区的涡结构主要为马蹄涡和肋状涡,在出口位置存在涡环。研究结果为深入分析撞击流反应器流体运动规律和优化反应器提供了理论参考。     

浸没式撞击流反应器流场涡特性的数值研究

利用大涡模拟（LES）方法研究了撞击流反应器内流场涡特性,分析撞击区域流体流动特征。改变进口速度、喷嘴间距,讨论流场速度、涡量和平面涡能量分布规律,并分析了流场流型、涡演化过程和涡核形式。在反应器内靠近撞击驻点的涡尺寸小、脉动性高,随着撞击距离的增加,流体速度逐渐减小,涡影响范围变大。平均涡量和平均涡能量随进口速度的增加,先增加后减小。结合Q判据分析了反应器内涡的演化过程和流体流型。根据径向射流涡的演变过程,得到径向射流两侧涡演化的周期,在0.15~0.20 s之间。撞击区的涡结构主要为马蹄涡和肋状涡,在出口位置存在涡环。研究结果为深入分析撞击流反应器流体运动规律和优化反应器提供了理论参考。     

场协同对PP/PS复合体系传热传质性能的影响

从动量和能量守恒方程出发,提出速度场与温度场的协同耦合作用对聚合物流动与换热具有重要影响,并以此为依据设计了新型扭转螺杆结构和场协同螺杆。以聚丙烯(PP)和聚苯乙烯(PS)为原料制备PP/PS复合材料体系,通过挤出试验对比研究了常规螺杆与场协同螺杆的传热传质性能。结果表明,相对于常规螺杆,设置有扭转螺杆结构的场协同螺杆的数均粒径更小、粒径分布密度和平均停留时间更大,提高了挤出机的混合能力;场协同螺杆的对流换热系数更大、径向温差更小,提高了挤出机的换热能力。     

Sc-CO2辅助聚合物挤出中螺杆结构对熔体压力的影响

采用Polyflow软件对双螺杆挤出机内不同螺杆元件所形成流道中的熔体流动进行了模拟,分析了不同螺杆元件的建压能力。结合超临界二氧化碳(Sc-CO2)辅助挤出的螺杆组合原则,对不同螺杆元件组合的螺杆结构进行了挤出实验;并通过对熔体压力的模拟值与实际测量值的比较,分析了不同螺杆结构对熔体压力的影响;另外还针对不同材料的挤出加工确定了相应的螺杆结构。     

Electrical monitoring of crack propagation during quasi-static loading and ballistic impact of alumina plates

An electrical methodology has been developed to monitor crack advance during ballistic impact. Velocities of radial cracks can be measured and information about development of the crack pattern can be obtained. This is done via a grid of thin gold tracks, printed onto the front or back surfaces of the sample. These elements were incorporated into an electrical circuit and high speed data acquisition was carried out during impact. The fracture behaviour of two grades of alumina has been examined. Hard spherical projectiles were used, with a range of impact velocities. An increase in radial crack velocity was detected with increasing impact velocity, within this range. Differences in crack velocities were also noted between the two types of alumina. Crack speed values are in good agreement with data reported previously for similar systems, obtained using high speed photography. The methodology proposed here looks to be reliable, convenient and economically attractive.     

垂直螺旋输送机中颗粒速度的分布

采用离散元方法对垂直螺旋输送机中颗粒的流动进行模拟计算,得到颗粒圆周速度、轴向速度、自转角速度在螺旋空间中的分布. 结果表明,颗粒圆周速度最大在叶片工作面径向位置33 mm处,右下区域内圆周速度变化明显,颗粒受周向剪切力较大. 随径向距离增加,底层颗粒与叶片的圆周速度差变大,至近料槽壁处达峰值,导致叶片远端和边缘处磨损较大. 较大颗粒轴向速度区域在颗粒床中层,最大值在近料槽壁处叶片工作面上方22 mm处. 在左下角出现颗粒流滞流区,颗粒轴向速度小于0.1605 m/s,有些甚至小于0.09625 m/s. 螺旋叶片工作面附近颗粒自转角速度较大,最大值在叶片边缘,且高自转角速度的颗粒都集中在颗粒群下层.     

