The flow of the viscoelastic polymer melt in gas-assisted extrusion die was investigated by using Polyflow software,and the equation to propose the extrusive swell ratio of product diameter was obtained. The simulation results showed that the relation of the ratio of the average residence time that polymer melt settled in the slippage stage to material relaxation time with the extrusive swell ratio was decayed exponentially. By the experiment of gas-assisted extrusion HDPE,the experimental diameter agreed w...