Effects of die shape on polymer processing characterized in a new light

When it comes to processing polymers, forces that destabilize polymer flow and cause melt fracture phenomena are a major limiting factor to faster production. The geometry of the die used to cast the polymer product and the molecular weight of the polymer itself have been identified as key aspects that significantly affect the flow instability phenomena in polymer processing. A new paper published in Physics of Fluids showcases a new technique manufacturers can use to characterize production outcomes and potentially save costs by analyzing these properties before large scale productions.

Using widely available capillary rheometers for analyzing flow characteristics, the team was able to evaluate the processability of linear low-density polyethylenes as they flow through rod-producing capillary dies, film-producing slit dies and tube-producing annular dies. The researchers used parallel-plate rheometry and optical microscopy to determine the rheological flow characteristics of three polyethylenes and detect defects in the resulting products respectively, allowing them to evaluate how each die performed at different production rates.

Their results revealed that at high production rates slit dies performed better than capillary dies, while annular dies with their ringlike geometry were found to not experience flow instabilities until higher shear rates and to eliminate stick-slip defects entirely. Their work revealed the polymers’ molecular weight characteristics correlated well with critical conditions and thus allowed for inferences to be made about their performance at higher production scales. They hope to continue their work with rapid polymer evaluation and expand their findings to other types of polymers.

Source: “Melt fracture of linear low-density polyethylenes: Die geometry and molecular weight characteristics,” by Marzieh Ebrahimi, Tanja Tomkovic, Guochang Liu, Antonios A. Doufas, and Savvas G. Hatzikiriakos, Physics of Fluids (2018). The article can be accessed at https://doi.org/10.1063/1.5029380 .