Polymer viscosity alone is important in the analysis and only through the flow index (m). Therefore, we can expect that for a given die design, that the extrusion of any different polymer with the same value of m (m= 1/n) will yield similar flow distributions while the substitution of a polymer with a different power-law index would be expected to give a different flow distribution, and therefore, thickness profile, across the die exit. We can conclude that a die manifold designed for LLDPE would be expected to give essentially the same layer-distribution results with EVOH, but we would expect that PP, HDPE and PMMA would give different layer distributions. Conversely, a die manifold designed for PP should give similar layer distributions for PMMA and HDPE but not for LLDPE or EVOH.
A key learning from this analysis is that for any given film width, a center-feed die will give better overall uniformity due to the decrease in manifold width as the results show that flow uniformity decreases with increasing manifold width, W.
Control of melt, temperature uniformity is vital
Die-design calculations are predicated on an isothermal, homogeneous melt stream of uniform flow rate entering the die, so of critical important is the process control of the polymer extrusion process to deliver a melt of uniform rate and melt temperature with time. Machine-direction variations in layer uniformity are not related to die design but only to extrusion pressure variations caused by rate or melt temperature variations. If melt temperature variations exist in the melt stream entering the die, they will cause layer-thickness distribution problems and must be eliminated. But once the extrusion process is under control and supplying a uniform melt stream to the die, it is the resin-flow properties and the die-manifold design which determine the layer distribution and, hence, the film-property uniformity across a film.
So, we have learned that the only material property that gives control of layer distribution in a die is controlled by the shape of the flow curve, or is viscosity versus shear rate behavior. This is a characteristic of the material and must be measured to ensure a sound die design.