Study On Optimization Of Common PVC-U Pipe Extrusion Die

In recent years,the demand for PVC pipes in the domestic and foreign building materials markets has gradually increased,among which PVC-U pipes account for 3/4 of the total demand for PVC pipes,and the requirements for the quality and production efficiency of PVC-U pipes have gradually increased.As an important component of PVC-U pipe extrusion molding,the extrusion die is mainly used to transform the rotating plastic melt into parallel linear motion to form products with the required cross-sectional shape.Whether the geometric structure and parameters of the die flow channel are reasonable is critical to the quality and efficiency of PVC-U pipe extrusion.In this paper,the flow mechanism of the melt in the flow channel of simple geometric cross-section was analyzed,and the flow channel model of the common extruder head was established in combination with the empirical design formula,and the flow field was simulated numerically by using Polyflow software.On this basis,the influence between the structural parameters of the die,the flow field distribution and the rheological properties of the melt was analyzed,optimized the structural parameters of the die flow channel,and verified the reliability of die flow channel extruding pipes at higher inlet flow rates.By analyzing the flow mechanism of PVC melt in circular,rectangular and annular flow channels,the expressions of melt velocity and flow was obtained.On this basis,combined with the empirical design formula and the existing PVC-U pipe die,a PVC-U pipe common extrusion die was designed,which can be applied to the extrusion of PVC-U pipes with different pipe diameters and wall thicknesses.According to the conservation of mass,conservation of momentum,conservation of energy and flow boundary conditions,the finite element model of the die flow channel was established,and the distribution of the pressure field,velocity field and shear rate field in the flow channel was analyzed.On this basis,with the optimization goal of improving the quality and efficiency of pipe extrusion,the influence between flow channel structural parameters,flow field distribution and melt rheological characteristics in the flow channel of die was analyzed,the geometric parameters of support section,transition shape of compression section and length of forming section in die flow channel was optimized.For a PVC-U pipe with an outer diameter of 90mm and a wall thickness of 3.5mm,the entrance angle is 60°,the exit angle is 30°in the support section,the transition geometry of compression section is circular type,and the length of the forming section is 80mm in optimized die flow channel.Under the same boundary conditions,the melt pressure drop in the optimized die flow channel increased by 2.1MPa,the flow velocity in the forming section increased by2.4mm/s,the maximum shear rate decreased by about 23.3s^-1,and the uniformity of flow velocity and shear rate at the outlet of die flow channel was significantly improved.The products extruded from the flow channel of the optimized die had higher density,more uniform wall thickness,higher mechanical strength at the weld line and smoother pipe surface.Verified the reliability of the optimized flow channel of die to extrude the pipe at a higher inlet flow rate,and compared the distribution of the flow field in the flow channel of die when the inlet volumetric flow rate increased by 5×10^-6m³/s.It was found that the pressure drop of the melt in the optimized runner increased by about 1MPa,the flow velocity at the forming section increased by 7.16mm/s,and the shear rate of the melt at the wall of the forming section increased by about 10.25s^-1 when the inlet flow rate increased by 5×10^-6m³/s.When the flow rate at the inlet of flow channel increased to 1.5×10^-5 m³/s,the phenomenon of shark skin may appear on the surface of extruded product.Therefore,increasing the inlet flow appropriately increased the compactness of extruded product and reduced the residence time of melt in the flow channel while improving the extrusion efficiency of the die flow channel.