In this work we present a detailed description of a die design methodology aimed to improve the flow balance of complex extrusion dies, totally based on open source computer codes. The work encompasses the description of the overall design methodology, the implementation of the energy conservation equation in an existing OpenFOAM® solver, which then will be capable of simulating the steady non-isothermal flow of incompressible generalized Newtonian fluids, and its application to two industrial case studies. The new design methodology, is composed by a sequence of stages involving the use of different open-source software packages, namely: i) Free CAD – for the generation of the flow channel geometry; ii) Salome – the previous geometry is imported to this package where its boundary is divided in different groups to allow applying different boundary conditions. This boundary regions are exported as STL ASCII files from Salome; iii) SnappyHexMesh – based on the previous STL ASCII files from Salome, this package is used to generate, in a semi-automatic manner, the corresponding unstructured mesh; iv) Developed solver (OpenFOAM®) – is used to perform the numerical simulation in the tentative flow channel geometry and is followed by the post-processing, in which the predicted results are used to access the flow distribution quality. The methodology encompasses two alternative approaches to improve the flow distribution: modification of the flow channel geometry and/or boundary conditions. For each approach, an industrial case study is presented in detail. The results obtained with these case studies evidence the capabilities and practical usefulness of the developed methodology, and demonstrate that the computational design aid is an excellent alternative to the experimental trial-and-error procedure commonly used in industry.