The basic structure of a nozzle holder assembly consists of a nozzle holder and injection nozzle. The injection nozzle consists of the nozzle body and nozzle needle. The nozzle needle is guided into the guide bore on the nozzle body. However, it is securely sealed against high injection pressures. The lower end of the nozzle needle features a seal cone that pushes the static compression spring against the conical surface of the nozzle body. The two seal cones have slightly different opening angles to produce a line contact with a high compression and good sealing effect between them.
The diameter of the needle guide is larger than the seat diameter. The hydraulic pressure of the injection pump acts on the differential surface between the needle cross-section and the surface covered by the seat.
If a combination of the sealing surface and pressure exceeds the force of the compression spring in the retaining element, the injection nozzle opens. Since the pressurized surface on the nozzle needle abruptly increases in size around the seat surface during this process, the injection nozzle "snaps" open extremely quickly if the delivery rate of the unit pump is insufficient. It only closes again when the pressure value falls below the closing pressure, which is lower than the opening pressure. This hysteresis effect is extremely important for the hydraulic stability of injection systems. The opening pressure of a nozzle holder assembly is adjusted by inserting washers under the compression springs.
The closing pressure is then determined by the geometry of the injection nozzle and the needle guide diameter to seat diameter ratio, also referred to as the pressure level.