A molding process and a mold for manufacturing a toothbrush of plastic material with a resistant inner core, is disclosed. It is provided to manufacture a resistant inner core, and to introduce it into a mold cavity having opposite footprints belonging to a lower half-mold and an upper half-mold and to subsequently inject molten plastic material into the mold cavity for manufacturing at least part of the toothbrush handle. A resistant metal core is placed into a respective footprint of the lower half-mold in contact with at least two support platforms, adapted to support the core in the proximity of a central line of the mold cavity, and before the injection of structural plastic material, to couple the lower half-mold with an upper half-mold having at least one ribbing to contact with the metal core at least in a position between the support platforms and in a position opposite thereto.