This work shows a design and simulation methodology of a production process to manufacture injection pieces in automobile industry, elaborated to optimise the work in a real plant, and presented through a real example. The example consists in two inserts of an instrument panel for an automobile.The departure data are the design plane of the piece, the materials and additives used, the final specifications of the piece, and the production. The simulation methodology allows us to establish the rules to develop the mold, to specify the number of cavities, to determine the solidification and the cycle times, to locate the pieces and the tapping in the mold, to calculate the clamp force necessary in the injector machine and the dimensions of the mold, to analyse the results of the simulation (with Mold-Flow), to choose the appropriate injector machine and to obtain the necessary information to develop the mold.