Theoretical And Experimental Study On Gas-assisted Extrusion Forming Of Plastic Micro-tubes

In order to solve the problems of traditional no gas-assisted extrusion forming of plastic micro-tubes,such as extrudate swell,melt fracture,and extrudate distortion,the gas-assisted technique is employed in this dissertation.By optimally designing the gas-assisted extrusion die and reasonably controlling the process parameters of plastic micro-tubes,the stable gas layers can be established between the melts and the die's inner wall,as well as between the melts and the mandrel's outer wall.Under the assistance of gas layers,the polymer melts of plastic micro-tubes can be extruded with the full-slip condition,the stress concentrations of melts on the wall of die and at the outlet of die can be greatly decreased,which finally eliminates the extrusion problems of plastic micro-tubes.In order to verify the feasibility and availability of gas-assisted extrusion for plastic micro-tubes,the mechanism of gas-assisted extrusion of plastic micro-tubes is deeply and systemically studied by using the theoretical analysis and experimental methods.The main study contents of this dissertation are summarized as follows:1.Numerical simulations of plastic micro-tubes gas-assisted extrusion forming are performed by using finite element method.Moreover,the effects of melt's material parameters,process parameters,and gas length on the gas-assisted extrusion forming of plastic micro-tubes are numerically investigated.In addition,the effects of different gas-assisted modes on gas-assisted extrusion forming of plastic micro-tubes are also studied.Numerical results show that: 1)The extrusion problems of plastic micro-tubes can be well eliminated by using the double gas-assisted technique;2)Compared with no gas-assisted extrusion,there are some differences at the effects of material parameters and process parameters on the gas-assisted extrusion forming of plastic micro-tubes;3)The extrudate swell effect is eliminated by properly extending the length of gas-assisted section.2.The influences of size effects on the gas-assisted extrusion forming of plastic micro-tubes are numerically studied.The size effects including the extrudate swell and viscosity-size effect,surface tension effect,melt's compressibility,slip effect,and viscous dissipation effect.Numerical results show that: 1)Compared with the without gas-assisted extrusion,the influences of size effects on the extrusion forming of plastic micro-tubes at a certain extent are eliminated by the gas-assisted method;2)For the slip effect,the influence of slippage at the gas/wall interface on gas-assisted extrusion forming of plastic micro-tubes is little,but the influence of slippage at the gas/melt interface is very prominent;3)When the several size effects are simultaneously considered,it is found that the outer and inner diameter of gas-assisted plastic micro-tubes are shrinked toward inner cavity,but the wall thickness is swelled.3.The gas-liquid-gas mode of gas-assisted extrusion forming for plastic micro-tubes is established.By using the finite element method,the effects of gas and melt process parameters on the gas-assisted extrusion forming for plastic micro-tubes are numerically studied.At the same time,the influences between the gas and melt are also investigated.Numerical results show that: 1)the influences between the gas and melt can be well represented by using the model of gas-liquid-gas multiple stratified flow;2)With the increasing of the gas length and width,the stress influences of gas layers on the annual melt increase,which leads to increase the interference for the annual melts flow of plastic micro-tubes.Therefore,the reasonable choosing of gas length and width are also the very key factors of ensuring the stability of gas-assisted extrusion forming of plastic micro-tubes;3)The impacts of inner and outer gas layers on the melt flow at the gas inlet are eliminated by using the small gas inlet angle between the gas inflow and melt flow;4)In the numerical simulations of gas-assisted extrusion forming of plastic micro-tube,the precision of numerical results can be improved by considering the gas compressibility.4.The gas-assisted extrusion die of plastic micro-tubes is optimally designed and manufactured.Meanwhile,the shape and size of the gas-assisted extrusion mode of plastic micro-tubes is gotten.The optimal parameters values of four key geometric structures,i.e.,division angle,compression angle,compression ratio,and the length of without gas-assisted section,are determined by using the orthogonal experiment combined with finite element numerical simulationmethod.Based on the optimized die,the influence laws of the process parameters on the gas-assisted extrusion of plastic micro-tubes are studied by using the finite simulation method,and the optimal process parameters are obtained.5.The experimental system of the gas-assisted extrusion forming for plastic micro-tubes is established.Then,the effects of gas and melts process parameters on the gas-assisted extrusion forming of polypropylene plastic micro-tubes are experimentally investigated.Moreover,the effect of four kinds of different gas-assisted modes on the extrusion forming of plastic micro-tubes are performed and analyzed.Experimental results show that: 1)When other process parameters are not changed,with the increasing of the screw speed,the outer diameter,inner diameter and wall thickness of gas-assisted extruded plastic micro-tubes increase.With the increasing of traction speed,the outer and inner diameters and wall thickness linearly decrease.With the increasing of outer gas-assisted pressure,the surface of gas-assisted plastic micro-tubes appears the waveform phenomenon.With the increasing of inner gas-assisted pressure,the outer and inner diameters increase but the wall thickness decrease.At the same time,the impacts of melt and gas' s temperatures on the smoothness,transparency and gas-assisted stability of plastic micro-tubes are prominent;2)Based on the orthogonal experiments,the optimal process parameters are obtained,i.e.,the temperatures of gas and melt are 230°C,screw speed is 5.55r/min,traction speed is 27mm/s,inner gas-assisted flow rate is about 0.055m3/h,outer gas-assisted flow rate is about 0.8m3/h,inner gas-assisted pressure is about 65 k Pa,outer gas-assisted pressure is about 150 k Pa;3)The profiles and surface qualities of gas-assisted plastic micro-tubes are obtained by using the macroscopic and microcosmic method,which proves that the gas-assisted method well eliminates the extrudate swell,melt fracture and extrudate distortion problems of traditional without gas-assisted extrusion forming of plastic micro-tubes,and the product qualities are greatly improved.