Abstract: A threedimensional isothermal viscoelastic numerical model for PE-LD and PE-HD was established with PTT constitutive equation and finite element method. Using the model, conventional and gasassisted coating coextrusions and the extrusion swell of gasassisted coating coextrusion were simulated. It showed that in the gasassisted coating coextrusion the whole die swell, core and shell die swell kept at zero with the relaxation time when the unit volume flow rate of the core and shell melt were equal. However, when the core and shell melt unit volume flow rate were different，the melt with large unit volume flow rate swelled, and that with small one shrank.

Key words: viscoelastic flow, gas-assisted coat, coextrusion, die swell, die shrinkage, polyethylene