An injection apparatus for injecting molten plastic into a mold of articles made of thermoplastic material including bottle preforms. The injection apparatus includes an injection nozzle and a shutter for controlling flow of the molten plastic through the injection nozzle. A cylinder-piston system moves the shutter from open and closed positions of the injection nozzle. The cylinder-piston system includes a first chamber with a first passage for introducing fluid into the first chamber to open the shutter, and a second passage, different from the first passage, for discharge of fluid and of possible particles from the first chamber. A fluid distribution device with a first channel is connected to the first passage, and a second channel different from the first channel is connected to the second passage. The cylinder-piston system includes a second chamber having a third passage for introducing fluid into the second chamber to close the shutter.