Instabilities During Flow Through Spinneret

Two types of flow instabilities are observed when polymer fluid flows through a cylindrical path and is extmded from 

Very high extmdate swell results in improper flow of the material in the spinning line, and therefore must be minimized. It may also result in fusing of filaments if the spinneret holes are placed very close to each other. To reduce the extmdate swell, the flow conditions must be adjusted to control the elastic character of the polymer fluid. 

Solutions of polymer show much lower extrudate swell because the polymer chains are placed apart from each other due to the presence of the solvent around the chains. The elasticity of polymer solutions is very low compared to that of polymer melts. Therefore, both the diameters and the l/d ratios in spinnerets for solution spinning are often small, that is, 0.05 mm and 1, respectively. Also the spinneret holes are spaced close together. In a polymer solution, the extrudate swell increases with an increase in the polymer concentration in addition to the other factors discussed above. 

Application of the downward tensile force in the spinning zone also helps in reducing the extrudate swell of an extruded filament. This is possibly because of the applied spinning forces on the polymer network that facilitate the polymer relaxing more in the spinning direction than in the lateral directions. 

The value of critical shear rate for melt fracture of a polymer depends on its ability to move at a given set of conditions. The faster a polymer network is able to relax, the higher is its value of critical shear rate at those conditions. The relaxation time of the polymer decreases with the following parameters [6]  

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