Meaning
Loss of hydraulic force that occurs as molten plastic is forced through the injection unit of a moulding machine defines the initial energy required for filling. In thermoplastic processing, the nozzle drop pressure represents the difference in melt pressure between the injection cylinder and the sprue bushing. This loss is caused by the restriction and friction encountered by the polymer as it passes through the nozzle assembly.
Accurate calculation of this variable is necessary to ensure that the machine has sufficient pressure capacity to fill the mould cavity.
Machine Calibration
Sufficient hydraulic pressure must remain available to pack the part after the initial flow path is completed. If the nozzle drop pressure is too high, the press cannot transfer enough energy to the cavity to prevent sink marks. Processors must measure this loss during dry-cycle air shots to determine the true pressure delivered to the mould.
Runner Design
The size of the flow channel in the nozzle must match the runner system of the tool. A poorly matched orifice increases the nozzle drop pressure and causes localized shear heating. This mismatch can lead to premature material degradation before the polymer even enters the runner.
Viscosity Relationship
Melt temperature and resin selection directly influence the resistance encountered during injection. A lower melt temperature increases the nozzle drop pressure because the polymer is more viscous. This change requires higher injection force to maintain the target fill rate during production.
