The diameter of a micro-tube is very small and its wall thickness is very thin. Thus, when applying double-layer gas-assisted extrusion technology to process a micro-tube, it is necessary to find the suitable inlet gas pressure and a method for forming a stable double gas layer. In this study, a double-layer gas-assisted extrusion experiment is conducted and combined with a numerical simulation made by POLYFLOW to analyze the effect of inlet gas pressure on micro-tube extrusion molding and the rheological properties of the melt under different inlet gas pressures. A method of forming a stable double gas layer is proposed, and its formation mechanism is analyzed. The research shows that when the inlet gas pressure is large, the viscosity on the inner and outer wall surfaces of the melt is very low due to the effects of shear thinning, viscous dissipation, and the compression effect of the melt, so the melt does not easily adhere to the wall surface of the die, and a double gas layer can be formed. When the inlet gas pressure slowly decreases, the effects of shear thinning and viscous dissipation are weakened, but the gas and the melt were constantly displacing each other and reaching a new balanced state and the gas and melt changed rapidly and steadily in the process without sudden changes, so the melt still does not easily adhere to the wall of the die. Thus, in this experiment, we adjusted the inlet gas pressure to 5000 Pa first to ensure that the melt do not adhere to the wall surface and then slowly increased the inlet gas pressure to 10,000 Pa to reduce the viscosity of the melt. Lastly, we slowly decreased the inlet gas pressure to 1000 Pa to form a stable double gas layer. Using this method will not only facilitate the formation of a stable double gas layer, but can also accurately control the diameter of the micro-tube.
Keywords: double gas layer; gas-assisted extrusion; micro-tube forming; shear thinning; viscous dissipation.