By Dr. Eldridge M. Mount
Extrusion-die manifold design
So, how is layer distribution uniformity across a film designed in? The initial analysis of the cross-head die was performed by Carley  for isothermal, Newtonian and power law fluids. Layer uniformity, or layer distribution across the film, is primarily controlled by the design of the extrusion-die manifolds as shown in Equation 1. A complete discussion of the flow analysis  and its importance to manifold design can be found in Reference 2.
The expression for UI, the Uniformity Index, is a function of the dimensionless group aW and is dependent only on the dimensions of the die (W = manifold width, H = land gap, R = manifold radius and L = land length) and m, the flow index (or the power law index as m= 1/n) and not the fluid viscosity or the manifold pressure.
The result of this flow-uniformity analysis has all of the essential features of subsequent die-design analyses. The Newtonian and power law results show that flow uniformity is independent of the fluid viscosity and the manifold pressure and dependent only on die geometry and flow index.
So, we see that the only resin property important for controlling die-manifold design for layer uniformity is the resin flow and, in particular, the shape of the flow curve as a function of shear rate, which determines the value of m=1/n.
1. J.F. Carley, “Flow of Melts in “Crosshead”-Slit Dies; Criteria for Die Design,” J. Appl. Phys., 25, 1118-1123, (1954).
2. Mount III, Eldridge M., “Coextrusion equipment for multilayer flat films and sheets”, Chapter 7, Multilayer Flexible Packaging: second edition, J. R. Wagner ed., Elsevier, pp 99- 122, (2016).