| The unique high viscoelasticity of the polymer melt makes the melt subject to greater shear and tensile stress during processing.In the extrusion process of extruded products such as rods,pipes,and square profiles,problems such as extrusion swel ing,deformation and rough surface quality of polymer melt appear.In the coating and extrusion of wires,the main problems are rough coating layer,poor appearance and the naked core wires.These problems are most likely caused by the excessive adhesion between polymer melt and die surface.Gas-assisted extrusion technology is one of the effective means to reduce the adhesion friction between melt and die surface.In this paper,the gas-assisted extrusion technology is applied to the wire coating extrusion.The principle is to establish a stable gas layer between the inner wall of the die and the polymer melt,so as to reduce the adhesion friction close to zero and ensure the quality of the coating layer.And in the process of dragging the core wire,the positive friction force between core wire and melt is greater than the negative friction force between die wall and melt,which prevents the core wire from being exposed when the melt is separated from the core wire.This paper uses numerical simulation method to analyze the 2.5 mm~2 wire coating gas-assisted extrusion process.The main work is as follows:(1)The PTT constitutive equation is used to establish a finite element analysis model for wire-coated gas-assisted extrusion,and the comparison and analysis of wire-coated gas-assisted extrusion and traditional extrusion processes are mainly related to the pressure field,velocity field and shear stress field.Studies have shown that:Compared with traditional extrusion,wire-coated gas-assisted extrusion can reduce the pressure drop of the melt die,reduced the radial velocity of the melt,and reduced the surface of the melt.Therefore,gas-assisted extrusion can reduce the energy consumption and improve the cladding quality of the wire.(2)The process parameters,physical parameters and die size of wire-coated gas-assisted extrusion are analyzed.The results show that when the dragging velocity of the core wire is equal to the average flow velocity of the melt in the die,the melt is just full of the gas-assisted section,and the wire coated with the gas-assisted extrusion has a significant effect on the decrease of pressure drop,radial velocity and shear stress of melt.The study of physical parameters shows that the longer melt relaxation,the smal er melt viscosity or the smal er proportion of the viscosity in the zero shear viscosity,the melt has lower pressure drop in the die.The research on the size of die shows that the length of the gas-assisted section should be greater than 7.5 mm and the coating thickness should be greater than 1.4 mm and less than 1.8 mm for the wire with the specification of 2.5 mm~2,which exactly meets the standard coating thickness.According to the research on the size of the die,a reasonable design of the wire-coated gas-assisted extrusion die is carried out.(3)Navier's model is used to study the influence of wire-coated gas-assisted extrusion on core slip.Two influencing factors of slip coefficient and core wire dragging velocity are analyzed.Compared with traditional extrusion,the results indicate that the extrudate swell ratio is the largest under the condition of the complete slip between core wire and melt during traditional extrusion process.The increase of core wire dragging velocity can lead to the increase of slip velocity,the decrease of pressure and the increase of shear stress of melt.Gas-assisted extrusion can eliminate the negative effects caused by the slip of core wire or the increase of core wire dragging velocity.Therefore,gas-assisted extrusion can reduce the energy consumption,improve the cable coating layer quality and increase the production efficiency during extrusion process. |
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