Influence of the flow rate on the die swell for polymer micro coextrusion process

Abstract A numerical model for a polymer double-layer single lumen microtubes coextrusion flow was constructed and the finite element method was used for numerical solution. The distribution of the velocity, pressure, shear rate, etc., were analyzed, as well as the relationship between the normal stress difference, secondary flow, and die swell were explored. Then, the influence of the flow velocity difference on the die swell of the double-layer microtubes was revealed. The research results indicate that, in the traditional coextrusion process, the melts will undergo expansion and deformation after leaving the die. And the velocity difference caused by the increase in the outer layer melt velocity will cause the outer layer to squeeze the inner layer, resulting in an uneven wall thickness of the composite microtubes. In the gas-assisted coextrusion process, the distribution of the velocity, pressure, shear rate, etc., can be effectively improved. However, the extrusion deformation between the melt layers caused by the velocity difference cannot be eliminated. That is, the gas-assisted technology can effectively improve the overall shape and size accuracy of composite microtubes, but cannot solve the problem of uneven wall thickness caused by the velocity difference.