Nozzle piston and method of operating said nozzle piston

A molding system for hydraulic blow molding of a container from a preform, the molding system comprising:

• a pressurization unit that is configured to receive liquid molding medium from a source;
• a fill head unit coupled to the pressurization unit to receive the liquid molding medium therefrom;
• a mold assembly having mold halves defining a mold cavity in the shape of the container, the mold assembly being configured to receive the preform therein, wherein the fill head unit is moveable to engage at least one of the mold assembly and the preform;
• and
• the fill head unit including a housing, a nozzle piston and a seal pin, the nozzle piston being moveably received within a bore of the housing and being moveable within the bore between retracted being disengaged from the mold assembly or preform and extended position engaged with the mold assembly or preform, the seal pin being slideably received within the nozzle piston for movement between an opened position and a closed position, in the closed position an outlet of the nozzle piston being closed off by the seal pin, the nozzle piston further defining a receiving chamber located between the nozzle piston and the housing that is coupled to the pressurization unit to receive the molding medium, the receiving chamber having a constant volume during movement of the nozzle piston between the retracted and extended positions within the housing, in the open position the outlet of the nozzle piston being unobstructed by the seal pin and in fluid communication with the receiving chamber, the nozzle piston further including a leading head in liquid sealing engagement with the bore of the housing during movement of the nozzle piston between the retracted and extended positions.