Influence of Gas in the Inner Gas Layer on Gas-Assisted Extrusion of Plastic Micro-Tube

The size of the plastic micro-tube is extremely small. When the melt exits the die during extrusion, it is still in molten state. The gas in the inner gas layer still flows in the cavity of micro-tube after leaving the die, which will affect the micro-tube forming. Therefore a double layer gas-assisted extrusion model considering the gas flow in the cavity of plastic micro-tube was established, focused on the influence of the flow of the gas in the inner gas layer after leaving the die on the micro-tube forming. Through the finite element numerical simulation and experimental research on the micro-tube extrusion process under the double gas layer inside and outside the tube wall, the micro-tube shape, velocity, pressure drop and first normal stress difference were obtained. The analysis show that when the gas leaves the die, the gas in the external gas layer enters the air and then decompresses instantaneously, and the gas in the inner gas layer enters the cavity of micro-tube after leaving the die, and there is still a certain pressure. The inner air pressure of the wall is larger than the outer air pressure, which leads to the diameter of the micro tube gradually increasing. The first normal stress difference is generated at the exit of the die and after the exit of the die, and the analysis shows that there is a large correlation between the micro-tube morphology, wall thickness, flow velocities and pressure drop. The geometric model considering gas flow in the cavity of micro-tube is more in line with the phenomenon that the wall surface of the micro-tube is corrugated, the wall of the micro-tube at the exit of the die is rapidly thinned and the wall of the micro-tube after the exit of the die is gradually thined when the gas pressure is large. © 2020, Editorial Board of Polymer Materials Science & Engineering. All right reserved.