Mechanics of the Solid Bed in Single Screw Extruders

除了输送和均质外，单螺杆塑化挤出机的任务是将聚合物从固体状态转化为熔融状态。在该过程中，聚合物在通过筒体的热传导和耗散的作用下熔化，以便沿着螺杆通道不断增加熔体含量。一方面，目的是及时完全熔化塑料，以确保在剩余工艺长度内产生的熔体随后均匀化。另一方面，相对于螺杆长度，完全熔化不应过早发生，否则熔体将被热损坏。只有这样，才能在挤出机的下游设备中生产出质量完美的产品。为了能够通过计算预测所需的熔化长度，过去已经开发了大量的模型。这里通常假设在被压实的固体和加热的料筒之间形成一层熔体膜，当径向厚度足够时，被螺旋飞行刮掉，流入所谓的熔池。固体床、筒体上的熔膜和熔池以及螺杆通道和筒体之间的相互作用是模拟熔化过程的必要条件。本研究项目的目的是提高单螺杆挤出机熔炼行为的计算精度，特别考虑固体床的力学。因此，本文对迄今为止在理论层面上尚未得到充分澄清的两个问题进行了研究。一方面，假设Maddock熔化模型的有效性，存在单螺杆挤出机压缩段固体床的速度如何变化的问题。为此，进行了实验研究，在实验中，不同的螺杆几何形状和颗粒尺寸对固体床速度有特殊的影响。另一方面，当该模型的有效范围取决于工艺参数而必须假设分散熔化模型时，问题就出现了。利用已有的理论模型，将挤压过程中固体床上的载荷与可通过实验确定的大块材料样品上的强度进行比较，研究其有效性范围。在环剪试验机上进行工艺条件下的强度测定，本课题对环剪试验仪进行了改进。

In addition to conveying and homogenization, the task of a single-screw plasticizing extruder is to convert the polymer from the solid to the molten state. In this process the polymer is melted under the effect of heat conduction through the barrel as well as by dissipation in order to continuously increase the melt content along the screw channel. On the one hand, the aim is to completely melt the plastic in time to ensure subsequent homogenization of the generated melt within the remaining process length. On the other hand, complete melting should not take place too early in relation to the screw length, otherwise the melt will be thermally damaged. Only in this way qualitatively perfect products can be produced in the downstream equipment of the extruder. In order to be able to predict the required melting length by calculation, a large number of models have been developed in the past. Here it is usually assumed that a melt film forms between the compacted solid and the heated barrel, which is scraped off by the screw flight when the radial thickness is sufficient and flows into the so-called melt pool. The interactions between the solid bed, the melt film on the barrel and the melt pool as well as the screw channel and barrel are essential for modeling the melting process.The aim of this research project is to improve the calculation accuracy of the melting behaviour in single-screw extruders with special consideration of the mechanics of the solid bed. Therefore, two issues that have so far been insufficiently clarified on a theoretical level are investigated.On the one hand, there is the question of how the velocity of the solid bed changes in compression sections of single-screw extruders if the validity of the Maddock melting model is assumed. For this purpose, experimental investigations are carried out, in which the solid bed velocity is specifically influenced by means of different screw geometries and pellet dimensions.On the other hand, the question arises when the validity range of this model ends depending on the process parameters and a disperse melting model has to be assumed instead. The validity range is to be investigated by comparing the loads on the solid bed in the extrusion process by means of an existing theoretical model with strengths on bulk material samples which can be determined experimentally. The strengths are determined under process-related conditions in a ring shear tester, which is modified in this research project.