双螺杆挤出机的技术难点

近几年来，同向旋转双螺杆挤出机在国内外得到了突飞猛进的发展，其产量、扭矩和转速大幅度地提高，应用也日益广泛。目前，我国生产的同向平行双螺杆挤出机多为中小型机，螺杆直径约在φ25～φ100之间，国内所需的大型挤出机全部依赖于进口。国内大部分厂家生产的双螺杆挤出机总体性能较低，应用范围较窄，在很多行业的应用领域还尚未开拓；对双螺杆挤出机理的研究才刚起步，在脱挥理论的研究方面还处于空白状态；在双螺杆的设计、计算以及制造技术方面仍有不少难点需要克服，这些都制约了我国双螺杆挤出技术与装备的发展。鉴此，我们结合国家“九五”重大技术装备攻关项目——“高效多功能双螺杆挤出造粒机组”的研究，对开发研制中的技术难点和脱挥理论进行了重点研究。
本课题以国外第六代双螺杆挤出机的水平为赶超目标，围绕双螺杆挤出机高的生产效率、强的混炼塑化能力、良好的操作稳定性、低的能耗以及多功能、多用途等六项主要技术性能指标，开发研制成功了国内第一台φ30高效多功能双螺杆挤出造粒机组。在高扭矩、高转速双螺杆专用传动装置、高粘度齿轮计量泵、组合式推力轴承装置的设计与制造以及激光测试技术等方面取得了关键性的突破；完善了双螺杆挤出机的核心部件——螺纹元件的几何造型程序，实现了三维模型的参数化设计，图形精度和计算速度有显著的提高，程序的适用性和可操作性大为增强。这些都可为大型同向旋转双螺杆挤出机的国产化以及多功能双螺杆挤出机的系列化开发提供经验。
在广泛查阅国内外有关聚合物的脱挥机理以及各种螺杆挤出脱挥理论和实验研究的文献基础上，对同向旋转双螺杆挤出机的脱挥机理进行了深入研究。通过分析物料在挤出过程中低分子挥发分气体形成和排出过程，研究了加入物料物性、挥发分含量、挤出工艺条件、螺纹元件几何参数和组合形式对气体挥发物脱挥效率的影响，分析了气泡形成条件和加速气泡破裂的各种因素。
借鉴静止液体中气泡均匀成核和非均匀成核速率表达式，提出了在一定的温度和螺杆转速下同向旋转双螺杆挤出机中的气泡成核速率表达式，并由实验得出模型中含有的相关参数。在此基础上，结合物料在同向旋转双螺杆挤出机中的流动特性，利用结构微发泡中气泡长大模型，建立了同向旋转双螺杆挤出机的起泡控制脱挥和扩散控制脱挥的有限膜物理和数学模型。采用国际上先进的大型商业有限元软件ANSYS对非牛顿流体三维流动进行了数值计算，得到了物料在双螺杆挤出机内的压力、速度和粘度分布以及螺杆特性曲线，求得了物料在双螺杆挤出机内的有效充满长度、用于脱挥的未充满段长度和脱挥传质面积。这些数据的获得对脱挥效率的计算是必不可少的。
为了验证本研究所建立的同向旋转双螺杆挤出机脱挥问题的物理、数学模型和理论计算值的正确性，我们进行了一系列实验。在此基础上用回归分析法求得了起泡脱挥时气泡群密度计算公式中的若干经验参数。通过对实验数据的整理分析，得到如下结论：
1、主螺杆转速、加料量以及机筒设定温度是双螺杆挤出机中脱挥过程的主要影响因素。这些因素的变化又会影响物料温度、螺槽充满度、停留时间以及有效充满长度，从而多方面地影响脱挥。对于特定的工艺，有一个最佳工作点，在稳定工作的情况下，可以得到最高的脱挥效率。
2、提高螺杆转速，有利于气泡形成、长大和破裂，有利于降低物料在螺槽中的充满长度、增强物料质量传递表面的更新作用，可以提高脱挥效率；但过高的转速，使物料在脱挥段的停留时间急骤减少，脱挥效率反而下降。
3、适当降低喂料量可以减少排气段的充满率，使脱挥效率提高；但过低的喂料量不仅使挤出量减少并产生波动，而且由于充满率太低，不足以形成熔池，使起泡脱挥效率下降，因此喂料量必须适中。
4、提高熔体温度，使扩散系数和Henry常数增大，熔体粘度下降，有利于脱挥过程的进行。
5、增加物料在脱挥段的停留时间、增加脱挥段长度可以提高脱挥效率，为此在螺杆结构设计中可以考虑增加排气段长度和采用多阶排气。
综上所述，φ30新型高效多功能双螺杆挤出造粒机组的开发研究成功，既可为我国聚合物混合、改性造粒工艺提供最新的装备，又可为后续大型双螺杆造粒机组的开发研制在技术和理论两方面积累经验。实验结果还表明，本研究建立的同向旋转双螺杆挤出机起泡控制和扩散控制脱挥模型应用于生产实际是可行的。它对聚合物生产过程的下游工序如清洗、凝固、脱挥、挤压脱水、干燥和造粒以及反应挤出等工艺参数选择、大型多阶排气式双螺杆挤出机的开发研究有重要参考价值。
During the past few years, the co-rotating twin-screw extruder has achieved great development. Its throughput, torque and screw speed has been increased to a higher lever, and its application has been extended. The co-rotating twin-screw extruders made in our country are middle or small size machines, whose diameters are from 25mm to 100mm. The large-scale twin-screw extruders are all imported. The performance of most homemade extruders is low, and applications are narrow. The study of twin-screw extruding mechanism in China has just started, and the research on devolatilization theory is still blank. There are a lot of technical difficulties in twin-screw design, calculation and manufacture to be overcome. All these problems hamper the development of twin-screw extruding technology and devices of our country. So based on"National Nine-Five Plan"subject—the development of high performance, multifunction twin-screw extruder, we studied some technical difficulties and emphasized on research of devolatilization theory for co-rotating twin-screw extruder.
