In this study, to eliminate the extrudate swell, melt fracture and extrudate distortion problems of polypropylene (PP) micro-tube, the experiments of gas-assisted extrusion forming was firstly carried out. Meanwhile, a set of modified gas-assisted extrusion system was set-up, and a kind of novel gas-assisted die was also designed and manufactured. Under the role of gas-assisted extrusion system, PP micro-tubes were successfully prepared. Moreover, the macroscopic and microscopic qualities of PP micro-tubes for gas-assisted extrusion were obtained and compared with those of the traditional extrusion. Experimental results show that gas-assisted extrusion not only overcomes extrudate swell, but also greatly improves the smoothness and transparency of PP micro-tube. In addition, to ascertain the mechanisms of gas-assisted extrusion of PP micro-tube, numerical simulations were also performed. The swell ratios, flow velocities, pressure and stresses distributions of melt for gas-assisted extrusion and traditional extrusion were analyzed and compared. Moreover, the effects of inner cavity’s gases pressures and traction force on the gas-assisted extrusion of PP micro-tube were numerically investigated and discussed with the traditional extrusion. Numerical results show that the swell ratio of PP micro-tube increases with the inlet volume flow rate for traditional extrusion, but it is nearly equal to 0 for the gas-assisted extrusion. Moreover, due to the assistance of gas-layers, not only the axial flow velocity of melt is uniformed and radial flow velocity is removed, but also the pressure, shear stress and first normal stress difference of melt are all greatly reduced, especially at the outlet of die. These weakened pressure and stresses diminish the elastic energy storage and tensile stress of melt, and finally result in the elimination of extrudate swell, melt fracture and extrudate distortion. With the increase of inner cavity’s pressure, the inner and outer radiuses of the gas-assisted extrusion of PP micro-tube all increase, but the wall thickness decreases because the swell ratios of inner radius are all larger than that of the outer radius. In addition, under the same traction force, the sizes of gas-assisted extrusion of PP micro-tube are smaller than that of the traditional extrusion, although the sizes for the both extrusions of PP micro-tube all exponentially decrease with the increase of traction force. Moreover, the extrudate radial swell and melt fracture problems for the traditional extrusion are always existed at the outlet of die although the radial swell of melt decreases with the increase of traction force. However, the extrudate swell and melt fracture problems are greatly removed by the gas-assisted extrusion.

为了消除聚丙烯微管的挤出胀大，熔体破裂和挤出变形问题，本研究首先进行了气体辅助挤出成型的实验。同时，建立了一套改进的气辅挤出系统，并设计制造了一种新型的气辅模具。在气体辅助挤出系统的作用下，成功制备了PP微管。此外，获得了用于气体辅助挤出的PP微管的宏观和微观质量，并将其与传统挤出的相比较。实验结果表明，气助挤出不仅克服了挤出胀大，而且大大提高了PP微管的光滑度和透明度。另外，为了确定PP微管的气辅挤出机理，还进行了数值模拟。分析和比较了气体辅助挤出和传统挤出的熔体膨胀比，流速，压力和应力分布。此外，利用传统的挤出技术，对内腔气压和牵引力对PP微管气体辅助挤出的影响进行了数值研究。数值结果表明，传统挤塑工艺中PP微管的溶胀率随进口体积流量的增加而增加，而气体辅助挤塑的膨胀率几乎等于0。而且，由于气体层的辅助，不仅熔体的轴向流速均匀，径向流速被消除，而且熔体的压力，剪切应力和第一法向应力差都大大减小，特别是在模具出口处。这些减弱的压力和应力减小了熔体的弹性能量存储和拉伸应力，并最终导致消除了挤出物溶胀，熔体破裂和挤出物变形。随着内腔压力的增加，PP微管气体辅助挤出的内径和外径均增大，但由于内径的膨胀率均大于外径的膨胀比，壁厚减小。此外，在相同的牵引力下，PP微管的气体辅助挤出的尺寸比传统挤出的要小，尽管PP微管的两种挤出的尺寸都随着牵引力的增加而呈指数减小。力。此外，尽管随着牵引力的增加，熔体的径向膨胀减小，但是传统挤出的挤出物径向膨胀和熔体破裂问题始终存在于模具的出口。但是，气体辅助挤出大大消除了挤出物的溶胀和熔体破裂问题。