Numerical Simulation And Experimental Study On Gas-assisted Extrusion Process Of Crystalline Polymer U-shaped Parts

Polymer profiled cross-section parts have become an indispensable part of our lives,however,the quality and efficiency of extrusion are still restricted by the phenomena of die expansion,shrinkage,distortion and droop after extrusion.In order to study the mechanism of the effect of extrusion process on the quality of special-shaped parts,crystalline polymer PP,composite PP + 10% GF and PP + 20%GF were selected,the heat transfer and polymer crystallization kinetics were considered in the extrusion process of U-shaped section,and the method of numerical simulation and experimental study was used,the following activities were carried out:(1)The extrusion experiment system of U-shaped part was established.The u-section gas-assisted extrusion experiment was carried out.In the experiment,the melt temperature,gas temperature,die temperature,traction and other processing parameters were adjusted.The effects of different extrusion processing parameters on the quality of extruded samples were compared and analyzed.The experimental results show that the gas-assisted extrusion of PP and its composites can improve the extrusion efficiency,reduce or eliminate the swelling and drooping of the sample.The deformation of the sample and the distribution of the fiber on the sample can be controlled by adjusting the process parameters.(2)The extrusion forming of U-shaped part was numerically simulated.Based on the principle of heat transfer,a gas-assisted extrusion model of solid-gas-liquid three-phase was established,and the influence of three-phase temperature setting on the velocity field and stress field distribution in die was studied by numerical analysis.It is found that the gas-assisted extrusion technology can make the melt flow in die more balanced and reduce the droop trend of the sample melt at the die exit.And the gas layer in the gas-assisted extrusion molding plays a buffer role between the die and the melt,and the melt temperature in the die is more stable,the abrupt change of shear rate also transferred from the inner corner region of the U-shaped specimen to the gas layer,which reduced the possibility of late deformation of the specimen.(3)The flow around the corner of the U-shaped part is simulated.The die-melt two-phase model was constructed,and numerical simulation was carried out for U-shaped part with different opening arm length or melt inlet flow rate.The simulation results are as follows: the larger the inlet flow rate or the shorter the opening arm,the larger the shear rate in melt.At the front of the die,the velocity in x and y directions is stable,but at 5-10 mm near the exit of the die,the velocity in all directions changes rapidly,even reverses.The variation is related to the opening wall length and the inlet flow rate,which also reflects the extent of die swell and sample droop.So it is necessary to lubricate the die wall or adopt gas-assisted extrusion technology about 5-10 mm before the die exit to improve the extrusion flow and the quality of the sample.(4)The flow factors and crystallinity were analyzed.The cross-section model is simplified,the heat transfer between the free surface of the sample and the outside is taken into account,and the heat transfer between solid-gas-liquid three-phase is taken into account,the flow factors and crystallinity of samples at different positions were analyzed.It is found that the velocity,pressure,shear rate,temperature,first normal stress and crystallinity at different positions can be well controlled by gas-assisted extrusion.(5)The optimization scheme of die structure for U-shaped part was put forward.Based on the equilibrium of mould flow,the solid-gas-liquid three-phase model of U-shaped part is established,and the cavity of the die is optimized,the influence of different thickness of inner and outer gas layer and different dip angle of side wall on the quality of extruded U-shaped parts was discussed.It is found that the melt flow in the die is smoother and the extrusion efficiency can be improved by adopting a certain slope at the entrance of the die or gas-assisted molding technology.Adjusting the different thickness of the inner and outer gas layers can improve the inclination of the opening arm of the U-shaped extrusion after cooling.The inclination of the open wall can be reduced by compensating the inclination of the side wall of U-shaped part,but the variation of the inclination has a certain effect on the x-direction velocity,that is to say,it has a certain effect on the expansion of the sample.That is to say,optimizing the cavity structure of the die,modifying the inclination angle of the opening wall or adjusting the thickness of the inner and outer gas layers in the process of die manufacture and installation can improve the inclination of the open arm of the U-shaped part.