Mechanics of the Solid Bed in Single Screw Extruders

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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.