Mechanics of the Solid Bed in Single Screw Extruders

除了输送和均化外，单螺杆增塑挤出机的任务是将聚合物从固体转化为熔融状态。在这一过程中，聚合物在通过筒体的热传导以及散热的影响下熔化，以便沿着螺杆通道不断增加熔体含量。一方面，目标是及时完全熔化塑料，以确保随后在剩余工艺长度内将产生的熔体均化。另一方面，相对于螺杆长度，完全熔化不应发生得太早，否则熔体将受到热损伤。只有这样，才能在挤出机的下游设备中生产出质量上完美的产品。为了能够通过计算来预测所需的熔化长度，过去已经建立了大量的模型。这里通常假设在压实的固体和加热的筒体之间形成熔膜，当径向厚度足够时，该熔膜被螺杆刮掉并流入所谓的熔池。为了提高单螺杆挤出机熔融行为的计算精度，特别考虑了固体床的力学特性，研究了固体床、熔体上的熔体膜和熔池之间的相互作用以及螺杆通道和螺杆之间的相互作用。因此，本文研究了两个理论上尚未得到充分阐明的问题：一方面，在假设Maddock熔融模型成立的前提下，单螺杆挤出机压缩段内固体床的速度如何变化；为此，进行了实验研究，其中不同的螺杆几何形状和颗粒尺寸对固体床速度有特定的影响，另一方面，当该模型的适用范围随着工艺参数的变化而结束时，就会出现问题，必须假设为分散熔融模型。通过将现有的理论模型与可通过实验确定的块状材料样品的强度进行比较，研究挤压过程中固体床上的载荷的有效范围。强度是在与工艺相关的条件下在环状剪切试验机上测定的，该试验机在本研究项目中进行了改进。

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.