Aimed at the level of the international 6〓 generation co-rotating twin-screw extruder, featured in high efficiency, strong mixing & plasticating capacity, fine stability, multifunction and low consumption, we successfully developed the first homemade high performance, multifunction φ 30 co-rotating twin-screw pelletizing line. In many areas, such as high torque, high screw speed twin-screw gearbox, high viscosity gear metering pump, series thrust bearings design, manufacture and laser testing technology, we have gained great breakthrough. We consummated the geometrymodeling program of heart part of twin screw extruder—the screw element, and realized its 3D parametric design. The graphic precision and compute speed has been remarkably improved. The applicability and operability of the program have been also enhanced. All these establish valuable foundation for localization of large-scale co-rotating twinscrew extruder and development of a series of multifunction co-rotating twin-screw extruders.
Based on the analysis of references about polymer devolatilization mechanism and all kinds of screw extruders'devolatilization theory and experiments, the devolatilization mechanism of co-rotating twin-screw extruder was deeply studied. By analyzing the forming and discharging of volatile gas during extrusion, the effects of material properties, volatile content, operation parameters, screw element's geometry parameters and combination on devolatilization efficiency were discussed. The conditions for bubble nucleation and the causes for accelerating bubble rupture were qualitatively analyzed.
Referring to the expressions of bubble homogeneous and heterogeneous nucleation rate within a quiescent liquid, we propose an expression of bubble nucleation rate of corotating twin-screw extruder at limited temperature and screw speed, and get relative parameters in the expression via experiment. Integrating with the flow characteristics of co-rotating twin-screw extruding, referring to bubble growth model in microstructure foam, we built the physical and mathematical models of foam-controlled devolatilization and diffusion-controlled devolatilization. We have done extensive 3Dflow analysis of non-Newtonian fluid in screw element by using commercial finited element analysis software—ANSYS. Then we got the distribution of pressure, velocity, viscosity of the filled material in co-rotating twin-screw channels and the characteristic curve of the screw element. Finally we calculated the filled length, unfilled length for devolatilization and area for mass transfer in extruder. All these data are necessary for calculation of devolatilization efficiency.
In order to test and verify the correctness and the reasonableness of the physical model and the mathematics model, a series of experiments have been done. Based on the experiment data collection, we regress the parameters in the bubble density expression. By the experiment data arranging and analyzing, the following conclusions can be obtained:
1. The screw speed, throughput and temperature settings of barrel are the main factors that affect the devolatilization efficiency. They affect the devolatilization efficiency through changing the temperature of material, the degree of filling, residence time and filled length of screw. These affections are various and interactive. To a specified process condition, there is an optimal working point, which can reach maximum devolatilization efficiency at stable working condition.
2. Screw speed increase makes for bubble nucleation, growth and rupture, and reduces the degree of filling, increases polymer surface renewal, thus enhances devolatilization efficiency. But overhigh screw speed sharply reduces the residence time, which decreases the devolatilization efficiency.
3. Adequacy feeding rate decrease enhances devolatilization efficiency by reducing the degree of filling in venting zone. But overlow feeding rate will reduce throughput and cause fluctuation. Because of overlow fillage, the rolling pool of polymer cannot be formed, thus decreases devolatilization efficiency. So the feeding rate must be moderate.
4. The increase of material temperature gives rise to a decrease in melt viscosity, an increase in diffusion coefficient and Henry's law constant, thus enhances devolatilization efficiency.
5. As residence time and unfilled length for devolatilization increases, the devolatilization efficiency increases. Thus we can consider increasing devolatilization length and adopting multi-vent at screw structure design.
To summarize, the successful development of the high performance, multi-function φ 30 co-rotating twin-screw extruding line not only supplied a new generation device for polymer mixing and pelletizing process, but also founded the technical and theoretical basis for developing large-scale co-rotating twin-screw extruder. From the results of experiment, it can be seen that the foam-controlled and diffusion-controlled models we built are feasible. They can be used to help selecting of process parameters of flushing, degassing, dewatering etc. , and to developing large-scale twin-screw extruder with multi-stage vents.