Most systems don’t leak because of poor design, but instead because they are operated outside of their designed operating window.

The most common location for leakage is at the seal-off between the nozzle and the manifold. The design criteria for a typical hot runner system in which the nozzles have a solid skirt design, requires the height of the component stack to be less than the actual pocket depth in the hot runner plates. The difference between the dimensions – known commonly as the cold clearance – is necessary to protect the components from collapsing due to thermal expansion when the system is at operating temperature.

For example: A 60-mm thick manifold with a components stack height of 40 mm (total 100 mm) will grow by 0.26 mm from room temperature to operating temperature (450xF). If there were no cold clearance the thermal expansion would cause the skirt of the nozzle to collapse.

The fact that there is no positive seal in the cold condition is the main reason for hot runner leakage. In order for the system to seal (nozzle to manifold) the operating temperature must be reached in order to create enough force (20,000 pounds, for example) to resist the injection forces that want to push the two components apart. Inexperienced operators might not wait to reach full operating temperature or, in the worst case, may even forget to turn the system’s heat on. A hot runner system designed with a cold clearance that has not reached full operating temperature will not have enough surface pressure to prevent leakage.

Leakage also can occur after the hot runner is accidentally overheated. Again, solid skirt design nozzles have little flexibility for thermal expansion. If we use the previous example with the manifold nozzle stack height of 100 mm and the system is accidentally heated to 800°F, the components will grow an additional 0.24 mm.

The force then is more than 40,000 pounds, which is above the yield strength of the skirt, ultimately causing it to deform permanently. Once an overheated system returns to its operating temperature (450xF) it will not develop the necessary seal-off pressure to prevent leakage. In addition to the damage caused by leakage of the molten plastic, all of the nozzles will need to be replaced due to the permanent damage caused by over-compression.

Since the correct preload on nozzles and the manifold is so critical it is understandable that the dimensions and tolerances provided by the hot runner supplier must be strictly adhered to in order to avoid leakage of the system. Moldmakers should carefully inspect all stack heights and pocket dimensions against the hot runner manufacturer’s print. Any questions should be discussed immediately with the hot runner manufacturer.