A process for producing plastic molded articles with thickened and strengthened walls, being said process a hybrid process combining conventional plastic molding techniques with the injection of a structural plastic foam of thermoplastic materials, the process comprising the steps of: a) designing a plastic article having at least two previously molded parts (150, 190; 410', 420) forming at least one cavity or hollow zone (152, 430) to be filled out with strengthening foamed thermoplastic material (40, 440), and at least one specific injection gate; b) premolding the plastic article having at least two molded parts (150, 190; 410', 420) designed in step a), by means of a conventional molding process, wherein at least one of the at least two molded parts (150, 190; 410', 420) is premolded to at least partially define at least one cavity or hollow zone (152, 430); c) fastening together the at least two previously molded parts (150, 190; 410', 420), forming at least a molded cavity or hollow zone (152, 430) with the walls thereof; d) filling by injecting the strengthening thermoplastic material with an expanding or foaming agent (40, 440), by means of a low-pressure injection equipment, through said at least one specific injection gate, into said at least one previously molded cavity or hollow zone (152, 430), in order to fill up said cavity and in order for the foaming agent to fastening the at least two previously molded parts (150, 190; 410', 420) and to strengthen the wall thickness; e) withdrawing the plastic molded article from the low-pressure injection equipment, providing for the expansion of said strengthening thermoplastic material with an expanding or foaming agent (40, 440); and f) carrying out the cooling down of the produced plastic molded article in a storage zone or warehouse, wherein the at least one molded cavity or hollow zone (152, 430) comprises means for closing to prevent the expanding or foaming agent (40, 440) previously injected from running out of said at least one molded cavity or hollow zone (152, 430), obtaining a plastic molded article with the capability of obtain strengthened walls with thickness of from 6.0 mm to 12.0 mm. a) designing a plastic article having at least two previously molded parts (150, 190; 410', 420) forming at least one cavity or hollow zone (152, 430) to be filled out with strengthening foamed thermoplastic material (40, 440), and at least one specific injection gate; at least two previously molded parts (150, 190; 410', 420) forming at least one cavity or hollow zone (152, 430) to be filled out with strengthening foamed thermoplastic material (40, 440), and at least one specific injection gate; b) premolding the plastic article having at least two molded parts (150, 190; 410', 420) designed in step a), by means of a conventional molding process, wherein at least one of the at least two molded parts (150, 190; 410', 420) is premolded to at least partially define at least one cavity or hollow zone (152, 430); c) fastening together the at least two previously molded parts (150, 190; 410', 420), forming at least a molded cavity or hollow zone (152, 430) with the walls thereof; d) filling by injecting the strengthening thermoplastic material with an expanding or foaming agent (40, 440), by means of a low-pressure injection equipment, through said at least one specific injection gate, into said at least one previously molded cavity or hollow zone (152, 430), in order to fill up said cavity and in order for the foaming agent to fastening the at least two previously molded parts (150, 190; 410', 420) and to strengthen the wall thickness; e) withdrawing the plastic molded article from the low-pressure injection equipment, providing for the expansion of said strengthening thermoplastic material with an expanding or foaming agent (40, 440); and f) carrying out the cooling down of the produced plastic molded article in a storage zone or warehouse, wherein the at least one molded cavity or hollow zone (152, 430) comprises means for closing to prevent the expanding or foaming agent (40, 440) previously injected from running out of said at least one molded cavity or hollow zone (152, 430), obtaining a plastic molded article with the capability of obtain strengthened walls with thickness of from 6.0 mm to 12.0 mm.