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Porthole die life is often limited in practice by tool deformation, resulting in profiles out of tolerances. In some cases premature tool cracking may even occur. The aim of this study is to understand the in-service die stresses and strains in order to generate guidelines for die design. Numerical calculations were carried out on an industrial porthole die of a two-strand ladder profile and validated with industrial data. A fully-coupled, 3-D calculation of the aluminum flow and resulting die stresses remains a major challenge, and here the calculations were simplified by conducting 2-D flow calculations for a number of die configurations using the FORGE2 finite element code. Two different approaches were used for the 2-D flow calculations, conserving either the local extrusion ratio or the local section area/perimeter ratio. The results were used to estimate the die loading boundary conditions in 3-D. Finally, the 3-D elastic-plastic calculation for the die was performed using the MARC finite element code. The sensitivity of the results to the mesh, boundary conditions and plastic behavior was evaluated. The effects of overloading, unloading and loading-unloading cycles on the tool were also simulated. The results are compared with observations on industrial dies. Linear elastic calculations in the tool give stresses well in excess at the steel flow stress, so significant plastic deformation must be considered. The residual stresses generated in the first loading cycles are of major importance, reducing plastic deformation in subsequent cycles.