Dynamics of steady and unsteady melt spinning

The present work involves an overall study of isothermal melt spinning, including both steady state and unsteady state spinning ("draw resonance"). A systematic study of two commercial polypropylenes was carried out as a function of the variables which one normally associates with draw resonance and in addition the variables which have to do with the die flow. An integral viscoelastic analysis for steady spinning developed by Matsui and Bogue, which includes the shear rate in the die as a parameter, was solved numerically for comparing with data from steady spinning and also as a guide for correlating data in unsteady spinning.

The computer analysis gives predictions for the die swell and the velocity distribution along the spinline which are in good agreement with the experimental results and also with several differential analyses based on a convected Maxwell model. The computer solution is a more general solution than those based on the differential models, since the latter do not include the effect of the shear rate in the die. An important result from the steady analysis is that a maximum force occurs as the draw ratio is increased, provided that the rheological parameter which governs "thinning behavior" is sufficiently large.

The experimentally observed draw ratios for the onset of draw resonance and also the periods of the fluctuations are in the range of existing theoretical predictions. Despite this general agreement, however, there are clearly deviations which are related to the shear flow in the die. In order to deal with this die flow effect, several correlating schemes were investigated. The most successful involved a ratio between the tensile force in the die and that in the spinline which somehow reflects the "mismatch" between the die flow and the spinline flow; the larger this ratio, the less stable is the spinline.