Meaning
Intentional clearance between the hot runner nozzle and the manifold ensures a tight seal when the system is hot. This nozzle preload gap is machined into the assembly to account for the thermal expansion of the steel. As the manifold reaches its operating temperature, it grows and closes the gap, compressing the nozzle against its seat.
It prevents the leakage of molten plastic into the internal spaces of the hot runner.
Thermal Expansion
Predictable growth of the metal components is the basis for the design of the assembly. The nozzle preload gap is calculated based on the length of the nozzle and the temperature difference between the room and the melt.
Sealing Force
Compression of the nozzle creates the pressure needed to stop resin from bypassing the seals. If the nozzle preload gap is too large, the expansion will not be enough to close it and the system will leak. If it is too small, the force will be excessive and can crush the nozzle or the manifold plate.
Installation Accuracy
Measuring the actual gap during the assembly of the tool is a critical quality control step. Technicians use feeler gauges or depth micrometres to confirm that the nozzle preload gap matches the design specifications. This verification prevents failures during the first heat-up of the mould.
Correct gaps ensure that the manifold floats freely until the target temperature is reached. Digital measurement tools provide the precision required to verify these clearances.
