NECKING IN HIGH-SPEED SPINNING REVISITED

The conditions required for ''necking'' in melt-spinning are discussed. The adopted criterion defines necking as a filament-radius profile R(x) exhibiting two inflexion points (R'' = 0), one characterized by a negative, the other one by a positive, third gradient R'''. Analysis of the dynamics of spinning of a non-isothermal corotational Maxwell fluid leads to the conclusion that the conditions of necking require either long relaxation times (Deborah number > 1/2), or a large, positive tension gradient F' -possibly controlled by inertia. In inertialess Newtonian filaments, the necking condition can also be satisfied by local reduction of viscosity. The above three mechanisms of necking, viscoelastic, inertial, and viscous, are analyzed and discussed.