Die drool theory

When molten plastic is extruded from a die, it sometimes collects on the open face of the die. Known as die drool, this phenomenon costs plastics manufacturers by requiring die cleaning. This has been attributed to many causes, but none of these has led to an equation for the drool rate. In this work, we provide an exact analytical solution for the drool rate, and we base this solution on a postulate of a cohesive slip layer near the die walls. We thus attribute die drool to cohesive failure within the fluid at an internal surface where the fluid slips on itself. We adimensionalize the drool rate with the production rate, and call this the buildup ratio, BR. We provide an exact analytical solution for BR when the cohesive slip layer either slips at the die wall, or when it does not. We examine two important extrusion geometries: slit (which we then extend to pipe) and tube flow. We identify two new experiments: one to measure BR as a function of pressure drop, and another as a function of the die aspect ratio, and we then use our new theory to design droolometers.