1999 Volume 11 Issue 11 Pages 937-944
An experimental visual observation and theoretical study were carried out to investigate the dynamic behavior of bubble nucleation and growth in a foam extrusion die. In this study, a mixture of polypropylene and isobutane was extruded through a slit die with glass windows, and gas bubbles in the flow channel were observed using a CCD camera and a high-speed video with a long-distance microscope.
Some new facts were found about the macroscopic foaming behavior and the microscopic bubble nucleation behavior. Macroscopically, the foaming position, where bubbles started to grow, moved downstream towards the die exit with increasing extrusion pressure, and the foaming pressure was lower than the polymer/gas equilibrium pressure. Microscopically, bubbles started to grow at several points in the die, and there seemed two typical styles of gas bubble nucleation; formation of bubbles accompanied with a wedge shaped gas film and formation of single bullet shaped bubbles.
We also constructed a new model to simulate the bubble nucleation and growth simultaneously in a shear flow field. A threshold value in nucleation rate was introduced to explain bubble nucleation under supersaturation conditions. Numerical results showed good agreement with experiments.