A parallel coextrusion technique for simultaneous measurements of radial die swell and velocity profiles of a polymer melt in a capillary rheometer

This article proposes a new experimental technique to simultaneously measure radial die swell and velocity profiles of polystyrene melt flowing in the capillary die of a constant shear rate rheometer. The proposed technique was based on parallel coextrusion of colored melt‐layers into uncolored melt‐stream from the barrel into and out of the capillary die. The size (thickness) ratio of the generated melt layers flowing in and out of the die was monitored to produce the extrudate swell ratio for any given radial position across the die diameter. The radial velocity profiles of the melt were measured by introducing relatively light and small particles into the melt layers, and the times taken for the particles to travel for a given distance were measured. The proposed experimental technique was found to be both very simple and useful for the simultaneous and accurate measurement of radial die swell and velocity profiles of highly viscous fluids in an extrusion process. The variations in radial die swell profiles were explained in terms of changes in melt velocity, shear rate, and residence time at radial positions across the die. The radial die swell and velocity profiles for PS melt determined experimentally in this work were accurate to 92.2% and 90.8%, respectively. The overall die swell ratio of the melt ranged from 1.25 to 1.38. The overall die swell ratio was found to increase with increasing piston speed (shear rate). The radial extrudate swell profiles could not be reasoned by the shear rate change, but were closely linked with the development of the velocity profiles of the melt in the die. The die swell ratio was high at the center (～1.9) and low (～0.9) near the die wall. The die swell ratio at the center of the die reduced slightly as the piston speed was increased. Polym. Eng. Sci. 44:1960–1969, 2004. © 2004 Society of Plastics Engineers.     

自转螺旋扭带管内三维流动与传热数值模拟

建立了内置自转螺旋扭带管的三维流动模型,采用RNG k-ε湍流模型对管内流场进行了数值模拟,得到了流体的流动和传热特性.研究表明：自转扭带管内的流动是复杂的三维螺旋流动.流体在扭带与管内壁环缝区域的轴向速度比光管的大;流体在与扭带宽度等直径范围内的切向速度随半径的增大而增大,流体在扭带与管内壁环缝区域内也存在明显的切向速度,但随半径的增大而减小;而光管内流体的流动只有随机的切向运动,且其切向速度要比扭带管内的切向速度小2个数量级;扭带管流体的径向速度和湍流度也比光管的大.扭带管内流体近管壁区域轴向、切向、径向速度和湍流度的加大,强化了管内的对流传热,其表面传热系数大于光管.另外,速度场的模拟值与激光测试值进行了比较,二者吻合较好.     

LDV measurements of particle velocity distribution in an annular stripper

Particle descent velocities in an annular stripper were measured by a laser Doppler velocimetry (LDV) system. In the radial direction, particle descent velocity was relatively constant in the mid-region of the stripper and increased towards the walls on both sides, exhibiting an anti-U-shaped distribution. Particle descent velocity in the radial mid-region increased with the increase of superficial gas velocity, and the maximum in the outer wall region increased significantly with the increase of solid mass flux. Superficial stripping gas velocity had stronger effect on particle velocity distributions near the stripper gas distributor, and such effect weakened with the increase of the distance from the distributor. Local particle velocity and its radial profiles could be adjusted by changing the superficial stripping gas velocity. Empirical formulas were established to describe the relationships between the local particle velocity and cross-sectional averaged velocity based on the effects of operating conditions and measuring positions. The result showed that the predicted data was in good agreement with the experimental value.     

Measurement technique on the diffusion coefficient of powder flow in a screw feeder by X-ray visualization

Measuring the diffusion coefficient of powder flow, which shows the magnitude of mixing, using X-ray visualization has been developed with the help of image processing techniques. Injecting minute metallic tracers into the powder yielded flow characteristics using the X-ray penetration images. The diffusion coefficient of the flow was determined by tracer figure deformation with time. Using this method, we evaluated four kinds of powder flows transported using different shaped screws in a screw feeder unit. The following results were obtained from this investigation: (i) Increasing the pitch width of the screw yielded decreased transport velocity and a larger diffusion coefficient. (ii) Double screw provided the largest transport velocity and the smallest diffusion coefficient.In addition, uncertainty analysis was applied to the method of measuring the diffusion coefficient to achieve greater bias limit.     

