Rheology in hollow fiber drawing process

Among the many stretching processes, the continuous drawing of melt-spun fibers is an important step in the fiber forming process, and the optimization of drawing process to achieve desired final properties has been an essential part of the process development studies. In continuous drawing, the machine variables such as applied draw ratio and drawing velocity do not have a unique correlation with the final fiber properties, since the fiber elements actually go through the various state of stress-strain-strain rate along the drawing path for a given machine variables. The local variables, thus, at every position of drawing path must be investigated to understand the deformation behavior and control the final properties of drawn fibers. Melt spinning is a kind of technique to manufacture hollow fibers and it is usually followed by drawing process in a consecutive or separated operation. In drawing of hollow fibers, not only are the drawing force and stress distribution along the drawing path which affect the microstructure of fibers of particular interest, but also the profile development and hollowness of final drawn fiber are of significant value. The cold stretching of melt-spun polypropylene hollow fiber is particularly important due to its special end use such as filters. Thus, the analysis on the deformation mechanism of continuous drawing is required, and the prediction of local variables with respect to material properties can be used to account for the drawing behavior of PP hollow fibers with different initial microstructures. In the present work, in order to predict the trends of strain localization in the continuous drawing as a function of material properties of undrawn fibers, simple model describing continuous drawing process was presented. The numerical analysis was performed to find out the relation between the rheological property of undrawn filament and the significant drawing parameters in continuous drawing: drawing force, intensity of strain localization and position of inflection point. Experiments showed that the initial microcrystalline structure of undrawn PP hollow fibers affects the neck geometry, and this result was discussed with the numerical data in terms of strain rate sensitivity and strain hardening effects. The Hollowness that is the main characteristic of the hollow fibers was also observed from experiments.