Studies on structural foam processing. III. Bubble dynamics in foam extrusion through a converging die

An experimental and theoretical study was carried out to achieve a better understanding of bubble dynamics in foam extrusion through a converging die. For the experimental study, a number of converging channels were constructed of aluminum, with glass windows on both sides. Bubble dynamics in the flow channel were recorded on movie film as a gas‐charged molten polymer was extruded. The dies had various converging angles (30, 45, 60, 90, and 150 degrees), and the polymer was polystyrene. As blowing agent, sodium bicarbonate (generating CO 2 ) was used. It was found that the gas bubbles moving along the centerline of the channel grow initially at the upstream end of the die, and then start to collapse as the gas‐charged molten polymer approaches the exit plane of the die. In order to help interpret the experimental results, a theoretical analysis was made of bubble dynamics in a converging channel, in which a thread‐like bubble was assumed to flow along the centerline of the converging channel and the Coleman‐Noll second‐order fluid model was assumed to describe the rheological behavior of the polymer melt. Some mathematically convenient simplifying assumptions not‐withstanding, the theoretical analysis corroborates the experimental observations. The practical significance of the present investigation is discussed in connection with controlling the cell structure in extruded foam products.