动态旋叶压滤机滤室内流体径向速度的研究

对采用动态过滤原理的旋叶压滤机滤室内的流体流动,在有流通量和双面渗滤的实际工作条件下进行了理论分析.采用有限元方法计算了平板形转盘结构的滤室内流体的径向速度,并用激光测速方法进行了实验验证与研究,得到了径向速度与过滤半径、流通量之间的关系.结果表明有限元计算值与实验值基本相符,计算方法是可靠的.在有渗滤条件下,流体径向速度随渗滤比的增大而减小;随流通量的增大而增大.同时对平板形、沟槽形、叶片形三种不同转盘结构的滤室内流体的径向速度进行了激光测定,结果表明三种转盘形式下径向速度的分布及操作参数对径向速度的影响趋势是相似的;条件相同时,叶片形转盘和沟槽形转盘滤室内流体的径向速度远大于平板形转盘滤室内流体的径向速度,三种转盘结构中叶片形转盘对滤室内流体的径向速度影响最大.因此,实际应用时应该选用叶片形转盘或沟槽形转盘.     

Structure of powder flow in a planetary mixer during wet-mass granulation

Wet massing granulation, a widely used industrial process, is difficult to monitor and control and the structure of the flow is poorly understood. Flow patterns in a planetary mixer were investigated using positron emission particle tracking. Both dry and wet powders of a model pharmaceutical formulation were studied to develop understanding of the influence of moisture content on the flow structure during granulation. The flow structure was characterised using the distributions of the velocity components in different cross-sections of the mixer. Fourier analysis showed that the dry system is essentially dissipative and disordered whereas the wet system, being more inertial, shows signs of being more ordered with a periodic recirculation within the bowl. In both systems, radial and axial displacements are strongly correlated. For the dry system, within a central radial core region, the behaviour of the particle was determined by the rapid movement of the agitator, forming a single toroidal recycling cell. The radial and axial velocities of the tracer were up to two orders of magnitude lower than the tangential component. However, in the regions close to the wall, the particle was found to exhibit small movements dictated by the planetary rotation. For wet systems these two main regions were again observed. However, velocity field and velocity distribution showed the presence of two toroidal circulation loops, one above the other. In the wall region, the small movements governed by the planetary motion were again found, but with the amplitude of the displacements reduced by an order of magnitude.     

啮合同向双螺杆挤出过程组合流道(捏合块 螺纹元件)三维流场分析

利用ANSYS有限元分析软件对啮合同向双螺杆挤出机由捏合盘和螺纹元件组合而成的流道进行了三维等温非牛顿模拟分析 ,得到了组合流道的速度场和压力场。并对螺纹元件流道、捏合块流道及组合流道的流量、回流量、拉伸速率、剪切速率及剪切应力进行了比较。     

注塑机止逆螺杆头三维等温流动的数值模拟

使用聚合物流动分析软件Polyflow，模拟了注塑机止逆螺杆头中的三维等温流场。选用聚苯乙烯（PS）作为模拟流体，分析了塑化和注射两个过程中注塑机止逆螺杆头流道内熔体速度和剪切应力的分布情况，并将无止逆环螺杆头与止逆螺杆头的流场进行了比较。数值模拟结果表明：塑化时熔体速度和剪切应力的最大值均出现在沟槽处，注射时二者均出现在流道出口处；止逆螺杆头沟槽处熔体的回泄量和回泄速度较无止逆环螺杆头小得多，且沟槽处回泄的熔体到达止逆环时已被完全消除。这说明止逆环的存在可以有效地防止熔体回泄。     

啮合同向双螺杆挤出机不同螺杆流场仿真分析

根据啮合同向双螺杆几何学,运用Solid Works三维建模软件建立螺杆的实体模型。采用190℃聚乳酸熔体作为流体,通过ANSYS软件划分网格得出有限元模型,然后在Polyflow里进行仿真,分别得到了两种不同截面的螺纹元件的流场模拟结果。分析对比了压力场,速度矢量场、剪切速率场和黏度场,得出螺棱厚度对流场的影响。结果表明,I型截面螺杆的建压能力强于II型截面螺杆。II型截面的剪切能力强于I型截面的剪切能力,II型截面增强了螺杆的分散混合能力。