The dissertation is devoted to the study of the movement of granular polymeric materials in the feeding zone of the screw extruder and its influence on the structural and technological parameters of this zone, which is necessary for the effective design and operation of extrusion equipment in general.
An analytical review of the current state of research of the process of feeding of screw extruder with polymer raw materials has been carried out. The existing methods of modeling the equipment and the process of feeding of single-screw extruder with polymer raw materials and the peculiarities of its processing have been considered. The analysis of existing methods and devices for researches of tribotechnical properties of polymeric raw materials and influence of such parameters as loading, temperature and speed of a mobile working body on them has been carried out.
According to the results of the review, it has been established that the closest to the actual behavior of polymer granules in the working channel of the screw extruder is the description of the flow of bulk medium based on the Discrete Element Method (MDE), which is studying the interaction of a single particle with other particles of the bulk material layer and the boundaries of the calculation area. In this case, to perform the appropriate numerical calculations, it is necessary to study the friction coefficients and lateral pressure coefficients of the processed materials and their dependences on the design and technological parameters of the feeding zone of the extruder.
It should also be noted that studies of the tribotechnical properties of polymeric materials required for numerical calculations are usually performed for monolithic samples and not for granular material, although the design accuracy of extrusion equipment for processing polymeric raw materials depends on the accuracy of the external friction coefficient and the lateral pressure coefficient of bulk material. In addition, there is no single method for the experimental determination of external friction coefficients and lateral pressure coefficients, and most of the existing devices for the determination of tribotechnical properties do not allow to conduct research for granular polymeric materials.
To modeling the processes of bulk material movement, a mathematical model of discrete description of bulk material motion based on the Discrete Element Method was used, which allowed to consider the polymer in the form of individual granules and take into account the influence of shape and size of individual granules on the interaction of polymer with walls of working bodies of the equipment. The Hertz – Mindlin visco-springy model was used to describe the interaction among the granules, which assumes that the spherical particles do not deform during contact, but overlap each other by a certain amount, forming a contact spot.
The study of the interaction among the granules of the following four polymers has been conducted: high-density polyethylene of brand Marlex HHM 5502BN (HDPE), copolymer of ethylene with vinyl acetate (sevilene) of brand 11104-030 (CEV), polystyrene of brand Denka Styrol MW-1-301 (PS), polyvinyl chloride of brand SorVyl G 2171/9005 11/01 (PVC). The research has been carried out for the problems of forming the angle of natural slope (APS) and the movement of the flow of granules on the example of a screw feeder.
The use of a discrete approach made it possible to take into account the influence of the shape and size of individual granules on the nature of the interaction of the polymer with the walls of the working bodies of the equipment. A comparison of two approaches to modeling the shape of granules has been carried out: under the condition of considering granules in the form of spheres and in the form of multispheres, when the calculated form of granules is as close as possible to the real one.
The obtained results showed that the MDE model with the parser in the form of multispheres adequately reproduces the behavior of bulk materials, in contrast to the analysis of granules in the form of spheres. Compared to the full-scale experiment, the MDE model for calculating the APS using the multisphere method in the case of HDPE and CEV gives an error 3 times less than the MDE model using the sphere method, in the case of PS - the error is 6 times less, in the case of PVC - the error is 9 times less. The difference is primarily due to the complex shape of the granules, which directly affects the formation of the APS of the layer formed by them.
Additional verification of the conformity of the behavior of the real material to the simulated one has been carried out by studying the mass productivity of the screw feeder by the methods of full-scale, numerical experiments and analytical calculation and their comparison. In case of the numerical simulations granules simulated by multispheric method were used.
