System for operating a fluid actuator

System comprising:

• an actuator that is provided with a piston movably mounted to be moved linearly by the thrust of a fluid within two chambers separated by the piston,
• a needle shutter connected to the piston for regulating the injection flow of molten material from a nozzle into a mold,
• so that the shutter can move from a closing position, wherein there is no passage of molten material through the nozzle, to an opening position, wherein there is passage of molten material through the nozzle, and vice versa;
• a supply circuit for bringing pressurized fluid from a reservoir to the chambers with a flow direction and an opposite flow direction, the two fluid flow directions corresponding to a forward and a backward movement of the piston, respectively;
• and
• a solenoid valve, connected to the supply circuit and configured \so that in resting condition it leaves open a fluid drainage path that connects the chambers and the reservoir to allow a flow of fluid between the chambers and the reservoir determining a movement of the piston that brings the shutter to a closing position.