Modeling the process of polymers processing in twin-screw extruders

Abstract

Models for the processing of polymers in co-and counter-rotating twin-screw extruders based on the generalized mathematical model of screw extrusion were developed . A base of the proposed models is the analysis of an allocated C-shaped volume, which is limited by one turn of cutting of each of screws and in which there is a certain volume of a processed polymer. Such model gives possibility to describe the process of processing both in the case of complete and partial filling of an operation channel with processed material. This is especially important in case of dosed feeding of an extruder with a polymer, which is typical for modern processing equipment.In addition, the proposed models take into account real boundary conditions on operation surfaces of rotating screws and a stationary barrel, which gives possibility to choose parameters of thermal stabilization systems of operation elements of an extruder unambiguously.The process of melting of polymer granules in an operation channel of an extruder screw was studied. The results of the study showed that it is necessary to use mixing elements at an increase of deviation of the behavior of processed material from the Newtonian behavior in an operation channel of screws. We established that intensity of energy dissipation at certain points in the volume of a processed polymer determines temperature fields in the volume of processed material. We substantiated that, firstly, an intensive external energy supply to screws and a barrel with their subsequent gradual cooling is necessary during the processing of a polymer on twin-screw extruders in contrast to single-screw extrusion.The adequacy of the developed model was verified by comparison of the results of the numerical modeling with the experimental data in processing of a composition based on high-density polyethylene filled with aluminum hydroxide in a twin-screw extruder Ø83×30 with co-rotation screws. We carried out studies for the industrial production of 50 kg/h and the rotational frequencies of screws of 25 and 40 rpm.The proposed model of twin-screw extrusion gives possibility to determine main parameters of the equipment and the process and to estimate temperature heterogeneity of melt at the design calculation for a given productivity and the required final temperature of polymer melt. The parameters include the geometry of operation elements, frequency of rotation of screws and required minimum drive power. Itis possible to determine rotational frequency of screws and thermal conditions of extruder working elements at verification calculation of an extruder for a given geometry of screws.