The present invention relates to a hybrid process for producing molded plastic articles by reinforcing the walls thereof through the injection of a foamed thermoplastic material, thus increasing the thickness of the wall in previously determined zones in order to improve the mechanical characteristics thereof. According to the invention, said hybrid process comprises to strengthen previously defined zones of the plastic articles formed by any of the already known processes, such as: injection, extrusion, extrusion-blowing, injection-blowing, thermoforming, roto-molding, or any combination thereof; and afterwards the injection of a foamable thermoplastic material to fill up said strengthening predesigned zones.