In a plunger assembly for forming ribbons of molten glass, at least one extruder plunger is guided in a vertical direction from and towards a tank adapted to contain a mass of molten glass from a guide head carried by a support arm kept fixed during the forming of the bead. The extruder plunger is displaced by alternated cyclic motion with respect to the support arm according a predefined law of motion from one drive unit displaced above the support arm and at least partly above the tank.