Injection system with closing rod (2) provided with a cleaning device (6, 7, 8) suitable for removing the powder of molten plastic that builds up in the movement areas of the closing rod (2), in particular in the thrust bearing (3), during the normal operation of a molding machine. Advantageously the system according to the invention avoids the limitation of the piston stroke and the subsequent blockage with time and thus also the blockage of the closing rod, considerably reducing the number of maintenance operations and thus increasing the productivity of the molding machine.