Mathematical Modeling and Experimental Study of Dynamics and Structure Development during High Speed Melt Spinning of Nylon-6

A mathematical model was developed to describe the high speed melt spinning behavior of nylon-6. This model is based on the equations of continuity, momentum and energy. These equations are combined with a Newtonian model for rheology to describe the process dynamics and models for the development of birefringence, molecular orientation and crystallization kinetics to describe the structure formation during high speed melt spinning of nylon-6. Physical property relationships from the literature are input and used together with the processing conditions to calculate velocity, diameter, temperature, stress, birefringence and crystallinity profiles along the spinline.

Online experimental techniques were used to obtain actual temperature, diameter and birefringence profiles. Two different molecular weight resins and two different mass throughputs were studied. Comparison of model predictions with experimental profiles showed that the trends predicted by the model were consistent qualitatively, but there were quantitative disagreements.

To obtain quantitative agreement, an analytical inversion procedure was developed in which online experimental data were utilized to generate key relationships for elongational viscosity and heat transfer coefficients. Utilization of these relationships in the model, provided a good agreement between experimental and predicted diameter and temperature profiles in the absence of crystallization. Subsequent adjustment in the parameter that controls the effect of orientation on crystallization rate provided good agreement between experimental and predicted diameter, temperature and birefringence profiles, including the effects of crystallization with molecular orientation.

The model predictions and online experimental profiles indicate that high spinning speeds produced high spinline stresses and increased molecular orientation which induced crystallization in the spinline. Increasing the molecular weight of the resin produced higher spinline stresses (due to high melt viscosity) and induced online crystallization at a lower critical take-up velocity or at a higher temperature and smaller distance from the spinneret at chosen high take-up velocities. Under some spinning conditions (high speeds and lower throughput) the energy released by crystallization increased the local filament temperature which decreased the melt viscosity, allowing rapid local deformation or neck formation in the spinline