This paper investigates the critical plunger velocity in high-pressure die casting during the slow phase of the piston motion and how it can be determined with computational fluid dynamics (CFD) in open source software. The melt-air system is modelled via an Eulerian volume-of-fluid approach, treating the air as a compressible perfect gas. The turbulence is treated via a Reynolds-averaged Navier Stokes (RANS) approach that uses the Menter SST k-ω model. Two different strategies for mesh motion are presented and compared against each other. The solver is validated via analytical models and empirical data. A method is then presented to determine the optimal velocity using a two-dimensional (2D) mesh. As a second step, it is then discussed how the results are in line with those obtained for an actual, industrially relevant, three-dimensional (3D) geometry that also includes the ingate system of the die.