MODELING OF POLYMER MELT FLOW WITH THE PRESENCE OF A LOW VISCOSITY NEAR-WALL LAYER

Abstract

Investigation of polymers movement process in the channels of processing equipment showed that the polymer layer near channel walls warms up faster and therefore acquires different properties from the main mass of the material - increased temperature, low molecular weight, and then reduced viscosity. A number of factors determines the nature of the wall layer: the amount of wall friction, the speed of the material, the mode of heating of the raw material. Such near-wall layers differ from those of ordinary low-molecular liquids, since they have a different nature.An analysis of these works showed that with a thin relative to the main flow of the boundary layer, the distribution of velocities in it can be considered close to linear. The purpose of the article is to analyze the results of mathematical modeling of the flow of polymer melts with imitation of the presence of low viscosity wall layer.Of particular interest is the study of the nature of polymer flows with a wall layer in the channel of the head of the device for forming by the method of spatial printing (3D printer). A device based on the method of layer-by-layer surfacing of the shape of the future product is selected as the basis the channel of the working body usually has a diameter of from 0.1 mm to 1.2 mm. For small nozzle diameters, the polymer properties throughout the channel volume correspond to the properties of the near-wall layer. However, as the diameter of the nozzle increases, the polymeric material becomes cooler and more viscous closer to the central axis.A feature of the conducted numerical experiments is that, since the value of the velocity of the polymer at the wall is not known in advance, instead of it, a conditional thin layer of liquid is introduced, with properties different from those of the base material. This simulates the presence of low molecular weight fractions in the surface layer of the polymer, or the use of lubricating impurities during extrusion.It has been established that with an increase in productivity, the pressure increases along a flatter nonlinear curve, in contrast to this dependence in the absence of a near-wall layer. Therefore, the introduction of a lubricating wall layer increases the productivity of processing equipment.The obtained dependences of the distribution of the flow parameters of the polymer in the channel on the presence and properties of the near-wall layer, as well as the dependence of the viscosity of the near-wall layer on the velocity and temperature. The simulation results help to clarify the results of calculations of the parameters of extrusion, 3D printing and other polymer processing processes, as well as recommend the use of lubricant additives to increase productivity and prevent.Prospects for further research may be modeling flow with a wall of melts with highly elastic properties.