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The high degree of thickness uniformity of an extruded polymer tube is one of its most important geometric features. A factor that has a direct impact on this is the die, especially the mandrel. Correctly designed, it ensures, among other things, a high degree of homogenization of the processed polymer and uniform distribution of the volume flow rate along the circumference, which has a key impact on the quality of the extruded film. Due to the number of variables affecting the flow of polymer through the die, a number of simulations are required to find the optimal design. This paper presents the validation of a tool based on Chris Rauwendaal's mathematical model for performing fast numerical calculations to determine the geometric parameters of the spiral mandrel to ensure the thickness of the extruded film is as uniform as possible. Simulations were carried out based on 13 geometric and processing variables, from which 3 key geometric parameters were selected. In order to select a set of parameters ensuring the optimal distribution of polymer flow at the exit, a series of simulations were carried out for several levels of values of the aforementioned key parameters. The obtained set of geometric parameters was used to manufacture a spiral mandrel, which was used in the process of validation of the tool by experimental method.