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 .